Korrelationstage 2025

We introduce the concept of "tetris chains", which are linear arrays of 4-site molecules that differ by their intermolecular hopping geometry. We investigate the fermionic symmetry-protected topological Haldane phase in these systems using Hubbard-type models. The topological phase diagrams can be understood via different competing limits and mechanisms: strong-coupling $U\gg t$, weak-coupling $U\ll t$, and the weak intermolecular hopping limit $t'\ll t$. Our particular focus is on two tetris chains that are of experimental relevance. First, we show that a "Y-chain" of coarse-grained nanographene molecules (triangulenes) is robustly in the Haldane phase in the whole $t'-U$ plane due to the cooperative nature of the three limits. Secondly, we study a near-homogeneous "Y$^{\prime}$-chain" that is closely related to the electronic model for poly(p-phenylene vinylene). In the latter case, the above mechanisms compete, but the Haldane phase manifests robustly and is stable when long-ranged Pariser-Parr-Popple interactions are added. The site-edged Hubbard ladder can also be viewed as a tetris chain, which gives a very general perspective on the emergence of its fermionic Haldane phase. Our numerical results are obtained by large-scale, SU(2)-symmetric tensor network calculations. We employ the density-matrix-renormalization group as well as the variational uniform matrix-product state (VUMPS) algorithms for finite and infinite systems, respectively. The numerics are supplemented by analytical calculations of the bandstructure winding number.

In the Mpemba effect a system prepared at a higher temperature cools down faster to a target equilibrium state than the same system prepared at a lower temperature. Lately the search for quantum analogoues of this effect has attracted great attention, especially in the context of Markovian open quantum systems. We investigate the occurence of such a Markovian quantum Mpemba effect in spin systems coupled to lossy optical cavities. In this setting the coupling to photon modes enables high energy states to cool down resonantly to the ground state, while states lower in energy can remain trapped in local minima of the systems energy landscape, leading to intriguing anomalous relaxation behaviours.

We extend the concept of conventional multiferroicity—where ferroelectric and ferromagnetic orders coexist—to include multipolar degrees of freedom. Specifically, we explore how this phenomenon emerges in $4d^2/5d^2$ Mott insulators with strong spin-orbit and Hund's couplings. Our study uncovers the origin of magnetic multipolar interactions in these systems and demonstrates that a combination of quadrupolar and octupolar magnetic order can simultaneously induce both electrical quadrupolar moments and ferroelectric polarization. By expanding the multiferroic framework to higher-order multipoles, we reveal the possibility of coexisting multipolar orders of different or same ranks, paving the way for different functional properties in a large class of strongly correlated materials.

We identify the many-body counterpart of flat bands, which we call many-body caging, as a general mechanism for non-equilibrium phenomena such as a novel type of glassy eigenspectrum order and many-body Rabi oscillations in the time domain. We focus on constrained systems of great current interest in the context of Rydberg atoms and synthetic or emergent gauge theories. We f ind that their state graphs host motifs which produce flat bands in the many-body spectrum at a particular set of energies. Basis states in Fock space exhibit Edwards-Anderson type parameters in the absence of quenched disorder, with an intricate, possibly fractal, distribution over Fock space which is reflected in a distinctive structure of a non-vanishing post-quench long-time Loschmidt echo, an experimentally accessible quantity. In general, phenomena familiar from single-particle flat bands manifest themselves in characteristic many-body incarnations, such as a reentrant ‘Anderson’ delocalisation, offering a rich ensemble of experimental signatures in the abovementioned quantum simulators. The variety of single-particle flat band types suggests an analogous typology–and concomitant phenomenological richness to be explored–of their many-body counterparts.

Kondo screening of a magnetic impurity in a Fermi gas can be suppressed if the fermion-bath density of states follows a pseudogap power law. This leads to a Kondo-breakdown quantum phase transition showing non-Fermi liquid behavior due to critical local-moment fluctuations in both spin and charge sectors. Here we study a two-impurity Anderson model with a pseudogap where the interactions between the two impurities can drive additional transitions into phases with inter-moment order. We utilize perturbative renormalization-group techniques to map out the phase diagram and to study the various quantum phase transitions. For the critical fixed points, we obtain analytical results for correlation-length exponents and anomalous dimensions of physical observables. We discuss possible connections to heavy-fermion systems.

