Past Seminars
Optically driven effective electron-electron attraction in a model with nonlinear electron-phonon interaction
June 15, 2023 (Thurs.) at 3:00PM (ET)
University of Ljubljana, Slovenia
We investigate a Holstein-like model with two electrons nonlinearly coupled to quantum phonons. Using an efficient method based on full quantum approach [1-4] we simulate the dynamical response of a system subject to a short spatially uniform optical pulse that couples to dipole-active vibrational modes. Nonlinear electron-phonon coupling can either soften or strengthen the phonon frequency in the presence of electron density. In the atomic limit, both cases lower the energy of the doubly occupied site compared to two singly-occupied ones [5]. When two electrons are free to propagate on a lattice subject to non-linear coupling to phonons that soften phonon frequency, an external optical pulse with well tuned frequency can induce attraction between electrons. Electrons remain bound long after the optical pulse is switched off. Changing the frequency of the pulse the attractive electron–electron interaction can be switched to repulsive. Two sequential optical pulses with different frequencies can switch between attractive and repulsive interaction.
References:
[1] J. Bonca, S. A. Trugman, and I. Batistic , Phys. Rev. B 60, 1633 (1999).
[2] J. Bonca, T. Katrasnik, and S. A. Trugman, Phys. Rev. Lett. 84, 3153 (2000).
[3] D. Golez, J. Bonca, L. Vidmar, and S. A. Trugman, Phys. Rev. Lett. 109, 236402 (2012).
[4] J. Kogoj, M. Mierzejewski, and J. Bonca, Phys. Rev. Lett. 117, 227002 (2016).
[5] D. M. Kennes, E. Y. Wilner, D. R. Reichman, and A. J. Millis, Nature Physics 13, 479 (2017).
Excitonic condensation and superconductivity in kagome metals
May 10, 2023 (Wed.) at 1:30PM (ET)
Boston University
The kagome lattice is a network of corner-sharing triangles. Historically, the kagome structure has been closely studied in the context of insulating materials; due to the geometric frustration of the lattice, such systems are expected to host exotic magnetic states. Recent interest in metallic kagome systems has undergone a crescendo with the discovery of topological metal AV3Sb5, a superconductor with Tc ~ 2.5 K and a variety of novel ordered states at higher temperatures, including charge density wave (CDW) and nematic order. Initial theoretical work proposed that the charge density wave state may break time-reversal-symmetry (TRS), but experimental reports have conflicted – with some experiments indicating the presence of CDW but no TRS breaking.
Recent ARPES measurements have demonstrated the existence of twofold van Hove singularities near the Fermi level – a pair of saddle points with opposite concavity – the result of which are two hexagonal Fermi surfaces, one electron-like and the other hole-like. In this talk I will discuss some theoretical consequences of the Fermi surface structure in these materials [1]. The presence of an electron and hole Fermi surface results in a strong tendency towards the formation of a condensate of excitons, i.e. electron-hole pairs, which I show may coexist with charge density wave order. The dominant excitonic state is chiral d-wave, resulting in the spontaneous breaking of TRS. I shall discuss some phenomenological properties of this state and argue that these materials exhibit a coexisting phase of CDW and excitonic order, explaining the apparent independence of TRS breaking and CDW order.
Time permitting, I will discuss some possible mechanisms for superconductivity arising from the topological properties of these materials. The presence of Dirac points results in a novel interference effect between states of opposite momenta, through which the system can lower the energy due to Coulomb repulsion by forming a superconducting state [2]. A likely candidate phase is a multi-band extended s-wave state, which exhibits higher-order topology.
[1] H. S. Scammell and J. Ingham et al., Nature Communications 14 605 (2023).
[2] T. Li, M. Geier, J. Ingham and H. S. Scammell, 2D Materials 9 (1), 015031 (2022); H. S. Scammell, J. Ingham, M. Geier, and T. Li, Phys. Rev. B 105 (11), 115302 (2022).
Realizing Z2 Quantum Spin Liquids in Quantum Simulators
April 19, 2023 (Wed.) at 1:30PM (ET)
Boston University
Quantum spin liquids (QSLs) have gathered significant attention due to their unique properties that cannot be characterized by traditional Landau theory and spontaneous symmetry breaking. Their ground states lack magnetic ordering, but instead display topological order. However, they have not been unambiguously observed in the natural materials, and an active search of QSLs in the labs is still ongoing. We propose to simulate Z2 QSLs using programmable quantum devices. In this talk, I will discuss how to simulate Z2 quantum spin liquids using combinatorial gauge symmetry -- a framework that allows for the exact Z2 local symmetry in QSLs using only local two-body interactions. I will then show how to use D-Wave machine to realize Z2 QSLs states in its classical limit at the endpoint of quantum annealing protocol. Finally, I will discuss how to probe the fractional statistics of the quasiparticle excitations in Z2 QSLs using quantum simulators. Observing the fractional statistics serves as a clear indication of realizing QSLs.
Long-time tails and short-wavelength oscillations in quantum hydrodynamics
April 18, 2023 (Tue.) at 1:30PM (ET)
University of Maryland
"Quantum hydrodynamics" is the coarse-grained transport of conserved quantities in strongly interacting quantum systems. For lattice systems at high temperature, this transport is generically diffusive. But at long times, nonlinear corrections to the diffusion equation can substantially modify the dynamics, leading to so-called hydrodynamic long-time tails, and at short wavelengths, experiments show oscillatory behavior inconsistent with diffusion simpliciter. I will discuss numerical simulations of the dynamics of a 1D non-integrable spin chain that treat both of these regimes. After briefly discussing the two numerical methods, "density matrix truncation" and "operator size truncated dynamics", I will give numerical evidence that long-time tails do not affect computations of the diffusion coefficient in systems with only one conserved quantity, and give a heuristic explanation in terms of the hydrodynamic effective theory. I will then show that the numerics give short-wavelength oscillatory behavior, explain this behavior in terms of the hydrodynamic effective theory, and give a toy model for the microscopic origin of these oscillations.
Quantum order and criticality from measurement and feedback
Mar. 22, 2023 (Wed.) at 1:30PM (ET)
Perimeter Institute
Recent advancement of near-term quantum devices has provided a bottom-up approach to explore fascinating quantum phenomena which may be challenging to find in materials. In this talk, I will discuss how to efficiently realize quantum orders and criticality with experimentally-available ingredients, namely, local unitary operations and mid-circuit measurements. Our construction relies on adaptive protocols, which involve measurements and unitary feedback conditioned on the measurement outcomes. In the first part, I will introduce three classes of adaptive protocols inspired by three distinct physical insights, including tensor networks, entanglement renormalization, and parton construction. These protocols allow for the deterministic preparation of a large class of topological orders in constant time, and critical states/non-abelian topological orders in depth scaling logarithmically with system size. In the second part, I will show how to design quantum channels based on adaptive protocols to realize mixed-state orders and criticality. As an illustration, symmetry-protected topological order can be universally converted into mixed-state long-range order, which can undergo a mixed-state phase transition. Furthermore, I will provide a notable example where novel mixed-state quantum criticality can emerge from a gapped state of matter in constant depth via fermion occupation measurements and feedback.
Probing the topological and geometric excitations of fractional quantum Hall systems on digitized quantum computers
Mar. 29, 2023 (Wed.) at 1:30PM (ET)
Western Washington University
ntermediate-scale quantum technologies provide new opportunities for scientific discovery. Yet, they also pose the challenge of identifying problems that can take advantage of such devices despite their present-day limitations. In solid-state materials, fractional quantum Hall phases continue to attract attention as hosts of charged topological excitations, exhibiting anyonic statistics and neutral geometrical excitations analogous to gravitons. However, direct implementation of anyonic braiding and detection of the graviton mode remain challenging. Here, we identify a quasi-one-dimensional model that allows us to simulate (1) quasihole braiding and (2) a geometric quench on the IBM quantum computer. The quantum algorithms are executed on the IBM device, with advanced error mitigation, providing experimental signatures of the two phenomena above.
Linear Growth of Complexity in Brownian Circuits
Mar. 15, 2023 (Wed.) at 1:30PM (ET)
Brandais University
Generating randomness efficiently is a key capability in both classical and quantum information processing applications. For example, Haar-random quantum states serve as primitives for applications including quantum cryptography, quantum process tomography, and randomized benchmarking. How quickly can these random states be generated? And how much randomness is really necessary for any given application? In this talk, I will address these questions in Brownian quantum circuit models, which admit a large-$N$ limit that can be solved exactly. Using path integrals methods I demonstrate that Brownian quantum systems have circuit complexity that grows linearly with time. In particular, I present a calculation of the $k$th Frame Potential in this model and show that it comes within $\epsilon$ of the Haar value after a time of order $t \sim k N + k \log k + \log \epsilon^{-1}$. This implies that the Brownian circuits come very close to a unitary $k$-design after a time of order $t \sim k N$. These same Brownian circuit models are also applicable to other salient problems in many-body dynamics, including measurement-induced phase transitions, dissipative quantum state engineering, and the design of new continuous-variable quantum error-correcting codes. I will conclude by reviewing some of these applications and suggesting some interesting future directions.
RG quantum circuits
Feb. 8, 2023 (Wed.) at 1:30PM (ET)
Ethan Lake
MIT
Suppose one is handed the ground state wavefunction of a local Hamiltonian. How does one determine the phase of matter this ground state belongs to? In this talk, I will describe a way of answering this question for 1d symmetry protected topological phases using `RG quantum circuits': unitary circuits that test whether or not their inputs belong to a particular phase. These circuits are constructed from only a small amount of universal data, and simulate renormalization group flow with the help of a simple error correction protocol. This talk is based on arXiv:2211.09803 with Shankar Balasubramanian and Soonwon Choi.
Finite energy properties with tensor networks
Feb. 22, 2023 (Wed.) at 1:30PM (ET)
Munich Center for Quantum Science and Technology
Ground state and thermal equilibrium properties of local quantum many-body systems can be explored with tensor networks, thanks to their area law entanglement. But highly excited states or out-of-equilibrium setups are much harder.
Energy filters allow us to access properties of the system at finite energy densities. They can be efficiently realized by quantum simulators or computers, which simulate the quantum dynamics, combined with classical filtering and sampling. But also replacing the quantum evolution by its classical simulation with tensor networks provides a new tool to classically compute dynamical properties of much larger systems than allowed by other methods.
Fractonic Behavior in Two Dimensions
Feb. 15, 2023 (Wed.) at 1:30PM (ET)
Guilherme Delfino Silva
Boston University
In this talk I provide a pedagogical introduction to fractons, a class of topologically ordered systems where fractionalized excitations with restricted mobility emerge. Fractonic systems present unusual physics, as several physicists’ favorite tools (as the renormalization group) cannot be applied to them. These systems fail to possess decoupled UV and IR sectors - a phenomenon called UV/IR mixing - where the ground state degeneracy (IR physics) is sensible to lattice details (UV information). We find the literature several works providing evidence that we cannot have fractons in 2D, as there are strong bounds on how the ground state degeneracy can scale with the lattice size. We illustrate however, with an exactly solvable example, that quasi-fractons can still emerge in 2D, displaying several features akin to their three-dimensional cousins.
Weak ergodicity breaking, many-body scars and superdiffusive energy transport in the PXP model
Dec 7, 2022 (Wed.) at 1:30PM (ET)
Zlatko Papic
University of Leeds
Universal nonequilibrium properties of isolated quantum systems are typically probed by studying transport of conserved quantities, such as charge or spin, while transport of energy has received considerably less attention. I will present results of our recent study [arXiv:2210.01146] of infinite-temperature energy transport in the kinetically-constrained PXP model describing Rydberg atom quantum simulators. The numerics in large systems reveal the existence of two distinct transport regimes. At moderate times, the energy-energy correlation function displays periodic oscillations due to families of eigenstates forming different representations of su(2) algebra, hidden within the spectrum. These families of eigenstates generalise the quantum many-body scarred states found in previous works and leave an imprint on the infinite-temperature energy transport. At later times, we observe a broad superdiffusive transport regime that we attribute to the proximity of a nearby integrable point. Intriguingly, strong deformations of the PXP model by the chemical potential do not restore diffusion, but instead lead to a stable superdiffusive exponent close to the Kardar-Parisi-Zhang value.
Embedding schemes for correlated quantum systems away from equilibrium
Nov. 30, 2022 (Wed.) at 1:00PM (ET)
SUNY Buffalo
The nonequilibrium dynamics of correlated systems remains the subject of intense activity both from the experimental and from the theoretical point of view. On the one hand, the emergence of novel phases of matter that occur when systems are driven away from equilibrium and, the development of time-dependent spectroscopy experiments that drive the system away from equilibrium with a strong pump and then assess the resulting excitations with a probing field, require tools that are valid beyond the linear response regime. On the other hand, strongly correlated systems away from equilibrium exhibit intriguing fundamental behaviors. Furthermore advances in the ability to trap and control ultracold atomic gases in optical lattices enable the simulation of various nonequilibrium models. In this talk, we will first discuss the application of the nonequilibrium dynamical mean field theory (DMFT) to DC field-driven systems. Next, we will introduce our recent extension of a combination of DMFT and the coherent potential approximation (CPA) for the treatment of nonequilibrium dynamics of a correlated system in the presence of disorder. Overall, these embedding schemes are powerful tools to understand properties of correlated quantum systems away from equilibrium in general and, in particular, the interplay of the interaction and disorder in the dynamics of these systems.
