Day one, August 17, 2021
Chair: I. Burmistrov; starts at 3:00pm Moscow time
Format: 12+3
Title: Kinematic dynamo in chaotic flows
Abstract: I present kinematic dynamo theory for turbulent conductive fluids. I describe how inhomogeneous magnetic fluctuations are generated below the viscous scale of turbulence. The statistics of magnetic field demonstrates both spatial and temporal intermittency. The case of two-dimensional flow which is subject of famous Zeldovich's anti-dynamo theorem is also discussed.
Format: 12+3
Title: Search for Bose condensation in nuclei
Abstract: Bose condensation is a fascinating Quantum phenomenon of phase transition to quantum condensed phase. It was predicted 100 years ago by Bose and Einstein and has been observed experimentally in many atomic and solid-state systems over the last 25 years. Naturally, the idea that Bose condensation may be possible in systems composed of nucleons has been around for a long time. Nucleons are fermions and to form Bose condensate they have to pair-up into bosons first, such as alpha-particles. Experimental and theoretical search for alpha-condensate in nuclei has been an active area of research in nuclear physics for several decades. In this talk I will briefly review the current status of this search.
Format: 12+3
Title: К истории одной публикации. (On a story of one publication.)
Abstract: TBA
Format: 12+3
Title: Avoided criticality and the phase transition in a striped two-dimensional system: some experimental facts.
Abstract: In ultrathin Fe films on Cu(100), the Fe spins are directed perpendicularly to the film plane. This produces a magnetic ground state that consists of alternating stripes with opposite perpendicular spin orientation. Here we report two aspects of the phase transition in this system. First, we find that the width of the stripes remains finite well above the temperature at which the stripes acquire a mobility, without displaying any singularity as a function of temperature. Second, albeit the conventional ferromagnetic critical point itself is eliminated by the presence of the stripes, the conventional scaling laws are recovered in a temperature and magnetic-field range sufficiently away from the presumed critical point, over up to 80 orders of magnitude of the suitable scaling variable. These two aspects seem to define an interesting and never observed previously situation of “avoided criticality”. (N. Saratz, A. Vindigni, U. Ramsperger, _D. Pescia_, Departement of Physics, ETH Zurich, 8093 Zurich, Switzerland.)
Format: 12+3
Title: Generalized holography with no strings attached
Abstract: The continuing quest into the tantalizing conjecture of some generalized ('beyond the AdS/CFT') holographic correspondence between certain pairs of systems of different dimensionalities remains an active, yet largely unverified (barring those few low-dimensional examples where the effective dimensions are same), endeavor. In that regard, the Wigner function representation offers the natural way of establishing a holography-like correspondence between the descriptions of a generic quantum system in the phase space ('bulk') picture versus its spacetime ('boundary') counterpart. In some cases the former can further reduce to the classical dynamics of a local metric-like variable while the latter takes on the form of bosonized collective field hydrodynamics. This intrinsic (pseudo-)holographic duality neither relies on any particular symmetry of the systems in question, nor does it imply a hypothetical underlying 'string theory', as in the various 'bottom-up' holographic scenarios.
Break 15 minutes
Chair: A. Finkelstein; starts at 4:30pm Moscow time
Format: 12+3
Title: Ferromagnet-Superconductor Hybrids
Abstract: Phenomena discussed in this presentation are based on interaction between spatially separated, but closely located ferromagnets and superconductors. They are called Ferromagnet-Superconductor Hybrids (FSH). These systems include coupled smooth and textured Ferromagnetic and Superconducting films, magnetic dots, wires etc.The interaction may be provided by the magnetic flux from magnetic textures and supercurrents. The magnetic flux from magnetic textures or topological defects can pin vortices or create them, changing dramatically the properties of the superconductor. On the other hand, the magnetic field from supercurrents (vortices) strongly interacts with the magnetic subsystem leading to formation of coupled magnetic-superconducting topological defects. We discuss experimental realization of the FSH.
Format: 12+3
Title: Hot spots along Fermi contour in layered cuprates -- a traditional explanation
Abstract: Angle resolved photoemission spectroscopy unambiguously demonstrates hot spots along the Fermi contour in high-*T*c cuprates. Together with linear
temperature dependence of the resistivity those properties are considered as a hint of nonconventional behavior of high-*T*c cuprates. We give a
traditional explanation of those phenomena as a consequence of the usual s-d exchange interaction applied to copper and oxygen orbitals. Between
highly conducting CuO2 planes it is necessary to take into account also the thermal fluctuations of the electric field. The same s-d Hamiltonian naturally explains the d-wave gap anisotropy as well.
Format: 12+3
Title: Dynamical vortices in superconductors with strong electron-electron repulsion
Abstract: We analyze the structure of a superconducting gap in systems with electron-phonon attraction and electron-electron repulsion. Earlier works have found that superconductivity develops despite strong repulsion, but the gap necessarily changes sign along the Matsubara axis. We analyze the sign-changing gap function from a topological perspective using the knowledge that a nodal point is a center of a dynamical vortex. We consider two models with different cutoffs for repulsive interaction and trace vortex positions along the Matsubara axis and in the upper frequency half plane upon changing the relative strength of the attractive and repulsive components of the interaction. We discuss how the presence of dynamical vortices affects the gap structure along the real axis, detectable in ARPES experiments.
