Disordered Systems and Neural Networks · Cross-listed from nlin.CD
Online training of Echo State Networks to predict chaotic dynamical systems
Astero Provata, Markos Syroukis and Johanne Hizanidis
Echo State Networks (ESNs) are fundamentally important from the conceptual perspective of AI, because they provide simple and tractable frameworks for understanding temporal information processing,
memory, and learning in dynamical systems. They are composed by an interconnected reservoir whose connectivity
matrix remains constant over time and only the output weights are updated during the training period. In particular, ESNs employing online training allow the incremental adaptation of the output weights as data is received, making them particularly suitable for applications in which new input data becomes available over time (non-stationary environments). A key objective in the study of such learning structures is to determine which statistical properties of the reservoir enhance computational efficiency and maximize prediction performance. To this end, we investigate the effects of reservoir
topology on learning performance, considering weight
matrices generated from Erdos-Renyi random networks and Watts & Strogatz small-world networks. As test data we use the Lorenz and Rossler models in the chaotic regime and the FitzHugh-Nagumo model in the limit cycle regime. In the case of random Erdos-Renyi reservoirs, our simulations show that best performances are obtained for sparse reservoirs and especially when the link-density is near and immediately above the critical point of mean-field bond percolation. Watts & Strogatz reservoirs are shown to demonstrate best performance also for small values of link-density, compatible with the mean-field bond percolation when taking into account the specific nonlocal ring structure of the Watts & Strogatz reservoirs. These studies on the influence of the reservoir architecture and hyperparameters on enhancing the accuracy of ESN predictions provide an efficient framework for real-time forecasting in dynamically evolving systems.
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Strongly Correlated Electrons · Cross-listed from physics.optics
Near-field contributions to cavity-mediated radiative heat transfer
Antonio Mastropasqua, Martin Eckstein, Daniele Fausti et al. (+1 more)
Embedding materials in optical cavities has emerged as a powerful strategy for controlling their physical properties. In this context, increasing
attention has been devoted to the thermal Purcell effect, which describes how the radiative heat transfer between a material and its photonic environ- ment can be modified by the electromagnetic boundary conditions imposed by the cavity. Here, we investigate near-field effects in cavity-mediated radiative heat transfer in planar cavities composed of realistic, dissipative gold mirrors, extending previous work that considered ideal mirrors. We iden- tify the conditions under which near-field effects lead to a substantial enhancement of heat transfer between the material and a photonic thermal bath relative to free space, owing to the strong modi- fication of the photonic local density of states by evanescent modes within the cavity. In particular, we find that radiative heat flow can be enhanced by several orders of magnitude in cavities ranging from nanometer to micrometer scales, depending on the absorber frequency and dipole orientation. These findings suggest that near-field effects may be relevant to polariton chemistry experiments and other systems operating within this cavity length-scale regime. Our results add to the idea that cavities can modify material properties not only by engineering
vacuum fluctuations in the strong light-matter coupling regime, but also by controlling radiative dissipation and heat transfer.
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All authors Antonio Mastropasqua, Martin Eckstein, Daniele Fausti, Francesca Fassioli
Strongly Correlated Electrons
Disorder-Induced Griffiths like Phase and Charge Localization in Sr2FeRuO6
Koustav Pal, Shuvendu Ghosh and I. Das
A comprehensive understanding of complex transition-metal oxides is key to designing next-generation spintronic materials with tailored
magnetic and transport functionalities. We investigate the interplay among structural disorder, frustrated magnetism, and localized charge transport in the 3d-4d hybrid double perovskite Sr2FeRuO6. The compound adopts a monoclinic structure with significant B-site antisite disorder that drives its unconventional behavior.
Magnetic measurements uncover a robust Griffiths phase extending well above the ordering temperature, evidencing stable short-range ferromagnetic clusters within a disordered
matrix. Electrical transport remains strongly localized, transitioning from small-polaron hopping at high temperatures to variable-range hopping at intermediate temperatures. These results position Sr2FeRuO6 as a representative 3d-4d hybrid system in which quenched disorder stabilizes
spin inhomogeneity, while polaronic and hopping transport are also observed.