Generating long-range entanglement through a stochastic measurement-only circuit
Nov. 16, 2022 (Wed.) at 1:30PM (ET)
Harvard University
Realizations of long-range entangled states such as quantum spin liquids are challenging because of numerous factors of complications in solid state materials. Digital quantum simulators, on the other hand, have recently emerged as a promising platform to controllably simulate exotic phases. I will talk about a constructive design of long-range entangled states in this setting, and exploit competing measurements as a new source of frustration to generate spin liquid. Specifically, we consider random projective measurements of the anisotropic interactions in the Kitaev honeycomb model. The monitored trajectories can produce analogues of the two phases in the original Kitaev model: (i) a topologically-ordered phase with area-law entanglement and two protected logical qubits, and (ii) a “critical” phase with a logarithmic violation of area-law entanglement and long-range tripartite entanglement. A Majorana parton description permits an analytic understanding of these two phases. Extensive numerical simulations of the monitored dynamics confirm our analytic predictions.
Driven Magnets: Archimedean screws, time crystals and flying domain walls
Nov. 9, 2022 (Wed.) at 1:30PM (ET)
University of Cologne, Germany
We explore the physics of magnets driven out of equilibrium by magnetic fields oscillating in time. Even for small driving amplitudes, magnets with Goldstone modes are converted into "active matter", showing a range of qualitatively new phenomena. For example, chiral magnets with a helical magnetic order are transformed into Archimedean screws able to pump charge, spin or energy. Domain walls start to fly, and lattices of magnetic whirls, so-called magnetic skyrmions, start to rotate [2]. When the amplitude of the oscillating field is increased we show theoretically how this leads to the emergence of magnonic time crystals.
[1] Nina del Ser, Lukas Heinen, Achim Rosch, SciPost Phys. 11, 009 (2021).
[2] Phoebe Tengdin et al., Imaging the ultrafast coherent control of a skyrmion crystal, arXiv:2110.04548.
Probing topology and correlations of quantum materials in strong laser fields
Nov. 2, 2022 (Wed.) at 1:30PM (ET)
Denitsa Baykusheva
Harvard University
The interaction of intense ultrafast electromagnetic fields with matter brings about a variety of interesting phenomena. One of them is high harmonic generation (HHG), which has been extensively harnessed in the past decade to interrogate the structure and dynamics of atoms, molecules, and, more recently, solids, on a (sub-)femtosecond time scale. Focusing on the paradigmatic 3D topological insulator Bi2Se3, I will first discuss the potential of HHG driven by circularly-polarized fields as a probe of topological band structures. I will then examine how light-induced changes to the band structure dynamically modify the HHG emission spectrum. HHG in solids has traditionally been interpreted within the single-particle picture, where the electronic bands are assumed to remain “frozen” during the light-matter interaction. However, recent theoretical work [1] has challenged this notion and revealed that the onsite Coulomb repulsion in strongly correlated systems (Hubbard U) can be substantially modified by strong non-resonant laser fields and lead to a dramatic reshaping of the HHG spectrum. By combining time-resolved x-ray absorption experiments and exact diagonalization calculations, I will illustrate how intense femtosecond pulses selectively induce a transient renormalization of the Hubbard U in two prototypical cuprate superconductors – the quasi-1D compound Sr2CuO3+d, and the quasi-2D La2-xBaxCuO4. This result has far-reaching implications for HHG, for the control of superconductivity and magnetism, as well as for the realization of other long-range-ordered phases in light-driven quantum materials.
[1] N. Tancogne-Dejean, M. A. Sentef, and A. Rubio, Phys. Rev. Lett. 121, 097402 (2018)
Interplay of Topology and Geometry in Fractional Quantum Hall Liquids
Oct. 26, 2022 (Wed.) at 1:30PM (ET)
Florida State University and National High Magnetic Field Lab.
Fractional Quantum Hall Liquids (FQHL) are the ultimate strongly correlated electron systems, and the birth place of topological phase of matter. Early theoretical work has emphasized the universal or topological aspects of quantum Hall physics. More recently it has become increasingly clear that there is very interesting bulk dynamics in FQHL, associated with an internal geometrical degree of freedom, or metric. The appropriate quantum theory of this internal dynamics is thus expected to take the form of a “quantum gravity”, whose elementary excitations are spin-2 gravitons. After briefly reviewing the topological aspect of FQHL, I will discuss in this talk how to probe the presence of this internal geometrical degree of freedom experimentally in the static limit, and detect the graviton excitation in spectroscopic measurements, in particular how to reveal its chirality. Comparison will be made with recent experimental and numerical work, and discussions on future experimental probe of the graviton chirality as well as its significance will be presented.
Topological Liquids from Non-Equilibrium Dynamics, or: How I Learned To Stop Worrying and Love the Small Gap
Oct. 19, 2022 (Wed.) at 1:30PM (ET)
Harvard University
While spin Hamiltonians can form topological liquids in their ground states, their requirements are rather demanding. In particular, such two-body Hamiltonians typically have small gaps above the ground state, implying both a sensitivity in tuning parameters as well as a challenge for cooling. In this talk, we show how a small energy gap can be made into a feature rather than a bug: non-equilibrium dynamics enables a much more straightforward preparation of liquid states of particular non-thermodynamic sizes. More generally, we will explore how quantum dynamics can effectively implement the projection operator, thereby transforming trivial states into exotic ones. Time permitting, we use this to elucidate the emergence of a Z2 spin liquid-like state in recent Rydberg atom tweezer array experiments, which also naturally points the way to vast generalizations.
Engineering superconducting quantum circuits robust to noise
Sept 28, 2022 (Wed.) at 1:30PM (ET)
Agustin Di Paolo
Research Laboratory of Electronics, Massachusetts Institute of Technology
Artificial atoms realized by superconducting circuits offer unique opportunities for storing and processing quantum information with high fidelity. Ideally, these artificial atoms should behave robustly against environmental noise by implementing a noise-protected qubit subspace. Engineering noise-protected systems is, however, challenging, as it requires simultaneously guarding against errors while ensuring universal high-fidelity qubit control. While partial noise protection is possible in single-mode superconducting circuits, such as the transmon or the fluxonium qubit, achieving complete noise insensitivity necessitates circuits of larger complexity. In this talk, we discuss principles for designing noise-protected qubits and describe some of the recent theoretical and experimental work in this direction.
Tuning magnetic symmetries and topology with bicircular light
June 1st, 2022 (Wed.) at 1:30PM (ET)
Iowa State University
Light-matter interaction is a powerful tool to manipulate the electronic properties of materials. Coherent light can be carefully tailored to break selected symmetries and stabilize phases of matter otherwise absent. This technique is known as Floquet engineering and has become a central topic in condensed matter physics. In this talk, I will focus on the effects of shining bicircular light on Dirac semimetals and topological insulators. This novel and versatile type of light consists of a superposition of two circularly polarized lights with an integer frequency ratio and traces out a rose pattern in real space. As a result, bicircular light allows for the simultaneous break of time-reversal and spatial inversion symmetry, leading to the realization of sought-after magnetic topological states. I will also show that bicircular light enables a dynamical modulation of the gyrotropic magnetic effect.
Superconductivity mediated by soft ferroelectric modes
May 25, 2022 (Wed.) at 1:30PM (ET)
ISC-CNR and Sapienza University of Rome
Superconductivity in doped quantum paraelectric materials develops in proximity to a ferroelectric phase. An intimate relation between the superconducting and ferroelectric phases has been experimentally established in SrTiO3, a model incipient ferroelectric. Quantum critical fluctuations of the ferroelectric order open the possibility of a novel pairing mechanism. In this talk I will present a minimal microscopic model for the linear coupling between electrons and the ferroelectric mode (a soft transverse optical phonon) in a system with spin-orbit coupling. Remarkably, the Cooper pairing interaction resulting from this Rashba-like coupling is attractive in even-parity (s-wave) as well as odd-parity (p- wave) channels. I will show how to estimate the bare Rashba coupling and BCS pairing coupling constant with the aid of first-principles computations in SrTiO3. This approach can be easily extended to other incipient ferroelectrics and interfaces.
Coupling and entangling solid-state spin qubits for quantum information science
May 11 2021 (Wed.) at 1:30PM (EST)
Qubits are a quantum – and more powerful – version of the classical bit, the basis of information in all digital devices. Recently, spin centers of semiconductor defects [1,2] have emerged as a room temperature long-lived spin qubit with the interesting property of being optically-initialized and read. One example is the negatively-charged nitrogen-vacancy (NV) spin center in diamond, with applications to quantum information processing, quantum networking and nanoscale high precision sensing of electric fields, magnetic fields and charge related phenomena [1]. Unfortunately, due to their atomic scale and the weak dipole-dipole interaction, NV spin centers do not have the ability to couple to each other over optically resolvable distances – a required ingredient for most of the quantum technologies. Hence the importance of proposing and creating hybrid quantum systems for coupling and entangling spin centers, aiming to push forward their applications in quantum information science. In this work, we propose a practical hybrid quantum system to realize the NV-NV strong coupling mediated by magnon excitations of a magnetized ferrimagnetic materials [3,4,5], thus describing a practical pathway for single-spin-state-to-single-magnon-occupancy transduction and for entangling NV centers over micron length scales [3,4]. Our works serve as an alternative path for future experiments to engineer on-chip entangling gates between spin defects.
[1] Denis R. Candido and Michael E. Flatté, arXiv:2112.15581.
[2] Denis R. Candido and Michael E. Flatté, PRX Quantum 2 (4), 040310 (2021).
[3] Denis R. Candido, Gregory D. Fuchs, Ezekiel Johnston-Halperin and Michael E. Flatté, Mater. Quantum Technol. 1 011001 (2021).
[4] Masaya Fukami, Denis R. Candido, David D. Awschalom, Michael E. Flatté, PRX Quantum 2 (4), 040314 (2021).
[5] Huma Yusuf, Michael Chilcote, Denis R Candido, Seth Kurfman, Donley S Cormode, Yu Lu, Michael E. Flatté, Ezekiel Johnston-Halperin, AVS Quantum Sci. 3, 026801 (2021).
Multiscale dynamical modeling of correlated electron systems
May 2, 2022 (Wed.) at 1:30PM (ET)
Gia-Wei Chern
University of Virginia
In this talk, I will present our recent efforts on multi-scale dynamical modeling of functional electron materials, and in particular correlated electron systems. In the first part, I will discuss the utilization of machine-learning (ML) methods to achieve large-scale dynamical simulations on two canonical examples of correlated electron systems: the double-exchange and the Falicov-Kimball models. The central idea is to develop deep-learning neural-network models that can efficiently and accurately predict generalized forces required for dynamical evolutions based on local environment. The large-scale simulations enabled by the ML method also reveal new phase-ordering dynamics in these correlated electron systems. In the second part, I will discuss a new type of quantum molecular dynamics (QMD) methods based on advanced many-body techniques, such as Gutzwiller/slave-boson and dynamical mean-field theory, that are capable of modeling strong electron correlation phenomena. We apply our new QMD to simulate the correlation-induced Mott transition in a metallic liquid, and the nucleation-and-growth of Mott droplets in Hubbard-type models.
Simulation of topological phases in driven quantum dot arrays
April 27, 2022 (Wed.) at 1:30PM (ET)
Gloria Platero
Instituto de Ciencia de Materiales de Madrid, CSIC
The fabrication and control of long semiconductor quantum dot arrays [1] opens the possibility to use these systems for transferring quantum information between distant sites. Interestingly, it also opens the possibility of simulating, in quantum dot arrays, complex hamiltonians as 1D topological insulators. An example of them is the Su-Schrieffer-Hegger (SSH) model, a chain of dimers, which presents chiral symmetry and bond ordering of nearest-neighbor couplings and displays two topological phases. In a finite chain, the presence of protected edge states, allows to transfer electrons between edges, and therefore their implementation is promising for quantum information transfer. However, it does not account for long range hopping which should occur in real systems and which can destroy the topological properties and the edge states formation [2]. In this presentation I will show that, by applying an ac-driving protocol, all hopping amplitudes can be modified at will, imprinting bond-order and effectively producing structures such as dimers chains. Importantly, our protocol allows for the simultaneous suppression of all the undesired long-range hopping processes, enhancement of the necessary ones, and the appearance of new topological phases with increasing number of edge states. I will discuss the dynamics of two interacting electrons in a 12-QD array with different number of edge states. The correlated dynamics, which can be experimentally detected with QDs charge detectors, allows to discriminate between different topological phases and importantly, it opens a new avenue for quantum state transfer protocols [3].