Format: 12+3
Title: Meetings with Valery: space-time landscapes
Abstract: Recollections on meetings and interactions with Valery in different places starting with Academgorodok, 1959.
Break 55 minutes
Chair: M. Feigelman; starts at 6:25pm Moscow time
Format: 12+3
Title: Road to room temperature superconductivity
Abstract: Room temperature superconductivity (RTS) was envisioned shortly after the discovery of the phenomenon in 1911. Increasing temperature washes out the superconducting state. At present, the most promising route to RTS derives from the use of high pressure. Recently, RTS was revealed in the C-H-S compound[1], created by strong electron-phonon coupling[2,3] to high-frequency hydrogen vibrations[4,5]. At present the main goal is to identify systems with RTS at a lower pressure, and ultimately at ambient pressure[6]. Another candidate RTS family is that of two-dimensional nanosystems, namely, π-electrons in aromatic molecules such as coronene[5,7]. In these systems size quantization overcomes the thermal smearing, and Josephson tunneling between molecules can give rise to macroscopic RTS.
References.
1. E.Snider, N.Dozenbrook-Gammon, R.McBride,M.Debebar, H.Vindana, K.Vencatasamy,,K.Karler,A.Salamat,R.Dias,Nature 586,376 (2020)
2. V.Pokrovsky,JETP,13,447 (1961)
3. B.Geilikman and V.Kresin, Sol.State 7, 8659 (1966)
4. L.Gor’kov and V.Kresin, Rev. of Mod. Phys. 90, 011001 (2018)
5. V.Kresin, S.Ovchinnikov,S.Wolf, Superconducting State; Mechanisms and Materials, OUS, Oxford, UK (2021)
6. H.Song,D.Duan,T.Cui,V.Kresin, Phys.Rev.B 102, 014510 (2020)
7. V.Kresin, Y.Ovchinnikov, Annals Phys. 417, 168141 (2020)
Format: 30+5
Title: Slow inter-minimum relaxation and its consequence for BEC of magnons
Abstract: Recent experiment [1] showed that the condensate magnons in Yttrium Iron Garnet film repulse each other. Another experiment [2] demonstrated
that the numbers of magnons in two symmetric minima of magnon spectrum are roughly equal. Both results contradict to theoretical predictions [3, 4] based on assumption that the equilibrium distribution is established in the low-energy magnon gas. We analyze coherent processes that transfer magnons from one minimum of energy to another and show that the time for inter-minimum relaxation is much longer than the lifetime of magnons. Therefore, the inter-minima equilibrium is not established, though it is established within each minimum. We analyze what are immediate consequences from this fact for the process of BEC of magnons under permanent pumping and consider the further prospect of theory and experiment.
References
[1] I.V. Borisenko, B. Divinskiy, V.E. Demidov, G. Li, T. Nattermann, V.L. Pokrovsky and S.O. Demokritov, Nat. Comm. 11, 1691 (2020).
[2] I.V. Borisenko, V.E. Demidov, V.L. Pokrovsky and S.O. Demokritov, Sci. Rep. 10, 14881 (2020).
[3] F. Li, W. Saslow and V.L. Pokrovsky, Sci. Rep. 3, 1372 (2013).
[4] I.S. Tupitsyn, P.C.E. Stamp, and A.L. Burin, Phys. Rev. Lett. 100, 257202 (2008).
Format: 25+5
Title: Superfluid Helium 3 and Beyond
Abstract: The discovery experiments of the superfluid phases of liquid helium 3 at Cornell University which took place almost half a century ago are discussed briefly. The experimental apparatus will be described along with the NMR properties of these phases and their interpretation. A flurry of theoretical and experimental activity followed and lasted for many years. The Cooper pairs associated with these new phases have orbital angular momentum l=1 in contrast to the l=0 angular momentum of ordinary superconductors. A discussion of other systems exhibiting l > 0 will be given including neutron stars, l=2 (d-wave) high temperature superconductors, and more recently uranium based superconductors in very high magnetic fields. Finally, collective ultra sound modes in superfluid helium 3 can be related to an analogue of the Higgs field in particle physics.
Format: 25+5
Title: TBA
Abstract:
Format: 25+5
Title: Quantum Hall states as a quantum rotating superfluid
Abstract: Since early days of superfluidity quantum liquids were central to understanding collective quantum matter. Apart from superconductors, two quantum liquids: superfluid Helium and (fractional) quantum Hall states are the most studied and measured. The aim of this talk is to emphasize an intimate relation between the two. In short, the theory of fast rotating superfluid is formally equivalent to that of FQHE under identifying the frequency of rotation of the superfluid with the Larmor frequency, vortices with electrons and the ratio between vortices and the number of atoms in superfluid with the filling fraction in FQHE. The physical difference between the two is that the hydrodynamics of the superfluid helium is semiclassical, while that of FQH states is ultra quantum.
