Posters

(F) = First week (Tuesday 18/07)

(S) = Second week (Monday 24/07)

Ramgopal Agrawal: Nonequilibrium critical dynamics of surface code statistical models         (F)

The statistical mechanics models are often used to determine the error correction threshold for various surface codes. The +\- J Ising model is one of them, where the exchange couplings independently take the discrete value -J with probability p and +J with probability 1-p. Here, we investigate the nonequilibrium critical behavior of the bi-dimensional +\- J  Ising model, after a quench from different initial conditions to a critical point T_c(p) on the paramagnetic-ferromagnetic (PF) transition line, especially, above, below and at the multicritical Nishimori point (NP). The dynamical critical exponent z_c seems to exhibit non-universal behavior for quenches above and below the NP, which is identified as a pre-asymptotic feature due to the repulsive fixed point at the NP. Whereas, for a quench directly to the NP, the dynamics reaches the asymptotic regime with z_c \simeq 6.02(6). We also consider the geometrical spin clusters (of like spin signs) during the critical dynamics. Each universality class on the PF line is uniquely characterized by the stochastic Loewner evolution (SLE) with corresponding parameter \kappa. Moreover, for the critical quenches from the paramagnetic phase, the model, irrespective of the frustration, exhibits an emergent critical percolation topology at the large length scales.

Marco Biroli: TBA         (S)

We consider N Brownian motions diffusing independently on a line, starting at x0>0, in the presence of an absorbing target at the origin. The walkers undergo stochastic resetting under two protocols: (A) each walker resets independently to x0 with rate r and (B) all walkers reset simultaneously to x0 with rate r. We compute analytically the mean first-passage time to the origin and show that, as a function of r and for fixed x0, it has a minimum at an optimal valuer r∗>0 as long as N<Nc. Thus resetting is beneficial for the search for N<Nc. When N>Nc, the optimal value occurs at r∗=0 indicating that resetting hinders search processes. Continuing our results analytically to real N, we show that Nc=7.3264773... for protocol A and Nc=6.3555864... for protocol B, independently of x0. Our theoretical predictions are verified in numerical Langevin simulations.

Liam J. Bond: Fast quantum many-body state preparation using non-Gaussian variational ansatz and quantum optimal control         (F)

We show how to exploit a variational ansatz based on non-Gaussian wavefunctions for fast non-adiabatic preparation of quantum many-body states using quantum optimal control. We demonstrate this on the example of spin-boson model, where we determine the optimal time variation of the Hamiltonian parameters to prepare (near) critical ground states in times which clearly outperform optimized adiabatic protocols. To this end we use the time dependent variational principle and polaron-like ansatz states, which correspond to generalized many-body squeezed cat states of the bosonic modes coupled to the spin.

Adam Y. Chaou: TBA         (S)

Higher-order topological insulators (HOTI) have anomalous boundary signatures of codimension greater than one. Motivated by the susceptibility of boundaries to crystalline symmetry breaking disorder we investigate the role of disorder that preserves the symmetry on average on the boundary signatures of HOTIs with order-two crystalline symmetries. We extend existing classifications of second-order HOTIs with disordered codimension-two boundaries to include disordered boundaries of all codimension and classify third-order HOTIs with disorder on boundaries of all codimension. In so doing we prescribe which anomalous boundary signatures survive the introduction of weak disorder. We further find that strong disorder can drive the system through an extrinsic higher-order, topological transition (while leaving the intrinsic invariant unchanged) and discuss an example of a system where this occurs.

Jeanne Colbois: Extreme value theory and localization in random spin chains         (F)

Despite a very good understanding of single-particle Anderson localization in one- dimensional quantum disordered systems, many-body effects are still full of surprises, a famous example being the interaction-driven many-body localization (MBL) [1, 2]. The non-interacting limit of this problem already shows non-trivial multiparticle physics, which allows to probe some general mechanisms (such as a many-body-induced chain breaking mechanism [3, 4]) using large-scale exact diagonalizations. Here, I will discuss how extreme value theory – best known to predict disasters, for example in hydrology to anticipate floods, in epidemiology to quickly identify emerging diseases – can help us understand some non-trivial effects in the statistics of extreme spin polarizations in two standard models of quantum localization, the XX and the Heisenberg spin chains in a random field [5]. Supported by state-of-the-art numerical simulations at infinite temperature, this analysis leads to the striking observation of a sharp "extreme-statistics transition" in the Heisenberg chain as the disorder changes, which may coincide with the recently debated MBL transition. 

