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Physics Condensed Matter

cond-mat Watch Condensed Matter

Wednesday 28 January

Strongly Correlated Electrons

Stripe antiferromagnetism and chiral superconductivity in tWSe$_2$

Erekle Jmukhadze, Sam Olin, Allan H. MacDonald et al. (+1 more)

The layer-dependent Hamiltonians of parallel-stacked MoTe$_2$ and WSe$_2$ homobilayer moiré materials are topologically non-trivial, both in real space and in momentum space, and have been shown to support integer and fractional quantum anomalous Hall states, as well as antiferromagnetic and superconducting states. Here, we address the interplay between the antiferromagnetic and superconducting states observed in tWSe$_2$ when the Fermi level is close to its $M$-point van Hove singularity and the displacement field is small. We combine DFT with path-integrals to construct a minimal moiré band model that accounts for lattice relaxation along the $c$-axis and perform Hartree-Fock calculations to identify competing charge and spin ordered states. For tWSe$_2$ at $\theta=2.7^\circ$ and $\theta=3.65^\circ$, we find that a layer antiferromagnet (AFM), a stripe spin-density-wave (SDW), and the ferromagnetic Chern insulator (FM) are the primary candidates for the ground state at zero displacement field, and argue that antiferromagnetic spin interactions on the next neighbor bond $J_2$ can induce a time-reversal symmetry breaking chiral superconducting state.
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All authors Erekle Jmukhadze, Sam Olin, Allan H. MacDonald, Wei-Cheng Lee

Other Condensed Matter

Real-Space Dynamic Electron Correlation in Beryllium

Rudra B. Bista, Yuya Shinohara, Wojciech Dmowski et al. (+6 more)

Electron correlation in solid has a major impact on material properties. However, it has been studied mainly by theory, with very limited direct experimental investigations. Here, we demonstrate that dynamic electron correlation function can be experimentally measured using inelastic X-ray scattering on polycrystalline beryllium. The data are expressed as the energy-resolved dynamic pair-distribution function. Our results confirm the size of the exchange-correlation hole as ~2 Å, consistent with theoretical expectations. However, at the plasmon energy of ~21 eV, the exchange-correlation hole is extended up to 4-5 Å, suggesting a unique influence of the dynamic plasmon state.
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All authors Rudra B. Bista, Yuya Shinohara, Wojciech Dmowski, Chae Woo Ryu, Jung Ho Kim, Mary Upton, Hlynur Gretarsson, Martin Sundermann, Takeshi Egami

Superconductivity

Fingerprinting superconductors by disentangling Andreev and quasiparticle currents across tunable tunnel junctions

Petro Maksymovych, Sang Yong Song, Benjamin Lawrie et al. (+2 more)

Tunneling Andreev reflection (TAR) spectroscopy offers a powerful new approach to fingerprint superconducting pairing symmetry at the atomic scale. By leveraging the exponential sensitivity of excess tunneling decay rate to Andreev reflection, TAR robustly distinguishes between s-wave, d-wave, and more complex order parameters, overcoming limitations of traditional conductance-based techniques. Here, using atomistic superconducting transport simulations, we show that the additivity of excess decay rate enables clear separation of Andreev and quasiparticle currents. In particular, we reveal how their competition as well as higher-order scattering processes shape both the decay rate spectra and their dependence on the coupling strength. We show that this phenomenology stems from the fact that Andreev reflection dominates mid-gap conductance for s-wave superconductors, it is suppressed for the d-wave, and it coexists with quasiparticle tunneling in sign-changing symmetries if the expectation value for the superconducting gap remains finite. These distinct spectral fingerprints pave the way for atomically resolved identification of unconventional superconducting states.
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All authors Petro Maksymovych, Sang Yong Song, Benjamin Lawrie, Wonhee Ko, Jose L. Lado

Soft Condensed Matter

Controlling the snap-through behavior of an elastica strip by cuts

Eszter Fehér

This work examines the snap-through behavior of a clamped-clamped elastica made from initially flat, thin, cut sheets under increasing vertical concentrated load acting at midspan. A two-parameter, symmetric pattern is introduced to conduct a numerical parameter analysis across three different support distances. When the support distance is one-quarter of the total length of the sheet, the structure loses stability at a symmetry point bifurcation over a wide range of parameters. Additionally, there exists a small range of parameters where limit point bifurcation occurs. In this case, the cuts can induce symmetry in the stability loss. For larger support distances (half or three-quarters of the total length), limit point bifurcation occurs only for small cuts, and there is a range of cut parameters that leads to monotonic monostability, indicating that no stability loss occurs. These findings are supported by experimental data. Overall, our research demonstrates that carefully designed cut patterns can either control the mode of stability loss in elastica strips or suppress it entirely.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Neural Quantum States in Mixed Precision

Massimo Solinas, Agnes Valenti, Nawaf Bou-Rabee et al. (+1 more)