In the talk I review recent development in quantum hydrodynamics suitable for the fractional quantum Hall effect.
Day two, August 18, 2021
Chair: A. Kitaev; starts at 3:00pm Moscow time
Format: 25+5
Title: Non-equilibrium Sachdev-Ye-Kitaev model with quadratic perturbations
Abstract: We consider a non-equilibrium generalization of the mixed SYK4+SYK2 model and calculate the energy dissipation rate W(ω) that results due to periodic modulation of random quadratic matrix elements with a frequency ω. We find that W(ω) possesses a peak at ω close to the polaron energy spliting E1 found recently in [1], demonstrating physical significance of the energy E1. Next, we study the effect of energy pumping with a finite amplitude A1 at the resonance frequency ω = E1 and calculate, in presence of pumping, non-equilibrium boson distribution function and dissipation rate due to low-frequency parametric modulation. At sufficiently strong pumping, low-frequency dissipation rate recovers its large value, pertinent to the pure SYK4 limit.
[1] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.196602
Format: 25+5
Title: Quasi-static and dynamic properties of magnon Bose-Einstein condensate in a potential well: evidence for spin-fluid behavior
Abstract: Bose-Einstein condensation (BEC) of magnons is one of few macroscopic quantum phenomena observed at room temperature. Since its discovery, it became an object of intense research, which led to the observation of many exciting phenomena such as interference of two condensates, quantized vortices, and second sound. However, it remained unclear, what physical mechanisms is responsible for the spatial stability of the magnon condensate. Indeed, since magnons are believed to exhibit attractive interaction, it is generally expected that the condensate is unstable with respect to the real-space collapse, contrarily to experimental findings. Here I will discuss experiments on BEC in non-uniform applied magnetic field provided a clear evidence of repulsive magnon-magnon interaction in the condensate, resulting in a suggestion that the condensate behaves as a spin-fluid with a finite compressibility. This suggestion is confirmed by the condensate dynamics in a potential well: we observe oscillations of the condensate density. Space- and time-resolved studies reveal that the oscillating mode is non-uniform with periodical redistribution of the condensate density from the middle of the well to its edges and vice versa.
Format: 25+5
Title: From magnon Bose-Einstein condensation to a magnonic qubit
Abstract: There is an enormous need for faster and more efficient information processing. Quantum computing is widely discussed as future computing technology, especially with regard to computing power and scaling properties.
Macroscopic quantum states of matter such as Bose-Einstein condensate (BEC) are excellent candidates for quantum information processing, particularly due to their inherent coherency. The wave function of the BEC describes a highly populated boson state and thus justifies a semiclassical approach.
In this talk, I will start with the principles of the magnon BEC. I will show that using the raping cooling mechanism as a new and universal approach enables BEC of magnons in nanostructured systems [1]. We can also prepare the magnon BEC in confined systems employing parallel parametric pumping [2].
Further, I will present a way to enable room-temperature quantum computing functionalities using a two-wavevector component magnon BEC in magnetic films [3]. It is based on the fact that the dispersion characteristics of the magnons in an in-plane magnetized magnetic film have two energy minima at finite opposite wavevectors, where the magnon condensation occurs. Similar to a qubit in quantum computing, the superposition of a two-component magnon BEC can be described as a state on the surface of a Bloch sphere. We developed novel methods to initialize and manipulate such magnon BECs as a qubit representative. For instance, I will show that the magnon qubit can be initialized by wavevector selective parallel pumping enabling the population of both BECs with pre-defined occupation number ratio. Furthermore, the concept of Rabi oscillations is translated into a magnonic qubit scenario and can be used for qubit manipulation. The applicability of the proposed methods is demonstrated by numerical simulations [3].
Our investigations greatly extend the freedom to study the dynamics of magnon BEC in confined systems and to design integrated circuits for magnon BEC-based applications at room temperature. Moreover, they bridge the fields of quantum computing and macroscopic quantum states of magnons.
Financial support of ERC within the Advanced Grant 694709 “SuperMagnonics” is gratefully acknowledged.
[1] M. Schneider et al., Bose–Einstein condensation of quasiparticles by rapid cooling, Nat. Nanotechnol. 15, 457 (2020).
[2] M. Mohseni et al., Bose-Einstein condensation of nonequilibrium magnons in confined systems, New J. Phys. 22, 083080 (2020).
[3] M. Mohseni et al., Towards quantum computing using magnon Bose-Einstein condensates, In preparation (2021).
