Nordic UK Condensed Matter Seminars

Entanglement is central to modern theoretical studies of quantum many-body systems but remains experimentally elusive due to its exponential complexity. Predictions of entanglement phase transitions in monitored quantum systems have further highlighted its importance in far-from-equilibrium states and quantum simulators. However, experimental exploration about manybody entanglement is hindered by the exponential costs of quantum-state tomography and, in monitored systems, by an additional experimentally complex bottleneck of postselection. To address these challenges, I will show how charge fluctuations—or more generally, reduced fluctuations of subsystem observables—can serve as scalable proxies for entanglement entropy, bypassing the need for full-state tomography. For monitored dynamics, I will also discuss a steering protocol that applies adaptive corrective feedback to approximate postselected pure-state trajectories with mixed ensembles of steered states, avoiding full postselection. Numerical simulations and theoretical arguments demonstrate the scalability and efficiency of these methods, with only a polynomial overhead.

The energy spectrum of lead-halide perovskites (LHPs) near the band gap resembles that of a Dirac equation. This observation establishes a fruitful analogy between condensed matter and high-energy physics. On the one hand, it allows the use of well-established theoretical techniques to study strong-field effects in LHPs. On the other hand, it paves the way for the first experimental observations of textbook phenomena in high-energy physics. The talk will first introduce LHPs as Dirac semiconductors, laying the foundation for discussing recent observations of tunneling ionization in LHPs and their interpretation in terms of the Schwinger effect [arXiv: 2406.05032]. Besides being of general interest, this observation implies several useful applications, particularly in detecting strong electric fields, as demonstrated in the laboratory. At the end of the presentation, I will briefly argue that LHPs can potentially be employed to study Dirac-Pauli physics—an extension of the Dirac equation due to Pauli, which has found limited use in high-energy physics [arXiv:2407.04450].

Rare and extreme events are notoriously hard to handle in any complex stochastic system: They are simultaneously too rare to be reliably observable in experiments or numerics, but at the same time often too impactful to be ignored. Large deviation theory, corresponding to a saddle-point approximation of the stochastic path integral, provides a classical way of dealing with events of extremely small probability, but generally only yields the exponential tail scaling of rare event probabilities. In this talk, I will discuss theory, and algorithms based upon it, that improve on this limitation, yielding sharp quantitative estimates of rare event probabilities from a single computation and without fitting parameters. Notably, these estimates require the computation of determinants of differential operators, which in relevant cases are not traceclass and require appropriate renormalization. We demonstrate that the Carleman-Fredholm operator determinant is the correct choice, corresponding to the renormalized fluctuation determinant around the instanton of the corresponding stochastic field theory. Throughout, I will demonstrate the applicability of these methods to high-dimensional real-world systems, for example coming from atmosphere and ocean dynamics.

The past few years have seen a revived interest in quantum geometrical characterizations. Although the metric tensor has been connected to many geometrical concepts for single bands, the exploration of these concepts to a multi-band paradigm still promises a new

 field of interest. I will discuss a new route involving Pl\"ucker embeddings to represent arbitrary classifying spaces, being the essential objects that encode all the relevant topology for any multi-band system. While I will argue that this tool can be applied

 in contexts that range from response theories to finding quantum volumes and bounds on superfluid densities as well as possible quantum computations, I will in particular also show that they can be used to formulate projector Hamiltonians with projected entangled

 pair ground (PEPS) states that have a finite topological invariant, the Euler class, circumventing many no-go theorems. We further demonstrate the versatility of our model states by applying a shallow quantum circuit, producing interacting PEPS and simple

 parent Hamiltonians in the Euler phase. These model states moreover pinpoint to new interacting physics.

The normal liquid 3He conforms to Landau’s Fermi liquid picture, but only at very low temperature (<20 mK). We have recently shown that its thermal conductivity [1] can be accounted for in the whole temperature range (0.01 K-3 K), by assuming that it is the sum of two contributions: one by quasi-particles (varying as the inverse of temperature) and another by a hydrodynamic sound mode (following the square root of temperature) [2].  The first component has been known for decades. The second is a collective [sound] mode [3] with a ~2kFwave-vector in the hydrodynamic limit. Our expression for it, derived from Landauer’s formula, is a quantum version of the Bridgman equation for thermal conductivity of classical liquids. The quadratic electrical resistivity of Fermi liquids offers its own puzzles [4]. The deviation from this behavior in strongly correlated Fermi liquids well below the Fermi temperature may be accounted for by the presence of such a collective mode.

[1] D. S. Greywall, Phys. Rev. B 29, 4933 (1984).
[2] K. Behnia & K. Trachenko, Nature Commun. 15, 1771 (2024).
[3] F. Albergamo et al., Phys. Rev. Lett. 99, 205301 (2007).

[4] X. Lin, B. Fauqué & K . Behnia, Science 349, 6251 (2015).

Quantum matter out of equilibrium emerge as an important platform to induce correlations and transient orders. Modern techniques of coherent and fast light sources enable this evolution. Nonequlibrium orders have been seen in driven cold atoms, spins, magnetism and superconductivity. To place these ideas in a broader context I will introduce the concepts of rectified quantum orders and quantum print as our approach to cotrolled quantum states. I will present recent theory predictions and experimental observations of induced orders: magnetization induced in paraelectric and light induced vortex excitations in supercondcutors.

While a significant fraction of topological materials has been characterized using symmetry requirements, the last couple of years have witnessed the rise of new topologies that arise in multi-level system and go beyond all previously-known classification. Topological transitions occur via band touching points and as such Dirac or Weyl nodes have been familiar and attracting a lot of attention in various contexts. Here, I will show that in systems with C2T or PT symmetry, band singularities can be characterised by a non-Abelian frame charges. I will demonstrate momentum space braiding of these topological singularities and discuss their observable signatures including the peculiarities of the associated edge states. In particular,  I will show that the combination of periodic driving and these recent multi-gap topological insights galvanize a new direction, arriving at new anomalous topological phases in out-of-equilibrium settings that have no static counterparts. I will identify observable signatures within reach of state-of-the-art quantum simulators ranging from optical lattices to kicked quantum rotors, and present specific interferometry schemes.