We investigate the effects of Holstein polarons formed due to the electron-phonon (e-p) coupling on the quantum spin Hall (QSH) phase of a pseudospin-$1$ fermionic $\alpha-T_3$ lattice. The pristine scenario possesses tunability from graphene ($\alpha=0$) to a dice ($\alpha=1$) lattice, and a flat band persists for all $\alpha\neq 0$. The parameter $\alpha$ and the e-p coupling strength $\lambda$ have an interesting interplay, which demonstrates that at smaller values of $\alpha$, there is a single transition from a topological to a trivial phase as a function of $\lambda$, while the larger $\alpha$ values host two gap closing transitions, namely, trivial-topological-trivial transitions, accompanied by a narrow semi-metallic phase in between. The topological properties are characterized by computing the $\mathbb{Z}_2$ invariant, which confirms the existence of topological (trivial) phases, which are hence verified against the presence (absence) of counter-propagating helical edge modes in a nanoribbon. Furthermore, on the introduction of a planar magnetic field into the system, the emergence of a second-order topological phase is observed. The in-plane field causes gapping out of the first-order edge states while maintaining the topological phase of the bulk intact, subsequently leading to the emergence of robust corner modes under \textit{suitable} open boundary conditions. This resultant phase is adequately designated by computing the projected spin Chern number, a well-established invariant for the TRS broken QSH phase. Further, we show that the e-p coupling yields a complete disruption of the corner modes as we tune it beyond a certain critical strength, giving rise to a second-order topological phase transition.

We propose a theoretical framework for the realization of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing in a helical Shiba chain subjected to an out-of-plane Zeeman field, analyzed through a self-consistent Bogoliubov-de-Gennes (BdG) mean-field formalism approach. A chain of magnetic adatoms with helical spin texture deposited on the surface of a common $s$-wave superconductor, has emerged as a pivotal platform for realizing topological Majorana zero modes (MZMs). Our study reveals the crucial role of finite momentum pairing of Cooper pairs in the form of FFLO state which also supports topological MZMs at the ends of the chain. Interestingly, we demonstrate that FFLO pairing facilitates non-reciprocal charge transport, giving rise to superconducting diode effect in our system where both time-reversal and inversion symmetries are broken. Such diode effect stems directly from the presence of finite Cooper pair momentum of the FFLO ground state. Our comprehensive analysis highlights the intricate interplay between the richness of helical Shiba chain, the out-of-plane Zeeman field, and FFLO pairing in the emergence of MZMs and driving the superconducting diode effect. These findings offer valuable insights into the design and realization of topological superconducting devices with diode-like properties, potentially advancing technological applications in quantum computing and superconducting electronics.

We study optical manifestations of multigap band topology in multiband superconductors with a non-trivial topological Euler class. We introduce a set of lattice models for non-Abelian superconductors with the Euler invariant signified by a non-trivial quantum geometry. We then demonstrate that the topological Bogoliubov excitations realized in these models provide for a characteristic first-order optical response distinct from those of the other known topological superconductors. We find that the spectral distribution of the optical conductivity universally admits a topologically quantized jump and naturally differs from the features induced by the quantum geometry in the non-interacting bands without pairing terms. Further to uncovering observable signatures in first-order optical conductivities, we showcase that the higher-order nonlinear optical responses of the non-Abelian Euler superconductor can result in enhanced steady dc currents that fingerprint the exotic topological invariant. Finally, by employing a diagrammatic approach, we generalize our findings beyond the specific models of Euler superconductors.

In this second contribution, we expand on key results of the quantum skyrmion Hall effect. We survey the variety of results in lattice models supporting the quantum skyrmion Hall effect and their connections to experiments, discussing each of the three sets of topological states in lattice models within the framework of the quantum skyrmion Hall effect. We also discuss characterization methods, in particular the concept of observable-enriched entanglement and related entanglement measures, as well as algebraic generalizations of topological invariants and conditions for realizing non-trivial results for these invariants.

Superconductivity in infinite-layer nickelates is mainly of interest due to its potential similarity to cuprate high-$T_C$ superconductivity. However, it is a priori no clear which impact additional states beyond the cuprate-like band of $x^2-y^2$ have, and several additional contributions have been proposed. We check the relevance of a number of them: ligand-oxygen states, other Ni-orbitals and a second band of combined rare-earth and Ni character. For this last contribution, careful Wannier downfolding and the constrained random-phase approximation suggest that Hund-coupling is negligible while inter-site Coulomb repulsion might be significant. We thus investigate charge order and its competition with antiferromagnetism.

We theoretically study the nonreciprocal Josephson current in the quantum dot (QD)--based Josephson junction (JJ), for the conventional and unconventional superconducting leads. Considering two BCS s-wave superconductors, we have explored the JJ with the tiniest possible weak-link, i.e. a QD. To achieve the required symmetry-breaking phenomena to establish the Josephson diode effect (JDE), we break the time-reversal symmetry through the Zeeman field and the inversion symmetry is broken by Rashba spin-orbit interaction. We calculate the Josephson current using the Keldysh nonequilibrium Green’s function technique. On the contrary, we have also shown the possibility of achieving JDE by exploring the Coulomb correlation in the inversion symmetry broken QD junction, which may generate an intrinsic magnetic moment in the system, resulting in the possibility of achieving an external field-free nonreciprocity in Josephson current. The QD--based JJ is explored in the context of unconventional superconductors by comprising two periodically driven Kitaev chains coupled with the QD. The asymmetric Floquet drive in the two Kitaev chains results in a Majorana-QD coupled nonreciprocal Josephson current, which shows anomalous properties. Our proposed QD--based JDs have the potential to be efficient superconducting device components due to their strong tunable property to achieve a large rectification coefficient by modulating the external magnetic field, Rashba interaction, Floquet frequency or drive asymmetry and external gate voltage.