[1] D.M. Zajac et al., Phys. Rev. App., 6, 054013 (2016),
[2] B. Pérez-González et al., Phys Rev. B, 99, 035146 (2019)
[3] B. Pérez-González et al., Phys. Rev. Lett., 123, 126401 (2019)
Hydrodynamic thermoelectric transport near charge neutrality
April 20, 2022 (Wed.) at 1:30PM (ET)
University of Wisconsin-Madison
We study hydrodynamic electron transport in Corbino and Hall bar graphene devices. In Corbino geometry due to the irrotational character of the flow, the forces exerted on the electron liquid are expelled from the bulk. We show that in the absence of Galilean invariance, force expulsion produces qualitatively new features in thermoelectric transport: (i) it results in drops of both voltage and temperature at the system boundaries and (ii) in conductance measurements in pristine systems, the electric field is not expelled from the bulk. We obtain thermoelectric coefficients of the system in the entire crossover region between charge neutrality and high electron density regime. The thermal conductance exhibits a sensitive Lorentzian dependence on the electron density. The width of the Lorentzian is determined by the fluid viscosity. This enables determination of the viscosity of electron liquid near charge neutrality from purely thermal transport measurements. In general, the thermoelectric response is anomalous: it violates the Matthiessen's rule, the Wiedemann-Franz law, and the Mott relation. For Hall bar devices subject to long-range inhomogeneities we show that the effective electrical conductivity of the system may significantly exceed the intrinsic conductivity of the electron liquid.
April 6, 2022 (Wed.) at 1:30PM (ET)
Institute for Complex Systems, National Research Council, Italy
The interest in quantum technologies have shifted the attention from the steady state regime of driven interacting quantum system to the previously considered uninteresting “transient” regime before decoherence takes place. We systematically studied a periodically driven BCS system in this regime, finding a remarkably rich phase diagram: i) When the frequency of the drive is higher than the gap frequency (2Δ) a Rabi-Higgs mode appears[1,2] in which a subset of quasiparticles synchronizes performing collective Rabi oscillations. ii) For subgap excitation[3], we demonstrate that the combined effect of drive and interactions results in emerging parametric resonances, analogous to a vertically driving pendulum. In particular, Arnold's tongues appear when the driving frequency matches 2Δ/n, with n being a natural number. iii) Inside the Arnold's tongues, we find a commensurate time-crystal condensate that retains the U(1) symmetry breaking of the parent superfluid/superconducting phase and shows an additional time-translational symmetry breaking. iv) Outside the tongues, the synchronized collective Higgs mode found in quench protocols [4] is stabilized without the need for a strong perturbation. v) Also gapless phases appear. Our results are directly relevant to cold-atom, cavity simulators and condensed-matter systems with prospective applications in several quantum technologies like parametric amplification and sensing.
References
[1] H. P. Ojeda Collado, José Lorenzana, Gonzalo Usaj, and C. A. Balseiro, Phys. Rev. B 98, 214519 (2018).
[2] H. P. Ojeda Collado, Gonzalo Usaj, José Lorenzana, and C. A. Balseiro, Phys. Rev. B 101, 054502 (2020).
[3] H. P. Ojeda Collado, Gonzalo Usaj, C. A. Balseiro, Damián H. Zanette, and José Lorenzana, Phys. Rev. Research 3, L042023 (2021).
[4] H. P. Ojeda Collado, Gonzalo Usaj, José Lorenzana, and C. A. Balseiro, Phys. Rev. B 99, 174509 (2019).
Dynamical properties of a polaron coupled to dispersive optical phonons
March 23, 2022 (Wed.) at 1:30PM (ET)
University of Ljubljana, Slovenia
In the first part I will present the study of static and dynamic properties of an electron coupled to dispersive quantum optical phonons in the framework of the Holstein model defined on a one–dimensional lattice [1]. Calculations are performed using the Lanczos algorithm based on a highly efficient construction of the variational Hilbert space. Even small phonon dispersion has a profound effect on the low energy optical response. While the upward phonon dispersion broadens the optical spectra due to single phonon excitations, the downward dispersion has the opposite effect. With increasing dispersion a multi–phonon excitation (MPE) state becomes the lowest excited state of the system at zero momentum and determines the low–frequency response of the optical conductivity where the threshold for optical absorption moves below the single–phonon frequency. Low–energy MPEs should be observable in systems with strong optical phonon dispersion in optical as well as angle resolvedphotoemission experiments.
In the second part I will discuss Holstein polaron spectral function using the finite–temperature (T) Lanczos method [2]. With increasing T additional features in the spectral function emerge even at temperatures below the phonon frequency. We observe a substantial spread of the spectral weight towards lower frequencies and the broadening of the quasiparticle (QP) peak. In the weak coupling regime the QP peak merges with the continuum in the high-T limit. In the strong coupling regime the main features of the low–T spectral function remain detectable up to the highest T used in our calculations. The effective polaron mass shows a non–monotonic behavior as a function of T at small phonon frequency but increases with T at larger frequencies. We have derived analytical expressions for the first few frequency moments, their values agree with those extracted from numerical calculations in a wide-T regime. If time permits, I will also discuss some relaxation properties of the electron coupled to various bosonic excitations [3].
References
[1] J. Bonča, S. A. Trugman, Phys. Rev. B 103, 054304 (2021)
[2] J. Bonča, S. A. Trugman, and M. Berçiu, Phys. Rev. B 100, 094307 (2019).
[3] J. Kogoj, M. Mierzejewski and J. Bonča, Phys. Rev. Lett., 117, 227002 (2016).
March 9, 2022 (Wed.) at 1:30PM (ET)
University of Gent, Belgium
The spectral function is one of the most important quantities that link theory and experiment in the field of strongly-correlated quantum matter, but its numerical evaluation for a given microscopic model is a notoriously hard problem. For one-dimensional systems, the formalism of matrix product states (MPS) has been instrumental for obtaining accurate spectral functions. These MPS techniques can be used in two dimensions by placing the system on a cylinder, but this requires huge computational resources for getting accurate results on relatively narrow cylinders. In this talk, we show how the efficiency of MPS methods can be improved by evaluating spectral functions directly in momentum space. We show that we can simulate the time evolution after applying a momentum operator to the ground state of an infinite (quasi) 1-D system, and that the entanglement growth is considerably smaller than after applying a real-space operator. This allows us to simulate the time evolution to much longer times with the same computational cost, which leads to much more precise spectral functions after transforming to frequency space. We show applications of these state-of-the-art MPS methods for spectral functions of one- and two-dimensional models with fractionalized excitations.
Detection and manipulation of Andreev states in hybrid nanowire Josephson junctions
February 23, 2022 (Wed.) at 1:30PM (ET)
Universidad Autonoma de Madrid
Although the existence Andreev bound states in SNS Josephson junctions was predicted by Kulik already in the 70’s, it was not until rather recently that direct evidence of these phase sensitive states was obtained experimentally using different techniques. On the other hand, the combination of high quality hybrid nanostructures and circuit QED techniques are allowing to explore the physics of Andreev states in novel conditions with unprecedented accuracy.
In this presentation I’ll give an overview on the theory that we have developed [1-5] to describe Andreev states in semiconducting nanowire Josephson junctions and their detection using circuit-QED techniques. Using a simple multichannel model we are able to identify the main effects on the Andreev states due to spin-orbit interactions [1,2]. Our theory allows to understand the line intensities in their microwave absorption spectrum and the absence of selection rules as observed in recent experiments [3,4]. Furthermore, I’ll discuss the signatures of electron-electron interactions that can be identified in the Andreev spectrum [5]. Finally, I’ll briefly discuss the prospects of using these states as a platform for different type of qubits [6].
[1] S. Park and A. Levy Yeyati, Phys. Rev. B 96, 125416 (2017).
[2] L. Tosi, C. Metzger, M. F. Goffman, C. Urbina, H. Pothier, Sunghun Park, A. Levy Yeyati, J. Nygård, and P. Krogstrup, Phys. Rev. X 9, 011010 (2019).
[3] S. Park, C. Metzger, L. Tosi, M. F. Goffman, C. Urbina, H. Pothier, and A. Levy Yeyati, Phys. Rev. Lett. 125, 077701 (2020).
[4] C. Metzger, S. Park, L. Tosi, C. Janvier, A. A. Reynoso, M. F. Goffman, C. Urbina, A. Levy Yeyati, and H. Pothier, Phys. Rev. Research 3, 013036(2021).
[5] F. J. Matute Cañadas, C. Metzger, Sunghun Park, L.Tosi, P. Krogstrup, J. Nygård, M. F. Goffman, C. Urbina, H. Pothier, A. Levy Yeyati, arXiv:2112.05625.
[6] M. Hays, V. Fatemi, D. Bouman, J. Cerrillo, S. Diamond, K. Serniak, T. Connolly, P. Krogstrup, J. Nygård, A. Levy Yeyati, A. Geresdi, M. H. Devoret, Science 373, 6553, 430-433 (2021)
Light-Control of Strongly Correlated Electrons
February 16, 2022 (Wed.) at 1:30PM (ET)
CNRS, Jeunes Equipes de l'Institut de Physique, Collège de France and IPhT/CEA, Saclay
The increased control over light-matter interactions, both at the classical level as well as in the genuine quantum regime, has turned the electromagnetic radiation from a traditional spectroscopic probe to an invaluable tool to control and manipulate complex quantum many body systems. A striking example is provided by light-induced superconductivity, observed in a number of compounds at temperatures far higher than in thermal equilibrium. An exciting new frontier is to take advantage of the quantum nature of light in solid state experiments to enhance transport or to dress, cool and control selected collective excitations of solids.
In this talk I will review our work on prototype models of strongly correlated electrons coupled to classical or quantum light. First I will show how a time-periodic modulation of the Hubbard interaction, mimicking a continuous pump excitation, can turn a Mott insulator into a non-thermal state with unusual nonequilibrium properties, including a population inversion of the doublon/holon band which can lead to superconducting correlations associated to the elusive eta-pairing state[1,2]. In the second part of the talk I will focus on the quantum regime of light-matter coupling and I will discuss the role of gauge invariance for the existence of coherent (super radiant) phases of photons in equilibrium [3].
References:
[1] F. Peronaci, M. Schiro’, O. Parcollet, Phys. Rev. Lett. 120, 197601 (2018)
[2] F. Peronaci, O. Parcollet, M. Schiro’, Phys. Rev. B. 101, 161101 (2020)
[3] O. Dmytruk and M. Schiro’, Phys. Rev. B 103 075131(2021)
Induced non-equilibrium states by measurements in quantum systems
February 9, 2022 (Wed.) at 1:30PM (ET)
University of Virginia
In quantum mechanics, the role of an observer is fundamentally different from that of a classical observer. The quantum mechanical observer necessarily plays an active role in the dynamics of the system that it is observing. This apparent difficulty may be turned into a tool to drive an initially trivial system into a complicated quantum many-body state simply by observing it. I will present two remarkable examples of states induced by measurement. In the first, we examine the role of a moving density measuring device interacting with a system of fermions, and in particular, show that it would leave behind a wake of purely quantum origin. In the second example, inspired by topological Floquet insulators, we will see how a suitably chosen set of density measurements, repeated periodically, will induce robust chiral edge motion of fermions. These examples show how quantum mechanical observation can be added as a versatile tool to the arsenal of quantum engineering in condensed matter systems.
New results on the Kondo effect in systems with atoms and molecules on metallic surfaces
February 2, 2022 (Wed.) at 1:30PM (ET)
Centro Atomico Bariloche, CONICET, Argentina
After a brief introduction to the Kondo effect, I will briefly describe the plausible explanations for the presence of a peak or a dip in the differential conductance dI/dV near V=0 observed by STM. Recently, interpreting experiments for Co on Cu(111), in which an antiresonance in dI/dV at V=0 AND a resonance near the bottom of the conduction band are observed, we conclude by NRG calculations that the STM tip “sees” mainly the Co s electrons and the dip is due to interference effects [1]. This is confirmed by recent ab initio calculations [2]. In addition, from other NRG calculations in a spin-1 two-channel Kondo model with anisotropy D(Sz)^2 we find a topological quantum phase transition in which the Kondo peak is suddenly turned to a dip with increasing D [3]. For large D the system is in a “non-Landau” Fermi liquid phase not adiabatically connected to a non-interacting system. Extending the theory to non-equivalent orbitals and non-zero magnetic field we can explain several relevant experiments in Fe phthalocyanine [4].
[1] J. Fernández, P. Roura-Bas, and A. A. Aligia, Phys. Rev. Lett.
126, 046801 (2021)
[2] M. S. Tacca et al., Phys. Rev. B 103, 245419 (2021).
[3] G. G. Blesio, L. O. Manuel, P. Roura-Bas, and A. A. Aligia, Phys.