[1] F. Alet and N. Laflorencie, “Many-body localization: An introduction and selected topics”, Comptes Rendus Physique 19, 498 (2018); [2] D. A. Abanin, E. Altman, I. Bloch, and M. Serbyn, “Many-body localization, thermalization, and entanglement”, Rev. Mod. Phys. 91, 021001 (2019); [3] N. Laflorencie, G. Lemarié, and N. Macé, “Chain breaking and Kosterlitz-Thouless scaling at the many-body localization transition in the random-field Heisenberg spin chain”, Phys. Rev. Res. 2, 042033 (2020); [4] M. Dupont, N. Macé, and N. Laflorencie, “From eigenstate to hamiltonian: prospects for ergodicity and localization”, Phys. Rev. B 100, 134201 (2019); [5] J. Colbois and N. Laflorencie, “Breaking the chains: extreme value statistics and localization in random spin chains”, arXiv:2305.10574 (2023)

Lorenzo Correale: Probing semi-classical chaos in the spherical p-spin glass model         (S)

We investigate the semiclassical dynamics of a quantum p-spin glass model within microcanonical shells. Our findings reveal a pronounced maximum of the Lyapunov exponent at a specific energy, indicating heightened chaotic behavior. Simultaneously, we observe a peak in the complexity of the energy landscape, coinciding with the energy of maximal chaoticity. At lower energies, we uncover indications of ergodicity breaking through analysis of the correlation function and fidelity susceptibility. 

Lukas Debbeler: Non-Fermi liquid behavior at flat hot spots at the onset of density wave order         (F)

We analyze quantum fluctuation effects at the onset of charge or spin density wave order with a 2k_F wave vector Q in two-dimensional metals – for the special case where Q connects a pair of hot spots situated at high symmetry points of the Fermi surface with a vanishing Fermi surface curvature. We compute the order parameter susceptibility and the fermion self-energy in one-loop approximation. The susceptibility has a pronounced peak at Q, and the self-energy displays non-Fermi liquid behavior at the hot spots, with a linear frequency dependence of its imaginary part. The real part of the one-loop self-energy exhibits logarithmic divergences with universal prefactors as a function of both frequency and momentum, which may be interpreted as perturbative signatures of power laws with universal anomalous dimensions. As a result, one obtains a non-Fermi liquid metal with a vanishing quasiparticle weight at the hot spots, and a renormalized dispersion relation with anomalous algebraic momentum dependencies near the hot spots.

Jonas F. Karcher: The effect of disorder and nonlinearity on topological slow light         (S)

In photonic crystal waveguides, light can be significantly slowed at wavelengths near the Brillouin zone edge, where the group velocity approaches zero. This has the effect of making light interact more strongly with matter, potentially leading to significant enhancement of nonlinear processes such as frequency comb generation and entangled pair creation. A significant shortcoming of these waveguides is that such slow light devices suffer from a narrow bandwidth and increased backscattering due to fabrication disorder, leaving them prone to Anderson localization. Photonic topological insulators exhibit chiral edge states that are protected from backscattering. These modes typically cross the bulk band gap over a single Brillouin zone. Recently [1], it was proposed that engineering the edge termination of a photonic Chern insulator circumvents this problem by winding the topological edge state many times around the Brillouin zone. This makes these structures suitable to host robust slow light propagation - free of Anderson localization - over a broad range of frequencies. Here, we analytically and numerically study the stability of transport properties along such edges against disorder and nonlinearity. 

[1] J. Guglielmon and M.C. Rechtsman, PRL 122, 153904

Wen-Han Kao: Spin- and Flux-gap Renormalization in the Random Kitaev Ladder         (S)

Recently, the discovery of second-generation Kitaev materials from chemical ion-exchange reactions points out a new route for realizing the Kitaev spin liquid phase. The inevitable quenched disorder in those materials can enhance the level of frustration but at the same time makes the low-energy physics enigmatic. Numerous theoretical studies have considered the weak-disorder effect in the exactly-solvable model, however, the fate of Kitaev systems in the strong-disorder limit is less explored. To address this rather difficult problem, as a first step, we present a strong-disorder renormalization group (SDRG) study on the random Kitaev ladder with the spin-spin couplings pertaining to random distributions. In the Kitaev ladder, themagnetic frustration comes from the bond-directional interactions, where the bonds on the legs (rungs) are covered by alternating x- and y-type (z-type) Ising couplings. Moreover, plaquette fluxes as conserved quantities can be defined on the ladder and remain static under duality transformation, which turns the ladder model into a XY spin chain with transverse fields and fluxes. This provides a proper playground for the interplay between Kitaev physics and the strong-disorder effect since SDRG is well-established in one dimension. In the Ising limit, the behavior of the pseudospin gap is consistent with the familiar random transverse-field Ising chain with accessible analytic solutions, but the flux gap is dominated by the additional y-couplings. In the XX limit, while the x- and  y-couplings are renormalized simultaneously, the z-couplings are not renormalized drastically and lead to non-universal disorder criticality at low-energy scales. Our work points out a new complexity in understanding the strong-disorder effect in frustrated spin systems with local conserved quantities.