Scientific computing has long relied on double precision (64-bit floating point) arithmetic to guarantee accuracy in simulations of real-world phenomena. However, the growing availability of hardware accelerators such as Graphics Processing Units (GPUs) has made low-precision formats attractive due to their superior performance, reduced memory footprint, and improved energy efficiency. In this work, we investigate the role of mixed-precision arithmetic in neural-network based Variational Monte Carlo (VMC), a widely used method for solving computationally otherwise intractable quantum many-body systems. We first derive general analytical bounds on the error introduced by reduced precision on Metropolis-Hastings MCMC, and then empirically validate these bounds on the use-case of VMC. We demonstrate that significant portions of the algorithm, in particular, sampling the quantum state, can be executed in half precision without loss of accuracy. More broadly, this work provides a theoretical framework to assess the applicability of mixed-precision arithmetic in machine-learning approaches that rely on MCMC sampling. In the context of VMC, we additionally demonstrate the practical effectiveness of mixed-precision strategies, enabling more scalable and energy-efficient simulations of quantum many-body systems.
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All authors Massimo Solinas, Agnes Valenti, Nawaf Bou-Rabee, Roeland Wiersema

Mesoscale and Nanoscale Physics

A Purely Magnetic Route to High-Harmonic Spin Pumping

Ousmane Ly

Spin pumping provides a fundamental route for dynamical spin transport, yet in its conventional form it produces only linear spin responses at the driving frequency. Recent studies have shown that spin-orbit coupling (SOC) can lift these restrictions and enable strongly nonlinear spin and charge currents. Here we propose a distinct mechanism for high-harmonic spin pumping that operates independently of spin-orbit interactions. We demonstrate that purely magnetic structures can sustain high-harmonic spin currents when an additional static magnetic order parameter is introduced perpendicular to the cone axis of a precessing magnetization. This secondary magnetic order qualitatively reshapes the dynamical response, giving rise to a cascade of higher harmonics in the pumped spin current in the absence of SOC. Our results establish a SOC-independent route to ultrafast and nonlinear spin pumping rooted solely in magnetic structure and dynamics.
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Materials Science

Millisecond spin coherence of electrons in semiconducting perovskites revealed by spin mode locking

Sergey R. Meliakov, Evgeny A. Zhukov, Vasilii V. Belykh et al. (+4 more)

Long spin coherence times of carriers are essential for implementing quantum technologies using semiconductor devices for which, however, a possible obstacle is spin relaxation. For the spin dynamics, decisive features are the band structure, crystal symmetry, and quantum confinement. Perovskite semiconductors recently have come into focus of studies of their spin states, notivated by efficient optical access and potentially long-living coherence. Here, we report an electron spin coherence time $T_2$ of the order of 1 ms, measured for a bulk FA$_{0.95}$Cs$_{0.05}$PbI$_3$ lead halide perovskite crystal. Using periodic laser pulses, we synchronize the electron spin Larmor precession about an external magnetic field in an inhomogeneous ensemble, the effect known as spin mode locking. It appears as a decay of the optically created ensemble spin polarization within the dephasing time $T_2^*$ of up to 20 ns and its revival during the spin coherence time $T_2$ reaching the millisecond range. This exceptionally long spin coherence time in a bulk crystal is complemented by millisecond-long longitudinal spin relaxation times $T_1$ for electrons and holes, measured by optically-detected magnetic resonance. These long-lasting spin dynamics highlight perovskites as promising platform for the quantum devices with all-optical control.
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All authors Sergey R. Meliakov, Evgeny A. Zhukov, Vasilii V. Belykh, Dmitri R. Yakovlev, Bekir Turedi, Maksym V. Kovalenko, Manfred Bayer

Strongly Correlated Electrons

Observation of Dipolar Spin-ice–like Correlations in the Quantum Spin Ice Candidate Ce$_2$Sn$_2$O$_7$

Bo Yuan, M. Powell, X. Liu et al. (+9 more)

We report diffuse scattering measurements on the dipolar-octupolar pyrochlore, Ce$_2$Sn$_2$O$_7$, to elucidate its magnetic correlations within the correlated paramagnetic regime above its ordering temperature, $\mathrm{T_N}\sim 40~$mK. The magnetic transition at $\mathrm{T_N}$, observed in recent specific heat measurements, has been interpreted within the nearest-neighbor XYZ model as a transition into an all-in-all-out (AIAO) magnetic order. However, the observed diffuse scattering above $\mathrm{T_N}$ argues against an AIAO magnetic order by exhibiting $no$ short-range order at the AIAO wave-vectors, inconsistent with the nearest neighbor model. Instead, the observed diffuse scattering strongly resembles that observed in classical dipolar spin ice, Dy$_2$Ti$_2$O$_7$, and suggests the presence of short-range correlated structures such as chains, or clusters in the low energy manifold of Ce$_2$Sn$_2$O$_7$, favored by further neighbor interactions. We discuss how chain-like states qualitatively account for the observed H-T phase diagram under a [1,1,0]-field.
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All authors Bo Yuan, M. Powell, X. Liu, J. Ni, E. M. Smith, R. Schäfer, R. Moessner, F. Ye, J. Dudemaine, A. D. Bianchi, J. W. Kolis, B. D. Gaulin

Materials Science

Multiscale Numerical Modelling of Ultrafast Laser-Matter Interactions: Maxwell Two Temperature Model Molecular Dynamics (M-TTM-MD)