Break 30 minutes
Chair: K. Tikhonov; starts at 5:00pm Moscow time
Format: 25+5
Title: Magnetic Skyrmions
Abstract: In the 1970s Valery Pokrovsky coined the term “skyrmion” for topological distinct spin textures in two-dimensional magnetic films.1 Subsequently, in the 1990’s, he showed that uniaxial anisotropy is beneficial for stabilizing skyrmions in real magnetic films.2 Thus he was well ahead of his time, since it took almost two more decades, before the investigation of skyrmions in magnetic thin films captured the broader imagination of the scientific community, and some of Valery’s ideas were demonstrated in practice.3 I will use this presentation to highlight some of our own contributions investigating magnetic skyrmions in thin films. Using magnetic multilayers we demonstrated that inhomogeneous charge currents allow the generation of skyrmions at room temperature in a process that is remarkably similar to the droplet formation in surface-tension driven fluid flows.4 Micromagnetic simulations reproduce key aspects of this transformation process and suggest a second mechanism at higher currents that does not rely on preexisting magnetic domain structures.5 Indeed, we demonstrated this second mechanism experimentally using non-magnetic point contacts.6 Using this approach, we demonstrated that the topological charge gives rise to a transverse motion on the skyrmions, i.e., the skyrmion Hall effect.7
This work was supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division.
References:
V. L. Pokrovsky, Adv. Phys. 28, 595 (1978).
Ar. Abanaov and V. L. Pokrovsky, Phys. Rev. B 58, R8889 (1998).
W. Jiang, et al., Phys. Rep. 704, 1 (2017).
W. Jiang, et al., Science 349, 283 (2015).
O. Heinonen, et al., Phys. Rev. B 93, 094407 (2016).
Z. Wang, et al., Phys. Rev. B 100, 184426 (2019).
W. Jiang, et al., Nature Phys. 13, 162 (2017).
Format: 25+5
Title: Interferrometric measurements of fractional charge and statistics in quantum Hall systems
Abstract: In 1989, I had the privilege of coauthoring a paper with Valery Pokrovsky. The abstract reads: "We propose various experimental circumstances in which the longitudinal resistance of a two dimensional electron gas in a high transverse magnetic field depends, in a simple and characteristic way, on the charge of the quasiparticle excitations. We propose that experiments of this sort could be used to directly measure the charge of the quasiparticle excitations which carry the dissipative part of the current. While it has been persuasively argued by Laughlin that the Hall conductance itself measures the quasiparticle charge, the connection is indirect, since the Hall current is carried by the condensate, not by the quasiparticles.” As I will discuss, this past year, definitive interferrometric measurements - extensions of some of those we discussed in our paper - have been carried through, directly confirming the theoretically expected values of both the quasi-particle charge and braiding statistics.
Format: 25+5
Title: Bose Einstein Condensation and Superfluidity in Magnons and in Quantum Hall Bilayers
Abstract: Long wavelength electronic particle-hole excitations behave like bosons and can condense at low temperatures.
I will discuss exciton condensation and superfluidity in semiconductor and two-dimensional electron system bilayers, both at B=0 and
In a strong magnetic field, comparing these phenomena with analogous phenomena in magnetic thin films.
Break 60 minutes
Chair: D. Pescia; starts at 7:30pm Moscow time
Format: 25+5
Title: Disordered superconductors as a platform for odd-frequency pairing.
Abstract: I will discuss the possibility of the odd frequency pairing in disordered superconductors. I will show that in these materials the instability to the odd-frequency paired state develops in the vicinity of the conventional superconducting state. The pairing mechanism in this exotic state is owing to the combined effect of the superconducting fluctuations in the s-wave channel and those of the spin-density. We believe that disordered superconductors are the systems where this pairing might have already been observed. In particular, this may explain the existence of the pseudo-gap and the insulating behavior near the vicinity of the superconducting domain in the strongly disordered superconducting films.
The work is done in collaboration with Dr. Vladimir Zyuzin
Format: 25+5
Title: Topological Defects and Phase Transitions
Abstract: This talk reviews some of the applications of topology and topological defects in phase transitions in two-dimensional systems for which Kosterlitz and Thouless split half the 2016 Physics Nobel Prize. The theoretical predictions and experimental verification in two dimensional superfluids, superconductors and crystals will be reviewed because they provide very convincing quantitative agreement with topological defect theories.
Format: 25+5
Title: Turbulence in one dimension
Abstract: 1d turbulence provides a useful toy model for the general theory, much like the Ising model for critical phenomena. I plan to briefly discuss old and new results, including analogy with Bose gas, turbulence and butterflies, non-Gaussian distribution of the velocity and other topics if time permits (it usually doesn’t).
Format: 25+5
Title: Superconducting-like response in driven systems near the Mott transition
Abstract: Over the past decade, pump-probe experiments have revealed superconducting-like response at temperatures much above Tc in three systems: Cuprate superconductor, organic superconductor and Buckyball K3C60. What these materials have in common is that they are all in proximity to a Mott insulator. We analyze the effect of a periodic drive on both the spin liquid and the metal close to the Mott transition. On both sides of the transition, we propose that the periodic drive parametrically couples to a fractionalized charge e boson field, producing a non-equilibrium condensate. The condensate contributes a Drude peak to the physical conductivity with a width much smaller than the equilibrium scattering rate, below the experimental resolution. On the metallic side, if the material is originally close to the Mott transition, the periodic drive can induce a charge gap, moving the sample into the Mott insulator phase. We apply our theory to the organics, and we propose that the apparent gap is not a superconducting gap, but an induced Mott insulator gap.