[1] W.-H. Kao and N. B. Perkins, Phys. Rev. B 106, L100402 (2022)

Arindam Mallick: Intermediate super-exponential localization with Aubry-André chains         (S)

It is known that similar to the random-disorder-induced Anderson localization, a quasi-period potential on a chain localizes a quantum particle exponentially in its insulating phase. On the other hand in a metallic phase when the quasi-periodic potential is smaller or comparable to the kinetic energy, the particle wave function extends all over the chain. We demonstrate the existence of a previously unobserved intermediate super-exponential localization regime of eigenstates in a quasi-period Aubry-André chain and connect the results with a Wannier-Stark ladder. In that regime, the eigenstates localize factorially. The super-exponential decay emerges on intermediate length scales for large values of the winding length---the quasi-period of the Aubry-André potential. This intermediate localization is present both in the metallic and insulating phases of the system. In the insulating phase, the super-exponential localization is periodically interrupted by weaker decaying tails to form the conventional asymptotic exponential decay predicted for the Aubry-André model. In the metallic phase, the super-exponential localization is followed by a super-exponential growth into the next peak of the extended eigenstate. By adjusting the parameters it is possible to arbitrarily extend the validity of the super-exponential localization. A similar intermediate super-exponential  localization regime is observed in quasi-periodic discrete-time unitary maps.

Francesco Mattiotti: Multifractality in the interacting disordered Tavis-Cummings model         (F)

When quantum emitters and a cavity mode coherently exchange energy at a rate faster than their decay, hybrid light-matter states emerge. Such hybrid states are superpositions composed of “bright” emitter modes and cavity photons, while numerous remaining emitter states have no photon contribution, i.e., remain “dark” [1]. The hybridization of N emitters with a single cavity mode is well captured by the Tavis-Cummings (TC) model. Recently, an extensive study of the single-excitation TC model has shown multifractality of all the eigenfunctions for any strength of the light-matter coupling [2]. Multifractality is well known to be a meaningful feature of critical wave functions at Anderson transitions, with a multifractal spectrum that characterizes the universality class of the transition [3-5]. Here we show that multifractality in the TC model is not limited to single-excitation, analyzing the system at half filling [6]. We demonstrate that a poissonian level statistics coexists with eigenfunctions that are multifractal (extended, but non-ergodic) in the Hilbert space, for all strengths of light-matter interactions, an effect observed in some power-law random matrix models [7]. This is associated with a lack of thermalization for a local perturbation, which remains partially localized in the infinite-time limit. We argue that these effects are due to the combination of finite interactions and integrability of the model. When a small integrability-breaking perturbation (nearest-neighbour hopping) is introduced, typical eigenfunctions become ergodic, seemingly turning the system into a near-perfect conductor, contrary to the single-excitation non-interacting case. We propose a realization of this model with cold atom platforms [8].

[1] T. Botzung, D. Hagenmüller, S. Schütz, J. Dubail, G. Pupillo, and J. Schachenmayer, Phys. Rev. B 102, 144202 (2020); [2] J. Dubail, T. Botzung, J. Schachenmayer, G. Pupillo, and D. Hagenmüller, Phys. Rev. A 105, 023714 (2022); [3] F. Evers and A. D. Mirlin, Rev. Mod. Phys. 80, 1355 (2008); [4] B. L. Altshuler, E. Cuevas, L. B. Ioffe, and V. E. Kravtsov, Phys. Rev. Lett. 117, 156601 (2016); [5] N. Macé, F. Alet, and N. Laflorencie, Phys. Rev. Lett. 123, 180601 (2019); [6] F. Mattiotti, J. Dubail, D. Hagenmüller, J. Schachenmayer, J.-P. Brantut, and G. Pupillo, arXiv:2302.14718 (2023); [7] W. Tang and I. M. Khaymovich, Quantum 6, 733 (2022); [8] N. Sauerwein, F. Orsi, P. Uhrich, S. Bandyopadhyay, F. Mattiotti, T. Cantat-Moltrecht, G. Pupillo, P. Hauke, and J.-P. Brantut, arXiv:2208.09421 (2022), accepted for publication on Nature Physics.

Sen Mu: Kardar-Parisi-Zhang Physics in Two-Dimensional Localized Wave Packets         (S)

In this study, we reveal that Anderson localization of wave packets in two dimensions belongs to the Kardar-Parisi-Zhang (KPZ) universality class. The KPZ universality class refers to a class of non-equilibrium stochastic systems displaying scale-invariant fluctuations characterized by a set of universal critical exponents. First introduced by Kardar, Parisi, and Zhang in the study of surface growth phenomena, KPZ physics was found to describe different types of classical systems subjected to noise or disorder. Recently, numerical and experimental observations of KPZ physics in certain quantum systems have attracted widespread attention: quantum magnets, random unitary circuits, polaritons and finally Anderson localization. In this study, we explore the analogy between two-dimensional Anderson localization and KPZ physics in the context of wave packet dynamics, as recently studied experimentally with cold atoms or ultrasounds. In a disordered medium, an initially peaked wave packet undergoes dynamic evolution, resulting in exponential localization at long times and large distances r. We examine the fluctuations of the wave density in this regime where the envelope of the wave-packet is stationary. We present evidence that − ln |ψ(r)|2 corresponds to a surface height, with the distance r serving as the equivalent of time, in the KPZ process, shown in Fig.1. We find the same critical exponents and statistical distributions controlling KPZ physics. This analogy reveals a new regime of universal fluctuations for Anderson  localization. It could make it possible to use the very elaborate analytical understanding of KPZ physics to describe the still imperfectly understood properties of Anderson localization in dimension two and above.