Neural Quantum States (NQS) have emerged as a reliable and efficient method for numerically simulating the ground states of two-dimensional quantum systems. Of particular interest for current research are lattice gauge theories, which are known to host topologically ordered states. A coupling to U(1) dynamical matter further enriches the physics and makes these systems, in principle, experimentally feasible. However, the simultaneous simulation of both gauge and matter fields poses a significant challenge for state-of-the-art numerical methods. In this study, we demonstrate that NQS can accurately represent such models, enabling the exploration of regimes beyond the limitations of current numerics.

With the advent of 2D moiré materials, Dirac fermion models have yet again emerged as promising candidates to describe putative quantum critical points in these systems. The presence of gapless fermions provides an avenue towards criticality beyond the conventional universality classes because it profoundly alters the quantum critical behavior, also giving rise to non-Fermi liquid behavior. We investigate the onset of nematic order in Dirac systems with hexagonal symmetry. Owing to the sixfold rotational symmetry, the nematic director selects among three equivalent orientations and the associated order parameter is described by a 3-state Potts model coupled to the Dirac fermions via a Yukawa interaction. In the ordered phase, the fermions remain gapless but the Dirac points split, dynamically breaking rotational symmetry. At the mean-field level, the transition is of first order, which we demonstrate using a minimal lattice model. We further employ a functional renormalization group approach to investigate the influence of the Dirac fermions on the Potts model and the nature of the transition due to a possible fermion-induced continuous quantum critical point.

For strongly correlated quantum systems, fundamental questions about the formation and stability of Floquet-Bloch sidebands (FBs) upon periodic driving remain unresolved. Here, we investigate the impact of electron-electron interactions and perturbations in the coherence of the driving on the lifetime of FBs by directly computing time-dependent single-particle spectral functions using exact diagonalization (ED) and matrix product states (MPS). We study interacting metallic and correlated insulating phases in a chain of correlated spinless fermions. At high-frequency driving we obtain clearly separated, long-lived FBs of the full many-body excitation continuum. However, if there is significant overlap of the features, which is more probable in the low-frequency regime, the interactions lead to strong heating, which results in a significant loss of quantum coherence and of the FBs. Similar suppression of FBs is obtained in the presence of noise. The emerging picture is further elucidated by the behavior of real-space single-particle propagators, of the energy gain, and of the momentum distribution function, which is related to a quantum Fisher information that is directly accessible by spectroscopic measurements. refs: [1] Karun Gadge and Salvatore R. Manmana, "Stability of Floquet sidebands and quantum coherence in 1D strongly interacting spinless fermions", arXiv preprint arXiv:2502.12643. [2] Marco Merboldt, Michael Schüler, David Schmitt, Jan Philipp Bange, Wiebke Bennecke, Karun Gadge, Klaus Pierz, Hans Werner Schumacher, Davood Momeni, Daniel Steil, Salvatore R. Manmana, Michael Sentef, Marcel Reutzel, Stefan Mathias, "Observation of Floquet states in graphene", Nature physics-2025.

Strong enough interactions induce a semimetal-to-insulator transition in Dirac materials, which can be viewed as the solid-state analogue of the chiral phase transition in quantum chromodynamics. Moiré Dirac materials such as twisted bilayer graphene offer a new opportunity to study this transition because they facilitate tuning the effective interaction via a twist angle. Motivated by this, we explore the quantum phase transition of a $(2+1)$-dimensional Dirac material at $T = 0K$ which spontaneously develops a gap that breaks an Ising symmetry. It is still an open question what is the structure of the phase diagram at finite chemical potential. To explore it, we study a Gross-Neveu-Yukawa model for the phase transition using both a mean-field theory and a functional renormalization group approach. Interestingly, we find an intermediate state between semi-metal and insulator where a homogeneous solution appears to be unstable.

Strong electronic correlation arises from a substantial increase in interaction between electrons compared to their kinetic energy. Disorder can enhance correlation effects by localizing electrons and suppressing screening, thereby driving the system away from single-particle behavior and leading to emergent phenomena. Using scanning tunneling microscopy and spectroscopy, we switch the ionization state of individual surface impurities on black phosphorus and discover nanoscale charge orders. These local charge orders break the symmetries of the host material underneath and spontaneously emerge from the otherwise uniform two-dimensional electron system. Furthermore, they appear with an anisotropy distinct from that of the Fermi surface, which is evidence against a Fermi-surface-related, weak-coupling interpretation for the local order. Our work demonstrates a microscopic approach for creating and manipulating strongly correlated electrons in two-dimensional systems even with weak intrinsic interactions.