Rev. B 98, 195435 (2018), Phys. Rev. B 100, 075434 (2019)
[4] R. Zitko, G. G. Blesio, L. O. Manuel, and A. A. Aligia, Nature Commun. 12, 6027 (2021)
Ultrafast dynamics of symmetry-broken states: From mean-field theory to a microscopic description of inhomogeneous disorder
January 26, 2022 (Wed.) at 1:30PM (ET)
University of Erlangen-Nuremberg (Germany)
Using ultrashort laser pulses, it has become possible to probe the dynamics of long-range order in solids on microscopic timescales. In the conventional description of symmetry-broken phases within time-dependent Ginzburg-Landau theory, the order parameter evolves coherently along an average trajectory. Recent experiments, however, indicate the profound effect of order parameter fluctuations on the dynamics. An extreme scenario is ultrafast inhomogeneous disordering, where the average order parameter is no longer representative of the state on the atomic scale. While this has a profound effect on the dynamics, a theoretical approach which takes into account atomic scale inhomogeneities of both the electronic structure and the order parameter is challenging. In my talk, I will report on results for the Holstein model, which are based on a nonequilibrium generalization of statistical dynamical mean-field theory, coupled to stochastic differential equations for the order parameter [1]. The results show that ultrafast disordering can occur already in this minimal model for the Peierls charge-density wave transition. Similar techniques may help in future to solve the coupled electron lattice dynamics for strongly interacting electrons.
[1] Antonio Picano, Francesco Grandi, Martin Eckstein, arXiv:2112.15323.
Z2 or not Z2? — Symmetry protection of triplet band-topology
December 15, 2021 (Wed.) at 3PM (ET)
University of California, Irvine (UCI)
Magnetic excitations offer a natural path to realize analogs of topologically nontrivial bands characterized by Z and Z2 indices. In this presentation, we will explore the fate of the Z2 band-topology realized by non-Kramers magnetic excitations in a bilayer system.
Z2 bands formed by magnetic excitations have been studied in bilayer honeycomb and kagome models involving magnons[1] and triplons[2]. In these models, the magnetic excitations form time-reversal (TR) partners, which play the role of the Kramers pairs of electrons in the model of Kane and Mele. However, bosonic excitations do not enjoy the same symmetry-protection as Kramers pairs of electrons and can be mixed by anisotropy terms. The consequences of such mixing for the Z2 topological phases of a quantum paramagnet remains an open question.
Here we show that a symmetric-exchange anisotropy — allowed by the symmetries of most bilayer models — can destroy the Z2 band-topology. We investigate possible symmetries that can provide an analog to the TR symmetry protecting the Kramers doublets in the case of electron bands. Furthermore, we examine the TR-breaking case and the formation of Chern bands.[3]
[1] H. Kondo, Y. Akagi, and H. Katsura, Phys. Rev. B 99, 041110 (2019)
[2] D. G. Joshi and A. P. Schnyder, Physical Review B 100, 020407 (2019)
[3] A. Thomasen, K. Penc, N. Shannon, and J. Romhányi Phys. Rev. B 104, 104412 (2021)
Photoinduced spinful excitons in Hubbard systems with magnetic superstructures
November 24, 2021 (Wed.) at 3PM (ET)
Georg-August-Universität Göttingen, Germany
The possibility to form excitons in photo-illuminated correlated materials is central from fundamental and application oriented perspectives. We show how the interplay of electron-electron interactions and a magnetic superstructure leads to the formation of a peculiar spinful exciton, which can be detected in ARPES-type experiments and optical measurements. We study this by using matrix product states (MPS) to compute the time evolution of single-particle spectral functions and of the optical conductivity following an electron-hole excitation in a class of one-dimensional correlated band-insulators, simulated by Hubbard models with on-site interactions and alternating local magnetic fields. An excitation in only one specific spin direction leads to an additional band in the gap region of the spectral function only in the spin direction unaffected by the excitation and to an additional peak in the optical conductivity. Recombination of the excitation happens on much longer time scales than the ones amenable to MPS. We discuss implications for experimental studies in correlated insulator systems.
Surprises when exploring electronic correlated systems at intermediate couplings
November 17, 2021 (Wed.) at 3PM (ET)
Elbio Dagotto
University of Tennessee, Knoxville and Materials Science and Technology Division, Oak Ridge National Laboratory
Recent results in the area of many-body physics will be discussed. In particular, employing mainly computational techniques, I will address the several surprising states that emerge in regions of parameters space with competing tendencies. Specifically, I will first focus on low dimensional chains and ladders, where the density matrix renormalization group technique is an accurate tool. For this reason, without the bias inevitable of mean field or variational approximations, the computer can reveal a variety of exotic phases difficult to anticipate. This new states involve spin staggered arrangements of ferromagnetic blocks [1], as well as spirals that become the ground state of some models at intermediate range couplings [2], without any obvious source of frustration. Predictions for inelastic neutron scattering for block states will be presented and discussed [3].
In the context of these spirals, coupling them to a garden-variety s-wave superconductor induces in the spiral both a singlet and triplet pairing components and, more interestingly, Majorana states at the edges [4]. This type of spirals, now in two dimensions, can also originate when including spin-orbit coupling and a magnetic field, creating a regularly spaced array of skyrmions, a so-called “skyrmion crystal” [5]. I will argue that this crystal can also be the platform for Majoranas as well [6].
Finally, time allowing, I will discuss progress with regards to pairing in multi-orbital models, employing a two-orbital Hubbard version of the Haldane chains [7], as well as low dimensional versions of models for iron superconductors [8]. The tendencies to hole pair found in this context are complementary to the gap equation methodologies of planar geometries.
Work supported by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division.
[1] See for example N. Patel et al, Commun. Phys. 2, 64 (2019); M. Sroda, E. Dagotto, and J. Herbrych, PRB 104, 045128 (2021), and references therein.
[2] J. Herbrych et al, Proc. Natl. Acad. Sci. USA 117, 16226 (2020).
[3] J. Herbrych et al., Nat. Comm. 9, 3736 (2018); J. Herbrych et al. PRB 102, 115134 (2020).
[4] J. Herbrych et al, Nat. Comm. 12, 2955 (2021).
[5] N. Mohanta et al., Phys. Rev. B 100, 064429 (2019) (Editor’s choice). See also N. Mohanta et al., Commun. Phys. (Nature) 3, 229 (2020).
[6] N. Mohanta et al., Commun. Phys. (Nature) 4, 163 (2021).
[7] N. D. Patel et al., npj Quantum Mater. 5, 27 (2020).
[8] B. Pandey et al., PRB 103, 214513 (2021) and references therein.
Nonequilibrium dynamical mean field theory
November 10, 2021 (Wed.) at 3PM (ET)
University of Fribourg
Recent experiments on laser-driven solids have revealed interesting nonequilibrium effects such as light-induced superconducting states [1,2] or switching into long-lived metastable states with novel structures and electronic properties [3]. To investigate and understand such phenomena, new theoretical and computational tools need to be developed. I will present the dynamical mean field approach, which over the past 15 years has been extended into a powerful framework for the simulation of real-time dynamics in correlated lattice systems [4]. After introducing this nonequilibrium Green’s function based technique, I will discuss benchmarks against cold-atom simulators [5], and present recent applications to laser-driven lattice models. These investigations demonstrate the possibility of effectively cooling correlated electron systems [6], and inducing magnetic, superconducting or excitonic order in long-lived nonequilibrium states [7,8]. I will comment on the implications of these findings for the experiments on light-induced superconductivity.
[1] S.Kaiser, C. R. Hunt, D. Nicoletti, W. Hu, I. Gierz, H. Y. Liu, M. Le Tacon, T. Loew, D. Haug, B. Keimer, and A. Cavalleri, Phys. Rev. B 89, 184516 (2014).
[2] M. Mitrano, A. Cantaluppi, D. Nicoletti, S. Kaiser, A. Perucchi, S. Lupi, P. Di Pietro, D. Pontiroli, M. Ricco, S. R. Clark, D. Jaksch, and A. Cavalleri, Nature 530, 461 (2016).
[3] L. Stojchevska, I. Vaskivskyi, T. Mertelj, P. Kusar, D. Svetin, S. Brazovskii, and D. Mihailovic, Science 344, 177 (2014).
[4] H. Aoki, N. Tsuji, M. Eckstein, M. Kollar, T. Oka, and P. Werner, Rev. Mod. Phys. 86, 779 (2014).
[5] K. Sandholzer, Y. Murakami, F. Goerg, J. Minguzzi, M. Messer, R. Desbuquois, M. Eckstein, P. Werner, and T. Esslinger, Phys. Rev. Lett. 123, 193602 (2019).
[6] P. Werner, M. Eckstein, M. Mueller, and G. Refael, Nature Comm. 10, 5556 (2019).
[7] P. Werner, J. Li, D. Golez, and M. Eckstein, Phys. Rev. B 100, 155130 (2019).
[8] P. Werner and Y. Murakami, Phys. Rev. B 102, 241103(R) (2020).
Nevanlinna Analytical Continuation
November 3, 2021 (Wed.) at 3:00PM (EST)
University of Michigan, Ann Arbor
Simulations of finite temperature quantum systems provide imaginary frequency Green’s functions that correspond one-to-one to experimentally measurable real-frequency spectral functions. However, due to the bad conditioning of the continuation transform from imaginary to real frequencies, established methods tend to either wash out spectral features at high frequencies or produce spectral functions with unphysical negative parts. Here, we show that explicitly respecting the analytic ’Nevanlinna’ structure of the Green’s function leads to intrinsically positive and normalized spectral functions and we present a continued fraction expansion that yields all possible functions consistent with the analytic structure. Application to synthetic trial data shows that sharp, smooth, and multi-peak data is resolved accurately. Application to the band structure of silicon demonstrates that high energy features are resolved precisely. Continuations in a realistic correlated setup reveal additional features that were previously unresolved. By substantially increasing the resolution of the real frequency calculations, our work overcomes one of the main limitations of finite-temperature quantum simulations.
Anyons and Fractional Quantum Hall Effect in Fractal Dimensions
October 27, 2021 (Wed.) at 3PM (ET)
Anne Nielsen
Aarhus University
Anyons are quasiparticles that are neither fermions nor bosons, and they appear in two-dimensional quantum systems under certain conditions, such as the fractional quantum Hall effect. Here, we show that anyons and the fractional quantum Hall effect can also be realized in fractal dimensions. We do this by constructing fractional quantum Hall models on different fractals and demonstrating that quasiparticles created in the models display anyonic charge and braiding statistics. Some of the models have a fractional quantum Hall trial state as exact ground state, which allows us to consider large system sizes. We also identify a simple Hamiltonian producing fractional quantum Hall physics and anyons on a small fractal lattice, which is suitable for realizations in ultracold atoms in optical lattices.
Three-particle mechanism for pairing and superconductivity
October 20, 2021 (Wed.) at 3PM (ET)
MIT
I will present a new mechanism and an exact theory of electron pairing from repulsive interaction in doped insulators. When the kinetic energy is small, the dynamics of adjacent electrons on the lattice is strongly correlated. By developing a controlled kinetic energy expansion, I will show that two doped electrons can attract and form a bound state. This attraction by repulsion can be understood as being mediated by the virtual interband transition of a third electron in the filled band. This three-particle pairing mechanism predicts a variety of novel phenomena in doped insulators, including spin-triplet superconductivity, pair density wave, BCS-BEC crossover and Feshbach resonance involving "trimers". Possible realizations in twisted bilayer graphene, ZrNCl and WTe2 will be discussed.
[1] V. Crepel and L. Fu, Science Advances 7, eabh2233 (2021)
[2] V. Crepel and L. Fu, arXiv:2103.12060
[3] K. Slagle and L. Fu, Phys. Rev. B 102, 235423 (2020)
[4] V. Crepel, T. Cea, L. Fu and F. Guinea, to appear
Lies my teacher told me about density functional theory: Can machine learning find the truth?
October 13, 2021 (Wed.) at 3PM (ET)
This seminar is about using machine learning to understand and improve modelling of strongly correlated systems. It will contain a broad overview of density functional theory, how DFT appears to fail when applied to strongly correlated systems, and how machine learning shows promise of helping improve the situation. Some of the work is in collaboration with Steve White, UCI physics, and Li Li, Google.
[1] Li, Li, Hoyer, Stephan, Pederson, Ryan, Sun, Ruoxi, Cubuk, Ekin D., Riley, Patrick and Burke, Kieron, Phys. Rev. Lett. 126036401 (2021)
Ultrafast excited-state dynamics with the exact factorization
October 6th 2021 (Wed.) at 3:00PM (EST)
Institut de Chimie Physique, University Paris-Saclay (France)
The interaction of molecular systems with visible/UV light is capable of triggering a strongly non-equilibrium, ultrafast response of such molecules, whose description often requires a theoretical framework beyond the Born-Oppenheimer approximation. This is the case for charge and energy transfer processes in photosynthetic and photovoltaic materials, as well as for the primary event in the process of vision. Computer simulations of those phenomena rely on the combination of advanced electronic structure and molecular dynamics techniques, in order to provide a complete dynamical picture of electrons and nuclei in molecules, including excited-state effects. In this talk, I will give an overview on the problem and on the state-of-the-art techniques for excited-state molecular dynamics simulations, in particular, focusing on the latest developments based on the exact factorization of the molecular wavefunction.