Goran Nakerst: Spectra of random sparse generators of Markovian evolution         (F)

The evolution of a complex multistate system is often interpreted as a continuous-time Markovian process. To model the relaxation dynamics of  such systems, we introduce an ensemble of random sparse matrices which can be used as generators of Markovian evolution. The sparsity is  controlled by a parameter φ, which is the number of nonzero elements per row and column in the generator matrix. Thus, a member of the ensemble is characterized by the Laplacian of a directed regular graph with D vertices (number of system states) and 2φD edges with randomly distributed weights. We study the effects of  sparsity on the spectrum of the generator. Sparsity is shown to close the large spectral gap that is characteristic of nonsparse random generators. We show that the first moment of the eigenvalue distribution scales as ∼φ, while its variance is ∼√φ.  By using extreme value theory, we demonstrate how the shape of the spectral edges is determined by the tails of the corresponding weight distributions and clarify the behavior of the spectral gap as a function of D. Finally, we analyze complex spacing ratio statistics of ultrasparse generators, φ=const, and find that starting already at φ⩾2, spectra of the generators exhibit universal properties typical of Ginibre's orthogonal ensemble.

https://journals.aps.org/pre/abstract/10.1103/PhysRevE.108.014102

Alessandro Pacco: TBA         (S)

We study statistical properties of high dimensional random energetic landscapes, in particular of the spherical p-spin model. We consider paths interpolating between two minima and inspect the average value of the energy profile le along these paths. For geodesic paths (in configuration space) we find that for most initial/final energies the energy encountered along the path reaches a maximum, that gives an upper bound of the typical energy barrier connecting the two minima. The maximum increases while diminishing the correlation between the minima in configuration space, and it is in most of the cases well above the threshold energy that separates the zone where minima proliferate (below) to where saddles proliferate (above). We improve this by considering deviations to these geodesic paths, constructed by imposing a starting direction given by the shallower direction of the Hessian at the initial point. We find that computing the energy pro le requires to tackle a random matrix problem where we need to study the correlations between the eigenvectors of the Hessian matrices at the initial and final configurations. Our results indicate that from the softest mode of the Hessian at the starting minimum the system “sees” a much easier landscape, full of paths that pass through points below the threshold energy.

Pranay Patil: Anomalous relaxation of density waves in a ring-exchange system         (F)

We present the analysis of the slowing down exhibited by stochastic dynamics of a ring-exchange model on a square lattice, by means of numerical simulations. We find the preservation of coarse-grained memory of initial state of density-wave types for unexpectedly long times. This behavior is inconsistent with the prediction from a low frequency continuum theory developed by assuming a mean-field solution. Through a detailed analysis of correlation functions of the dynamically active regions, we exhibit an unconventional transient long ranged structure formation in a direction which is featureless for the initial condition, and argue that its slow melting plays a crucial role in the slowing-down mechanism. We expect our results to be relevant also for the dynamics of quantum ring-exchange dynamics of hard-core bosons and more generally for dipole moment conserving models.

https://journals.aps.org/pre/abstract/10.1103/PhysRevE.107.034119

Konrad Pawlik: Many Body Localization phase transition, mobility edge and universality of spectral form factor in quantum sun model         (S)

It is currently of great scientific interest to study generic toy models of ergodicity breaking transition, in search for many-body localized (MBL) phase that survives in the thermodynamic limit [1]. Quantum sun model [2], which describes ergodic quantum dot coupled to particles with spin−1/2, is example of such toy model. Numerical simulations reveal that this model exhibits transition between ergodic and MBL phase and exactly at the transition properties of the system do not depend on the system size, suggesting stability of this transition in the thermodynamic limit. We extend analysis of [2] by considering a variant of this model with a conserved projection of the total spin and analysing the biggest symmetry sector. This makes the model more feasible for an experimental implementation using cold-atom platforms, as the total spin corresponds then to the particle number. We find that all hallmarks of the original model, like a universal shape of the spectral form factor independent of system size, are preserved in this variant with conservation up to systems with 24 spins. The biggest studied system sizes are calculated using Polynomially Filtered Exact Diagonalization (POLFED) algorithm [3]. Moreover, model with conservation exhibits the presence of the mobility edge (i.e. a boundary between localized and extended states in the spectrum) in the ergodic phase, which is observed by calculating energy spacing ratio r_n, and averaging it over many disorder realisations in different energy windows around rescaled energy.

[1] Sierant, P. & Zakrzewski, J. (2022), Challenges to observe many-body localization. Physical Review B, 105, 224203; [2]ˇSuntajs, J., & Vidmar, L. (2022). Ergodicity breaking transition in zero dimensions. Physical Review Letters, 129(6), 060602; [3] Sierant, P., Lewenstein, M., & Zakrzewski, J. (2020). Polynomially filtered exact diagonalization approach to many-body localization. Physical Review Letters, 125(15), 156601.