Supermoir\'{e} structures (SMS), formed by overlapping moir\'{e}-patterns in van der Waals heterostructures, display complex behaviour that lacks a comprehensive low-energy theoretical description. We demonstrate that these structures can form emergent fractals under specific conditions and identify the parameter space where this occurs in hexagonal trilateral SMS. This fractality enables a reliable calculation of low-energy band counts, which are crucial for understanding both single-particle and correlation effects. Using an effective Hamiltonian that includes in- and out-of-plane lattice relaxation, we analyze SMS in hBN-encapsulated single and bilayer graphene. We prescribe methods to experimentally verify these fractals and extract their fractal dimension through angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM).

We address Coulomb drag and near-field heat transfer in a double-layer system of incoherent metals. Each layer is modeled by an array of tunnel-coupled SYK dots with random interlayer interactions. Depending on the strength of intradot interactions and interdot tunneling, this model captures the crossover from the Fermi liquid to a strange metal phase. The absence of quasiparticles in the strange metal leads to temperature-independent drag resistivity, which is in strong contrast with the quadratic temperature dependence in the Fermi liquid regime. We show that all the parameters can be independently measured in near-field heat transfer experiments, performed in Fermi liquid and strange metal regimes.

In this work, we develop a non-equilibrium steady-state non-crossing approximation (NESS-NCA) impurity solver applicable to general impurity problems. The choice of the NCA as the impurity solver enables both a more accurate description of correlation effects with larger Coulomb interaction and scalability to multi-orbital systems. Based on this development, we investigate strongly correlated non-equilibrium states of a dissipative lattice system under constant electric fields. Both the electronic Coulomb interaction and the electric field are treated non-perturbatively using dynamical mean-field theory in its non-equilibrium steady-state form (NESS-DMFT) with the NESS-NCA impurity solver. We validate our implementation using a half-filled single-band Hubbard model attached to a fictitious free Fermion reservoir, which prevents temperature divergence. As a result, we identify metallic and insulating phases as functions of the electric field and the Coulomb interaction along with a phase coexistence region amid the metal-to-insulator transition (MIT). We find that the MIT driven by the electric field is qualitatively similar to the equilibrium MIT as a function of temperature, differing from results in previous studies using the iterative perturbation theory (IPT) impurity solver. Finally, we highlight the importance of the morphology of a correlated system under the influence of an electric field.

Moiré systems composed of van der Waals heterostructures provide an experimentally accessible platform to realize a wide range of strongly correlated electron phenomena. Using transition metal dichalcogenide materials, such as an AB-stacked MoTe$_2$/WSe$_2$ bilayer, gives rise to an effective multi-orbital Hubbard model on the honeycomb lattice, which can be tuned via doping and additional charge transfer energy through external voltages. Including strong Ising spin-orbit coupling leads to chiral Kondo exchange between localized and itinerant electrons in different layers near half-filling.[1] To gain a better understanding of experimentally observed phenomena, including magnetic ordering, numerical modeling is performed using the variational cluster approach. This methodology, closely related to cluster dynamical mean-field theory, has been proven effective for studying analogous systems exhibiting Kondo lattice behavior. [1] Guerci et al., Chiral Kondo Lattice in Doped MoTe$_2$/WSe$_2$ Bilayers, 2023

The spin-1/2 Heisenberg antiferromagnet on the diamond-decorated square lattice is a highly frustrated quantum spin system that exhibits rich physical phenomena. In the presence of a magnetic field, it displays various quantum phases including the Lieb-Mattis ferrimagnetic, dimer-tetramer, monomer-dimer, and spin-canted phases, in addition to the trivial fully saturated state [1]. We investigate the thermodynamic properties of this model using several complementary analytical and numerical methods such as exact diagonalization up to systems of 40 spins, an effective monomer-dimer description, sign-problem-free quantum Monte Carlo simulations for up to 180 spins, and a decoupling approximation. In this contribution, we focus on the parameter region favoring the dimer-tetramer phase [2]. This ground state can be represented by a classical hard-dimer model on the square lattice and retains a macroscopic degeneracy even under a magnetic field. However, the description of the low-temperature thermodynamics close to the boundary between the macroscopically degenerate dimer-tetramer and the non-degenerate monomer-dimer phases requires an extended classical monomer-dimer lattice-gas model. In the vicinity of the dimer-tetramer phase, we detect an enhanced magnetocaloric effect promoting an efficient cooling to absolute zero temperature under adiabatic demagnetization. [1] N. Çaçi, K. Karl'ová, T. Verkholyak, J. Strečka, S. Wessel, A. Honecker, Phases of the Spin-1/2 Heisenberg Antiferromagnet on the Diamond-Decorated Square Lattice in a Magnetic Field, Phys. Rev. B 107, 115143 (2023). [2] K. Karl'ová, A. Honecker, N. Çaçi, S. Wessel, J. Strečka, T. Verkholyak, Thermodynamic Properties of the Macroscopically Degenerate Tetramer-Dimer Phase of the Spin-1/2 Heisenberg Model on the Diamond-Decorated Square Lattice, Phys. Rev. B 110, 214429 (2024).