Zhehao Dai and Patrick Lee, arXiv:2106.08354
<https://arxiv.org/abs/2106.08354>
Day three, August 19, 2021
Chair: I. Kolokolov; starts at 3:00pm Moscow time
Format: 25+5
Title: Landau diamagnetism and de Haas - van Alphen effect versus Integer Quantum Hall effect
Abstract: In frame of a general statistical mechanics we investigate quantum oscillation phenomena in a two-dimensional metal bar. A very instructive derivation of free electron gas diamagnetic moment due to Felix Bloch [1] taking into account the consideration given by Edward Teller [2] gives the possibility of division the electrons on two groups: (i) occupying the quantum states in the bulk and (ii) in the surface of the metal. We show that both groups gives equal contribution to formation of the Landau diamagnetic moment. On the other hand, in neglect of chemical potential oscillations only bulk electrons produce the oscillating magnetisation that is the de Haas-van Alphen effect. However, in 2D case the surface electrons also participate in magnetisation oscillation. Only surface electrons are responsible for quantised Hall resistivity plateau and the existence of dissipationless longitudinal currents [3].
[1] F.Bloch ”Molekulartheorie des Magnetismus” in Handbuch der Radiologie v.2, Leipzig, 1934.
[2] E.Teller, Z. Phys. 67, 311 ((1931).
[3] V.P.Mineev, Phys. Rev. B 75, 193309 (2007) and Phys. Rev. B 76, 209902 (E) (2007).
Format: 25+5
Title: Superoscillations: some recent developments
Abstract: A bandlimited function can vary arbitrarily faster than its highest frequency. I will explain this, and describe some recent developments in what is now a large subject, possibly including: superoscillations and the quantum potential; semiclassical superoscillations; differentiation suppresses superoscillations; superoscillations in Aharonov-Bohm streamlines; superoscillations much rarer for vector than scalar waves.
Format: 25+5
Title: 2D Coulomb glass as a model of strong vortex pinning in thin-film superconductors
Abstract: A glass model of vortex pinning in highly disordered thin superconducting films in magnetic fields B << H_{c2} at low temperatures is proposed. Strong collective pinning of a vortex system realized in disordered superconductors that are close to the quantum phase transition to the insulating phase — such as InO_x, NbN, TiN, MoGe, nano-granular aluminium, and others — is considered theoretically for the first time. Utilizing the replica trick developed for the spin glass theory, we demonstrate that such vortex system is in non-ergodic state of glass type with large kinetic inductance per square L_K. Distribution function of local pinning energies is calculated, and it is shown that it possesses a wide gap, i.e. the probability to find a weakly pinned vortex is extremely low.
Break 30 minutes
Chair: Ar. Abanov; starts at 5:00pm Moscow time
Format: 25+5
Title: Superconducting Josephson diode
Abstract: We consider a SQUID with Josephson junctions possessing higher harmonics of the current-phase relation. At nontrivial values of the flux through the SQUID loop, the system represents a superconducting Josephson diode with different critical currents for the opposite directions of the current flow. We calculate the current-voltage characteristics and the Shapiro steps, paying special attention to asymmetry with respect to the sign of the current.
Format: 25+5
Title: Spin waves and skyrmions in a graphene quantized Hall state
Abstract: I shall review some recent work at Harvard on spin excitations in quantized Hall states in the lowest Landau level of monolayer graphene. In earlier work, it was found that spin waves were electrically generated in the = 1 state when a source-drain voltage was applied that exceeded the Zeeman energy [1]. Propagating spin waves can generate a non-local voltage between distant contacts. Spin waves were found to propagate readily across a region with = -1 and, to a lesser extent, across a region with = 0. More recent experiments, using measurements of the electron compressibility with a scanning SET tip, found that injected magnons could significantly reduce the = 1 energy gap, an effect that was argued to arise from the formation of skyrmions [2].