Arianna Poli: Transport exponents crossovers in interacting Weyl semimetals with band deviation from linearity         (F)

We study the quasi-particle and transport properties for a model of interacting Weyl semimetals in the presence of local Hubbard repulsion within DMFT-IPT. The linear dispersion of the bands is at the origin of the large exponents found for the temperature dependence of the resistivity [1], [2], [3]. While large exponents for the resistivity are actually found in M oP and in type−II MoP2 and W P2 materials (the measured longitudinal resistivity along a fixed direction [110] of the MoP for example has the form ρ_xx = ρ_0 + A T^2 + B T^5 [4], [5]) their value though large are not entirely consistent with a DMFT calculation. We then include deviation from the linearity of band dispersion through an intermediate scale energy cutoff Λ and focus on the semimetallic correlated phase. At the nodal point, the spectral weight renormalization modestly depends on Λ whereas it is sensible to the distance to the Mott transition. On the other hand, the quasi-particle scattering rate and resistivity show large temperature exponents that critically depend on cutoff Λ leading to a Weyl to Fermi liquid finite temperature crossover. A related density crossover is found as a function of the chemical potential using the Nenrst-Einstein relation for conductivity. Still, an energy crossover is found in the optical conductivity where the Γopt increases reducing the linear part of the bands.

[1] P. Hosur, S. A. Parameswaran, and A. Vishwanath, “Charge transport in weyl semimetals,” Phys. Rev. Lett., vol. 108, p. 046602, Jan 2012; [2] A. A. Burkov, M. D. Hook, and L. Balents, “Topological nodal semimetals,” Phys. Rev. B, vol. 84, p. 235126, Dec 2011; [3] N. Wagner, S. Ciuchi, A. Toschi, B. Trauzettel, and G. Sangiovanni, “Resistivity exponents in 3d dirac semimetals from electron-electron interaction,” Phys. Rev. Lett., vol. 126, p. 206601, May 2021; [4] N. Kumar, Y. Sun, N. Xu, K. Manna, M. Yao, V. S ̈uss, I. Leermakers, O. Young, T. F ̈orster, M. Schmidt, H. Borrmann, B. Yan, U. M. Zeitler, Shi, C. Felser, and C. Shekar, “Extremely high magnetoresitstance and conductivity in the type-ii weyl semimetals wp2 and mop2,” Nature Communications, vol. 8, no. 1, p. 1642, 2017; [5] N. Kumar, Y. Sun, M. Nicklas, S. J. Watzman, O. Toung, I. Leermakers, J. Hornung, J. Klotz, J. Gooth, K. Manna, V. S ̈ub, S. N. Giun, T. F ̈orster, M. Schmidt, L. Muechler, B. Yan, P. Werner, W. Schnelle, U. Zeitler, J. Wosnitza, S. S. P. Parkin, C. Felser, and C. Shekar, “Extremely high conductivity observed in the triple point topological metal mop,” Nature Communications, vol. 10, no. 1, p. 2475, 2019.

Mykhailo Rakov: Search for Bose liquid phase in Abelian gauge quantum Hamiltonians         (S)

Recent developments in experimental condensed matter physics allow for creation of Abelian gauge Hamiltonians on a variety of routinely controllable systems, for example: optical lattices, Josephson junction arrays, etc. This opened the prospect of simulation of the phenomena of compact quantum electrodynamics and quantum chromodynamics in the laboratory setup. An interesting example of such phenomena is quark confinement. We consider generalized Bose-Hubbard Hamiltonian on a two-dimensional lattice. This system possesses ZN symmetry, and it indeed is in a confined quantum phase at small values of N and the coupling amplitude g. However, field theory predicts that it exhibits many other interesting phases of matter when tuning N and g, namely: the gapped phase, one-dimensional Bose liquid phase, and gapless dipolar liquid phase. Our goal is detailed numerical scrutiny of the theoretical predictions. To this end, the von Neumann entanglement entropy is calculated as function of g for every value of N. The discontinuities of its first derivative signal the phase transitions, and the type of these transitions (or entire phases) is identified by the central charge. The Hamiltonian is re-written in the second order of the perturbation theory and transformed so that it acts on a dual cylinder lattice. Its ground state is approximated by matrix product state (MPS) in zigzag geometry, and evaluated using infinite-size density-matrix renormalization group (iDMRG). Our calculations for N < 6 indicate only one phase transition between two gapped phases. In particular, it is one-dimensional Ising-type transition at N = 2 with central charge c = 1/2. On the contrary, at N = 5 the correlation length at the transition point increases roughly exponentially with the transverse system size. 

Lee Reeve: TBA         (F)

A quasicrystal is a non-periodic structure with long range order, which provides a fascinating testing ground for a variety of phenomena. Its quasi-disordered nature allows for Anderson localization, as well as the formation of the novel Bose glass phase when interactions enter the picture. The long range order of the quasicrystal on the other hand allows for sharp coherence peaks in the superfluid phase, making the system ideal for probing the phase transition between superfluid and insulating phases. This poster will present our work on mapping out the phase diagram of the quasicrystal using ultracold atoms in an optical lattice, as well as our investigations into the properties of the phases of the system, and the dynamics of quenches across the Bose glass – superfluid phase transition. Our findings pave the way for further studying novel phenomena of disordered systems, such as many-body localization. Quasicrystals may be particularly well suited to this endeavour, as their long range order ensures a lack of rare regions.