Horstmann, Schnack Doing finite-temperature calculations on bigger spin systems is often limited by the size of the Hilbert spaces being too large for algorithms such as exact diagonalisation or finite-temperature Lanczos. In order to work around this problem White proposed a method based on the Density Matrix Renormalization Group (DMRG) in the late 90th which allows the calculation of bigger systems by applying multiple local optimisation steps while truncating the size of the Hilbert space by a large amount without loosing too much information about the system. This method works, but it is slow. Therefore, the whole method was translated into a tensor representation using matrix product states where the full system and its operators are described by a tensor network which allows faster linear algebra calculations [1]. In this contribution we will expand this method to finite-temperature calculations using imaginity-time evolution with TenPy [2] to calculate thermodynamic properties for larger spin systems. [1] Johannes Hauschild, Frank Pollamnn, doi:10.21468/SciPostPhysLectNotes.5 [2] Ulrich Scholwöck, doi:10.1016/j.aop.2010.09.012

Effective low-energy descriptions of strongly correlated condensed matter systems — such as 2D Dirac semi-metals — can be written as Gross-Neveu-Yukawa-type theories. These relativistic quantum field theories capture the universal behaviour near quantum critical points, and capture spontaneous symmetry-breaking throughout large parameter ranges such as temperature and even small deviations from half filling. The description in terms of relativistic fermions suggests the analogy to mechanisms in Quantum Chromodynamics (QCD), where strongly correlated fermions create a rich phase diagram at low energies. Functional Renormalization Group (fRG) methods provide a non-perturbative tool capable of accessing regimes which are obstructed by the sign-problem in quantum Monte Carlo simulations and are being actively developed to also detect e.g. color-superconducting or inhomogeneous phases. In my talk I want to discuss numerical tools developed for the fRG in a QCD context, specifically focusing on the quantitative resolution of first and second order phase transitions. I also want to highlight the versatility of generalised fRG techniques which were recently shown to quantitatively capture instanton-mediated tunnelling.

The spin-1/2 quantum Heisenberg model on the two-dimensional diamond-like decorated honeycomb lattice is a highly frustrated magnet exhibiting rich phenomena. Its ground-state phase diagram includes, in addition to the fully polarized state, a monomer-dimer phase, a Lieb-Mattis type ferrimagnetic phase, a spin-canted phase, and a macroscopically degenerate dimer-tetramer phase with finite residual entropy. Moreover, we consider the effects of distortions that enhance the couplings within the vertical dimers or along the zig-zag chains of the lattice structure. This model hosts several distinct ground states depending on the microscopic parameters. In particular, the isotropic version of the spin-1/2 quantum Heisenberg model exhibits a macroscopically degenerate dimer-tetramer phase, a small distortion can either completely lift the degeneracy, resulting in a dimer-tetramer crystal (DTC), or produce a dimer-tetramer liquid (DTL) phase with significant degeneracy, but zero residual entropy. Based on a mapping of the original quantum spin model onto a hard-dimer model on the hexagonal lattice, we predict Kastelyen physics above the DTC, which resembles features of a thermal phase transition from the Kasteleyn universality class. This scenario is assessed by a numerical treatment of the full quantum spin model.

Topologically ordered states have garnered significant interest in condensed matter physics, particularly since the discovery of the quantum Hall effect (QHE). This field investigates the intricate relationship between topology and electronic properties in materials, with promising applications in quantum computing and spintronics. In this work, we focus on a two-dimensional topological insulator—silicene—which describes non-interacting fermions on a staggered honeycomb lattice that exhibit the QHE without the need for Landau levels. Silicene undergoes a phase transition between a topological insulator and a trivial insulator, and can also show transition to a conducting phase depending on the applied electric field.

The weakly-doped Hubbard ladder is a widely studied paradigmatic model presenting unconventional superconducting correlations even in the presence of strongly repulsive interactions. Despite efforts, the study of this model is far from over. We find an enhancement of the superconducting correlations in the presence of a correlated hopping term. We explain this by showing how this terms can act as an effective attraction between pairs. Density-assisted hopping terms arise naturally in two dimensions during the mapping to the one-band Hubbard model, and it is motivated in ladders by possible realizations in ultracold atom experiments.

We consider the effect of an imposed boundary in the Kondo-Heisenberg ferromagnetic model on a square lattice strip. In absence of a boundary, this model presents three phases; namely a metallic ferromagnetic phase, a heavy-fermion ferromagnetic, and a Kondo insulating phase. We demonstrate that the imposition of a boundary into the system results in the stabilization of an intermediate phase where sites at the boundary of the system hybridize, yielding an insulating boundary, while the bulk of the system remains conductive. We further discuss the implication of our findings and the observation of this novel boundary phase in the context of the recently synthetized UAsS compound.

We will elucidate the nature of the linear-response tensors in planar-Hall and planar-thermal-Hall set-ups, where we subject a chiral 3d semimetal to the combined influence of an electric field (and/or temperature gradient) and a weak (i.e., nonquantising) magnetic field. We will explain why it is essential to include the effects of orbital magnetic moment in conjunction with the Berry curvature, in order to obtain a holistic picture of the effects of bandstructure-topology in linear response. Going beyond the well-studied example of Weyl semimetals, we will discuss the cases of their multifold as well as anisotropic cousins, harbouring higher Chern numbers. We will highlight how we can compute internode scatterings between nodes of higher-valued Chern numbers, where the associated nodal points can carry differing pseudospin representations. Finally, we will outline how to systematically compute the contributions from the so-called Lorentz-force operator --- it turns out that it furnishes in-plane components of response in addition to the out-of-plane ones, which has been overlooked so far.

We characterize the current-carrying nonequilibrium steady state (NESS) in a single-band Hubbard model confronted with a static electric field in the presence of quenched disorder. Beyond the linear response regime, the electric field amplitude must compensate for at least half of the band gap to have a non-negligible stationary current. As the disorder is not expected to dissipate the extra energy injected by the field, optical phonons assisted by a fermionic heat bath serve as dissipation channels for the current-induced Joule heat generated by the accelerated electrons. The NESS of the system is addressed employing the dynamical mean-field theory using the so-called auxiliary master equation approach as impurity solver. Disorder effects are treated locally via the coherent potential approximation (CPA) and the self-consistent Born (SCB) approach. In the regime in which the two schemes yield similar results, we employ the SCB as it is computationally cheaper than the CPA. We show that, in a purely electronic setup, the disorder-induced dephasing cannot contribute states within the gap but only smear out the edges of the Hubbard bands. When phonons are taken into account, the different nature of disorder-induced dephasing and phonon-related dissipation becomes clear. We show that although both disorder and electron-phonon interaction enhance the current at off-resonant fields, disorder effects play a marginal role since they cannot provide in-gap states which are instead brought about by phonons and represent the privileged relaxation pathway for excited electrons.

By means of a quantum-classical mapping, we show that the exact diffusion constants of a certain class of kinetically-constrained model can be obtained from non-interacting spin wave theory. In particular, we show that when the rate matrix governing the transitions between the states of a classical, kinetically-constrained stochastic process is expressed as a spin Hamiltonian, the equilibrium state of the classical process corresponds to the ground state of the spin system, and the near-equilibrium dynamics can thus be obtained from the low-energy spectrum of the spin model. We show that the low-momentum magnon dispersion is not renormalised when the constraints can be expressed in terms of whether a single site is occupied (or unoccupied), and thus the exact diffusion constant can be read off, regardless of the specifics of the lattice or the dimensionality of the model. We further present numerical simulations supporting this finding.

We study kagome lattices with on-site and extended spin-singlet s-wave superconducting pairing and show that the inclusion of Rashba spin-orbit (RSO) interaction allows time-reversal-invariant topological superconducting states which support helical Majorana pairs at the edge. We calculate the Z2 topological invariant as a function of the pairing parameters for different chemical potentials. The rich phase diagrams reveal topological, nodal, and trivial superconducting states depending on the system parameters. We also consider a 2X2 time-reversal symmetry-breaking chiral flux phase, which has been demonstrated to be energetically favorable in the AV3Sb5 family of superconductors. Incorporating such symmetry-breaking order in our model leads to chiral Majorana edge states defined by a Chern number. We show how the RSO interaction allows for topological phases with even and odd Chern numbers for different system parameters. This work demonstrates how a simple s-wave kagome superconductor with RSO interaction can support helical and chiral Majorana edge states, and motivates the search for Majorana fermions in kagome superconductors.

The experimental signature of topological charge density wave(CDW) has been reported on twisted mono - bilayer graphene(TMBG) at filling factor $\nu = 3/2$ and $\nu = 7/2$. Theoretically, this can be described by partially fill the Chern 2 band emerging from TMBG, which allows Coulomb interaction to break the moire translation symmetry and split into two Chern 1 bands. However, it is still not clear how the correlated ground states of TMBG between integer and half integer filling are connected. In this work, we study the competition between CDW and non-CDW states of the TMBG for positive $\nu = Z/2$ by performing unbiased Hartree - Fock calculation. We argue that CDW can be energetically favorable for entire positive $\nu = Z/2$, which affects the strength of other orders such as flavor polarized order, intervalley coherent order, etc. In addition, we study how the proximitized spin orbit coupling(SOC) effect induced by putting transition metal dichalcogenide material on top of TMBG can affect the competition between CDW and non - CDW orders. We found that even small SOC parameters described by Ising and Rashba terms can change the strength of orders dramatically, which still allows CDW to be energetically favored than non -CDW state.

Coulomb phases have been previously identified in certain $N$ state antiferromagnetic Potts models. We present a general theory for such Potts ices, and demonstrate how they generalise the familiar classical spin ice model, possessing multiple emergent gauge fields, and vector valued charged excitations. The properties of these models are understood through the connection between their microscopic degrees of freedom and the $SU(N)$ Lie algebras. We then further explore quantum generalisations of these models, and identify novel phenomena not present in the N=2 classical spin ice model, including generically first order phase transitions, flavour changing excitation scattering processes, and frustration of the emergent flux.

Nonlinear spectroscopy has emerged as a promising approach to probe the excitation spectra of quantum magnets in unprecedented detail, enabling to observe "hidden" features such as multipolar excitation from their signatures in higher-order responses. This poster provides an introduction to 2-dimensional coherent spectroscopy (2DCS) and reports on the implementation of 2DCS in the context of the Su(n)ny Julia package for modeling atomic-scale magnetism. Employing a semi-classical approach, we use it to study the properties of quadrupolar excitations, such as spin-1 single ion bound states (SIBS), in the context of various candidate materials.

Eigenstate multifractality, a hallmark of non-interacting disordered metals which can potentially be observed in many-body localized states as well, is characterized by anomalous slow dynamics and appears relevant for many areas of quantum physics from measurement-driven systems to superconductivity. We propose a novel approach to achieve non-ergodic multifractal (NEM) states without disorder by iteratively introducing defects into a crystal lattice, reshaping it from a plain structure into fractal geometry. By comprehensive analysis of the Sierpiński gasket case, we find a robust evidence of the emergence of NEM states that go beyond the conventional classification of quantum states and designate new pathways for quantum transport studies. We discuss potential experimental signatures of these states.

We present a method for performing a full graph expansion for light-matter systems, utilizing the linked-cluster theorem. This enables us to explore $1/N$ corrections to the thermodynamic limit $N\to \infty$, giving us access to the mesoscopic regime. Yet largely unexplored, this regime hosts intriguing features, such as the entanglement between light and matter, which vanishes in the thermodynamic limit [1–3]. We calculate ground-state properties, like the energy and entanglement entropy, of generalized Dicke models by accompanying the graph expansion by both exact diagonalization (NLCE [4]) and perturbation theory (pcst⁺⁺ [5]), benchmarking our approach against other techniques for the limiting cases of microscopic and macroscopic systems [6]. [1] J. Vidal, S. Dusuel; EPL 74 817 (2006) [2] K. Lenk, J. Li, P. Werner, M. Eckstein; arXiv:2205.05559 (2022) [3] A. Kudlis, D. Novokreschenov, I. Iorsh, I. Tokatly; Phys. Rev. A 108, L051701 (2023) [4] M. Rigol, T. Bryant, R. R. P. Singh; Phys. Rev. Lett. 97, 187202 (2006) [5] L. Lenke, A. Schellenberger, K. P. Schmidt; Phys. Rev. A, 108 (2023) [6] A. Schellenberger, K. P. Schmidt; SciPost Phys. Core 7, 038, (2024)

In frustrated magnetism, the empirically discovered quantum-to-classical correspondence (QCC) matches the real-space static susceptibility pattern of a quantum spin-1/2 model with that of its classical counterpart computed at an elevated effective temperature. This striking correspondence, first observed via bold-line diagrammatic Monte Carlo simulations in two and three dimensions, holds within error bars down to temperatures an order of magnitude below the exchange coupling J. In this talk, we explore the analytical foundation and inherent limitations of QCC using dynamic high-temperature series expansions (Dyn-HTE), carried out to twelfth order in J/T. We show that the static susceptibility of a variety of quantum Heisenberg models in d>1 dimensions is remarkably well approximated by a renormalized mean-field (MF) ansatz. This effective form arises from partial cancellations among high-order diagrams and retains its accuracy deep into the cooperative paramagnetic regime, thus providing an explanation for the surprising universality of QCC observed across a variety of frustrated lattices. We illustrate this framework across all models previously discussed in the context of QCC, including the recently experimentally studied S=1 material $\mathrm{K}_2\mathrm{Ni}_2(\mathrm{SO}_4)_3$.

Fractional quantum hall physics with vanishing magnetic fields has be- come an increasingly important research topic in recent years due to new findings in the field of moiré materials. Experimental signatures of these phases are often observed in combination with signatures of charge ordered or other symmetry broken phases. This indicates that band mixing might play an elementary role in the complete descrip- tion of this phase of matter. For this purpose, an extended Hubbard model on a triangular lattice with $\nu$ = 2/3 is considered. This allows the formation of bands of non-trivial topology as well as the formation of commensurate charge density waves. The analysis is carried out by exact diagonalisation. The charge density wave can be suppressed by artificially increasing the band gap, whereby the properties of the emerging competing phases are analysed.

We study the non-equilibrium dynamics of the one-dimensional transverse field Ising model under periodic driving. Using Floquet theory, we derive the steady states of the driven model for a fixed driving amplitude and identify Floquet modes that emerge from strong resonant dressing of the eigenstates of the undriven system. Studying the real time evolution and comparing it with Floquet theory, we find that the system evolves into superpositions of Floquet states, where the ramping rate of the driving amplitude influences the occupation of higher Floquet bands. We also compute the two-point correlation functions, which show oscillations in position space that can be tuned with the driving frequency. Our results highlight how periodic driving can be used to create complex non-equilibrium states.