Exploring the physics of one dimensional Kitaev magnets
September 29, 2021 (Wed.) at 3PM (ET)
In the past 15 years, since A. Kitaev's proposal of an exactly solvable spin one-half model on the two-dimensional (2D) honeycomb lattice displaying a quantum spin liquid ground state, we have witnessed a surge of experimental efforts in the search and characterization of "candidate" materials realizing Kitaev's model. Unsurprisingly, experiments have shown that inevitable non-Kitaev interactions play an important role, as most of these materials seem to display magnetic order rather than a quantum spin liquid ground state at low temperatures. At the same time, the lack of accurate theoretical methods for highly frustrated spin models in 2D has hindered progress in understanding the role of these "unwanted" interaction terms. In 1D, it is well known that many analytical as well as numerically accurate methods to solve the many-body problem exist, so it is natural to wonder whether progress can be made looking at the problem from a 1D perspective.
In this talk, I will show how, using the density matrix renormalization group method, field-theory bosonization, and spin-wave theory, the phase diagram of a 1D version of the Kitaev spin model including the effects of the relevant non-Kitaev interactions, e.g., the Heisenberg interaction and a symmetric off-diagonal exchange "Gamma" interaction, can be determined accurately [1]. Among the many distinct phases found, I will focus on the regions of parameter space relevant for real materials, showing that close to the antiferro-Kitaev point an ordered phase with spins oriented in a Neel-like pattern with 6 site periodicity appears [2,3], while a gapless phase with emergent SU(2) symmetry exists close the ferro-Kitaev point [4].
Finally, I will discuss the relevance of the results for the 2D case as well as possible material realizations, where 2D-1D dimensionality crossover could be achieved by tailoring 2D superlattices or by applying pressure/strain on some 2D candidate Kitaev materials.
[1] W. Yang, A. Nocera, and I. Affleck, Phys. Rev. Research 2, 033268 (2020)
[2] W. Yang, A. Nocera, E. S. Sørensen, H.-Y. Kee, and I. Affleck, Phys. Rev. B 103, 054437 (2021)
[3] W. Yang, A. Nocera, and I. Affleck, Phys. Rev. B 102, 134419 (2020)
[4] W. Yang, A. Nocera, T. Tummuru, H.-Y. Kee, and I. Affleck, Phys. Rev. Lett. 124, 147205 (2020)
Perturbing the Kitaev model
September 15, 2021 (Wed.) at 3PM (ET)
São Carlos Institute of Physics at the University of São Paulo
The exact solution of Kitaev's spin-1/2 honeycomb spin-liquid model has sparked an intense search for Mott insulators hosting bond-dependent Kitaev interactions, of which Na2IrO3 and α−RuCl3 are prime examples. Subsequently, it has been proposed larger spin analogs of Kitaev interactions may occur in materials with strong spin-orbit coupling. In the first part of the talk, I will discuss the Heisenberg-Kitaev Hamiltonian in an external magnetic field in a consistent 1/S expansion, with S being the spin size, as a minimal model to describe the ordered phases in these Kitaev materials. In the second part of the talk, I will investigate the effects of disorder in the Kitaev model, motivated by the H3LiIr2O6 iridate. I will discuss the pileup of low-energy states previously found in the literature, and its connection the experiments, and how the different flux backgrounds affect the chiral edge states in the presence of disorder.
Theory of the Kitaev model in a [111] magnetic field
Oak Ridge Natnl. Lab.
June 9, 2021 (Wed.) at 3PM (ET)
Recent numerical studies indicate that the antiferromagnetic Kitaev honeycomb lattice model undergoes a magnetic-field-induced quantum phase transition into a new spin-liquid phase. This intermediate-field phase has been previously characterized as a gapless spin liquid. By employing a novel variational approach based on the exact fractionalized excitations of the zero-field model, we demonstrate that the field-induced spin liquid is gapped and belongs to Kitaev's 16-fold way. Specifically, the low-field non-Abelian liquid with Chern number C=±1 transitions into an Abelian liquid with C=±4. The critical field and the field-dependent behaviors of key physical quantities are in good quantitative agreement with published numerical results. Furthermore, we derive an effective field theory for the field-induced critical point which readily explains the ostensibly gapless nature of the intermediate-field spin liquid.
June 2 2021 (Wed.) at 3:00PM (EST)
Uppsala University
Odd-frequency superconductivity is a remarkable superconducting phase appearing when electrons pair at unequal times, with the pair amplitude being odd under the exchange of the time coordinates, or equivalently, odd in frequency. Since odd-frequency pairing vanish at equal times it is, in contrast to conventional superconductivity, intrinsically non-local in time and represents a truly dynamical effect. Odd-frequency superconductivity has been realized to be the key to understand the surprising physics of superconductor-ferromagnet (SF) structures and has also enabled the emerging field of superconducting spintronics. More recent discoveries have identified odd-frequency superconductivity in a range of known superconductors, from doped topological insulators and multiband superconductors, such as Sr2RuO4 and UPt3, to superconducting heterostructures of Weyl semimetals, and also in light-driven conventional superconductors. In this talk I will provide a brief introduction to odd-frequency superconductivity followed by a review of a few systems and materials where odd-frequency superconductivity is important for our understanding of the superconducting state.
Many-body localization: When thermalization fails and how to experimentally observe it
May 26, 2021 (Wed.) at 3PM (ET)
Beijing Computational Science Research Center
The observation of many-body localization is a paradigmatic example of the amount of time an idea takes to get mature enough, and the numerical and experimental methods to sufficiently develop, in order to settle its existence. After the original study of Philip Anderson in 1958, demonstrating localization of non-interacting quantum particles in disordered settings, a natural question is on the resulting effects of the inter-particle interactions on this phenomenon. Only after 50 years, substantial theoretical progress was made in solving this puzzle and, in 2016 the first experimental observation of this phenomenon was realized. The advent of platforms involving ultracold atoms trapped by optical lattices allowed the inspection of an inherently dynamical quantum phase transition, that goes beyond the standard ground-state classification of the quantum matter, and its associated low-lying excitations. Instead, it is described by a high-energy phase transition, inherently manifested via the unitary dynamics of an isolated quantum system, wherein by tuning the strength of disorder, one is able to halt the onset of ergodic behavior and thermalization. In this talk, after introducing the general conditions where it occurs, and review the experiments tackling it so far, I will show numerical and experimental results using quantum circuits of superconducting qubits that shed light on yet two other debated aspects: the possible existence of many-body mobility edges and on the localization without quenched disorder.
May 19 2021 (Wed.) at 3:00PM (EST)
Lex Kemper
North Carolina State University
Quantum hardware has advanced to the point where it is now possible to perform simulations of physical systems and elucidate their topological and thermodynamic properties, which we will discuss in this talk. We present a perspective on thermodynamics of quantum systems ideally suited to quantum computers, namely the zeros of the partition function, or Lee-Yang zeros. We develop a quantum circuit to measure the Lee-Yang zeros, and use these to reconstruct the thermodynamic partition function of the XXZ model. The zeros qualitatively show the cross-over from an Ising-like regime to an XY-like regime, making this measurement ideally suitable in a NISQ environment. Next, we demonstrate how topological properties of physical systems can be measured on quantum computers. When applied to the Kitaev spin model, we show that the identification of a topological phase transition is possible for an 8-qubit calculation on NISQ hardware. A second approach is to leverage the holonomy of the wavefunctions to obtain a noise-free measurement of the Chern number, which we apply to an interacting fermion model.
Catalyzing Emergence in Quantum Materials with Light
May 5th, 2021 (Wed.) at 3PM (ET)
University of California, San Diego
Quantum materials manifest fascinating phenomena ranging from superconductivity to metal-insulator transitions. Many of these materials exhibit colossal changes to external perturbations which includes electromagnetic excitation. This opens up exciting possibilities for “on-demand” control of emergent properties using light. Following a global overview of this topic, I will present vignettes from my research group highlighting the potential of light to explore quantum materials. The primary focus will be on correlated transition metal oxides, including cuprates, manganites, and iridates. From equilibrium and non-equilibrium perspectives, such materials offer numerous possibilities for light-based discovery and control arising from delicate interplay between interactions and dimensionality.
Spin-phonon coupling in frustrated Heisenberg models: Peierls distortions in spin liquids and valence-bond crystals
April 28, 2021 (Wed.) at 3PM (ET)
University of Trieste
The existence and stability of spin-liquid phases represent a central topic in the field of frustrated magnetism. While a few examples of spin-liquid ground states are well established in specific models, recent investigations have suggested the possibility of their appearance in several Heisenberg-like models on frustrated lattices. An important related question concerns the stability of spin liquids in presence of small perturbations in the Hamiltonian. The magnetoelastic interaction between spins and phonons represents a relevant and physically motivated perturbation, which has been scarcely investigated so far. We study the effect of the spin-phonon coupling on prototypical models of frustrated magnetism. We adopt a variational framework based upon Gutzwiller-projected wave functions implemented with a spin-phonon Jastrow factor, providing a full quantum treatment of both spin and phonon degrees of freedom. The results on the frustrated J_1-J_2 Heisenberg model on one- and two-dimensional (square) lattices show that, while a valence-bond crystal is prone to lattice distortions, a gapless spin liquid is stable for small spin-phonon couplings.
Willian Natori
Institute Laue Langevin
April 21, 2021 (Wed.) at 3PM (ET)
The most general signatures of topological states of matter are emergent edge states due to the bulk-boundary correspondence. Rapid progress was made in the experimental research of charged topological phases due to surface-sensitive probes. However, direct observation of topological surface states is more challenging in quantum magnets because of the excitations' charge-neutral character. The need for such experimental settings has become more pressing due to the necessity of correctly diagnosing quantum spin liquid in compounds such as RuCl3 under moderate magnetic fields.
In this talk, we propose spin-polarized scanning tunneling microscopy as a spin-sensitive local probe to edge states in topological magnets. We show how the tunneling conductance is directly related to the local dynamical structure factor and the relative spin polarization of the metallic substrate and the microscope tip. In particular, we determine the expected tunneling conductance of the Kitaev honeycomb model with open boundaries. In doing so, we describe how such an experiment would probe the hypothesized quantum spin liquid in RuCl3.
Orbital-selective correlations, block magnetism, and pairing in low-dimensional iron-based superconductors
Adriana Moreo
University of Tennessee, Knoxville
April 14, 2021 (Wed.) at 3PM (ET)
The discovery of superconductivity in Fe-based two-leg ladder materials under high pressure [1] has open new directions to improve our understanding of pairing tendencies in iron-based superconductors. Computational calculations of strongly correlated electronic models can be performed with high accuracy in quasi one dimension, including the case of multi-orbital systems. Using numerical techniques, such as DMRG, we have studied multi-orbital models for various Hubbard and Hund couplings, and electronic densities. We have found clear indications of pairing in slightly doped chains [2] and ladders [3]. The magnetic properties observed in these systems are also very rich, in particular the observation of an ``orbital selective Mott Phase'' in a wide parameter range. The phase is characterized by the formation of magnetic block states in which a block of N spins up alternate with a block of N spins down [4,5,6,7]. We were able to calculate the dynamical spin structure factor and interpret the results in terms of a mixture of acoustic and optical modes [4,6,7] also observed in neutron scattering experiments. The complex behavior observed from accurate studies of models from the Fe-based superconductors in 1 dimension indicates that the physics of these materials could bring novel surprises.
References:
[1] H. Takahashi et al., Nat. Mater. 14, 1008 (2015); J. Ying et al., PRB 95, 241109(R) (2017).
[2] N. Patel et al., PRB 96, 024520 (2017)
[3] N. Patel et al., PRB 94, 075119 (2016)
[4] J. Herbrych et al., Nat. Comm. 9, 3736 (2018)
[5] J. Herbrych et al., PRL 123, 027203 (2019)
[6] J. Herbrych et al., PNAS 117, 16226 (2020)
[7] J. Herbrych et al., Phys. Rev. B 102, 115134 (2020).
Quantum oscillations in the zeroth Landau Level and the serpentine Landau fan
MIT
April 7, 2021 (Wed.) at 3PM (ET)
We identify an unusual mechanism for quantum oscillations in nodal semimetals, driven by a single pair of Landau levels periodically closing their gap at the Fermi energy as a magnetic field is varied. These `zero Landau level' quantum oscillations (ZQOs) appear in the nodal limit where the zero-field Fermi volume vanishes, and have distinctive periodicity and temperature dependence. We link the Landau spectrum of a two-dimensional (2D) nodal semimetal to the Rabi model, and show by exact solution that across the entire Landau fan, pairs of opposite-parity Landau levels are intertwined in a `serpentine' manner. We propose 2D surfaces of topological crystalline insulators as natural settings for ZQOs, and comment on implications for anomaly physics in 3D nodal semimetals.
References: T. Devakul, Y. H. Kwan, S. L. Sondhi, S. A. Parameswaran, arXiv:2101.05294.
Non-unitary dynamics via spacetime duality: Fractally-entangled steady states and more
Stanford University
Mar. 31, 2021 (Wed.) at 3PM (ET)
The extension of many-body quantum dynamics to the non-unitary domain has led to a series of exciting developments, including new out-of-equilibrium entanglement phases and phase transitions. I will present recent work [1] in which we show that a duality transformation between space and time on one hand, and unitarity and non-unitarity on the other, can be used to realize non-unitary steady states that exhibit a rich variety of behavior in the scaling of their entanglement with subsystem size — from logarithmic to extensive to fractal. I will discuss how these outcomes relate to the growth of entanglement in time in unitary circuits, and how they differ, by using an exact mapping to a problem of unitary evolution with boundary decoherence, in which information is “radiated away” from one edge of the system. This idea allows us to construct fractally-entangled steady states that lie outside the paradigms of area-, volume-, or log-law that typically characterize eigenstates or steady states of unitary dynamics. Finally, I will discuss how these ideas could be experimentally realized with present-day or near-term quantum technologies, and how spacetime duality allows us to mitigate (or eliminate altogether) the overhead from "postselection" of random measurement outcomes and directly access entanglement in these states [2].
[1] MI, T. Rakovszky, V. Khemani, arxiv:2103.06873
[2] MI, V. Khemani, PRL 126, 060501 (2021)
Dynamics of Coulomb quantum spin liquids
MIT
Mar. 24, 2021 (Wed.) at 3PM (ET)
Quantum spin liquids are phases of matter with fractionalized excitations and emergent gauge fields. However, distinct and unambiguous signatures of spin liquids are hard to come by, especially in the experimentally relevant scenario of non-zero temperatures.
We show that the dynamics of spinon production in Coulomb spin liquids, which realize an emergent QED, contain such distinct signatures due to dramatic interaction effects and unusual energy scales. These include Sommerfeld enhancement of the pair production cross section due to the emergent Coulomb interaction and Cerenkov radiation due to the presence of slow photons. Varying the temperature provides characteristic changes in these effects due to the bath of thermal photons and magnetic monopoles.
Our results are consistent with recent numerics in lattice models and point to the important role of interactions in understanding dynamics of spin liquids, while also showing spin liquids to be interesting playgrounds for gauge theories in unusual regimes.
Magnetic, superconducting, and topological surface states on Fe1+yTe1−xSex
Brookhaven National Lab
Mar. 10, 2021 (Wed.) at 3PM (ET)
The idea of employing non-Abelian statistics for error-free quantum computing ignited interest in recent reports of topological surface superconductivity and Majorana zero modes (MZMs) in FeTe0.55Se0.45. An associated puzzle is that the topological features and superconducting properties are not observed uniformly across the sample surface. Understanding and practical control of these electronic inhomogeneities present a prominent challenge for potential applications. Here, we combine neutron scattering, scanning angle-resolved photoemission spectroscopy (ARPES), and microprobe composition and resistivity measurements to characterize the electronic state of Fe1+yTe1−xSex. We establish a phase diagram in which the superconductivity is observed only at sufficiently low Fe concentration, in association with distinct antiferromagnetic correlations, while the coexisting topological surface state occurs only at sufficiently high Te concentration. We find that FeTe0.55Se0.45 is located very close to both phase boundaries, which explains the inhomogeneity of superconducting and topological states. Our results demonstrate the compositional control required for use of topological MZMs in practical applications.
Renaissance in the Ruthenates: some Ruminations on a Resolution
Mar. 3, 2021 (Wed.) at 3PM (ET)
University of Florida
Recent nuclear magnetic resonance studies [A. Pustogow et al., Nature 574, 72 (2019); Chronister, arXiv:2007.13730] have challenged the prevalent chiral triplet pairing scenario proposed for the canonical unconventional superconductor Sr$_2$RuO$_4$. I present a detailed theoretical study of spin-fluctuation mediated pairing for this compound, mapping out the phase diagram as a function of spin-orbit coupling, interaction parameters, and band-structure properties over physically reasonable ranges, comparing when possible with photoemission and inelastic neutron scattering data information. Even-parity pseudospin singlet solutions are found to dominate large regions of the phase diagram, leading to suggestions that accidentally degenerate representations may explain the data. In particular, we propose that an accidentally degenerate combination of extended s and d_xy pairing may explain experiments consistently, if the microscopic nodal structure of such states is accounted for. If time permits, I'll discuss the prospects of direct measurements of the superconducting gap by STM. Interpreting such experiments requires a knowledge of the reconstructed surface band structure.
Topological Josephson junctions
Feb. 24, 2021 (Wed.) at 3PM (ET)
Universidad de San Martin, Argentina
Topological superconductors are characterized by the existence of edge states described by Majorana fermions. For systems with time-reversal symmetry they appear in Kramers pairs. When two such superconductors with a phase difference in their pairing potential are connected through a Josephson junction, the edge modes hybridize to form topological Andreev states. Their peculiar properties lead to signatures of the topological phase in the behavior of the Josephson current.
In this seminar I will discuss how to use the information provided by the Josephson current to perform quantum tomography of these states in a particular platform for topological superconductivity based on wires with spin-orbit coupling, in proximity with ordinary superconductivity and with an applied magnetic field. I will also discuss peculiarities of Josephson junctions of time-reversal invariant topological superconductors.
Collective modes of magnetized spin liquids
Feb. 17, 2021 (Wed.) at 3PM (ET)
University of Utah
We show that Zeeman magnetic field enhances the interaction between spinons in spin- conserving U(1) spin liquids. This interaction shifts the two-spinon continuum up in energy and leads to the appearance of the collective spin-1 mode in the transverse dynamic susceptibility at small momenta. This general effect is checked by detailed analytical and numerical calculations for the best-understood spin liquid — the spin-1/2 magnetized Heisenberg chain. We show that antiferromagnetic next-nearest neighbor exchange interaction can be used to tune the spin chain between the interacting spinon liquid and non- interacting spinon gas regimes at a small magnetic field. In the high magnetization regime, as the Zeeman field approaches the saturation value, we uncover the appearance of two-magnon bound states in the transverse susceptibility. This bound state feature generalizes the one arising from string states in the Bethe ansatz solution of the integrable case. We also sketch how the Dzyaloshinskii-Moriya interaction can be used to detect the interaction-induced splitting of transverse spin modes at small momentum in the ESR experiments.
Phonon dynamics in the Kitaev spin liquid
Feb. 10, 2021 (Wed.) at 3PM (ET)
University of Minnesota
Recent years have seen remarkable progress in identifying candidate materials that can realize quantum spin liquid phases, a particularly fascinating class of frustrated magnets that have been a focus of condensed matter research since the initial proposal by P.W. Anderson. In particular, a significant experimental and theoretical effort has been devoted to the study of magnetic properties of spin-orbit coupled 4d and 5d magnets, which can potentially realize the celebrated Kitaev honeycomb model.
The search for fractionalization in quantum spin liquids largely relies on their decoupling with the environment. However, the spin-lattice interaction is inevitable in a real setting. In our recent work [1], we show that in the Kitaev spin liquid the study of phonon dynamics may serve as an indirect probe of fractionalization of spin degrees of freedom. In particular, we propose that due to the spin-lattice coupling the signatures of the fractionalization can be seen in the sound attenuation from the phonon scattering off the Majorana fermions and the Hall viscosity, which can be induced by the time-reversal breaking spin Hamiltonian.
1. Mengxing Ye, Rafael M. Fernandes, and Natalia B. Perkins, Phys. Rev. Research 2, 033180 (2020).
2. Kexin Feng, Mengxing Ye, and Natalia B. Perkins, in preparation.
Engineering a doped Mott insulator and chiral d+id superconductivity in a triangular adatom lattice on a silicon surface.
Feb. 3, 2021 (Wed.) at 3PM (ET)
University of Tennessee, Knoxville
A doped Mott insulator's behaviour lies at the heart of some of the most exotic physical phenomena in materials research. The adsorption of a one-third monolayer of Sn atoms on a Si(111) surface produces a triangular surface lattice with half-filled dangling bond orbitals. In this talk, I will show how modulation hole doping of these dangling bonds unveils clear hallmarks of Mott physics [1], including a spectral weight transfer and the formation of dispersive quasiparticles at the Fermi level. I will also discuss the recent observation of superconductivity [2] in the most heavily hole-doped monolayers and evidence for an unconventional d+id chiral order parameter obtained from quasiparticle interference imaging and state-of-the-art calculations in the dynamical cluster approximation. These observations are remarkably similar to those made in complex oxide materials, including high-temperature superconductors, but highly extraordinary within the realm of conventional sp-bonded semiconductor materials.
References:
[1] F. Ming et al., PRL 119, 266802 (2017).
[2] X. Wu et al., PRL 125, 117001 (2020).
Estimating Heating Times in Periodically Driven Quantum Many-Body Systems via Avoided Crossing Spectroscopy
University of Innsbruck
Jan. 27, 2021 (Wed.) at 3PM (ET)
Periodic driving of a quantum many-body system can alter the systems properties significantly and therefore has emerged as a promising way to engineer exotic quantum phases, such as topological insulators and discrete time crystals. A major limitation in such setups, is that generally interacting, driven systems will heat up over time and lose the desired properties. Understanding the relevant time scales is thus an important topic in the field and so far, there have only been few approaches to determine heating times for a concrete system quantitatively, and in a computationally efficient way. In this talk we present a new approach, based on building the heating rate from microscopic processes, encoded in avoided level crossings of the Floquet propagator. We develop a method able to resolve individual crossings and show how to construct the heating rate based on these. The method is closely related to the Fermi Golden Rule approach for weak drives, but can go beyond it, since it captures non-perturbative effects by construction. This enables our method to be applicable in scenarios such as the heating time of discrete time crystals or frequency dependent couplings, which are very relevant for Floquet engineering, where previously no efficient methods for estimating heating times were available.
Superdiffusion and KPZ hydrodynamics in isotropic spin chains
UMass Amherst
Jan. 20, 2021 (Wed.) at 3PM (ET)
Finite-temperature spin transport in the quantum Heisenberg spin chain is known to be superdiffusive, and has been conjectured to lie in the Kardar-Parisi-Zhang (KPZ) universality class. In this talk, I will review the numerical and experimental evidence for this surprising anomalous transport property, and propose a theory in terms of “giant”, soft quasiparticles stabilized by integrability. I will argue that anomalous transport is “superuniversal” in integrable spin chains with continuous non-abelian symmetry. Finally, I will discuss the stability of this phenomenon against integrability-breaking perturbations, and argue that it is surprisingly robust to perturbations preserving non-abelian symmetries.
Partial dislocations in higher order topological insulators
Weizmann Institute
Jan. 13, 2021 (Wed.) at 3PM (ET)
Nonzero weak topological indices are thought to be a necessary condition to bind a single helical mode to a lattice dislocation. I will show that higher-order topological insulators (HOTIs) can, in fact, host a single helical mode along screw or edge dislocations in the absence of weak topological indices. When this occurs, the helical mode is necessarily bound to a dislocation characterized by a fractional Burgers vector, macroscopically detected by the existence of a stacking fault. The robustness of a helical mode on a partial defect is demonstrated by an adiabatic transformation that restores translation symmetry in the stacking fault. Since partial defects and stacking faults are commonplace in bulk crystals, the existence of such helical modes can measurably affect the expected conductivity in these materials.
Finally I will describe a general framework towards the classification of symmetry breaking defects based on symmetry representations.
Refs:
Phys. Rev. Lett. 123, 266802 (2019) (arXiv:1809.03518)
arXiv:1908.00011
Emergent fractons in Elusive Bose Metal --- When IR theory blends with UV physics
Princeton University
Jan. 6, 2021 (Wed.) at 3PM (ET)
The entanglement pattern of a quantum many-body system can be characterized by quasiparticles and emergent gauge fields, much like those found in Maxwell's theory. My talk begins with the basic aspects of symmetry fractionalization and emergent gauge fields in strongly correlated systems. I will further extend this paradigm into a new type of quantum many-body state, dubbed "fracton phase," from a quantum melting transition of plaquette paramagnetic crystals. These exotic states contain fractionalized sub-dimensional quasiparticles with constraint motion and emergent higher-rank gauge fields. Such constraint dynamics of the quasiparticles bring about an intriguing Bose metal phase with quasi-long range order and yields non-local quantum entanglement. In particular, the key peculiarities of this phase is the UV/IR mixing, where the short wavelength physics controls the low energy theory and hence challenges the standard notion of the renormalization group perspective.
Probing excitations via quantum quenches: a fresh look at simple (or not so simple) models
Dec. 16, 2020 (Wed.) at 3PM (ET)
Impressive recent experiments on synthetic quantum matter (based on cold atoms, superconducting circuits, etc.) aim at coherently steering quantum many-body systems towards highly entangled states, at equilibrium or -- more commonly -- far away from it. The experiments operate in the absence of any thermal bath, putting the non-equilibrium Hamiltonian dynamics of many-body systems at the center stage. In particular they expose the nature of elementary excitations, and how their spectral features (in momentum and frequency) translate into the dynamics of correlations and entanglement in real space and in real time. From the theoretical point of view, the challenge of faithfully describing quantum many-body states away from equilibrium is a formidable one -- made even more fascinating by the simplicity and the tunability of the models realized in the experiments.
Field-driven spin liquid physics in Kitaev materials
Dec. 9, 2020 (Wed.) at 3PM (ET)
In the field of frustrated magnetism, Kitaev materials have attracted broad interest for their potential to realize spin liquid physics — long-range entangled states of matter that often manifest themselves in unique topological properties. Experimentally, a number of 4d and 5d systems have been widely studied including the honeycomb materials Na2IrO3, α-Li2IrO3, and RuCl3 as candidate spin liquid hosts — however, all of these materials magnetically order at sufficiently low temperatures.
In this talk, I will discuss the physics of Kitaev materials that plays out when applying magnetic fields. Experiments on RuCl3 indicate the formation of a chiral spin liquid that gives rise to an observed quantized thermal Hall effect. Conceptually, this asks for a deeper understanding of the physics of the Kitaev model in tilted magnetic fields. I will report on numerical studies that give strong evidence for a Higgs transition from the well known Z2 topological spin liquid to a gapless U(1) spin liquid with a spinon Fermi surface and put this into perspective of experimental studies. I will also discuss a recent experiment-theory collaboration probing the angular field-dependence of the quantized thermal Hall effect, which unambiguously demonstrate the formation of a Majorana Chern insulating state in RuCl3.
Photo-induced Phase Transitions in Charge Density Waves
MIT
Dec. 2, 2020 (Wed.) at 3PM (ET)
Upon excitation with an intense laser pulse, materials can undergo a non-equilibrium phase transition through pathways different from those in thermal equilibrium. The mechanism underlying these photoinduced phase transitions has long been researched, but many details in this ultrafast, non-adiabatic regime still remain to be clarified. To this end, we studied light induced phase transitions in two different charge density wave (CDW) systems. First, we investigated the photo-induced melting of a unidirectional CDW in LaTe3. Using a suite of time-resolved probes, we independently track the amplitude and phase dynamics of the CDW. We find that a fast (approximately 1 picosecond) recovery of the CDW amplitude is followed by a slower re-establishment of phase coherence dictated by the presence of topological defects in CDW. Furthermore, after the suppression of the original CDW by photoexcitation, a different, competing CDW along the perpendicular direction emerges. The timescales characterizing the relaxation of this new transient CDW and the reestablishment of the original CDW are nearly identical, which points towards a strong competition between the two orders. Secondly, I will also report the realization of optical chiral induction and the observation of a gyrotropically ordered CDW phase in 1T -TiSe2. Our results provide a framework for understanding other photoinduced phase transitions and for unleashing novel states of matter that are “trapped” under equilibrium conditions.
Correlated superconductivity: the bronze, the iron, and the nickel era
Rutgers University and Brookhaven National Lab.
Nov. 18, 2020 (Wed.) at 3PM (ET)
Surprising experimental discoveries of superconductivity in unexpected compounds have shaped the field of strongly correlated materials. We will discuss the normal state of the recently discovered infinite layer nickelate superconductors to ascertain the importance of Mott and Hund physics. We will compare the results to those of archetypical systems, such as the iron pnictides and chalcogenides and the copper oxide superconductors.
Photon-assisted tunneling at the atomic scale: Probing resonant Andreev reflections from Yu-Shiba-Rusinov states
Freie Universität Berlin
Nov. 11, 2020 (Wed.) at 3PM (ET)
Exchange coupling of magnetic adsorbates to a superconducting substrate leads to Yu-Shiba-Rusinov (YSR) states within the superconducting energy gap. These can be probed by scanning tunneling spectroscopy as a pair of resonances at positive and negative bias voltage and over a wide range of tunnel conductances. At low tunneling rates, the current is carried by single-electron processes, where each excitation is sufficiently quickly followed by a relaxation into the energetic continuum. Upon increasing the junction conductance, the relaxation rates suppress single-electron tunneling and resonant Andreev processes start to dominate the transport process. The cross-over of these processes is expressed in the variation of the ratio of YSR peak height at positive and negative bias voltage [1].
Here, we investigate these transport processes by photon-assisted tunneling. While applying high- frequency radiation to the tunneling junction, we record the differential conductance spectra in the low and high-conductance regime. At low conductance, the YSR states exhibit symmetrically spaced sidebands with their spacing directly evidencing single-electron tunneling. Surprisingly, at large junction conductance, the spacing remains the same while the patterns become asymmetric. We show that this asymmetry is direct evidence of a resonant Andreev reflection with tunneling threshold conditions imposed on its electron and hole component [2]. We suggest that photon-assisted tunneling can be a powerful tool for the determination of the nature of the charge carriers in a single tunneling event.
References
[1] M. Ruby et al., Phys. Rev. Lett., 115 087001 (2015). [2] O. Peters et al., Nature Phys. (2020).
Resonant Inelastic X-ray Scattering to study ultrathin quantum materials
Brookhaven National Laboratory
Nov. 4, 2020 (Wed.) at 3PM (ET)
The understanding of the interactions leading to the intriguing properties of quantum materials requires the investigation of their elementary excitations in energy and momentum space. In this context, Resonant Inelastic X-ray Scattering (RIXS) has emerged as a powerful probe with prime sensitivity to electronic (spin, orbital, and charge) and lattice degrees of freedom. Thanks to recent developments RIXS has been employed in the investigation of many different systems, including cuprates, Fe-based superconductors, and low dimensional magnets. One of the latest interests of our group at Brookhaven National Laboratory has been the investigation of ultrathin films and materials where the properties are markedly different than the bulk.
In my talk I will present our recent RIXS investigations in field of ultrathin films. I will focus on two different cases, the effect of confinement on the spin excitations of metallic iron, and the evolution of the spin fluctuations in FeSe from the bulk down to the monolayer. I will show how RIXS can identify the elementary excitations in samples as thin as a single unit cell and its advantage over other techniques such as neutron and Raman scattering. By comparison with the respective bulk materials, I will show how the limited thickness affects the spin excitations and the consequences in the description of the interactions of those systems. Finally, I will conclude by presenting a view on new perspective in the use of RIXS in ultrathin films and materials such as van der Waals, and strongly correlated electron systems.
Quantum Computation and Simulation – Spins inside
Lieven Vandersypen
QuTech and Kavli Institute of Nanoscience, TU Delft
Oct. 28, 2020 (Wed.) at 3PM (ET)
Excellent control of over physical 50 qubits has been achieved, but can we scale up quantum computers to solve relevant problems? Quantum bits encoded in the spin state of individual electrons in silicon quantum dot arrays have emerged as a highly promising avenue. In this talk, I will present our vision of a large-scale spin-based quantum processor, and our ongoing work to realize this vision. I will also show how the same quantum dot arrays offer a powerful platform for analog quantum simulation of Fermi-Hubbard physics and quantum magnetism.
Physics Today 72(8), 38 (2019), npj Quantum Information 3, 34 (2017), Nature 555, 633 (2018), Science 359, 1123 (2018), Phys. Rev. X 9, 021011 (2019), Nature 579, 528 (2020), Nature 580, 355 (2020)
Triangular-lattice antiferromagnets. Again.
University of California, Irvine
October 21, 2020 (Wed.) at 3PM (ET)
I will describe our efforts to understand the phase diagram of a model that combines paradigmatic geometrical frustration of spins on a triangular lattice with strong spin-orbit-induced interactions. This model is relevant to a growing family of rare-earth-based magnets and other related materials and our work sets up a consistent interpretation of the current and future experiments in them.
Eigenstate Thermalization, random matrices and Behemoths
Maynooth University
Oct 14, 2020 (Wed.) at 3PM (ET)
The eigenstate thermalization hypothesis (ETH) is a cornerstone in our understanding of quantum statistical mechanics. The extent to which ETH holds for nonlocal operators (observables) is an open question. I will address this question using an analogy with random matrix theory. The starting point will be the construction of extremely non-local operators, which we call Behemoth operators. The Behemoths turn out to be building blocks for all physical operators. This construction allow us to derive scalings for both local operators and different kinds of nonlocal operators.
Boston University
Oct. 7, 2020 (Wed.) at 3PM (ET)
I will discuss how a d dimensional quantum system responds to being driven by D incommensurate monochromatic waves. This generalizes the well-studied Floquet case where D=1. By introducing a mapping to synthetic dimensions, I will present a topological classification of the quasi-energy states and their associated quantized dynamical responses. I will then describe a recent experiment on a single nitrogen-vacancy center in diamond driven by D=2 tones which experimentally demonstrated the dynamical analog of the quantum Hall effect in the synthetic space.
Critical Temperature of the 2D Attractive Hubbard model
Universidade Federal do Rio de Janeiro
Sept. 30, 2020 (Wed.) at 3PM (ET)
Ultra cold atoms trapped in optical lattices are unique in being a clean and tunable platform to study strongly correlated fermionic systems; the Fermi-Hubbard Model with both repulsive and attractive interactions being one of the most studied hamiltonians. Progress in cooling ultra cold atoms in optical lattices is bringing experiments to lower temperatures where ordered states are available.
The two dimensional attractive Hubbard Model exhibits a Kosterlitz-Thouless transition to an s-wave superfluid with a pairing pseudogap above the critical temperature TKT. Here I discuss Quantum Monte Carlo studies that have calculated the TKT for the attractive Hubbard Model for a range of densities and interaction strengths relevant to cold atom experiments.
Magic Angle Bilayer Graphene - Superconductors, Orbital Magnets, Correlated States and beyond
ICFO (Barcelona)
Sept. 23, 2020 (Wed.) at 3PM (ET)
When twisted close to a magic relative orientation angle near 1 degree, bilayer graphene has flat moire superlattice minibands that have emerged as a rich and highly tunable source of strong correlation physics, notably the appearance of superconductivity close to interaction-induced insulating states. Here we report on the fabrication of bilayer graphene devices with exceptionally uniform twist angles. We show that the reduction in twist angle disorder reveals insulating states at all integer occupancies of the four-fold spin/valley degenerate flat conduction and valence bands, i.e. at moire band filling factors nu = 0, +(-) 1, +(-) 2, +(-) 3, and reveals new superconductivity regions below critical temperatures as high as 3 K close to - 2 filling. In addition we find novel orbital magnetic states with non-zero Chern numbers. Our study shows that symmetry-broken states, interaction driven insulators, and superconducting domes are common across the entire moire flat bands, including near charge neutrality. We further will discuss recent experiments including screened interactions, fragile topology and the first applications of this amazing new materials platform.
Anomalous excitation spectra of conventional magnets
Martin Mourigal
Georgia Tech
Sept. 16, 2020 (Wed.) at 3PM (ET)
One of the scientific frontier in quantum magnetism is the discovery and understanding of quantum entangled and topologically ordered states in real bulk materials. At the focal point of the experimental investigation of these quantum spin networks is the identification of fractionalized excitations in transport and spectroscopic measurements. Inelastic neutron scattering has proved a powerful technique to reveal such signatures in a variety of systems ranging from quasi-1D magnets to Kagome compounds and more. Recent and on-going developments with neutron scattering instrumentation have allowed the characterization of magnetic excitations in entire volumes of momentum-energy space with high resolution.
In this talk, I will discuss such experiments on two long-known materials, FeI2 and MgCr2O4, and show how high-fidelity modeling brings new insights on their spin dynamics. On FeI2 [1], I will describe the mechanism endowing low-energy quadrupolar fluctuations with large spectral weight and how these can be completely understood using a SU(3) representation of spin degrees of freedom. The work on MgCr2O4 [2] will showcase that a continuous magnetic excitation spectrum can emerge in a frustrated magnet in absence of quasiparticle fractionalization. Overall, these experiments uncover highly unusual magnetic responses hidden within conventionally ordered magnets and have implications to the search for quantum spin-liquids using neutron scattering. This work was supported by DOE/BES under award DE-SC-0018660
[1] https://arxiv.org/abs/2004.05623 (2020).
[2] Phys. Rev. Lett. 122, 097201 (2019).
Loop currents and anomalous Hall effect due to time-reversal-breaking superconductivity
Victor Yakovenko
Department of Physics, CMTC and JQI, University of Maryland, College Park
Sept. 9, 2020 (Wed.) at 3PM (ET)
It was found experimentally that superconductivity spontaneously breaks time-reversal symmetry (TRS) in such materials as Sr2RuO4, UPt3, URu2Si2, and Bi/Ni bilayers. For the latter material, we argue that the superconducting order parameter has the winding number of +-2 around the Fermi surface, thus making Bi/Ni bilayers a rare example of intrinsic 2D topological superconductivity [1]. The experimental evidence for TRS breaking comes from the polar Kerr effect, which is rotation of polarization of normally incident light upon reflection from the sample. For a clean superconductor, theoretical studies indicate that this effect is possible only if electrons have more than one band. To clarify these conditions, we study a model of chiral TRS-breaking superconductivity on the honeycomb lattice with pairing between different sublattices [2]. We show that the experimental manifestations of TRS breaking can be characterized using the TRS-odd commutator of the superconducting pairing potential and its Hermitian conjugate. It generates persistent loop currents around each lattice site and opens a topological mass gap at the Dirac points with the corresponding chiral edge states, as in Haldane's model of the quantum anomalous Hall effect. It also generates the intrinsic ac Hall conductivity in the absence of an external magnetic field, which determines the polar Kerr effect. We also speculate on a possibility of breaking Z2 time-reversal and U(1) gauge symmetries in two separate phase transitions.