Alessandro Rizzi: Algebraic solution method of Fokker-Planck equations         (S)

The aim is to describe baryon stopping and charged hadron thermalization in heavy ion collisions by means of a non-equilibrium statistical model. The particle's phase-space trajectories are treated as a drift-diffusion stochastic process, leading to a Fokker-Planck equation for the single particle probability distribution. The drift and diffusion coefficients are derived from the expected asymptotic state, and the resulting non-linear FPE is then numerically solved with a spectral eigenfunction decomposition.

Dario Rossi: TBA         (F)

Schwinger boson mean field theory is a powerful approach to study frustrated magnetic systems which allows to distinguish long range magnetic orders from quantum spin liquid phases, where quantum fluctuations remain strong up to zero temperature. In this work, we use this framework to study the Heisenberg model on the Kagome lattice with up to third nearest neighbour interaction and Dzyaloshinskii-Moryia (DM) antisymmetric exchange. This model has been argued to be relevant for the description of transition metal dichalcogenide bilayers [1] in certain parameter regimes, where spin liquids could be realized. By means of the projective symmetry group classification of possible ansätze, we study the effect of the DM interaction at first nearest neighbor and then compute the J2 -J3 phase diagram at different DM angles. We find a new phase displaying chiral spin liquid characteristics up to spin S = 0.5, indicating an exceptional stability of the state.

Adith Sai Aramthottil: Role of interaction-induced tunneling in the dynamics of polar lattice bosons         (S)

The Bose-Hubbard model stands tall in the novel research it has been amenable to in ultracold atoms. Here, for the 1-dimensional case we extensively map out the various scenarios concerning thermalization for different lattice depths and dipolar potentials of soft-core bosons. In particular, we identify that even dipolar strengths of larger magnitudes sustain ergodicity. In addition, we unveil a practical regime with strong ergodicity-breaking and a decoupling between the hard-core boson states and other states. These results have substantial implications for future experiments on optical lattices, with the identified ergodicity-breaking regime opening up the interests of a broader audience.

Justin Schirmann: Amorphous Kramer-Weyl Semimetals         (F)

While nearly half of all crystals exhibit topological properties, little is known about topology in amorphous materials. In this study, we developed a model of amorphous chiral Kramer-Weyl semimetals, where widely used topological markers such as the Bott index or the local Chern marker are trivially zero due to time-reversal symmetry. We thus proposed an alternative way to characterize the survival of Weyl fermions in strongly disorder systems. Our results indicate that the doubling Nielsen-Ninomyia theorem, which states that Weyl fermions must come in pairs of opposite chiralities on a periodic lattices, also holds in the absence of long-range lattice order.

Harald Schmid: Edge modes of the random-field Floquet quantum Ising model         (S)

Motivated by a recent experiment on a superconducting quantum processor [Mi et al., Science 378, 785 (2022)], we study the stability of edge modes in the random-field Floquet quantum Ising model and its ramifications for temporal boundary spin-spin correlations. The edge modes induce pairings in the many-body Floquet spectrum with splittings exponentially close to zero or pi. We find that random transverse fields induce a log-normal distribution of both types of splittings. In contrast, random longitudinal fields affect the zero and pi splittings in drastically different ways. While zero pairings are rapidly lifted, the pi pairings strengthen, with concomitant differences in the boundary spin-spin correlations. We explain our result within a low-order Floquet perturbation theory. The strengthening of pi pairings by random longitudinal fields may have applications in quantum information processing.

Nyayabanta Swain (1): Evidence of many-body localisation in two dimensions         (F)

We use the stochastic series expansion quantum Monte Carlo method, together with the eigenstate-to-Hamiltonian construction, to map the localized Bose glass ground state of the disordered two-dimensional Heisenberg model to excited states of new target Hamiltonians. The localized nature of the ground state is established by studying the participation entropy, local entanglement entropy, and local magnetization, all known in the literature to also be identifying characteristics of many-body localized states. Our construction maps the ground state of the parent Hamiltonian to a single excited state of a new target Hamiltonian, which retains the same form as the parent Hamiltonian, albeit with correlated and large disorder. We furthermore provide evidence that the mapped eigenstates are genuine localized states and not special zero-measure localized states like the quantum scar-states. Our results provide concrete evidence for the existence of the many-body localized phase in two dimensions. 