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Materials Science
Identification of carbon-related color centers in hBN by high-resolution ab initio photoluminescence modeling
Vytautas Žalandauskas, Marek Maciaszek, Marianne Etzelmüller Bathen et al. (+3 more)
Identification of optically active defects in hexagonal boron nitride (hBN) remains challenging because different defect candidates can produce similar calculated photoluminescence (PL) lineshapes. Here, we demonstrate that force-constant
embedding methodology can be extended to calculate high-resolution, vibrationally resolved PL lineshapes in the polar layered material hBN. Benchmarking against the carbon dimer defect, responsible for the 4.1 eV emission, we show that the methodology accurately reproduces the low-energy acoustic
phonons and fine spectral features that are inaccessible when using direct supercell calculations. Applying the methodology to neutral carbon trimers ($\mathrm{C_2 C_B}$ and $\mathrm{C_2 C_N}$), we find that the calculated PL lineshape and excitation energy of $\mathrm{C_2 C_N}$ closely match the low-temperature spectrum of the emitter with a zero-
phonon line at 1.54 eV. These results establish high-resolution
phonon-sideband modeling as a powerful tool for discriminating among candidate defects in polar materials.
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All authors Vytautas Žalandauskas, Marek Maciaszek, Marianne Etzelmüller Bathen, Julius Janušonis, Tadas Paulauskas, Lukas Razinkovas
Materials Science · Cross-listed from physics.optics
Polaritonic self-hybridized critical coupling by Brillouin zone folding induced quasi-bound states in the continuum in bulk WS2 metasurfaces
Kuidong Wang, Xiongwei Guo, Fu Liu et al. (+2 more)
Maximizing the absorption of
exciton-polaritons in transition-metal dichalcogenides (TMDCs) under critical coupling conditions is crucial for developing high-efficiency polariton-based photodetectors, solar
cells, and perfect absorbers. Here, we propose a polaritonic critical self-hybridized coupling between Brillouin zone folding driven quasi-bound states in the continuum (BZF-QBIC) mode and A
excitons in a bulk WS2 metasurface. This polaritonic critical coupling shows robust behavior with respect to the asymmetric parameters of the involved BZF-QBIC mode, in contrast to the asymmetry-sensitive conventional QBIC-based critical coupling. By changing the height of WS2 metasurfaces, we observe a typical anti-crossing behavior with a Rabi splitting of 246.5 meV. Moreover, the maximum absorption value of 0.5 for
exciton-polaritons is achieved at the polaritonic critical coupling condition. The large population of
excitons provided by bulk WS2 is essential for achieving polaritonic critical coupling and can further enhance polaritonic absorption. This work enables the realization of robust, high-absorption self-hybridized polaritonic critical coupling in TMDCs, and further expedite their applications in high-absorption low-dimensional polaritonic devices.
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All authors Kuidong Wang, Xiongwei Guo, Fu Liu, Jinhai Si, Xun Hou
Quantum Gases
Unconventional quantum spin mixing of two spin-1 atoms beyond the single-mode approximation
Yanhong Liu, Fulin Deng, Haoxue Qiao et al. (+1 more)
The single-mode approximation (SMA) has been widely employed in ultracold spinor gases, where all
spin components share the same spatial mode. By adopting generalized Van Vleck (GVV) perturbation theory, we achieve a relative error of $10^{-6}$ in the ground-state energy with a reduced effective
Hilbert space dimension of $\sim 10^3$ for two
spin-1
bosons confined in a one-dimensional hard-wall trap. With the accurate GVV method, we explore the validity of the SMA across ground-state properties and quench
spin dynamics for two $^{7}\text{Li}$, $^{87}\text{Rb}$, or $^{23}\text{Na}$ atoms. Numerical results show that the quadratic Zeeman energy $q$ provides a powerful control knob: by increasing $|q|$, the fidelity between the
spin components in the ground state gradually decreases, leading eventually to the breakdown of the SMA (due to the competition between the kinetic energy and the quadratic Zeeman energy). Quench
spin dynamics with abruptly increased $q$ further reveals multiple resonances, where the SMA-predicted micromotion and beyond-SMA macromotion appear simultaneously. These results point to a direction to investigate the
quantum spin-mixing dynamics beyond the SMA, particularly in recent experiments of strongly interacting
spin-1 atomic gases.