Othmane Benhayoun and Martin E. Garcia

In this work, we present a comprehensive numerical framework that couples numerical solutions of Maxwell's equations using the Finite-Difference Time-Domain (FDTD) approach, Molecular Dynamics (MD), and the Two-Temperature Model (TTM) to describe ultrafast laser-matter interactions in metallic systems at the atomic scale. The proposed Maxwell-Two-Temperature Model-Molecular Dynamics (M-TTM-MD) bridges the gap between electromagnetic field propagation, electron-phonon energy exchange, and atomic motion, allowing for a self-consistent treatment of energy absorption, transport, and structural response within a unified simulation environment. The calculated electromagnetic fields incorporate dispersive dielectric properties derived using the Auxiliary Differential Equation (ADE) technique, while the electronic and lattice subsystems are dynamically coupled through spatially and temporally resolved energy exchange terms. The changes in the material topography are then reflected in the updated grid for the FDTD scheme. The developed M-TTM-MD model provides a self-consistent numerical framework that offers insights into laser-induced phenomena in metals, including energy transport and surface dynamics under extreme nonequilibrium conditions.
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Materials Science

Morphological Stability of Metal Anodes: Roles of Solid Electrolyte Interphases (SEIs) and Desolvation Kinetics

Jin Zhang and Peter W. Voorhees

Achieving stable lithium metal anodes requires control over the solid-electrolyte interphase (SEI) and desolvation kinetics. Here, we develop a unified theoretical framework integrating ion transport, desolvation, charge transfer, and SEI breakdown to predict morphological instabilities during electrodeposition. Using linear stability analysis, we identify six dimensionless parameters that govern the onset and evolution of instabilities. We show that SEI transport and desolvation rate effectively modulate apparent reaction kinetics, shifting the system toward a stable, reaction-limited regime. Extending the classical limiting current concept, we demonstrate that a thick, poorly conductive SEI and sluggish desolvation significantly reduce the limiting current. We introduce an apparent Damköhler number to quantify the critical balance: suppressing diffusion-limited instabilities by reaction rate reduction, while maintaining a high limiting current. Our theory enables predictive mapping of electrodeposition morphologies across diverse materials and operating conditions, guiding the rational design of stable lithium metal anodes.
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Soft Condensed Matter

Star-like microgels vs star polymers: similarities and differences

Tommaso Papetti, Elisa Ballin, Francesco Brasili et al. (+1 more)

Star-like microgels have recently emerged as a promising class of thermoresponsive soft colloids, that have an internal architecture similar to that of star polymers. Here, we perform extensive monomer-resolved simulations to theoretically establish this analogy. First, we characterize the effective potential between star-like microgels, finding that it is Gaussian for an extended range of distances, in stark contrast to the Hertzian-like one of standard microgels, but almost identical to that of star polymers with a core partially covered by chains. Next, we investigate the ratio between gyration and hydrodynamic radii across the volume-phase transition, showing qualitative agreement with both star polymers and experimental data. Finally, we estimate the bulk modulus, finding star-like microgels significantly softer than standard microgels and comparable to star polymers. The present work thus demonstrates that star-like microgels behave as ultrasoft particles, akin to star polymers, paving the way for their exploration at high concentrations.
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All authors Tommaso Papetti, Elisa Ballin, Francesco Brasili, Emanuela Zaccarelli

Disordered Systems and Neural Networks

Directionality and node heterogeneity reshape criticality in hypergraph percolation

Yunxue Sun, Xueming Liu and Ginestra Bianconi

Directed and heterogeneous hypergraphs capture directional higher-order interactions with intrinsically asymmetric functional dependencies among nodes. As a result, damage to certain nodes can suppress entire hyperedges, whereas failure of others only weakens interactions. Metabolic reaction networks offer an intuitive example of such asymmetric dependencies. Here we develop a message-passing and statistical mechanics framework for percolation in directed hypergraphs that explicitly incorporates directionality and node heterogeneity. Remarkably, we show that these hypergraph features have a fundamental effect on the critical properties of hypergraph percolation, reshaping criticality in a way that depends on network structure. Specifically, we derive anomalous critical exponents that depend on whether node or hyperedge percolation is considered in maximally correlated, heavy-tailed regimes. These theoretical predictions are validated on synthetic hypergraph models and on a real directed metabolic network, opening new perspectives for the characterization of the robustness and resilience of real-world directed, heterogeneous higher-order networks.
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Soft Condensed Matter

From biting to engulfment: Target mechanics determines modes of phagocytosis through curvature–actin coupling

Shubhadeep Sadhukhan, Caitlin E. Cornell, Mansehaj Kaur Sandhu et al. (+10 more)