H. Tang, N. Swain, D. Foo, B. Khor, G. Lemarie, F. F. Assaad, S. Adam, P. Sengupta. arXiv: 2106.08587

Nyayabanta Swain (2): Universal features of conductance fluctuations in two dimensional strong Anderson                     localised regime         (S)

We present numerical studies of the conductance fluctuations and their distribution in two-dimensional Anderson model in the strongly localised regime. While lng shows a non-Gaussian distribution in this regime; the fluctuations of lng grow with lateral size as L^{1/3}, and follow universal Tracy-Widom distributions that depend on the type of leads attached to the system. We provide an in-depth analysis of this behaviour by showing the analogy of our model with the directed polymer in a random medium. Furthermore, using importance-sampling of fluctuations based on a Markov chain Monte Carlo method in the disorder, we are able to access large conductance fluctuations which are otherwise impossible to access via standard sampling procedure. As a result, we can distinguish between the fluctuations belonging to the Tracy-Widom GUE and GOE classes. 

Rafał Świętek: Average entanglement entropy of midspectrum eigenstates of quantum-chaotic                                          interacting Hamiltonians         (F)

To which degree the average entanglement entropy of midspectrum eigenstates of quantum-chaotic interacting Hamiltonians agrees with that of random pure states is a question that has attracted considerable attention in the recent years. While there is substantial evidence that the leading (volume-law) terms are identical, which and how subleading terms differ between them is less clear. Here we carry out state of the art full exact diagonalization calculations of clean spin-1/2 XYZ and XXZ chains with integrability breaking terms to address this question in the absence and presence of U(1) symmetry, respectively. We first introduce the notion of maximally chaotic regime, for the chain sizes amenable to full exact diagonalization calculations, as the regime in Hamiltonian parameters in which the level spacing ratio, the distribution of eigenstate coefficients, and the entanglement entropy are closest to the random matrix theory predictions. In this regime, we carry out a finite-size scaling analysis of the subleading terms of the average entanglement entropy of midspectrum eigenstates when different fractions \nu of the spectrum are included in the average. We find indications that, for \nu \rightarrow 0, the magnitude of the negative O(1) correction is only slightly greater than the one predicted for random pure states. For finite \nu, following a phenomenological approach, we derive a simple expression that describes the numerically observed \nu dependence of the O(1) deviation from the prediction for random pure states.

M. Kliczkowski, R. Świętek, L.Vidmar, and M.Rigol, Phys. Rev. E 107, 063119 (2023)

Tomasz Szołdra: Unsupervised detection of decoupled subspaces: many-body scars and beyond         (S)

Highly excited eigenstates of quantum many-body systems are typically featureless thermal states. Some systems, however, possess a small number of special, low-entanglement eigenstates known as quantum scars. We introduce a quantum-inspired machine learning platform based on a Quantum Variational Autoencoder (QVAE) that detects families of scar states in spectra of many-body systems. Unlike a classical autoencoder, QVAE performs a parametrized unitary operation, allowing us to compress a single eigenstate into a smaller number of qubits. We demonstrate that the autoencoder trained on a scar state is able to detect the whole family of scar states sharing common features with the input state. We identify families of quantum many-body scars in the PXP model beyond the Z2 and Z3 families and find dynamically decoupled subspaces in the Hilbert space of disordered, interacting spin ladder model. The possibility of an automatic detection of subspaces of scar states opens new pathways in studies of models with a weak breakdown of ergodicity and fragmented Hilbert spaces.

Anne Tanguy: Vibrations and Heat Transfers in Amorphous Materials and in Glass-Ceramics         (F)

Amorphous materials have high heat capacity but low thermal conductivity compared to crystals with the same composition. The understanding of these properties is based on the study of acoustic attenuation and more generally on the specific vibrational properties of glasses. Quantum as well as classical explanations have been proposed for these phenomena, depending on the temperature range considered. Moreover, phononic localization induced by disorder, similar to Anderson’s localization, makes the material, thermal insulator above the mobility edge. After a general description of the vibrational eigenmodes in disordered materials, we will show the link between the dynamics of vibrational wave packets and the expression of thermal conductivity at the atomic scale. We will then discuss the different acoustic attenuation mechanisms and their expression at different scales: from the atomic scale to the continuous scale. Finally, we will focus on the effect of crystal/amorphous interfaces on the thermal properties of nanostructured materials such as glass-ceramics. We will show how the presence of amorphous parts reinforces the diffusive contribution to heat transport, and allows controlling not only the orientation, but also the direction of the heat flux.

[1] Y. Beltukov, D. A. Parshin, V.M. Giordano and A. Tanguy Physical Review E 98 023005 (2018): Propagative and diffusive regimes of acoustic damping in bulk amorphous material; [5] P. Desmarchelier, A. Carré, K. Termentzidis and A. Tanguy Nanomaterials 11, 1982 (2021): Heat Transport in Nanocomposite: The Role of the Shape and Interconnection of Nanoinclusions; [6] H. Luo, V.M. Giordano, A. Gravouil and A. Tanguy Journal of Non-Crystalline Solids 583, 121472 (2022): A continuum model reproducing the multiple frequency crossovers in acoustic attenuation in glasses.