[1]. Di S. Wei, Toeno van der Sar, Seung Hwan Lee, Kenji Watanabe, Takashi Taniguchi, Bertrand I. Halperin, and Amir Yacoby, Electrical generation and detection of spin waves in a quantum Hall ferromagnet, arXiv:1801.08534, Science 362, 229-233 (2018)
[2]. Andrew T. Pierce, Yonglong Xie, Seung Hwan Lee, Patrick R. Forrester, Di S. Wei, Kenji Watanabe, Takashi Taniguchi, Bertrand I. Halperin, and Amir Yacoby, Thermodynamics of free and bound magnons in graphene, arXiv:2103:00015
Format: 25+5
Title: Dynamical phases in the "multifractal " Rosenzweig-Porter random matrix model
Abstract: We consider the return/survival probability in the Rosenzweig-Porter random matrix model with the tailed distribution of off-diagonal elements in a form of large-deviation (or multifractal) ansatz. We show that for a generic distribution of this type one may identify in the region of extended wave functions the following dynamical phases: (1) the exponential relaxation, (2) the stretch-exponential relaxation, (3) the frozen-dynamics phase with the polynomial decay of return/survival probability with time. We compute analytically the line of the corresponding phase transitions for the logarithmically-normal distribution of hopping. Furthermore, we show that the Anderson model on a random regular graph with the nearest-neighbor hopping and non-tailed (e.g. Gaussian) distribution of diagonal disorder is equivalent to the multifractal Rosenzweig-Porter model with a special symmetry. This symmetry leads to existence of only one extended dynamical phase, namely, the one with the stretch-exponential relaxation of return/survival probability which is equivalent to sub-diffusion. We expressed the stretch-exponent in this case in terms of the eigenvalue of the linearized transfer-matrix equation on the corresponding Cayley tree and computed it as a function of disorder. All the results are tested by exact diagonalization numerics.
Break 60 minutes
Chair: V. Kravtsov; starts at 7:30pm Moscow time
Format: 25+5
Title: Magnetization texture production by electric current.
Abstract: In recent years a new type of magnetic memory and logic devices have been proposed. These devices use electric or spin currents to manipulate various topological magnetization textures. In my talk, I will show how to produce new domain walls in nanowires and new skyrmions (skyrmion-antiskyrmion pairs) in magnetic films by electric current.
Format: 25+5
Title: Circuit QED: Encoding quantum information in harmonic oscillators
Abstract: Circuit QED is the application of ideas from non-linear quantum optics to microwave electrical circuits, with superconducting qubits playing the role of artificial atoms. Because of the very strong photon-qubit coupling, quantum control and state engineering at the single-photon level have become routine. In this talk I will describe recent theoretical and experimental advances in bosonic quantum error correcting codes that store quantum information in harmonic oscillators (microwave resonators). By defining symplectic lattices that effectively compactify the oscillator phase space to a torus with area 4\pi\hbar, one obtains a logical code space of two quantum states in which logical Pauli operators correspond to simple (non-commuting) translations. This so-called GKP error-correction code [1] has recently been realized in both ion-trap [2,3] and circuit QED[4] experiments using new theoretical protocols for stabilizing the code words [3,5].
This work was supported by ARO W911NF-18-1-0212 and the Yale Quantum Institute.
References:
Gottesman et al., Phys. Rev. A64, 012310 (2001).
Flühmann et al.,Phys. Rev. X 8, 021001 (2018).
de Neeve et al., arXiv:2010.09681.
Campagne-Ibarcq et al.,Nature 584, 368 (2020).
Royer et al., Phys. Rev. Lett. 125, 260509 (2020).
Format: 25+5
Title: Limit shape phase transitions. A merger of Arctic circles.
Abstract: A limit shape phenomenon in statistical mechanics is the appearance of a most probable macroscopic state. This state is usually characterized by a well-defined boundary separating frozen and liquid spatial regions. The earliest studies related to this phenomenon in the context of crystal shapes are in works by Pokrovsky and Talapov [1]. We propose a class of topological transitions in the limit shape problem of statistical mechanics. The problem is generally known as the Arctic circle problem. The considered transition can be visualized as merging two melted regions (Arctic circles). We establish the mapping, which identifies the transition as the transition known in lattice QCD and random matrix problems [2,3]. The transition is a continuous phase transition of the third order. We identify universal features of the limiting shape and its statistics close to the transition using free fermion description.
[1] V. L. Pokrovsky and A. L. Talapov, Phys. Rev. Lett. 42, 65 (1979). “Ground State, Spectrum, and Phase Diagram of Two-Dimensional Incommensurate Crystals.”
[2] D. J. Gross and E. Witten, Phys. Rev. D, 21 (2): 446, 1980.
[3] M. R. Douglas and V. A. Kazakov. Phys. Lett. B, 319 (1-3): 219–230, 1993. "Large n phase transition in continuum QCD2."
Format: 25+5
Title: Quantum dynamics of open many-qubit systems strongly coupled to a quantized electromagnetic field in dissipative cavities
Abstract: We study quantum dynamics of many-qubit fermion systems strongly coupled to a quantized electromagnetic cavity field, in the presence of decoherence and dissipation for both fermions and cavity photons. The analytic solutions are derived for a broad class of open strongly coupled quantum systems in the Lindblad approximation. They include identical qubits, an ensemble of qubits with a broad distribution of transition frequencies, multi-level electron systems, and parametrically coupled electron-photon-phonon systems. Explicit time dependence of parameters can be included in the solution as long as it is slower than the optical frequency. Compact analytic solutions for time-dependent quantum state amplitudes and observables become possible with the use of the stochastic equation of evolution for the state vector. We show that depending on the initial quantum state preparation, the systems can evolve into a rich variety of entangled states with destructive or constructive interference between the qubits. In particular, dissipation in a cavity can drive the system into the long-lived dark states completely decoupled from the cavity modes.