In this talk, I will introduce the dynamical phenomenon of timescale separation known as metastability in the context of Markovian open quantum systems. Metastability arises, among others, in proximity to first-order phase transitions and leads to the existence of multiple long-lived states whose changes are negligible over a pronounced period of time, before their unavoidable asymptotic relaxation to a unique stable state. I will discuss metastability both in the average and stochastic dynamics of open quantum systems, with special focus on signatures which can be uncovered in numerical simulations or experiments. In average dynamics, those signatures depend on whether metastable states can be characterised in terms of classical degrees of freedom, or a more general quantum description is needed. In the context of stochastic dynamics arising from continuous monitoring of open quantum systems, by considering quantum trajectories described by quantum reset processes, we will see both cases can have classical but distinct analogues. Classical metastable phenomenology (fast relaxation into distinct phases and slow transitions between) will be shown to hold for the former, while for the latter, a continuous evolution to occur within a slow manifold (that can also be made fast thanks to underlying quantum degrees of freedom).

Quantum materials are on the rise and during last years there have been a large amount of predictions and discoveries of strongly correlated 2D materials, their ordered phases, and the vast possibilities to manipulate them various ways. These materials are often so called van der Waals crystals that can be “peeled” atomic layer by layer from a 3D bulk material down to a few or even a single crystal layer. The low dimensionality enables unprecedented possibilities to externally tune their properties by external knobs such as e.g. electrostatic gating, by combining them together in heterostructures, or by placing ad-atoms on their surface. In this talk I will discuss when the correlated state gives rise to unconventional superconductivity and how it may behave in devices. Namely, size matters for unconventional superconducting devices. The presence of low-energy Andreev states can drive the superconductor into a new low-temperature phase different from that of its bulk parent state.

 The classification of point gap topology in all local non-Hermitian (NH) symmetry classes has been recently established. However, many entries in the resulting periodic table have only been discussed in a formal setting and still lack a physical interpretation in terms of their bulk-boundary correspondence. In my talk, I will derive the edge signatures of all two-dimensional phases with intrinsic point gap topology. While in one dimension point gap topology invariably leads to the NH skin effect, NH boundary physics is significantly richer in two dimensions. I will show that there are two broad classes of non-Hermitian edge states: (1) infernal points, where a skin effect occurs only at a single edge momentum, while all other edge momenta are devoid of edge states. (2) NH exceptional point dispersions, where edge states persist at all edge momenta and furnish an anomalous number of symmetry-protected exceptional points.

The emergence of topological modes in non-Hermitian systems has established a new strategy to create quantum modes in open quantum systems. While formidable progress has been obtained in non-Hermitian topological systems in the single particle limit, the effect of many-body interactions remains an open problem in quantum physics. I will show that in an engineered platform based on quantum dots, non-Hermitian physics with tunable interactions can be realized [1]. Using a non-Hermitian tensor network method, we demonstrate that topological modes remain all the way from the non-interacting to the fully interacting limit. Furthermore, I will show that dynamical interacting non-Hermitian modes can be visualized using a tensor-network kernel polynomial algorithm, allowing us to probe the many-body topology of strongly interacting non-Hermitian systems [2]. I will show that the local spectral functions computed with our algorithm reveal topological spin excitations in a non-Hermitian spin model, faithfully reflecting the non-trivial line gap topology in a many-body model even in the presence of the non-Hermitian skin effect. Our results demonstrate a tensor-network methodology to probe dynamical excitations in topological matter driven by the interplay of many-body interactions and non-Hermiticity.

References:

[1] Timo Hyart and J. L. Lado, Phys. Rev. Research 4, L012006 (2022)

[2] Guangze Chen, Fei Song, Jose L. Lado, Phys. Rev. Lett. 130, 100401 (2023)

Similarly to the ergodicity hypothesis in classical chaotic systems, in the quantum setting there is a similar concept, related to quantum thermalization and equipartition over degrees of freedom and dubbed as the eigenstate thermalization hypothesis. This concept is very useful as it provides a link between classical and quantum chaos.

The concept of multifractality of quantum wave-functions is a way to break the above ergodicity in terms of chaotization and equipartitioning over degrees of freedom in quantum systems. On the other hand, in quantum information theory it is the entanglement entropy which represents the main measure of ergodicity and thermalization. On the third side, in the eigenstate thermalization hypothesis the fluctuations of local observables and their scaling with the system size play the central role. 

In this talk I will represent an exact relation between the above three measures, namely multifractal dimensions, scaling of fluctuations of local observables and the (Renyi) entanglement entropy. I will show that the fractal dimension of the non-ergodic wave function puts an upper bound on its entanglement entropy [1]. I will also provide a couple of explicit examples demonstrating that the entanglement entropy may reach its ergodic (Page) value when the wave function is still highly non-ergodic and occupies a zero fraction of the total Hilbert space. If time permits I will briefly discuss some other possible deviations from ergodicity relevant for the chaotic many-body systems [2-4].

[1] G. De Tomasi, I. M. K., “Multifractality meets entanglement: relation for non-ergodic extended states”, Phys. Rev. Lett. 124, 200602 (2020) [arXiv:2001.03173]

[2] I. M. K., M. Haque, and P. McClarty, “Eigenstate Thermalization, Random Matrix Theory and Behemoths”, Phys. Rev. Lett. 122, 070601 (2019) [arXiv:1806.09631].

[3] M. Haque, P. A. McClarty, I. M. K. , “Entanglement of mid-spectrum eigenstates of chaotic many-body systems—deviation from random ensembles.” [arXiv:2008.12782].