Tommaso Tonolo: Marginal stability in the spherical spin-glass: on the competition between disorder and (ordered)               non-linearity         (S)

Since the 1970’s spin glasses have been a rich source of techniques and ideas that provided a theoretical foundation and universal paradigm for the emergence of ergodicity broken phases at low temperature in many-body systems with frustration. What was evident from the beginning is that the behaviour of disordered models in statistical mechanics depends on the nature of the variables, whether they are continuous or discrete. Let us consider for instance the celebrated Sherrington-Kirkpatrick model, which is a sort of mean-field Ising model with random Gaussian couplings. What is remarkable is the difference between the low temperature equilibrium phase of this model, characterized by a fractal free-energy landscape and by the so-called full replica-symmetry breaking scenario [”Infinite number of order parameters for spin-glasses”, G. Parisi, Phys. Rev. Lett. 43, 1754 (1979)], and the behaviour of a model with the same Hamiltonian, but where ”spins” are locally unbounded continuous variables, known as spherical spin glass [”Spherical Model of a Spin-Glass”, J. M. Kosterlitz, D. J. Thouless, and Raymund C. Jones, Phys. Rev. Lett. 36, 1217 (1976)], where a transition to a low temperature non-ergodic phase occurs at the same critical temperature of Sherrington-Kirkpatrick, but the nature of this low-temperature phase is completely different. It is a trivial spin-glass phase with only one big connected component of the phase space, which is not broken down into an infinite hierarchy of sub-clusters as in Sherrington-Kirkpatrick. The stability analysis of this ”trivial spin-glass” phase, which is ”marginally stable”, suggests that it might be driven to a different phase by arbitrarily small perturbations. The goal of the poster will be the description of how the addition of non-linear terms to the spherical spin-glass solved by Kosterlitz et al. in 1976 modifies the nature of the low-temperature phase. Motivated by the idea of investigating the competition between disorder and non-linearity, we have analytically studied the effects of different kinds of non-linear perturbations, i.e. those corresponding to the following kind of distributions for the random coefficients of the non-linear couplings: first, purely ordered coefficients; second, purely disordered coefficients; and third, a competition between ordered and disordered interactions. The main outcome of our investigation, detailed by the presentation of complete phase diagrams, is that the marginally stable trivial spin-glass phase of a model with two-body interactions (soft spins) cannot be destabilized by any perturbation: randomness looks like a necessary ingredient. In particular, consistently with previous works [”Spherical 2+ p spin-glass model: An exactly solvable model for glass to spin-glass transition”, A. Crisanti, L. Leuzzi, Phys. Rev. Lett. 93, 217203 (2004)], we find that the spherical spin glass with 2+4 body disordered interactions, is characterized by full replica symmetry breaking in its low temperature phase when the non-linearity is not too strong.

Vladislav Temkin: On the Coexistence of Localized and Delocalized States in the Anderson Model with                                Power-Law Hopping         (F)

We investigate the coexistence of localized and delocalized states in the Anderson impurity model with a power-law hopping amplitude J ∝ −r^−β with D < β < 3D/2, proposed by K.S. Tikhonov, A.S. Ioselevich, M. V. Feigel’man (2021). We demonstrate that, strictly speaking, the genuine localized states do not occur at E > 0, but, instead of them, quasi-localized ones arise in the vicinity of optimal fluctuations. We provide a derivation of the explicit form for the quasi-localized wave function in the presence of the optimal fluctuation potential, determine the behavior of the Inverse Participation Ratio Pq as a function of the energy and system size, and consider the effects of scattering at typical weak potential fluctuations.

Léo Touzo: Run-and-tumble particles with long-range interactions: The active Dyson Brownian motion         (S)

We introduce and study a model in one dimension of N run-and-tumble particles (RTP) which repel each other logarithmically in the presence of an external quadratic potential. This is an “active” version of the well-known Dyson Brownian motion (DBM) where the particles are subjected to a telegraphic noise, with two possible states ± with velocity ±v0. We study analytically and numerically two different versions of this model. In model I a particle only interacts with particles in the same state, while in model II all the particles interact with each other. In the large time limit, both models converge to a steady state where the stationary density has a finite support. For finite N , the stationary density exhibits singularities, which disappear when N → +∞. In that limit, for model I, using a Dean-Kawasaki approach, we show that the stationary density of + (respectively −) particles deviates from the DBM Wigner semi-circular shape, and vanishes with an exponent 3/2 at one of the edges. In model II, the Dean-Kawasaki approach fails but we obtain strong evidence that the density in the large N limit retains a Wigner semi-circular shape.

Indra Yudhistira: Apparent strange metal in small angle twisted bilayer graphene         (F)

Strange metals are an intriguing class of conductors that exhibit unconventional electronic properties.  These poorly understood materials are usually identified by a linear-in-temperature dependence of resistivity and a linear-in-field dependence of magnetotransport. We focus on the electronic transport properties of twisted bilayer graphene, a material that some claim exhibits strange metal behavior.  We determine the window in parameter space for the apparent strange metal behavior defined by the co-existence of linear-in-temperature resistivity and linear-in-field magnetotransport. Our findings highlight the potential for ordinary metals to imitate strange metal behavior and emphasize the need for careful interpretation of experimental results.