[1] X. Gong, M. Kargarian, A. Stern, D. Yue, H. Zhou, X. Jin, V. M. Galitski, V. M. Yakovenko, and J. Xia, "Time-reversal symmetry-breaking superconductivity in epitaxial bismuth/nickel bilayers", Science Advances 3, e1602579 (2017), arXiv:1609.08538
[2] P. M. R. Brydon, D. S. L. Abergel, D. F. Agterberg, and V. M. Yakovenko, "Loop currents and anomalous Hall effect from time-reversal symmetry-breaking superconductivity on the honeycomb lattice", Phys. Rev. X 9, 031025 (2019), arXiv:1802.02280
Quantum dynamics after geometric quenches and their emergent eigenstate solution
Penn State University
Sept. 2, 2020 (Wed.) at 3PM (EST)
The quantum dynamics of interacting many-body systems has become a unique venue for the realization of novel states of matter. We discuss how it can lead to the generation of time-evolving states that are eigenstates of emergent local Hamiltonians, not trivially related to the ones dictating the time evolution. We study geometric quenches in fermionic and bosonic systems in one-dimensional lattices, and provide examples of experimentally relevant time-evolving states [1,2] that are either ground states or highly excited eigenstates of emergent local Hamiltonians [3,4]. We also discuss the expansion of Mott insulating domains at finite temperature, which can be described by constructing Gibbs ensembles of the emergent local Hamiltonians (emergent Gibbs ensembles) [5]. Remarkably, the melting of Mott domains is accompanied by an effective cooling of the system, as reflected by an increasing correlation length. We explain this phenomenon analytically using the equilibrium description provided by the emergent Gibbs ensemble [5].
References:
[1] L. Vidmar et al., PRL 115, 175301 (2015).
[2] J. Wilson et al., Science 367, 1461 (2020).
[3] L. Vidmar, D. Iyer, and M. Rigol, PRX 7, 021012 (2017).
[4] Y. Zhang, L. Vidmar, and M. Rigol, PRA 99, 063605 (2019).
[5] L. Vidmar, W. Xu, and M. Rigol, PRA 96, 013608 (2017).
Machine Learning Quantum Emergence
Eun-ah Kim
Cornell University
Aug. 26, 2020 (Wed.) at 3PM (ET)
Decades of efforts in improving computing power and experimental instrumentation were driven by our desire to better understand the complex problem of quantum emergence. However, increasing volume and variety of data made available to us today present new challenges. I will discuss how these challenges can be embraced and turned into opportunities by employing machine learning. The rigorous framework for scientific understanding physicists enjoy through our celebrated tradition requires the interpretability of any machine learning essential. I will discuss our recent results using machine learning approaches designed to be interpretable from the outset. Specifically, I will present discovering order parameters and its fluctuations in voluminous X-ray diffraction data and discovering signature correlations in quantum gas microscopy data.
Dynamics of interacting fermions under spin-orbit coupling
CU Boulder and JILA
August 19, 2020 (Wed.) at 3PM (ET)
Understanding the behavior of interacting electrons in solids or liquids is at the heart of modern quantum science and necessary for technological advances. However, the complexity of their interactions generally prevents us from coming up with an exact mathematical description of their behavior. Precisely engineered ultracold gases are emerging as a powerful tool for unraveling these challenging physical problems. In this talk, I will present recent developments at JILA using alkaline-earth atoms (AEAs) --currently the basis of the most precise atomic clock in the world-- for the investigation of complex many-body phenomena and magnetism. I will discuss how to use AEAs dressed by laser fields to engineer analogs of spin-orbit coupled Hamiltonians and explore rich physics emerging from the interplay between many-body interactions and spin-orbit coupling in a fermionic optical lattice clock. In particular I will explain how local interactions in the Hubbard model in the presence of spin-orbit coupling, can generate spin-locking, prolong inter-particle spin coherence, and transform dephasing effects into an entangling process. These investigations fall into the new paradigm of using driven, non-equilibrium many-body systems to advance quantum metrology.
The strange quantum transition from the pseudogap metal to the Fermi liquid
Harvard University
August 12, 2020 (Wed.) at 3PM (ET)
Numerous experiments have explored the phases of the cuprates with increasing doping density p from the antiferromagnetic insulator. There is now strong evidence that the small p region is a novel phase of matter, often called the pseudogap metal, separated from conventional Fermi liquid at larger p by a quantum phase transition. I will describe recent numerical and theoretical results on a model with random and all-to-all electron hopping and exchange interactions. Remarkably, this simple model captures much of the observed phenomenology, and contains a deconfined quantum critical point which can exhibit the observed linear-in-temperature resistivity in an intermediate “strange metal” regime. I will also describe recent ideas on models without disorder, which use ancilla qubits to obtain a critical theory with emergent gauge fields and ghost Fermi surfaces.
New physics in flat Moire bands
Erez Berg
Weizmann
Aug 5 2020 (Wed.) at 3:00PM (ET)
Flat bands in Moire superlattices are emerging as a fascinating new playground for correlated electron physics. I will present the results of several studies inspired by these developments. First, I will address the question of whether superconductivity is possible even in the limit of a perfectly flat band. Then, I will discuss transport properties of a spin-polarized superconductor in the limit of zero spin-orbit coupling, where the topological structure of the order parameter space allows for a new dissipation mechanism not known from conventional superconductors. If time allows, I will also discuss the interpretation of new measurements of the electronic compressibility in twisted bilayer graphene, indicating a cascade of symmetry-breaking transitions as a function of the density of carriers in the system.
References:
https://arxiv.org/abs/2006.10073
Electron hydrodynamics in low-dimensional metals
Joel Moore
UC Berkeley
July 29 2020 (Wed.) at 3:00PM (EST)
Hydrodynamics can be defined as the effective description of how many-particle systems evolve from local equilibrium to global equilibrium. This online talk gives several examples of how electrons show different kinds of hydrodynamical behavior compared to an ordinary fluid. One-dimensional systems often show special behavior because of additional exact or approximate conservation laws, which can be verified by comparison to DMRG-type simulations. Higher-dimensional electron systems also show unique features, for example because they move in an environment of low symmetry, and we focus on how Hall viscosity and related conductivity properties are seen or might be seen in experiments.
Quantum matter under the microscope
July 22 2020 (Wed.) at 3:00PM (EST)
More than 30 years ago, Richard Feynman outlined his vision of a quantum simulator for carrying out complex calculations on physical problems. Today, his dream is a reality in laboratories around the world. This has become possible by using complex experimental setups of thousands of optical elements, which allow atoms to be cooled to Nanokelvin temperatures, where they almost come to rest. Recent experiments with quantum gas microscopes allow for an unprecedented view and control of such artificial quantum matter in new parameter regimes and with new probes. In our fermionic quantum gas microscope, we can detect both charge and spin degrees of freedom simultaneously, thereby gaining maximum information on the intricate interplay between the two in the paradigmatic Hubbard model. In my talk, I will show how we can reveal hidden magnetic order, directly image individual magnetic polarons or probe the fractionalisation of spin and charge in dynamical experiments. For the first time we thereby have access to directly probe non-local ‘hidden’ correlation properties of quantum matter and to explore its real space resolved dynamical features also far from equilibrium. I will also discuss our most recent experiments on the crossover from a quantum Antiferromagnet to a Fermi Liquid open strong doping of the two-dimensional systems. The special interplay of hole doping and magnetic correlations will be elucidated by probing highly non-local correlators between spin and charge.
Pair density wave at high magnetic fields in cuprates with charge and spin orders
Dragana Popovic
National High Magnetic Field Laboratory and Department of Physics, Florida State University
July 20 2020 (Mon.) at 3:00PM (EST)
In underdoped cuprates, the interplay of the pseudogap, superconductivity, and charge and spin ordering can give rise to exotic quantum states, including the pair density wave (PDW), in which the superconducting order parameter is oscillatory in space. The richness of experimental observations in cuprates has raised the possibility that putative PDW correlations may, in fact, be responsible for the pseudogap regime, the origin of which has been a long-standing mystery. However, the evidence for a PDW state has been inconclusive and its broader relevance to cuprate physics is an open question. In order to distinguish between different scenarios, the key issue to address is what happens in the limit of low temperatures (T->0) and high magnetic fields when superconducting order is destroyed to reveal the nature of the normal ground state. This talk will describe the results of a comprehensive series of experiments using several complementary electrical transport techniques on underdoped La-214 cuprates in which charge and spin orders appear in the form of stripes. Our results a) establish a robust phase diagram for superconducting vortices in underdoped cuprates [1], and b) provide much-needed transport signatures of the elusive PDW state in stripe-ordered cuprates in the regime where superconductivity is destroyed by quantum phase fluctuations [2]. The broader relevance of our findings to cuprate physics will be also discussed.
[1] Zhenzhong Shi, P. G. Baity, T. Sasagawa, and D. Popović, Sci. Adv. 6, eaay8946 (2020).
[2] Zhenzhong Shi, P. G. Baity, J. Terzic, T. Sasagawa, and D. Popović, Nat. Commun. 11, 3323 (2020).
Exact eigenstates in non-integrable systems: A violation of ETH
Princeton University
July 15 2020 (Wed.) at 3:00PM (EST)
We find that several non-integrable systems exhibit some exact eigenstates that span the energy spectrum from lowest to the highest state. In the AKLT Hamiltonian and in several others “special” non-integrable models, we are able to obtain the analytic expression of states exactly and to compute their entanglement spectrum and entropy to show that they violate the eigenstate thermalization hypothesis.
Non-equilibrium control of moire lattice band structures and magnetism via phonons
Greg Fiete
Northeastern University
July 8 2020 (Wed.) at 3:00PM (EST)
Recently, there has been heightened interest in non-equilibrium phenomena in condensed matter systems. An important class of non-equilibrium systems includes those subjected to a periodic drive, such as a material with an incident laser beam. In this talk, I will describe some of our recent theoretical work aimed at finding accurate effective Hamiltonians for different periodically driven systems. I will focus on two distinct cases: (1) Moire lattice band structures of bilayer and double bilayer graphene and (2) Control of magnetism via selective excitation of phonons in CrI3.
Real Space Berry curvature of itinerant electron systems with spin-orbit interaction
July 1, 2020 (Monday) at 3PM (EST)
By considering an extended double-exchange model with spin-orbit coupling (SOC), we derive a general form of the Berry phase γ that electrons pick up when moving around a closed loop. This form generalizes the well-known result valid for SU(2) invariant systems, γ = Ω/2, where Ω is the solid angle subtended by the local magnetic moments enclosed by the loop. The general form of γ demonstrates that collinear and coplanar magnetic textures can also induce a Berry phase different from 0 or π, smoothly connecting the result for SU(2) invariant systems with the well-known result of Karplus and Luttinger for collinear ferromagnets with finite SOC. By taking the continuum limit of the theory, we also derive the corresponding generalized form of the real space Berry curvature. The new expression is a generalization of the scalar spin chirality, which is presented in an explicitly covariant form. We finally show how these simple concepts can be used to understand the origin of the spontaneous topological Hall effect that has been recently reported in collinear and coplanar antiferromagnetic phases of correlated materials.
(*) Work done in collaboration with Shang-Shun Zhang, Hiroaki Ishizuka, Hao Zhang, Gábor B. Halász. Phys. Rev. B 101, 024420 (2020).
A superconducting circuit realization of combinatorial gauge symmetry
June 24, 2020 (Monday) at 3PM (EST)
We propose a superconducting wire array that realizes a family of quantum Hamiltonians that possess combinatorial gauge symmetry – a local symmetry where monomial transformations play a central role. This physical system exhibits a rich structure. In the classical limit its ground state consists of two superimposed spin liquids; one is a crystal of small loops containing disordered U(1) degrees of freedom, and the other is a soup of loops of all sizes associated to Z2 topological order. We show that the classical results carry over to the quantum case when fluctuations are gradually tuned via the wire capacitances, yielding Z2 quantum topological order. In an extreme quantum limit where the capacitances are all small, we arrive at an effective quantum spin Hamiltonian that we conjecture would sustain Z2 quantum topological order with a gap of the order of the Josephson coupling in the array. The principles behind the construction for superconducting arrays extends to other bosonic and fermionic systems, and offers a promising path towards topological qubits and the study of other many-body systems.
New perspectives on quantum matter: bringing together quantum simulations and machine learning
June 17, 2020 (Monday) at 3PM (EST)
I will discuss the idea of combing quantum simulators with machine learning to perform inference of NMR spectra for small biological molecules. I will also discuss applications of machine learning techniques to analyze snapshots of many-body states obtained from quantum gas microscopes.
Topological superconductivity in full shell proximitized nanowire
June 15, 2020 (Monday) at 3PM (EST)
I will discuss a new model system supporting Majorana zero modes based on semiconductor nanowires with a full superconducting shell. I will demonstrate that, in the presence of spin-orbit coupling in the semiconductor induced by a radial electric field, the winding of the superconducting order parameter leads to a topological phase supporting Majorana zero modes. The topological phase persists over a large range of chemical potentials and can be induced by a predictable and weak magnetic field piercing the cylinder. The system can be readily realized in semiconductor nanowires covered by a full superconducting shell, opening a pathway for realizing topological quantum computing proposals.