1. M. Tokman, M. Erukhimova, Y. Wang, Q. Chen, and A. Belyanin, Generation and dynamics of entangled fermion-photon-phonon states in nanocavities, Nanophotonics 10, 491 (2021). https://doi.org/10.1515/nanoph-2020-0353
2. Q. Chen, Y. Wang, S. Almutairi, M. Erukhimova, M. Tokman, and A. Belyanin, Dynamics and control of entangled electron-photon states in nanophotonic systems with time-variable parameters, Phys. Rev. A 103, 013708 (2021).
3. M. Tokman, Q. Chen, M. Erukhimova, Y. Wang, and A. Belyanin, Quantum dynamics of open many-qubit systems strongly coupled to a quantized electromagnetic field in dissipative cavities, https://arxiv.org/abs/2105.14674.
Day four, August 20, 2021
Chair: Y. Fominov; starts at 3:00pm Moscow time
Format: 25+5
Title: Anomalous elasticity of anisotropic flexible two-dimensional materials
Abstract: We develop the theory of anomalous elasticity in 2D flexible materials with orthorhombic crystal symmetry. We argue that in the long-wave limit, which is easily accessible experimentally, these materials possess anisotropic flat phases with anisotropic bending rigidity and Young's modulus which scale with absolute value of momentum in exactly the same way as in the isotropic flat phase. We demonstrate that these anisotropic flat phases are uniquely labeled by the ratio of absolute Poisson's ratios computed with respect to strains applied along two mutually perpendicular directions. We apply our theory to 2D black phosphorus.
Format: 25+5
Title: Quantum eigenstates from classical Gibbs distributions
Abstract: I will discuss how the language of wave functions (state vectors) and associated non-commuting Hermitian operators naturally emerges from classical mechanics by applying the inverse Wigner-Weyl transform to the phase space probability distribution and observables. In this language, the Schrödinger equation follows from the Liouville equation, with ℏ now a free parameter. Classical stationary distributions can be represented as sums over stationary states with discrete (quantized) energies, where these states directly correspond to quantum eigenstates. Interestingly, it is now classical mechanics which allows for apparent negative probabilities to occupy eigenstates. This correspondence is particularly pronounced for canonical Gibbs ensembles, where classical eigenstates satisfy an integral eigenvalue equation that reduces to the Schrödinger equation in a saddle-point approximation controlled by the inverse temperature. This correspondence by showing that some paradigmatic examples such as tunneling, band structures, Berry phases, Landau levels, level statistics and quantum eigenstates in chaotic potentials can be reproduced to a surprising precision from a classical Gibbs ensemble, without any reference to quantum mechanics. At the end I will mention some unpublished results on emergence of negative probabilities and associated doubling of the Hilbert space, which is similar to emergence of spin degrees of freedom.
Format: 25+5
Title: Vector Chiral Phases in the Frustrated 2D XY Model and Quantum Spin Chains
Abstract: The phase diagram of the frustrated 2D classical and 1D quantum XY models is calculated analytically. Four transitions are found: the vortex unbinding transitions triggered by strong fluctuations occur above and below the chiral transition temperature. Vortex interaction is short range on small and logarithmic on large scales. The chiral transition, though belonging to the Ising universality class by symmetry, has different critical exponents due to nonlocal interaction. In a narrow region close to the Lifshitz point a reentrant phase transition between paramagnetic and quasiferromagnetic phase appears. Applications to antiferromagnetic quantum spin chains and multiferroics are discussed.
Co-authors are H. Schenck. and V. L. Pokrovsky.
Break 30 minutes
Chair: D. Feldman; starts at 5:00pm Moscow time
Format: 25+5
Title: Thermal and thermoelectric transport in the two-dimensional disordered electron gas
Abstract: In correlated electron systems, electric, thermal and thermoelectric transport coefficients offer complementary information as they probe different aspects of the electron dynamics. In this talk, I will discuss thermal and thermoelectric transport in the disordered electron gas. I will start out with the thermal conductivity and describe our analysis of the heat density-heat density correlation function, which follows a two-stage procedure: a renormalization group calculation based on the Keldysh nonlinear sigma model in the presence of Luttinger’s gravitational potentials is supplemented with a perturbative study of scattering processes induced by the Coulomb interaction at low (sub-temperature) energies. These scattering processes are at the origin of logarithmic corrections violating the Wiedemann-Franz law. In contrast to electric and thermal transport, thermoelectric transport is very sensitive to particle-hole asymmetry. Since this effect is not accounted for by the conventional sigma model approach, we derive a minimal extension of the Keldysh nonlinear sigma model tailored for two-dimensional interacting systems. I will describe our analysis of the heat density-density correlation function based on this model including ongoing work on interaction corrections to the dynamical part, which gives the thermoelectric transport coefficient.