[4] A. Bäcker, I. M. K., M. Haque,, “Multifractal dimensions for chaotic quantum maps and many-body systems”, Phys. Rev. E 100, 032117 (2019) [arxiv:1905.03099].

The concept of space-evolution (or space-time duality) has emerged as a promising approach for studying quantum dynamics. The basic idea involves exchanging the roles of space and time, evolving the system using a space transfer matrix rather than the time evolution operator. The infinite-volume limit is then described by the fixed points of the latter transfer matrix, also known as influence matrices. To establish the potential of this method as a bona fide computational scheme, it is important to understand whether the influence matrices can be efficiently encoded in a classical computer. In this talk I will present a systematic characterisation of their entanglement -- dubbed temporal entanglement -- in chaotic quantum systems with special focus on dual-unitary circuits. Specifically, I will show that, although Rényi entropies with index larger than one are sub-linear in time, the von Neumann entanglement entropy grows linearly.

 Collective structural arrangements and cell migration are important physical processes underlying tissue development and regeneration. Understanding the complexity of cell-cell interactions and the emergence of collective behavior at the tissue scale presents formidable challenges both experimentally and theoretically.

In this talk, I will discuss recent theoretical work on the dynamical patterns that emerge at the tissue scale from localised rearrangements and topological defects. Using a multi-phase field model, we demonstrate that tissue fluidity stems from cell neighbor exchanges, serving as transient sources of vortical flow. This flow emerges from the relative dispersion of cells at a rate proportional to the frequency of rearrangements. Balancing collective migration with relative cell motion appears to be essential for maintaining tissue shape and fluidity. Using a cell-based model, we study the tissue's response to the presence of a vortex. While solid-like behavior tends toward conical shapes, localised fluidisation triggers the transition to a tube, which is fundamental in biological tissues.

 Electron pairing gives rise to a distinct state of matter: superconductivity, described by an order parameter bilinear in fermions <cc>.  In a series of works, we proposed a fluctuations-based theoretical mechanism for how electron quadrupling can occur (see, e.g. [1]). The prerequisites for this mechanism is strong fluctuations and multiple broken symmetries (for a review, including the review of early works, see [2]). Quadrupling gives a much richer spectrum of states of matter than pairing. Especially intriguing ones are the quadrupling states are associated with dissipationless counterflows, with order parameter <c1c1c2^\dagger c2^\dagger> while electric DC currents are dissipative. This results in a class of states principally different from superconductors and superfluid as they are described by an effective model different from Ginzburg-Landau and Gross-Pitaevskii functionals, but related to a Skyrme-like model [3], resulting in a plethora of new effects, ranging from unconventional magnetic properties to new type of hydrodynamics [9]. One such state where the dissipationless counterflows occur because electron quadrupling leads to spontaneous breaking of time-reversal symmetry was predicted to occur in Ba1-xKxFe2As2 [4]. Experimental evidence for that state was reported  in Ba1-xKxFe2As2  [5,6]. The first evidence came from transport, calorimetric, magnetic, thermal transport, and ultrasound measurements. The theoretical predictions of fermion quadrupling in materials like Ba1-xKxFe2As2 required the existence of quantum vortices carrying an arbitrary fraction of magnetic flux quantum  [7] in superconducting states of the compounds that support fermion quadrupling condensates. The recent experiment on Ba1-xKxFe2As2 [8] observed vortices violating that quantization, i.e. carrying flux, which is not a function of fundamental constants. If time permits, I will also briefly discuss a  topological counterpart of the electron quadrupling state [10]  

[1] E. Babaev A.Sudbo, N.W. Ashcrosft  Nature 431 (7009), 666-668 (2004). E. Babaev cond-mat/0201547

[2]  BV Svistunov, ES Babaev, NV Prokof'ev Superfluid states of matter. CRC press, available on ResearchGate (2015) Chapter 6.10

[3] J Garaud, E Babaev Physical Review Letters 129 (8), 087602 (2022)

[4] TA Bojesen, E Babaev, A Sudbø Physical Review B 88 (22), 220511 (2013) TA Bojesen, E Babaev, A Sudbø Physical Review B 89 (10), 104509 (2014)

[5] Vadim Grinenko, Daniel Weston, Federico Caglieris, Christoph Wuttke, Christian Hess, Tino Gottschall, Ilaria Maccari, Denis Gorbunov, Sergei Zherlitsyn, Jochen Wosnitza, Andreas Rydh, Kunihiro Kihou, Chul-Ho Lee, Rajib Sarkar, Shanu Dengre, Julien Garaud, Aliaksei Charnukha, Ruben Hühne, Kornelius Nielsch, Bernd Büchner, Hans-Henning Klauss, Egor Babaev Nature Physics 17 (11), 1254-1259 (2021)

[6] Ilya Shipulin, Nadia Stegani, Ilaria Maccari, Kunihiro Kihou, Chul-Ho Lee, Yongwei Li, Ruben Hühne, Hans-Henning Klauss, Marina Putti, Federico Caglieris, Egor Babaev, Vadim Grinenko Nature Communications 14 (1), 6734

[7] E. Babaev Phys. Rev. Lett. 89, 67001 (2002)

[8] Yusuke Iguchi, Ruby A Shi, Kunihiro Kihou, Chul-Ho Lee, Mats Barkman, Andrea L Benfenati, Vadim Grinenko, Egor Babaev, Kathryn A Moler Science Magazine  380, 6651 1244-1247 (2023)

[9] E Babaev, B Svistunov arXiv preprint arXiv:2311.04340

[10] E Babaev arXiv preprint arXiv:2401.02551

Moiré excitons promise a new platform with which to generate and manipulate hybrid quantum phases of light and matter in unprecedented regimes of interaction strength. We explore the properties in this regime, through studies of a Bose-Hubbard model of excitons coupled to cavity photons. We show that the steady states exhibit a rich phase diagram with pronounced bistabilities governed by multiphoton resonances reflecting the strong interexciton interactions. In the presence of an incoherent pumping of excitons we find that the system can realize single- and multiphoton lasers.

One standard assumption for classifying topological phases of matter is that the system has an (at least partial) energy gap. More precisely, the corresponding topological invariants become generically ill-defined when the system becomes gapless, i.e. at critical points. Nevertheless, it was observed that certain hallmarks of topological phases, in particular the presence of robust edge modes, survive at criticality.  In this talk, we discuss two distinct ways of how to generalize topological invariants to systems without an energy gap, as well as their properties. We will also introduce a relation between critical hermitian and gapped non-hermitian phases, using the example of one-dimensional systems with chiral symmetry.

Optomechanical systems, where light or a probe field interacts with a small mechanical element are among the largest and heaviest quantum systems that we can control in the laboratory to date. The large mass of optomechanical systems make them particularly interesting as detectors for sensing gravitational fields. In addition, the intrinsically nonlinear quantum dynamics of these systems offer interesting sensing advantages in terms of a favourable Heisenberg scaling. In my talk, I will outline the research direction of deriving the fundamental sensing limits of these systems and consider some applications, including to searches of modified gravity theories.

The classification of point gap topology in all local non-Hermitian (NH) symmetry classes has been recently established. However, many entries in the resulting periodic table have only been discussed in a formal setting and still lack a physical interpretation in terms of their bulk-boundary correspondence. In my talk, I will derive the edge signatures of all two-dimensional phases with intrinsic point gap topology. While in one dimension point gap topology invariably leads to the NH skin effect, NH boundary physics is significantly richer in two dimensions. I will show that there are two broad classes of non-Hermitian edge states: (1) infernal points, where a skin effect occurs only at a single edge momentum, while all other edge momenta are devoid of edge states. (2) NH exceptional point dispersions, where edge states persist at all edge momenta and furnish an anomalous number of symmetry-protected exceptional points.

Heterostructures involving magnetic insulators (MIs) and normal metals (NMs) have been of enormous interest for some time. One particularly appealing aspect of such system is the possibility to design superconducting systems by sandwiching NMs and MIs and proximity coupling them via an exchange interaction. In this talk I will present theoretical work on such spin-fluctuations mediated superconductivity in NMs coupled to antiferromagnetic MIs as well as to MIs with non-coplanar spin and novel unconventional magnetic ground states. Both types of systems feature a rich superconducting phase diagram involving unconventional superconductivity. The latter systems also feature the possibility of exhibiting topological superconductivity.

Statistical mechanics mappings provide key insights on quantum error correction. However, existing mappings assume incoherent noise, thus ignoring coherent errors due to, e.g., spurious gate rotations. We map the surface code with coherent errors, taken as X- or Z-rotations (replacing bit or phase flips), to a two-dimensional (2D) Ising model with complex couplings, and further to a 2D Majorana scattering network. Our mappings reveal both commonalities and qualitative differences in correcting coherent and incoherent errors. For both, the error-correcting phase maps, as we explicitly show by linking 2D networks to 1D fermions, to a ℤ2-nontrivial 2D insulator. However, beyond a rotation angle ϕth, instead of a ℤ2-trivial insulator as for incoherent errors, coherent errors map to a Majorana metal. This ϕth is the theoretically achievable storage threshold. We numerically find ϕth≈0.14π. The corresponding bit-flip rate sin^2(ϕth)≈0.18 exceeds the known incoherent threshold pth≈0.11.

Extreme Ultraviolet light sources based on high-order harmonic generation in gases are now used in many areas of science. The radiation consists of extremely short light bursts, in the 100 as range, allowing for outstanding temporal resolution. We will give an introduction to this field of research.

One application is the measurement of ultrafast electron dynamics in matter, using interferometric techniques. We will describe recent measurements of ultrashort temporal delays in photoionization and of the quantum state of the created electron wavepackets.

Turbulent aerosols are suspensions of small particles in a turbulent gas. The analysis of such highly non-linear and multi-scale dynamical systems poses formidable challenges. Laboratory experiments resolving the particle dynamics have only recently become possible, and direct numerical simulations of such systems are still immensely difficult. In this talk I describe recent progress in understanding the dynamics of turbulent aerosols using highly idealised  statistical models that  capture the relevant physics, and that allow systematic mathematical analysis using dynamical-systems theory. I summarise how this helps to understand fundamental mechanisms determining the particle dynamics: small-scale fractal clustering, caustic singularities, and anomalously large relative particle velocities. I highlight successes and failures of this approach, as well as the most important open questions. 

Superfluid phases of 3He possess unique variety of properties determined

by topology in real and momentum spaces. Novel topological phases of

this p-wave superfluid can be engineered by placing the fluid into

nanostructured confinement. We study experimentally the polar phase of

3He, stabilized between long nm-diameter solid strands. In the momentum

space of the polar phase, we demonstrate existence of the Dirac nodal

line in the energy spectrum of Bogoliubov quasiparticles and its

robustness to disorder introduced by impurities provided by extension of

the Anderson theorem [1]. The nodal line reduces Landau critical

velocity in the polar phase to zero, but we nevertheless observe stable

superflow as the nodal line transforms to topological Bogoliubov Fermi

surface under the flow [2]. In the real space of the polar phase, we

create half-quantum vortices (HQVs) [3] using in particular the

Kibble-Zureck mechanism controlled by a symmetry-breaking bias field

[4]. We then transfer HQVs through a sequence of transitions to other

superfluid phases [5] to realize composite topological objects suggested

by Kibble, Lazarides, and Shafi for cosmological phase transitions.

[1] T. Kamppinen et al, arXiv:1908.01645v4 (2022).

[2] S. Autti et al, Phys. Rev. Research 2, 033013 (2020).

[3] S. Autti et al, Phys. Rev. Lett. 117, 255301 (2016).

[4] J. Rysti et al, Phys. Rev. Lett. 127, 115702 (2021).

[5] J.T. Mäkinen et al, Nature Commum. 10, 237 (2019).

I will provide a rigorous operator algebraic framework of dynamics in locally interacting systems in any dimension. It is based on pseudolocal dynamical symmetries generalising pseudolocal charges. This generalization proves sufficient to construct a theory of all sufficiently local quantum many-body dynamics in closed, open and time-dependent systems, in terms of time-dependent generalized Gibbs ensembles. These ensembles unify seemingly disparate manifestations of quantum non-ergodic dynamics including quantum many-body scars, continuous, discrete and dissipative time crystals, Hilbert space fragmentation, lattice gauge theories, and disorder-free localization. In the process novel pseudo-local classes of operators are introduced: "restricted local", which are local only for some states, and "crypto-local", whose locality is not manifest in terms of any finite number of local densities. This proven theory is intuitively the rigorous algebraic counterpart of the eigenstate thermalization hypothesis and has implications for thermodynamics: quantum many-body systems, rather than merely reaching a Gibbs ensemble in the long-time limit, are always in a time-dependent generalized Gibbs ensemble for any natural initial state.

References:

Berislav Buca. Unified theory of local quantum many-body dynamics: Eigenoperator thermalization theorems. arXiv:2301.07091 (2023).

Benjamin Doyon. Thermalization and pseudolocality in extended quantum systems. Commun. Math. Phys. 351: 155-200 (2017).

Experimental realization and detection of majorana fermions (who are their own antiparticles) and non-Abelian anyons (which have non-Abelian exchange statistics) are among the major goals in physics.  In recent years, there have emerged a number of solid state material platforms predicted to host quasiparticles behaving like majorana fermions and non-Abelian anyons, with the latter attracting strong interests for potential applications in topologically protected quantum information.   In this talk, I will describe our recent experimental programs studying topological insulators interfaced with superconductors, where a “topological superconductor” is supposed to emerge that can host (itinerant) majorana fermions, which can become non-Abelian anyons when localized as majorana zero modes. I will describe efforts to develop relevant materials and heterostructures, characterize key properties and develop experimental probes (transport including those with spin-sensitive devices and phase-sensitive Josephson junctions, and even STM based local probes) that may help detect and manipulate majoranas and non-Abelian anyons in these material platforms.  If time allows, I may briefly discuss another emerging anyonic material platform -- candidate quantum spin liquid materials such as insulating RuCl3, where spin-based excitations are suggested to give a charge-less version of majorana fermions and non-Abelian anyons under appropriate conditions.

Materials with flat energy bands close to the Fermi level often exhibit extraordinarily high critical ordering temperatures for symmetry breaking orders. The currently most studied example is likely magic-angle twisted bilayer graphene where both superconductivity and other correlated phases appear, but also simpler carbon structures such as ABC-stacked graphite exhibits flat bands.   

In this talk I will first show how full-scale atomistic modelling of magic-angle twisted bilayer graphene generates an unexpected superconducting state. Specifically, solving self-consistently for superconductivity assuming local electronic interactions, mimicking closely those of the high-temperature cuprate superconductors, we find d-wave nematic ordering on both the atomic and moiré lattice length scales. Despite the d-wave nature, the superconducting state surprisingly has a full energy gap. These results show that the superconducting state in twisted bilayer graphene can be distinctly different from that of both monolayer graphene and the cuprate superconductors.    

Then I will show how flat bands generically give rise to a universal phase diagram with doping. In particular, the critical ordering temperature follow one of two universal phase diagram curves with doping away from the flat band, depending on it being superconducting or magnetic/charge ordering. Notably, we find that superconductivity survives to decisively higher doping, and thus, even if a magnetic or charge order initially dominates, superconducting domes are still likely to exist on the flanks of flat bands. This is consistent with the behavior of twisted bilayer graphene and, as an additional example, we illustrate how these results can be directly applied to the topological surface flat bands of rhombohedral or ABC-stacked graphite.      

The orbital and spin magnetization of a cavity-embedded quantum square dot array  defined in a GaAs heterostructure are calculated within quantum-electrodynamical density-functional theory (QEDFT). To this end, a gradient-based exchange-correlation functional recently employed for atomic systems is adapted to the hosting two-dimensional  electron gas (2DEG) submitted to an external perpendicular homogeneous magnetic field. The functional was derived through the adiabatic-connection fluctuation-dissipation theorem by J. Flick [1] and could facilitate the investigation of the effects of cavity photons on extended 2DEG systems. 

Numerical results reveal the polarizing effects of the cavity photon field  on the electron charge distribution and nontrivial changes of the orbital magnetization.  We discuss its intertwined dependence on the electron number in each dot,  and on the electron-photon coupling strength [2].

[1] J. Flick, Phys. Rev. Lett. 129, 143201 (2022).

[2] V. Gudmundsson et al, Phys. Rev. B 106, 115308 (2022).

A challenge in photonics is to create a scalable platform in which topologically protected light can be transmitted over large distances. I will talk about the design, modelling, and fabrication of photonic crystal fibre (PCF) characterised by topological invariants [1]. The fibre is made using a stack-and-draw technique in which glass capillaries are stacked, molten, and drawn to the desired size. Light propagates in glass cores, whose normal modes are analogous to atomic orbitals. Topological invariants emerge in the band structure of many coupled cores inside a periodic array. We directly measure the bulk winding-number invariant and image the associated boundary modes predicted to exist by bulk-boundary correspondence. The mechanical flexibility of fibre allows us to reversibly reconfigure the topological state. As the fibre is bent, we find that the edge states first lose their localization and then become relocalized due to disorder. We envision fibre as a scalable platform to explore and exploit topological effects in photonic networks.

[1] Nathan Roberts, Guido Baardink, Josh Nunn, Peter J. Mosley, Anton Souslov. Topological supermodes in photonic crystal fibre. Science Advances 8, add3522 (2022).

Fermi liquid theory is a cornerstone of condensed matter physics. However, Landau's formulation of Fermi liquid theory does not fit into the paradigm of effective field theory. We describe a new method that leads to a field-theoretical reformulation of Landau Fermi liquid theory. In this approach, a system with a Fermi surface is described as a coadjoint orbit of the group of canonical transformations. The method naturally leads to a nonlinear bosonization of the Fermi surface. The Berry phase that the Fermi surface acquires when changing shape is shown to be given by the Kirillov-Kostant-Souriau symplectic form on the coadjoint orbit. We show that the resulting local effective field theory captures both linear and nonlinear effects in Landau’s Fermi liquid theory. Possible extensions and applications of the theory are described. (Reference: Luca Delacrétaz, Umang Mehta, Yi-Hsien Du, DTS arXiv:2203.05004.)

In this talk, I will review how quantum fluctuations in mixtures of bosonic atoms can lead to the formation of self-bound droplets. Emphasis will be on persistent currents in ring-trapped binary mixtures as well as dipolar supersolids. For binary condensates with equal intra-component interactions but an unequal number of atoms in the two components, there is an excess part that cannot bind. A droplet then becomes amalgamated with a residual condensate. This results in particular rotational behavior that sheds new light on the coexistence of localization and superfluidity.

The jerky dynamics of domain walls driven by applied magnetic fields in disordered ferromagnets—the Barkhausen effect—is a paradigmatic example of avalanches or crackling noise. I’ll start by discussing our recent study of Barkhausen noise in disordered Pt/Co/Pt thin films with perpendicular magnetic anisotropy due to precessional motion of domain walls using full micromagnetic simulations, allowing for a detailed description of the domain wall internal structure consisting of topological defects known as Bloch lines [1]. The scaling features of the Barkhausen jumps appear to agree with those of the quenched Edwards-Wilkinson (qEW) equation, but since the reachable domain wall lengths in micromagnetic simulations are limited to a few micrometers, this result could be affected by finite-size effects. To overcome such limitations, I’ll discuss our very recently developed collective coordinate model of thin film domain walls able to account for the internal degrees of freedom of the domain wall while allowing us to reach much larger system sizes. The model exhibits a depinning phase transition transition with disorder-dependent scaling: For weak disorder, excitations of the internal magnetization are rare, and the depinning transition takes on exponent values of the quenched Edwards-Wilkinson equation. Stronger disorder results in disorder-dependent exponents concurrently with nucleation of an increasing density of Bloch lines within the domain wall [2]. 

[1] T. Herranen and L. Laurson, Phys. Rev. Lett. 122, 117205 (2019).

[2] A. Skaugen and L. Laurson, Phys. Rev. Lett. 128, 097202 (2022).

New scientific discoveries in the broad field of quantum nanoelectronics have always been linked to the development of cryogenic refrigeration technologies. Colder electron temperatures allow for the observation of novel phenomena on a lower energy scale, which is limited by the thermal energy.

In this talk, I will show our efforts of cooling down nanoelectronic devices well below 1 mK, and discuss the associated challenges with materials and metrology. Our results pave the way for quantum nanoelectronic devices with on-chip integration of electron cooling in the millikelvin and microkelvin regime.

References:

N. Yurttagul, M. Sarsby, A. Geresdi, Phys. Rev. Applied 12, 011005 (2019)

M. Sarsby, N. Yurttagul, A. Geresdi, Nature Communications 11, 1492 (2020)

N. Yurttagul, M. Sarsby, A. Geresdi, Journal of Low Temperature Physics 204, 143 (2021)

It is common lore to assume that the ground state of a given quantum field theory is uniform. Understanding the conditions under which a spatially nontrivial structure can arise from local translationally invariant interactions is an intriguing problem in quantum many-body physics. In the first part of the talk, I will review some known mechanisms for formation of nonuniform order. Then I will focus on the lattice of topological solitons that is induced in quark matter by strong magnetic fields. The universality of this chiral soliton lattice (CSL) order is underlined by the mathematical analogy between dense quark matter and chiral magnets. In the end, I will give a brief overview of the various realizations of CSL in quark matter, discovered in the recent years.

I will give an extended introduction to the physic of breakdown of thermalisation in closed quantum systems with focus on many-body localisation. I’ll explain the effective theory of many-body localisation given in terms of l-bits and then discuss recent results that show extremely slow growth of particle number entropy and what they may mean for localization.

Time translation and time reversal are accurate symmetries of fundamental laws, but they can be violated in interesting ways within material systems.  I will discuss some new developments on these themes: time crystals based on ferroelectrics and on Q-balls; the question of $T$ violation in biology; and a remarkable potential universality class related to (emergent) axion physics.

The flatbands of Moiré materials provide a rich playground for the study of strongly correlated phases of matter. I will discuss the emergence of high temperature fractional Chern insulators in these systems [1,2], as well as novel symmetry breaking competing states emerging as a consequence of the underlying quantum geometry of the flatbands [1,3]. Recent experiments are in agreement with our predictions [4].

[1] A. Abouelkomsan, Z. Liu and E.J. Bergholtz, PRL 124, 106803 (2020).

[2] Z. Liu, A. Abouelkomsan and E.J. Bergholtz, PRL 126, 026801 (2021).

[3] Work in preparation with A. Abouelkomsan and K. Yang.

[4] Xie et. al. (Yacoby’s group), Nature 600, 439 (2021).

At the interface between a ferromagnetic insulator and a superconductor there is a coupling between the spins of the two materials. We show that when a supercurrent carried by triplet Cooper pairs flows through the superconductor, the coupling induces a magnon spin current in the adjacent ferromagnetic insulator. The effect is dominated by Cooper pairs polarized in the same direction as the ferromagnetic insulator, so that charge and spin supercurrents produce similar results. Our findings demonstrate a way of converting Cooper pair supercurrents to magnon spin currents [1].

[1] L. G. Johnsen, H. T. Simensen, A. Brataas, and J. Linder, Phys. Rev. Lett. 127, 207001 (2021)

The charge carrier-dependent spectral function underpins the electronic and optical properties of materials. Here, I will demonstrate the powerful concept of unifying spectroscopic and transport measurements of the energy- and momentum-dependent spectral function using angle-resolved photoemission spectroscopy with nanoscale spatial resolution (nanoARPES). I will show that this technique allows for a noninvasive local measurement of composition, structure, many-body effects and carrier mobility of devices composed of two-dimensional heterostructures in the presence of high current densities and tunable charge carrier concentrations [1,2,3]. Future opportunities for the experiments will be discussed.

[1] R. Muzzio et al. Phys. Rev. B 101, 201409(R) (2020) 

[2] D. Curcio et al. Phys. Rev. Lett 125, 236403 (2020) 

[3] Nguyen et al. Nature 572, 220 (2019) 

I will talk about the conditions for the emergence of the preformed Cooper pairs in materials hosting flat bands. As a particular example, I will consider a semimetal with a pair of three-band crossing points at which a flat band intersects with a Dirac cone. I will argue that the nearly dispersionless nature of the flat band promotes local Cooper pair formation so that the system may be modeled as an array of superconducting grains. On the other hand, Andreev scattering between the grains, thanks to the dispersive bands, gives rise to the phase-coherent superconductivity at low temperatures. I will talk about how to calculate transition temperature between the preformed Cooper pair state and the phase-coherent state. 

Quantum geometry, namely quantities such as quantum metric, Berry curvature, and Chern number, have become increasingly important in understanding interacting many-body systems in solid state and ultracold gas quantum matter. We have shown that supercurrents and superfluidity in a flat band are governed by quantum geometry [1], which opens new prospects for achieving high temperature superconductivity. These findings have become relevant for superconductivity in twisted bilayer graphene and ultracold gas moiré materials [2]. We present our newest results on the topic, showing that to achieve the critical temperature enhancement, the flat band does not need to be isolated from other bands, which is promising from the experimental perspective [3]. We discuss also interesting flat band effects in the normal superconducting states could be observed [4-6]. Further, we show that quantum geometry governs the behaviour of bosonic condensates in flat bands as well, making quantum fluctuation effects remarkably strong [7].

References     

[1] S. Peotta, P. Törmä, Nature Communications 6, 8944 (2015)   

[2] P. Törmä, S. Peotta, B.A. Bernevig, arXiv:2111.00807, review article accepted for publication in Nat. Rev. Phys. (2022)   

[3] K.-E. Huhtinen, J. Herzog-Arbeitman, A. Chew, B.A. Bernevig, P. Törmä, arXiv:2203.11133 (2022)   

[4] K.-E. Huhtinen, P. Törmä, Phys. Rev. B 103, L220502 (2021)   

[5] P. Kumar, S. Peotta, Y. Takasu, Y. Takahashi, P. Törmä, Phys. Rev. A 103, L031301 (2021)   

[6] V.A.J. Pyykkönen, S. Peotta, P. Fabritius, J. Mohan, T. Esslinger, P. Törmä, Phys. Rev. B 103, 44519 (2021)   

[7] A. Julku, G.M. Bruun, P. Törmä, Phys. Rev. Lett. 127, 170404 (2021)     

Linear response functions are essential for classifying quantum materials' physical and chemical properties. However, there are more profound schemes in the nonlinear spectroscopies, such as the Raman effect, high-harmonic generation, and photovoltaic. In this talk, I will first introduce nonlinear optical spectroscopy. Then, I will discuss recent studies on the many-body formulation of THz nonlinear optics in graphene [1,2], highlighting the importance of a conserving theory where the self-energy and vertex correction are taken into account on the same footing. We show that the nonlinear terms are strikingly ruled by the appearance of a dominant two-photon vertex which is absent at the bare level and finite even in the weak-coupling limit. Afterward, I will discuss the importance of shear phonons in the heterostructure of two-dimensional (2D) materials and explain a recent work [3] on the light-induced displacive Raman force and its application to staking manipulation in van der Waals 2D materials. Finally, I will briefly talk about a current project on the characteristics of the nonlinear spin Hall effect in 2D WTe2 [4].   

[1] H. Rostami and E. Cappelluti, npj 2D Materials and Applications 5, Article number: 50 (2021).  

[2] H. Rostami and E. Cappelluti, Phys. Rev. B 103, 125415 (2021). 

[3] H. Rostami, arXiv:2204.08060 (2022) 

[4] P. Bhalla and H. Rostami, manuscript in preparation. 

The development of donor-acceptor architectures has been a successful strategy to tune the photophysics of polymeric compounds for applications in optoelectronics and energy conversion systems. The latter has gained a significant momentum in the last years thanks to the breakthroughs on power conversion efficiency of organic photovoltaics. In this presentation, the fundamentals of photo-induced electronic transitions and stability of excitonic states will be discussed by means of first-principles calculations (within the framework of density functional theory) on molecular oligomeric models (1,2). It will be argued that the fundamental gap should be calculated from the full Gibbs free energy to properly access the exciton binding energy (Eb). Here, the effect of the medium polarity and molecular geometry will be discussed. Finally, the correlation between Eb and some features of the electronic structure will be presented.   

(1) Leandro Benatto, C. F. N. Marchiori, C. Moyses Araujo and M. Koehlera J. Mater. Chem. C 7, 12180-12193 (2019)

(2) G. B. Damas, C. Marchiori and C. Moyses Araujo J. Phys. Chem. C 123, 25531-25542 (2019)

Abstract: After a short prelude on the recent experimental results on fractional statistics in the 𝜈=1/3 Quantum Hall fluid, I will present some recent results in composite boson theory. The first part is on an extension of the theory that allows for an interpolation between the physical theory, and a model where the mean field approximation can be convincingly justified. The second is about a method to use the the mean field composite boson theory to  derive abelian QH wave functions directly in terms of correlators in a conformal field theory, and in particular I will explain how this helps us to understand the origin of orbital spin.

Abstract: Shot noise in mesoscopic conductors has since long been identified as a valuable tool to analyze the characteristics of conductors. The measurement of noise has for example been used to identify correlations between charge carriers, also the fractional charge of quasiparticles in strongly correlated systems has been extracted from charge current noise. Recently, there have been two interesting novel directions in this “noise spectroscopy”. One one hand not only charge current noise, but also heat current noise and power fluctuations have started to raise interest. This additional observable increases the opportunities for spectroscopy, but is also of practical relevance in (quantum) thermoelectrics [1]. On the other hand, so called delta-T noise was experimentally observed, where charge current shot noise - also known as “partition noise” -  arises despite the absence of an average charge current [2,3] when a pure thermal bias is applied across a conductor with an energy-independent transmission. In this talk, I will present our recent results which generalize these observations. I will discuss charge current shot noise in the absence of charge currents as well as heat shot noise in the absence of heat currents in generic two-terminal conductors under generic non-equilibrium conditions [4]. In thermoelectric conductors, these zero-current shot noises, could correspond to the charge shot noise at the thermovoltage or the heat shot noise at the stopping voltage of a cooling device. In the case where the conductor’s transmission is energy-independent, I show that simple bounds can be found for both types of zero-current noises that can not be exceeded under any non-equilibrium condition. In contrast, as soon as the conductors become energy dependent, these bounds are broken! While the zero-current charge shot noise can then still be shown to never exceed the coexisting thermal noise, the heat shot noise can become arbitrarily much larger than the heat thermal noise. I will show examples for specific conductors for which the shot noise in comparison to the thermal noise can be maximized and I will analyze their characteristics.

[1] S. Kheradsoud, N. Dashti, M. Misiorny, P. P. Potts, J. Splettstoesser, P. Samuelsson: Power, Efficiency and Fluctuations in a Quantum Point Contact as Steady-State Thermoelectric Heat Engine. Entropy 21, 777 (2019)

[2] E. V. Sukhorukov, D. Loss: Noise in multiterminal diffusive conductors: Universality, nonlocality, and exchange effects. Phys. Rev. B 59, 13054 (1999)

[3] O. S. Lumbroso, L. Simine, A. Nitzan, D. Segal, O. Tal: Electronic noise due to temperature differences in atomic-scale junctions. Nature 562, 240 (2018)

[4] J. Eriksson, M. Acciai, L. Tesser, J. Splettstoesser: General bounds on electronic shot noise in the absence of currents. arXiv:2102.12988 (2021), accepted for publication in Phys. Rev. Lett.

Abstract: Describing quantum models on Bravais lattice structures has a long tradition in condensed matter physics. Progress in quantum engineering, however, motivates the study of non-periodic systems, where the choice of lattice structure is seen as a parameter. Here, we investigate the possibility to obtain topological models on fractals and quasicrystals. We show that anyons and the fractional quantum Hall effect can be realized in fractal dimensions between 1 and 2, and we discuss a new type of topological states in quasicrystals.

Abstract: The emergence of collective order in matter is among the most fundamental and intriguing phenomena in physics. In recent years, the ultrafast dynamical control and creation of novel ordered states of matter not accessible in thermodynamic equilibrium is receiving much attention. Among those, the theoretical concept of dynamical multiferroicity has been introduced to describe the emergence of magnetization by means of a time-dependent electric polarization in non-ferromagnetic materials. In simple terms, a large amplitude coherent rotating motion of the ions in a crystal induces a magnetic moment along the axis of rotation. However, the experimental verification of this effect is still lacking. Here, we provide the first evidence of room temperature magnetization in the archetypal paraelectric perovskite SrTiO3 due to this mechanism. To achieve it, we resonantly drive the infrared-active soft phonon mode with intense circularly polarized terahertz electric field, and detect a large magneto-optical Kerr effect. A simple model, which includes two coupled nonlinear oscillators whose forces and couplings are derived with ab-initio calculations using self-consistent phonon theory at a finite temperature, reproduces our experimental observations on the temporal and frequency domains.

Abstract: In his Nobel lecture, Louis Néel who won the 1970 Nobel prize in physics for prediction and discovery of antiferromagnetism, stated "[antiferromagnetic materials] are extremely interesting from the theoretical view point, but do not seem to have any applications”. In contrast to this prediction, the last decade advances in spintronics showed that not only antiferromagnets may have some applications but they are even better than their ferromagnetic counterparts. Now, antiferromagnetic spintronics offers a promising basis for next generation low-power, ultrafast, and dense processing and storage appliances. In this colloquium, I briefly review recent theoretical predictions and experimental progresses in the emerging field of antiferromagnetic spintronics. 

Abstract: Dynamical quantum phase transitions, defined in terms of the many-body Loschmidt echo, generalize the notion of phase transitions and criticality to many-body systems far from equilibrium. This rapidly advancing research field is bringing together themes from statistical physics, topological matter and strongly correlated systems. However, solving the dynamics of strongly interacting quantum systems is a notoriously difficult problem, which is hindering the progress in the field. To circumvent this impasse, I introduce a novel method to determine the singular dynamics by employing so-called Loschmidt cumulants [1]. I will demonstrate the remarkable power of the new method by solving dynamical phase transitions in interacting Kitaev chain and spin-1 Heisenberg chain.

[1] S. Peotta, F. Brange, A. Deger, T. Ojanen and, C. Flindt, Physical Review X 11, 041018 (2021).