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All authors Yanhong Liu, Fulin Deng, Haoxue Qiao, Wenxian Zhang
Materials Science
Sorption Molecular Dynamics of Point Defects in 2D Materials with a Machine Learning Potential
Nils-Ruben Skogemyr, Albin Sjöstedt, Isak Wernroth et al. (+6 more)
Defect engineering is a crucial tool for tuning and tailoring the properties of two-dimensional (2D) materials. A key challenge is identifying which defects are kinetically stable at room temperature and sufficiently promising to warrant further investigation. However, identifying stable defects with suitable properties using first-principles calculations, such as density functional theory (DFT) is computationally demanding, effectively limiting the scope of high-throughput studies. In this work, we investigate whether the machine learning interatomic potential MACE can reproduce sorption trends (adsorption vs. absorption) previously obtained from high-throughput DFT calculations and extend the analysis to elevated temperatures. A total of 17,624 defect structures from the ADAQ database are simulated using MACE-accelerated molecular dynamics (MD) at both 0 K and 300 K. The results show that MACE successfully reproduces the overall sorption trends observed in DFT calculations at 0 K at a fraction of the computational cost. Simulations performed at 300 K further indicate that the qualitative sorption trends remain largely the same when including finite-temperature effects. These findings demonstrate that MACE-driven MD can be used as an effective high-throughput prescreening tool for defect-related studies in 2D materials, specially related to sorption. While conventional first-principles methods, such as DFT, remain necessary for accurate and quantitative validation, the use of MLIPs enables MD simulations of finite-temperature defect behavior at a fraction of the computational cost of DFT-based MD.
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All authors Nils-Ruben Skogemyr, Albin Sjöstedt, Isak Wernroth, Isak Häggqvist, Isac Ljungwald, Hans-Ove Åkesson, Henricus R.A. ten Eikelder, Rickard Armiento, Joel Davidsson
Materials Science
Transverse magneto-gyrotropy driven by spin-induced ferroaxiality
Yoshihiro Okamura, Takumi Kagawa, Ayano Nakamura et al. (+5 more)
Photon chirality is inherently sensitive to structural handedness and magnetism, which break spatial inversion and time-reversal
symmetries, respectively. Ferroaxial order, by contrast, hosts two-dimensional chirality while preserving both
symmetries, thereby precluding its direct coupling to
photon chirality. Intertwining ferroaxiality with
magnetic order, however, may lift such constraints and enable unconventional coupling to
photon chirality. Nevertheless, the optical response arising from such intertwined order has yet to be demonstrated experimentally. Here, we demonstrate transverse magneto-gyrotropy arising from
spin-driven ferroaxiality in the Weyl magnet EuCd2Sb2. Whereas the conventional polar magneto-optical Kerr effect is governed by magnetization parallel to the light-propagation direction, we observe a gyrotropic response associated with magnetization transverse to the light-propagation direction. The optical rotation exhibits a characteristic 120-degree periodicity upon rotation of the in-plane
magnetic field, reflecting the threefold
symmetry of the emergent ferroaxial state and revealing the ferroaxial origin of the transverse gyrotropic response. This coupling to
photon chirality enables vectorial control of the gyrotropic response through the
magnetic field direction. Our findings establish an interaction between
photon chirality and transverse magnetization enabled by intertwined ferroaxial and
magnetic orders, demonstrating a novel form of multiferroicity with functional light-matter interaction.
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All authors Yoshihiro Okamura, Takumi Kagawa, Ayano Nakamura, Shinichi Nishihaya, Masaki Uchida, Susumu Minami, Ryotaro Arita, Youtarou Takahashi
Quantum Gases
Dicke model of open-shell atoms with tensor polarizability
Ziheng Zhu and Yu Chen
We study the extended Dicke model obtained from the scalar–vector–tensor polarizability of open-shell atoms coupled to a shared cavity mode. For a single electric-dipole channel, rotational
symmetry fixes the tensor to vector ratio; we use $^{168}\mathrm{Er}$ with $J=6$ as a product-state example. A circular pump produces a chiral Raman vertex whose tensor-dependent
matrix elements select the occupied Zeeman sector. The resulting first-order transition occurs before the empty-cavity branch loses local stability, and the ordered state is read out through $\langle\hat J_z\rangle$. A linear pump instead produces an empty-cavity $\pm x$ cat and a continuous transition into an even-$m$ cat-like state with order parameter $\langle\hat J_z^2\rangle$. With cavity loss alone, the circular trajectory emits a
photon burst and approaches a Raman-dark Zeeman pole rather than a finite-field condensate. In the linear geometry, cavity-bright modes are damped toward the cat-like branch while a dark
spin mode remains undamped. These are phenomena of that model; the vector-only controls show that they depend on the channel-fixed tensor coupling.