Phagocytosis is a core innate immune process that clears targets spanning a wide range of mechanical properties, yet the role of target mechanics in recognition and engulfment remains unclear. Here, we combine theoretical modeling and experiments to reveal how target stiffness governs distinct modes of phagocyte–target interaction. We develop a membrane-based simulation framework in which both the engulfing cell and its target are deformable and undergo large shape changes, while actin-driven protrusions are regulated by curvature-sensitive membrane complexes. The model predicts three mechanical regimes with increasing target stiffness: (i) biting (trogocytosis), where part of the target is extracted; (ii) pushing, where the target is displaced rather than engulfed; and (iii) complete engulfment. We validate these predictions in epithelial clearance of apoptotic targets in vivo and macrophage engulfment of Giant Unilamellar Vesicles (GUVs) and lymphoma cells. Together, our results identify target mechanics as a key regulator of clearance and cell–cell interactions.
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All authors Shubhadeep Sadhukhan, Caitlin E. Cornell, Mansehaj Kaur Sandhu, Marta Batet Palau, Youri Peeters, Stijn Hanssen, Samo Penič, Aleš Iglič, Daniel A. Fletcher, Valentin Jaumouillé, Daan Vorselen, Verena Ruprecht, Nir S. Gov

Materials Science

Universality of Type-II Multiferroicity in Monolayer Nickel Dihalides

Aleš Cahlík, Antti Karjasilta, Anshika Mishra et al. (+5 more)

The recent discovery of type-II multiferroicity in monolayer NiI${_2}$ indicated a new pathway for intrinsic magnetoelectric coupling in the two-dimensional limit. However, determining whether this phenomenon is a unique anomaly or a general, chemically tunable property of the material class remains unresolved. Here, we demonstrate the universality of type-II multiferroicity in the transition metal dihalides by visualizing the ferroelectric order in monolayer NiBr${_2}$. Using scanning tunneling microscopy (STM), we resolve atomic-scale ferroelectric domains and confirm their magnetoelectric origin through reciprocal manipulation experiments: reorienting magnetic order via electric fields and suppressing the electric polarization with external magnetic fields. Furthermore, we find that the multiferroic state in NiBr${_2}$ is energetically less robust than in its iodide counterpart, consistent with modified superexchange interactions and the reduced spin-orbit coupling. Our results establish the transition metal dihalides as a versatile platform where the stability of magnetoelectric phases can be engineered through chemical substitution.
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All authors Aleš Cahlík, Antti Karjasilta, Anshika Mishra, Robert Drost, Mohammad Amini, Javaria Arshad, Büşra Arslan, Peter Liljeroth

Statistical Mechanics · Cross-listed from hep-lat

A scalable flow-based approach to mitigate topological freezing

Claudio Bonanno, Andrea Bulgarelli, Elia Cellini et al. (+4 more)

As lattice gauge theories with non-trivial topological features are driven towards the continuum limit, standard Markov Chain Monte Carlo simulations suffer for topological freezing, i.e., a dramatic growth of autocorrelations in topological observables. A widely used strategy is the adoption of Open Boundary Conditions (OBC), which restores ergodic sampling of topology but at the price of breaking translation invariance and introducing unphysical boundary artifacts. In this contribution we summarize a scalable, exact flow-based strategy to remove them by transporting configurations from a prior with a OBC defect to a fully periodic ensemble, and apply it to 4d SU(3) Yang–Mills theory. The method is based on a Stochastic Normalizing Flow (SNF) that alternates non-equilibrium Monte Carlo updates with localized, gauge-equivariant defect coupling layers implemented via masked parametric stout smearing. Training is performed by minimizing the average dissipated work, equivalent to a Kullback–Leibler divergence between forward and reverse non-equilibrium path measures, to achieve more reversible trajectories and improved efficiency. We discuss the scaling with the number of degrees of freedom affected by the defect and show that defect SNFs achieve better performances than purely stochastic non-equilibrium methods at comparable cost. Finally, we validate the approach by reproducing reference results for the topological susceptibility.
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All authors Claudio Bonanno, Andrea Bulgarelli, Elia Cellini, Alessandro Nada, Dario Panfalone, Davide Vadacchino, Lorenzo Verzichelli

Materials Science

Manipulating ferroelectricity without electrical bias: A perspective

Bixin Yan, Valentine Gillioz, Ipek Efe et al. (+1 more)

Ferroelectric materials are established candidates for beyond complementary metal-oxide-semiconductor technology, owing to their non-volatile spontaneous electrical polarization. The recent boom in electric dipole texture engineering and manipulation in such materials has revealed exciting routes for controlling ferroelectric polarization, offering alternatives to the classical, sometimes challenging, application of electrical fields. In this short perspective, we shed light on electrode-free external stimuli enabling control over polar states in thin films. We bring awareness to the polarizing role of chemically-engineered surface contributions and provide insights into the combination of chemical substitution and mechanical pressure, complementing the polar state tuning capabilities readily enabled by flexoelectricity. Finally, we describe recent developments in the optical modulation of polarization. Thus, our perspective aims to stimulate the advancement of alternative means to act on polarization states and facilitate the development of ferroelectric-based applications.
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All authors Bixin Yan, Valentine Gillioz, Ipek Efe, Morgan Trassin

Strongly Correlated Electrons

Collective excitations in chiral spin liquid: chiral roton and long-wavelength nematic mode