References:
[1] G. Schwiete and A. M. Finkel’stein, Theory of thermal conductivity in the disordered electron liquid, JETP 122 (3), 567 (2016),
[2] G. Schwiete, Non-linear sigma model with particle-hole asymmetry for the disordered two-dimensional electron gas, PRB 103, 125422 (2021)
Format: 25+5
Title: Bimerons in Magnetic Media
Abstract: Skyrmions are topologically protected spin textures, which are proven to be excellent candidates for elements of future spintronic memory and logic devices. An interesting question is if one can find similar candidates in numerous in-plane magnetized materials. In this talk I will discuss bimerons -– in-plane analogues of skyrmions with the same topological charge. I will first describe their stability, static and dynamics properties in ferromagnets [1]. Then I will turn to antiferromagnets, where I will show that in analogy with skyrmions [2], these topological solitons possess no skyrmion Hall effect [3], and among other interesting properties demonstrate chaotic behaviour when driven by ac-currents in low-damping systems [4].
[1] B. Göbel, A. Mook, J. Henk, I. Mertig, and O.A. Tretiakov, Phys. Rev. B 99, 060407(R) (2019).
[2] J. Barker and O.A. Tretiakov, Phys. Rev. Lett. 116, 147203 (2016).
[3] K. Litzius, et al., Nature Phys. 13, 170 (2017).
[4] B. Göbel, I. Mertig, and O.A. Tretiakov, Phys. Rep. 895, 1 (2021).
Format: 25+5
Title: From Anderson localization on random graphs to many-body localization
Abstract: I will review the physics of Anderson localization on random regular graphs (RRG) and its connections to many-body localization (MBL) in disordered interacting systems. Properties of eigenstate and energy level correlations in delocalized and localized phases, as well at criticality, will be discussed. In the many-body part, models with short-range and power-law interactions are considered, as well as the quantum-dot model representing the limit of the “most long-range” interaction. Central themes—which are common to the RRG and MBL problems—include ergodicity of the delocalized phase, localized character of the critical point, strong finite-size effects, and fractal scaling of eigenstate correlations in the localized phase.
Break 60 minutes
Chair: S. Demokritov; starts at 7:30pm Moscow time
Format: 25+5
Title: The quantum theory of the laser applied to: Bose condensation, radiation from a black hole, the Frölich condensate and COVID19 virus dynamics
Abstract: The original motivation for developing the quantum master equation for the laser was provided by Glauber [1] who said:
“The only reliable method we have of constructing density operators, in general, is to devise theoretical models and to solve the equation of motion for the density operator. These assignments are formidable ones for the case of the laser oscillator and have not been carried out to date.”
In the present talk, the laser quantum master equation analysis [2], satisfying the Glauber assignment, will be shown to provide a useful tool for describing the Bose condensate as an “atom” laser [3] on the one hand and the radiation from atoms falling into a black hole [4] on the other. Furthermore, the dynamics of various problems in biophysics such as Frohlich [5] condensate of collective motion in proteins [6] and calculating the binding energy of the COVID 19 virus to the ACE sites in the body [7], will also be discussed using the quantum theory of the laser formalism.
[1] R. Glauber, Les Houches Lectures, 1964
[2] M. Scully and W. Lamb, PRL, 1964
[3] M. Scully, PRL, 1999
[4] M. Scully et al. PNAS, 2018
[5] Z. Zhang, G. Agarwal and M. Scully, PRL, 2019
[6] H. Fröhlich, Int. J. Quantum Chem., 1968
[7] R. Nessler et al. TBP
Dedicated to Distinguished Professor Valery Pokrovsky on the occasion of his 90th birthday!
Format: 25+5
Title: Thermal interferometry of anyons in spin liquids
Abstract: Aharonov-Bohm interferometry is the most direct probe of anyonic statistics in the quantum Hall effect. The technique involves oscillations of the electric current as a function of the magnetic field and is not applicable to Kitaev spin liquids and other systems without charged quasiparticles. Here, we establish a novel protocol, involving heat transport, for revealing fractional statistics even in the absence of charged excitations, as is the case in quantum spin liquids. Specifically, we demonstrate that heat transport in Kitaev spin liquids through two distinct interferometer geometries, Fabry-Perot and Mach-Zehnder, exhibits drastically different behaviors. Therefore, we propose the use of heat transport interferometry as a probe of anyonic statistics in charge insulators.
Format: 25+5
Title: Out-of-time-order correlators: a two-way approach.
Abstract: Chaos in quantum many-body systems is described by out-of-time-order correlators (OTOCs) of four operators. One may regard two of them as a perturbation source and two other as a probe. A standard approach is to replace the actual source with some classical perturbation and to solve for the probe correlation function on the double Keldysh contour. We show how to obtain the OTOC by combining two such solutions, for perturbations propagating forward and backward in time.
Format: 25+5
Title: Time fractals, incommensurabilities and criticality for Bloch electron dynamics
Abstract: