Condensed Matter Physics Seminars

Three dimensional higher order topological insulators can have topologically protected chiral propagating modes on their hinges. Hinges with two co-propagating chiral modes can serve as a "beam splitter" hinges with only a single chiral mode. Here we show how such a crystal, with Ohmic contacts attached to its hinges, can be used to realize a Mach-Zehnder interferometer. We present concrete calculations for a lattice model of a first-order topological insulator in a magnetic field, which, for a suitable choice of parameters, is an extrinsic second-order topological insulator with the required configuration of chiral hinge modes.

We will also use this opportunity to briefly discuss the organistion of the seminars and to pick a paper for our first journal club for 18th October.

Superfluidity and superconductivity are remarkable manifestations of quantum coherence at a macroscopic scale. The existence of superfluidity has been experimentally confirmed in many condensed matter systems, in 3He and 4He liquids, in nuclear systems including nuclei and neutron stars, in both fermionic and bosonic cold atoms in traps, and it is also predicted to show up in dense quark matter. The interplay between spin-polarization and superfluidity gives rise to fascinating phenomena manifested in Josephson-p junction or in exotic phases like FFLO or Sarma (interior gap) phase, which involve nontrivial behavior of the order parameter.

I will present certain aspects of superfluidity in nonequilibrium conditions, which originate from dynamics of the order parameter both in unpolarized and spin-imbalanced systems. In particular, I will discuss certain properties related to the structure and dynamics of quantum vortices in the context of quantum turbulence. Finally, I will describe the possibility of creation of various exotic structures: spin-polarized droplets (ferrons), disordered structures and supersolids in superfluid cold atomic gases.

Out-of-time-order correlations and Floquet dynamical quantum phase transition
Sara Zamani, R. Jafari, and A. Langari
Phys. Rev. B 105, 094304 (2022)

Out-of-time-order correlators (OTOCs) progressively play an important role in different fields of physics, particularly in the nonequilibrium quantum many-body systems. In this paper, we show that OTOCs can be used to probe the Floquet dynamical quantum phase transitions (FDQPTs). We investigate the OTOCs of two exactly solvable Floquet spin models, namely, Floquet XY chain and synchronized Floquet XY model. We show that the border of driving frequency range, over which the Floquet XY model shows FDQPT, is signaled by the global minimum of the infinite-temperature time averaged OTOC. Moreover, our results manifest that OTOCs decay algebraically in the long time, for which the decay exponent in the FDQPT region is different from that in the region where the system does not show FDQPTs. In addition, for the synchronized Floquet XY model, which reveals FDQPT at any driving frequency depending on the initial infinite or finite temperature, the imaginary part of the OTOCs becomes zero whenever the system shows FDQPT.

Studies of thermal properties at the nanoscale are much less common than the corresponding electrical and magnetic investigations. This is partly due to the lack of fast thermometers capable of tracking the thermal transients that occur when an electrical circuit is driven out of equilibrium due to, say, the rapidly changing current responsible for Joule heating or photons absorbed in a bolometer. Yet, a proper understanding of thermal processes (or more generally of the quasiparticle dynamics) is essential for the failure-free operation of a variety of nanodevices, including nanoscale calorimeters, bolometers, qubits, RSFQ logical circuits, microcoolers and single electron boxes (proposed as metrological standards of electric current). At a fundamental level, time-resolved nanothermometry opens up new possibilities for quantum thermodynamics, which deals with the creation exchange of heat at the level of single particles, involving electrons, phonons and photons, but also resulting from the manipulation of superconducting vortices in the time domain.

I proposed and experimentally demonstrated a new type of nanothermometry, which I dubbed the switching thermometry, for probing the electron temperature of nanostructures in thermal transients with unprecedented resolution approaching a single nanosecond. As a thermometer I use a superconducting nanobridge tested with short (≥1 ns) current pulses to measure the temperature-dependent probability of its transition from the superconducting to the normal state, a process known as switching [1,2]. Interestingly, the bridge probing protocol remains in the analogy with familiar experiment of tossing a coin.

My method has proved to be very attractive in studying fundamental heat exchange mechanisms, involving coupling electrons with phonons [2] and real-time visualization of hot electron diffusion in nanostructures [4]. The developed thermometry allowed also to explore in detail the temporal dynamics of overheated electrons and phonons in superconducting nanostructures at low temperatures [3,5]. Recently, we have measured the heat and subsequent thermal relaxation of the nanostructure arising from expulsion of a single magnetic field vortex with an electric current pulse. This demonstration is a first calorimetric detection of the dissipation in a superconductor due to a single moving vortex. Our experiments show that switching thermometry is a powerful tool for gaining new insights into the thermal physics of low temperatures on previously inaccessible time scales and offers a qualitatively new tool for exploring the emerging discipline of quantum thermodynamics.

1. M. Foltyn, M. Zgirski*, Gambling with Superconducting Fluctuations, Phys. Rev. Applied 4, 024002 (2015)
2. M. Zgirski*, M. Foltyn, A. Savin, K. Norowski, M. Meschke, J. Pekola, Nanosecond Thermometry with Josephson Junctions, Phys. Rev. Applied 10, 044068 (2018)
3. M. Zgirski*, M. Foltyn, A. Savin, K. Norowski, Flipping-Coin Experiment to Study Switching in Josephson Junctions and Superconducting Wires, Phys. Rev. Applied 11, 054070 (2019)
4. M. Zgirski*, M. Foltyn, A. Savin, A. Naumov, K. Norowski, Heat Hunting in a Freezer: Direct Measurement of Quasiparticle Diffusion in Superconducting Nanowire, Phys. Rev. Applied 14, 044024 (2020)
5. M. Zgirski*, M. Foltyn, A. Savin, K. Norowski, Stochastic thermal feedback in switching measurements of superconducting nanobridge caused by overheated electrons and phonons, Phys. Rev. B 104, 014506 (2021)

We investigate the topological phases of two one-dimensional (1D) interacting superconducting wires, and propose topological invariants directly measurable from ground state correlation functions. These numbers remain powerful tools in the presence of couplings and interactions. We show with the density matrix renormalization group that the double critical Ising (DCI) phase discovered in [1] is a fractional topological phase with gapless Majorana modes in the bulk and a one-half topological invariant per wire. Using both numerics and quantum field theoretical methods we show that the phase diagram remains stable in the presence of an inter-wire hopping amplitude t⊥ at length scales below ∼1/t⊥. A large inter-wire hopping amplitude results in the emergence of two integer topological phases hosting one edge mode per boundary, shared between both wires. At large interactions the two wires are described by Mott physics, with the t⊥ hopping amplitude resulting in a paramagnetic order.

https://arxiv.org/abs/2210.05024

Abstract: T.B.A.

Link to the paper. Abstract: Controlling the topological properties of quantum matter is a major goal of condensed matter physics. A major effort in this direction has been devoted to using classical light in the form of Floquet drives to manipulate and induce states with non-trivial topology. A different route can be achieved with cavity photons. Here we consider a prototypical model for topological phase transition, the one-dimensional Su-Schrieffer-Heeger model, coupled to a single mode cavity. We show that quantum light can affect the topological properties of the system, including the finite-length energy spectrum hosting edge modes and the topological phase diagram. In particular we show that depending on the lattice geometry and the strength of light-matter coupling one can either turn a trivial phase into a topological one or viceversa using quantum cavity fields. Furthermore, we compute the polariton spectrum of the coupled electron-photon system, and we note that the lower polariton branch disappears at the topological transition point. This phenomenon can be used to probe the phase transition in the Su-Schrieffer-Heeger model.

Link to paper. Abstract: 2H-NbSe2 is an archetypal system in which superconductivity and charge-density-wave (CDW) coexist and compete macroscopically with each other. In particular, this interplay also manifests in their dynamical fluctuations. As a result, the superconducting amplitude fluctuations (i.e. Higgs mode) is pushed below the quasiparticle continuum, allowing it to become a coherent excitation observable by Raman scattering. In the present study, we coherently drive the collective oscillations of the two orders and visualize their interplay in the driven states in the time domain. We find that both collective modes contribute to terahertz third harmonic generation (THG) and the THG signals interfere below Tc, leading to an anti-resonance of the integrated THG signal. The dynamical Ginzburg-Landau model suggests that around the anti-resonance a periodic energy transfer between the driven Higgs oscillations and the driven CDW oscillations is possible. In addition to 2H-NbSe2, we also studied an underdoped La2-xSrxCuO4 (x ~ 0.12) driven beyond the perturbative regime of THG. A similar interference between two sources of THG is observed below Tc. While there might be additional sources of THG in these experiments, our results illustrate the roles of coupled modes in the terahertz THG process and the tantalizing possibility of coherent control via such couplings.

Bosonic lattice systems with extended interactions constitute a unique platform to study new phases of matter. This work presents an analysis of the Bose-Hubbard model with density-induced tunneling. The U(1) quantum rotor method in the path integral effective action formulation is used. This approach enables the discovery of a second kind of superfluidity in physical systems (unavailable using averaging like methods): pair superfluidity, thus adding to the phases of matter. It also sheds light on the properties of singleparticle Bose-Einstein condensation (BEC) in optical lattice systems with higher inter-particle correlations. The derived effective phase Hamiltonian includes the residue of many-body correlations, providing information about phase transitions between the normal state and single-particle and pair superfluids at finite temperatures. The thermodynamical properties of the system are investigated. The impact of density-induced tunneling on single-particle BEC is also analyzed. The density-induced term supports single-particle coherence at high densities and low temperatures, improving the single BEC critical temperature. It is also responsible for dissipative effects, which are independent of the system's thermal properties.

Abstract: It is known that Alfvén and magnetoacoustic waves both contribute to the heating of the solar chromosphere and drive plasma outflows. In both cases, the thermalization of the wave energy occurs due to ion-neutral collisions, but the obtained rates of plasma heating cannot explain the observational data. The same is true for the magnitude of the outflows.

Aims. The aim of the present paper is to reexamine two-fluid modelling of Alfvén and magnetoacoustic waves in the partially ionized solar chromosphere. We attempt to detect variations in the ion temperature and vertical plasma flows for different wave combinations.

Methods. We perform numerical simulations of the generation and evolution of coupled Alfvén and magnetoacoustic waves using the JOANNA code, which solves the two-fluid equations for ions (protons)+electrons and neutrals (hydrogen atoms), coupled by collision terms.

Results. We confirm that the damping of impulsively generated small-amplitude waves negligibly affects the chromosphere temperature and generates only slow plasma flows. In contrast, waves generated by large-amplitude pulses significantly increase the chromospheric temperature and result in faster plasma outflows. The maximum heating occurs when the pulse is launched from the center of the photosphere, and the magnitude of the related plasma flows increases with the amplitude of the pulse.

Conclusions. Large-amplitude coupled two-fluid Alfvén and magnetoacoustic waves can significantly contribute to the heating of the solar chromosphere and to the generation of plasma outflows.

The presentation will focus on the latest advances in studies of nanoscopic Josephson junctions realized by combining semiconductor and superconducting materials. In the last decade, they have been intensively studied due to the possibility of controlling their superconducting properties via electrical means and implementing topological superconductivity phase. Typically, the properties of such junctions are elucidated indirectly from their supercurrent dependence on gating and/or magnetic field. In this talk, I will present the latest experimental and theoretical efforts on more direct measurement techniques that allow visualization of the supercurrent distribution in the junction through scanning gate microscopy and spectroscopic measurements that reveal the discrete spectrum of quasiparticle Andreev bound states that carry the supercurrent.

Abstract