Hongyu Lu, Wei Zhu and Wang Yao

Chiral spin liquid (CSL) is a magnetic analogue of the fractional quantum Hall (FQH) liquid. Collective excitations play a vital role in shaping our understanding of these exotic quantum phases of matter and their quantum phase transitions. While the magneto-roton and long-wavelength chiral graviton modes in the FQH and fractional Chern insulator (FCI) liquids have been extensively explored, whether CSLs host analogous or qualitatively different modes remains elusive. Here we explore the collective excitations in the SU(2) symmetric CSL phase. Combining exact diagonalization and time-dependent variational principle calculations, we identify two spin-singlet collective modes: a chiral p-wave roton mode at finite momentum, and a elliptically polarized d-wave nematic mode at zero momentum, both of which are prominent across the CSL phase. The chiral p-wave singlet roton has no counterpart in FQH of FCI systems, and the q = 0 d-wave mode also exhibits fingerprint distinct from those of FQH/FCI liquids. We also elucidate that both singlet modes are general for CSLs on various lattice models. By tuning J2, we find the nematic mode to be pronouncedly soft, together with the spin-triplet two-spinon bound states, potentially promoting strong nematic and spin stripe instabilities. Our work paves the way for further understanding CSL from the dynamical perspective and provides new spectroscopic signatures for future experiments of CSL candidates.
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Materials Science

Impact of O concentration on the thermal stability and decomposition mechanism of (Cr,Al)N compared to (Ti,Al)N thin films

Pauline Kümmerl, Ganesh Kumar Nayak, Felix Leinenbach et al. (+6 more)

The composition-dependent thermal stability of (Cr$_{0.47 \mp 0.03}$Al$_{0.53 \mp 0.03}$)$_{z}$(O$_{y}$N$_{1-y}$)$_{1-z}$ thin films with O concentrations of y = 0, 0.15, and 0.40 is investigated up to 1200 {\deg}C and then compared to (Ti$_{0.56}$Al$_{0.44}$)$_{z}$(O$_{y}$N$_{1-y}$)$_{1-z}$. X-ray diffraction reveals a thermal stability limit of 1150 {\deg}C independent of the O concentration, as witnessed by the formation of decomposition products, namely h-Cr$_{2}$N for (Cr$_{0.50}$Al$_{0.50}$)$_{0.49}$N$_{0.51}$ and c-Cr for both (Cr$_{0.48}$Al$_{0.52}$)$_{0.48}$(O$_{0.15}$N$_{0.85}$)$_{0.52}$ and (Cr$_{0.44}$Al$_{0.56}$)$_{0.46}$(O$_{0.40}$N$_{0.60}$)$_{0.54}$. Based on TEM and ERDA data, the thermal stability limit is extended to 1100 - 1150 {\deg}C. DFT calculations indicate that bond breaking limits the thermal stability. In (Cr,Al)N, N has the lowest activation energy for migration. Furthermore, the O vacancy formation energy is highest in (Cr,Al)(O,N). It has to be overcome to enable diffusion on the non-metal sublattice, which is necessary for forming decomposition products like w-AlN or c-Cr. However, once Cr-N bonds break, decomposition into h-Cr$_{2}$N and subsequent c-Cr together with N$_{2}$ is triggered. This results in N evaporation, generating sufficient non-metal vacancies that greatly enhance diffusion and render the extensive vacancy formation energies for non-metals irrelevant. This reduction of the activation energy for mass transport on the non-metal sublattice to the migration barrier causes the similar thermal stability in (Cr$_{0.47 \mp 0.03}$Al$_{0.53 \mp 0.03}$)$_{z}$(O$_{y}$N$_{1-y}$)$_{1-z}$. In contrast, Al bonds break first without creating non-metal vacancies in (Ti,Al)(O,N). Thus, the high O vacancy formation energy in (Ti,Al)(O,N) significantly increases the thermal stability compared to (Ti,Al)N as well as the here investigated films.
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All authors Pauline Kümmerl, Ganesh Kumar Nayak, Felix Leinenbach, Zsolt Czigány, Daniel Primetzhofer, Szilárd Kolozsvári, Peter Polcik, Marcus Hans, Jochen M. Schneider

Statistical Mechanics · Cross-listed from q-bio.PE

Noise-induced excitability: bloom, bust and extirpation in autotoxic population dynamics

Pablo Moreno-Spiegelberg and Javier Aguilar

Species populations often modify their environment as they grow. When environmental feedback operates more slowly than population growth, the system can undergo boom-bust dynamics, where the population overshoots its carrying capacity and subsequently collapses. In extreme cases, this collapse leads to total extinction. While deterministic models typically fail to capture these finite-time extinction events, we propose a stochastic framework, derived from an individual-based model, to describe boom-bust-extirpation dynamics. We identify a noise-driven, threshold-like behavior where, depending on initial conditions, the population either undergoes a “boom” or is extirpated before the expansion occurs. Furthermore, we characterize a transition between an excitable regime, where most trajectories are captured by the absorbing state immediately after the first bust, and a persistent regime, where most populations reach a metastable state. We show that this transition is governed by the noise strength and the ratio of environmental-to-population timescales. This framework provides a theoretical basis for understanding irreversible transitions in invasive species, plant succession, microbial dynamics, and the elimination of cancerous tumors.
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Quantum Gases

Double-exchange ferromagnetism of fermionic atoms in a $p$-orbital hexagonal lattice