The discovery of superconductivity in KTaO$_3$ (KTO) heterostructures and uncapped surfaces has recently triggered significant interest. Remarkably, the superconducting critical temperature $T_c$ shows high sensitivity on the crystallographic orientation, reaching values one order of magnitude larger than that of SrTiO$_3$ (STO) heterostructures [1]. As such, insights into the pairing mechanism are of great interest. We study a pairing mechanism based on spin-orbit assisted coupling between the $t_{2g}$ conduction electrons and the soft ferroelectric (FE) modes present in the material. The theoretical approach was developed for bulk STO and generalized to any incipient FE [2]. The presence of a soft phonon in incipient FE can open hopping channels otherwise prohibited by symmetries, leading to a Rashba-like electron-phonon coupling. By combining ab initio calculations and a microscopic model, we find a strongly anisotropic Rashba-like interaction as well as a strong electron-phonon coupling to the soft transverse FE mode characterized by non-spin conserving processes due to the strong SOC, and a strong interband coupling. [3,4]. [1] Changjiang Liu et al., Nat. Comm., 14, 951 (2023). [2] M. N. Gastiasoro, M.E. Temperini, P. Barone, and J. Lorenzana, Phys. Rev. R., 5(2), 023177 (2023). [3] G. Venditti, M.E. Temperini, P. Barone, J. Lorenzana, M. N. Gastiasoro, J. of Phys: Mater., 6(1), 014007 (2023). [4] G. Venditti, F. Macheda, P. Barone, J. Lorenzana, and M. N. Gastiasoro, In preparation (2025)

Artificial spin ice allows for the exploration of new geometries for spin ice with tunable couplings. Perhaps most directly, the study of planar lattices in 2D is a possibility. This motivates our exploration of the Sierpinski gasket fractal. This fractal with dimension $d=\ln(3)/\ln(2) \sim 1.5$ can host spin ice physics in fractional dimensions. We find that the magnetic monopoles are confined at zero temperature. However, Gauss's law on the fractal leads to flux bottlenecks that can lead to long-range spin correlations in the presence of multiple sources.

We study the effect Markovian dephasing has in the single impurity Anderson model. The quantum dot is attached to two metallic leads. In equilibrium, we investigate the spectral and distribution function and discuss the role of dephasing on Kondo physics. In the steady state, out of equilibrium, we compute the conductance as a function of applied voltage and find a universal scaling behavior, with a dephasing dependent Kondo scale.

We provide an intuitive understanding of a collective low-energy excitation in antiferromagnetic spin-1/2 chains, known as a spinon [1]. While signatures of spinons are experimentally visible in dynamic response functions, their fractionalized and collective nature makes it challenging to devise a simple protocol that creates one in a controllable way. In this contribution we demonstrate a geometrical origin of spinon by showing that spinon is excited by appropriate insertion of an extra lattice site into a 1D spin chain. This procedure was recently verified experimentally in length-tunable nanographene antiferromagnets [2] and provides new insights into the nature of fractionalised excitations in quantum magnets. In particular, we reveal the crucial role played by ground-state entanglement in the emergence of spinons. [1] T. Kulka, M. Panfil, M. Berciu, K. Wohlfeld, "Nature of spinons in 1D spin chains"; accepted in Phys. Rev. Lett. (2025). 2] C. Zhao et al., "Spin excitations in nanographene-based antiferromagnetic spin-1/2 Heisenberg chains", Nature Materials 24, 722 (2025).

Although the electric field is generally expected to be expelled from the bulk of a conductor, recent experiments suggest that a static electric field can influence superconductivity by reducing the supercurrent within the material. In this study, we examine a thin, homogeneous superconductor and solve Gor'kov’s equations, which provide a microscopic description of superconductivity in the presence of a static electric field. Our goal is to establish a theoretical framework that supports the experimental observations and offers an explanation for the Superconductve Field Effect.

The stripe phase, an intertwined order observed in high-temperature superconductors, is regarded as playing a key role in elucidating the underlying mechanism of superconductivity, especially in cuprates. Following Jan Zaanen's early scenario, %the fully-filled charge stripe the filled charge stripe, with one hole per unit cell of the charge order, can be taken as the interactive elastic quantum strings of holes, stabilized by $\pi$-phase shifts between neighboring magnetic domains. However, this scenario is challenging to explain, particularly in terms of electron pairing, which necessitates hole pairs. In this work, we propose a new effective model for describing the stripe phase in the hole-doped $t$-$J_z$ model. With respect to the antiferromagnetic background, the model comprises three types of color-labeled point-defects coupling to an effective spin field, so named as ``colored string". Comparing with numerical results from large-scale density matrix renormalization group (DMRG) simulations, we find semi-quantitative agreement in local hole density, magnetic moment, and the newly proposed spectrum features of the effective spin field. By systematically analyzing the hole-density distribution and the scaling of groundstate energy at different system sizes, we determine the effective core radius and the effective hopping amplitude of the quantum string. Furthermore, the local pinning field can be finely adjusted to drag the quantum string, offering a potential method for detecting it in optical lattices. At last, we further demonstrate the partially-filled stripe with less than one hole per unit cell of the charge order can also be well described by the effective theory.