Haoran Sun, Erhai Zhao, Youjin Deng et al. (+1 more)

A large class of correlated quantum materials feature strong Hund's coupling. Yet cold-atom quantum simulators have so far focused primarily on single-orbital Fermi-Hubbard systems near a Mott insulator. Here we show that repulsively interacting fermions loaded into the $p$-bands of a hexagonal lattice offer a unique platform to study the interplay of "Hundness" and "Mottness." Our theory predicts that the orbital degrees of freedom, despite geometric frustration, produce a rich phase diagram featuring a competing itinerant ferromagnetic (FM) metal and a spin-1 antiferromagnetic (AFM) insulator, with a surprising first-order transition between them controlled by density near half-filling. Ferromagnetism emerges at low fillings from the flat band and persists to stronger interactions and higher fillings via a double-exchange mechanism, where spins align to avoid Hund-rule penalties at the expense of Dirac-fermion kinetic energy. We further argue that the paramagnetic regime is a correlated "Hund metal." $p$-orbital Fermi gases thus provide an ideal experimental setting to investigate competing exchange mechanisms in multi-orbital systems with coexisting localized and itinerant spins.
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All authors Haoran Sun, Erhai Zhao, Youjin Deng, W. Vincent Liu

Mesoscale and Nanoscale Physics

Exchange-dominated frequency shift of spin-wave nonreciprocal dispersion relation in planar magnetic multilayers

Claudia Negrete, Attila Kákay and Jorge A. Otálora

Spin-wave nonreciprocity, manifested as a frequency difference between counterpropagating modes, underpins many proposed magnonic devices. While this effect is commonly attributed to dipolar interactions or interfacial chirality, the microscopic origin of the frequency shift in nonreciprocal dispersion in magnetic multilayers remains under debate. Here, we analyze the frequency shift of nonreciprocal spin-wave dispersion in planar multilayer heterostructures without Dzyaloshinskii-Moriya interaction. Using a frequency-shift dynamic matrix formalism, we show that the frequency asymmetry cannot generally be ascribed to dipolar effects alone. Instead, once counterpropagating modes differ in their geometric structure along the thickness, interlayer exchange dominates the frequency shift. Applied to representative multilayer systems, we find that the interlayer exchange contribution exceeds dipolar and intralayer exchange effects by up to two to three orders of magnitude over a broad wave-vector range. Our results establish interlayer exchange as the primary mechanism contributing to the frequency shift of nonreciprocal dispersion in multilayer magnonic systems and provide a quantitative framework for engineering large frequency shifts in nonreciprocal magnonic devices.
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Disordered Systems and Neural Networks

Variational Monte Carlo (VMC) with row-update Projected Entangled-Pair States (PEPS) and its applications in quantum spin glasses

Tao Chen, Jing Liu, Yantao Wu et al. (+2 more)

Solving the quantum many-body ground state problem remains a central challenge in computational physics. In this context, the Variational Monte Carlo (VMC) framework based on Projected Entangled Pair States (PEPS) has witnessed rapid development, establishing itself as a vital approach for investigating strongly correlated two-dimensional systems. However, standard PEPS-VMC algorithms predominantly rely on sequential local updates. This conventional approach often suffers from slow convergence and critical slowing down, particularly in the vicinity of phase transitions or within frustrated landscapes. To address these limitations, we propose an efficient autoregressive row-wise sampling algorithm for PEPS that enables direct, rejection-free sampling via single-layer contractions. By utilizing autoregressive single-layer row updates to generate collective, non-local configuration proposals, our method significantly reduces temporal correlations compared to local Metropolis moves. We benchmark the algorithm on the two-dimensional transverse-field Ising model and the quantum spin glass. Our results demonstrate that the row-wise scheme effectively mitigates critical slowing down near the Ising critical point. Furthermore, in the rugged landscape of the quantum spin glass, it yields improved optimization stability and lower ground-state energies. These findings indicate that single-layer autoregressive row updates provide a flexible and robust improvement to local PEPS-VMC sampling and may serve as a basis for more advanced sampling schemes.
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All authors Tao Chen, Jing Liu, Yantao Wu, Pan Zhang, Youjin Deng

Statistical Mechanics · Cross-listed from cs.CL

Single-Nodal Spontaneous Symmetry Breaking in NLP Models

Shalom Rosner, Ronit D. Gross, Ella Koresh et al. (+1 more)

Spontaneous symmetry breaking in statistical mechanics primarily occurs during phase transitions at the thermodynamic limit where the Hamiltonian preserves inversion symmetry, yet the low-temperature free energy exhibits reduced symmetry. Herein, we demonstrate the emergence of spontaneous symmetry breaking in natural language processing (NLP) models during both pre-training and fine-tuning, even under deterministic dynamics and within a finite training architecture. This phenomenon occurs at the level of individual attention heads and is scaled-down to its small subset of nodes and also valid at a single-nodal level, where nodes acquire the capacity to learn a limited set of tokens after pre-training or labels after fine-tuning for a specific classification task. As the number of nodes increases, a crossover in learning ability occurs, governed by the tradeoff between a decrease following random-guess among increased possible outputs, and enhancement following nodal cooperation, which exceeds the sum of individual nodal capabilities. In contrast to spin-glass systems, where a microscopic state of frozen spins cannot be directly linked to the free-energy minimization goal, each nodal function in this framework contributes explicitly to the global network task and can be upper-bounded using convex hull analysis. Results are demonstrated using BERT-6 architecture pre-trained on Wikipedia dataset and fine-tuned on the FewRel classification task.
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All authors Shalom Rosner, Ronit D. Gross, Ella Koresh, Ido Kanter

Other Condensed Matter · Cross-listed from quant-ph

Time complexity of a monitored quantum search with resetting

Emma C. King, Sayan Roy, Francesco Mattiotti et al. (+3 more)

Searching a database is a central task in computer science and is paradigmatic of transport and optimization problems in physics. For an unstructured search, Grover's algorithm predicts a quadratic speedup, with the search time $\tau(N)=\Theta(\sqrt{N})$ and $N$ the database size. Numerical studies suggest that the time complexity can change in the presence of feedback, injecting information during the search. Here, we determine the time complexity of the quantum analog of a randomized algorithm, which implements feedback in a simple form. The search is a continuous-time quantum walk on a complete graph, where the target is continuously monitored by a detector. Additionally, the quantum state is reset if the detector does not click within a specified time interval. This yields a non-unitary, non-Markovian dynamics. We optimize the search time as a function of the hopping amplitude, detection rate, and resetting rate, and identify the conditions under which time complexity could outperform Grover's scaling. The overall search time does not violate Grover's optimality bound when including the time budget of the physical implementation of the measurement. For databases of finite sizes monitoring can warrant rapid convergence and provides a promising avenue for fault-tolerant quantum searches.
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All authors Emma C. King, Sayan Roy, Francesco Mattiotti, Maximilian Kiefer-Emmanouilidis, Markus Bläser, Giovanna Morigi

Soft Condensed Matter

Ion-Modulated Polyelectrolyte Complexation of DNA and Polyacrylic Acid from Molecular Dynamics Simulations

Sisem Ektirici, Vagelis Harmandaris, Christos N. Likos et al. (+1 more)

The formation of complexes between like-charged polyelectrolytes challenges conventional electrostatic intuition and highlights the central role of ions in mediating macromolecular organization. Here, we investigate the salt-dependent association of DNA with poly(acrylic acid) (PAA) using atomistic molecular dynamics simulations in NaCl, MgCl$_2$, and CaCl$_2$ solutions. A time-resolved state classification scheme, based on heavy-atom distance and hydrogen-bond formation, was applied to distinguish bound and unbound configurations, enabling quantitative analysis of how ion valency modulates complex stability and structure. The results reveal a clear hierarchy of association strength with Ca$^{2+}$ promoting persistent complex formation through direct inner-sphere coordination between DNA phosphates and PAA carboxylates, Mg$^{2+}$ mediating weaker, transient bridging interactions and Na$^+$ exhibiting only electrostatic screening action with negligible bridge formation. Structural analysis shows that multivalent ions not only enhance complex stability but also reshape the molecular organization of both macromolecules. Ca$^{2+}$ induces expansion of DNA and compaction of PAA within a strongly bridged complex characterized by directional alignment and backbone-dominated binding, whereas Mg$^{2+}$ promotes more transient groove associations and Na$^+$ supports flexible, weakly correlated contacts. Our findings provide molecular-level insight into ion-specific mechanisms underlying polyelectrolyte organization and inform the design of responsive biomaterials and nucleic acid-based assemblies in multivalent ionic environments.
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All authors Sisem Ektirici, Vagelis Harmandaris, Christos N. Likos, Terpsichori S. Alexiou

Materials Science

Switchable Topological Polar Textures in Freestanding Ultrathin Ferroelectric Oxides

Franco N. Di Rino and Tim Verhagen

The remarkable advances achieved in two-dimensional materials are now being directly transposed to low-dimensional oxides. Here we show using first-principles-based atomistic simulations that ultrathin freestanding ferroelectric layers host a rich variety of polar states, from liquid-like ferroelectric domains with long-range orientational order to helix-wave and chiral bubbles configurations reminiscent of those observed in twisted freestanding oxide layers. Time-dependent electric fields enable reversible control, revealing freestanding oxide layers as ideal platforms to explore complex polar states and their potential applications in future ferroic devices.
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Disordered Systems and Neural Networks

Unified Symmetry Classification of Many-Body Localized Phases

Yucheng Wang

The Altland–Zirnbauer (AZ) tenfold way classifies disordered non-interacting systems by symmetry, but a unified symmetry classification of interacting many-body localized (MBL) phases has been lacking. Here we develop such a classification directly from the emergent local integrals of motion (LIOMs). Our scheme rests on two complementary criteria: whether a symmetry is compatible with a complete LIOM algebra, and whether it enforces many-body degeneracies. These criteria partition MBL phases into stable, fragile, symmetry-protected topological, and unstable classes, and unify AZ classes, onsite Abelian symmetries, and continuous non-Abelian symmetries within a single operator-algebraic framework. Our results provide a systematic foundation for understanding how symmetry constrains the stability and structure of MBL phases.
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Superconductivity

Scanning tunneling spectroscopy of superconducting nitridized aluminum thin films

Jose Antonio Moreno, Pablo García Talavera, Alba Torras-Coloma et al. (+5 more)

Nitride-based superconductors represent a family of superconducting thin film materials displaying higher quality than their corresponding bare superconductor when used in devices for applications such as cosmic radiation sensing. In recent times, Niobium-based and Titanium-based nitrides were used to improve the quality of superconducting devices in quantum technology applications. Recently, nitridized Aluminum (NitrAl) has been found to display higher critical temperatures and enhanced resilience to magnetic fields compared to those of Al, making it a new interesting candidate for superconducting quantum circuit applications. However, the microscopic properties of NitrAl remain highly unexplored. Here we use Scanning Tunneling Microscope (STM) to measure the superconducting density of states of a thin film sample of nitridized-Aluminum (NitrAl), with a room temperature resistivity between pure Al and fully insulating aluminum nitride. We show that the in-gap density of states is zero up to about $\hbar\omega=250~\mathrm{\mu eV}$ and that there is a distribution of values of the superconducting gap around $\Delta_0=360~\mathrm{\mu eV}$, close to the BCS expectation $\Delta=1.76 k_{\mathrm{B}}T_{\mathrm{c}}$. We also find varying superconducting gap values at the nanometer scale, by approximately 10%, when probing different regions of the sample. These results suggest a gap which is larger than the one of pure Al, and is spatially more homogeneous than the superconducting gap values often found in thin films. Our work demonstrates that STM is as a powerful tool to screen materials for quantum devices through the measurement of the spatial dependence of the superconducting density of states.
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All authors Jose Antonio Moreno, Pablo García Talavera, Alba Torras-Coloma, Gemma Rius, P. Forn-Díaz, Edwin Herrera Vasco, Isabel Guillamón, Hermann Suderow

Strongly Correlated Electrons

Magnetic states of the Kondo lattice Ce$_2$PdSi$_3$ and their pressure evolution

Yanan Zhang, Zhaoyang Shan, Jiawen Zhang et al. (+11 more)

Frustrated Kondo lattices are ideal platforms for exploring unconventional forms of quantum criticality, as well as magnetism and other emergent phases. Here we report the magnetic properties of the candidate frustrated heavy fermion compound Ce$_2$PdSi$_3$, and map their evolution upon applying magnetic fields and hydrostatic pressure. We find that at ambient pressure Ce$_2$PdSi$_3$ exhibits two distinct magnetic phase transitions, a ferromagnetic-like transition at $T_{\mathrm{M1}}=3.8$ K and an incommensurate antiferromagnetic transition at $T_{\mathrm{M2}}=2.9$ K. Upon applying pressure, $T_{\mathrm{M1}}$ is continuously suppressed and becomes undetectable above 4.2 GPa, whereas $T_{\mathrm{M2}}$ increases and remains robust up to at least 7.5 GPa. The observed pressure evolution of magnetic order in Ce$_2$PdSi$_3$ suggests the presence of competing magnetic orders, and cannot be simply encapsulated by the Doniach phase diagram, motivating further investigations for its origin, including discerning the role of geometric frustration.
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All authors Yanan Zhang, Zhaoyang Shan, Jiawen Zhang, Kaixin Ye, Yongjian Li, Dajun Su, Pascal Manuel, Dmitry Khalyavin, Devashibhai Adroja, Daniel Mayoh, Geetha Balakrishnan, Yu Liu, Michael Smidman, Huiqiu Yuan

Quantum Gases

Three-body scattering area of identical bosons in two dimensions

Junjie Liang, Hongye Yu and Shina Tan

We study the wave function $\phi^{(3)}$ of three identical bosons scattering at zero energy, zero total momentum, and zero orbital angular momentum in two dimensions, interacting via short-range potentials with a finite two-body scattering length $a$. We derive asymptotic expansions of $\phi^{(3)}$ in two regimes: the 111-expansion, where all three pairwise distances are large, and the 21-expansion, where one particle is far from the other two. In the 111-expansion, the leading term grows as $\ln^3(B/a)$ at large hyperradius $B=\sqrt{(s_1^2+s_2^2+s_3^2)/2}$. At order $B^{-2}\ln^{-3}(B/a)$, we identify a three-body parameter $D$ with dimension of length squared, which we term the three-body scattering area. This quantity should be contrasted with the three-body scattering area previously studied for infinite or vanishing two-body scattering length. If the two-body interaction is attractive and supports bound states, $D$ acquires a negative imaginary part, and we derive its relation to the probability amplitudes for the production of two-body bound states in three-body collisions. Under weak modifications of the interaction potentials, we derive the corresponding shift of $D$ in terms of $\phi^{(3)}$ and the changes of the two-body and three-body potentials. We also study the effects of $D$ and $\phi^{(3)}$ on three-body and many-body physics, including the three-body ground-state energy in a large periodic volume, the many-body energy and the three-body correlation function of the dilute two-dimensional Bose gas, and the three-body recombination rates of two-dimensional ultracold atomic Bose gases.
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