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Disordered Systems and Neural Networks
Deep Learning of Phase Transitions for Bilinear-Biquadratic Spin-1 Chains from Correlation Aspects
Ming-Chiang Chung, Yu-Ting Kuo, Ian McCulloch et al. (+1 more)
We investigate the role of different correlation functions in reconstructing the phase diagram of the
spin-1 bilinear-biquadratic chain, parametrized by the angle $\theta$ between the bilinear and biquadratic couplings, using our previously developed label-free machine-learning pipeline \cite{Ming21, Ming23}, in which an autoencoder compresses iDMRG correlation
matrices into latent features and K-means clustering with silhouette analysis groups them into phases. Without any prior labels, the scheme reveals four natural phases, the Haldane, dimerized, critical trimerized, and ferromagnetic phases, and accurately locates the Takhtajan-Babujian ($\theta=-\pi/4$), $SU(3)$-symmetric Uimin-Lai-Sutherland ($\theta=\pi/4$), and Ferro-Dimer ($\theta=5\pi/4$) transition points; a shallow supervised refinement further sharpens the phase boundaries. We find that different correlation functions play fundamentally different roles in phase identification. The $SU(3)$-symmetric correlation alone selects a four-cluster partition that can be associated with the four phases, whereas the
spin-
spin and dimer-dimer correlations must be combined to recover the complete phase diagram. In contrast, the trimer-trimer correlation behaves as a marginal correlation: it is insufficient for phase identification by itself but improves the precision of the predicted phase boundaries when combined with the other correlations. Furthermore, the latent representations and silhouette values reveal signatures that we relate to the $\theta$ dependence of the correlations within each phase and, tentatively, to the underlying renormalization-group structure. These results demonstrate that machine learning can not only identify
quantum phases but also uncover the physical information encoded in different correlation functions.
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All authors Ming-Chiang Chung, Yu-Ting Kuo, Ian McCulloch, Yuan-Hong Tsai
Statistical Mechanics
Absolute area under a scaled Brownian motion with resetting
Vicenç Méndez and Javier Cristín
We investigate the statistical properties and ergodic behavior of the absolute area below the trajectory of a particle subject to
stochastic resetting and moving according to a scaled Brownian motion with exponent $\beta$. We compare the contributions to the absolute area from the waiting times between resets and the backward recurrence time. Using the renewal formalism we derive the lower-order moments of the absolute area under both Poissonian and heavy-tailed power-law resetting waiting time distributions with exponent $\alpha$. For Poissonian resetting,
stochastic interruption suppresses fluctuations, driving the area to be ergodic, regardless of the value of $\beta$. In contrast, power-law resetting yields a rich spectrum of ergodic regimes. For $0 < \alpha < 1$, the system displays weak ergodicity breaking and the maximum contribution dominates the area accumulated during the waiting times. For $1+\beta < \alpha < 2$, we unveil an ergodic transition exclusive to the subdiffusive regime at $\alpha=1+\beta$, where ergodicity is restored. These results clarify the subtle interplay between anomalous
diffusion and power-law waiting times between resettings. Our results are tested by numerical simulations exhibiting an excellent agreement.
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Materials Science · Cross-listed from physics.chem-ph
High-Harmonic Generation Probes Quantum Nuclear Fluctuations in Liquid Water
George Trenins, Nicolas Tancogne-Dejean, Angana Mondal et al. (+5 more)
Nuclear
quantum effects are essential for an accurate description of the hydrogen-bonded structure of
liquid water, yet their spectroscopic signatures can be subtle and difficult to measure. Moreover, probing these effects in solution on femtosecond timescales (or below) is an open challenge that is pivotal for optical control of hydrogen bonds and ultrafast spectroscopy of
proton transfer. Here, we combine path-integral molecular dynamics with real-time time-dependent density-functional theory to explore high-harmonic generation (HHG) in
liquid water from first principles, rigorously accounting for
quantum nuclear delocalisation. Relative to classical-
nuclei simulations,
quantum sampling suppresses the harmonic yield around certain harmonics by up to an order of magnitude, inducing local structural minima. By analysing the ab initio simulations and reproducing the key effects with a simplified model
Hamiltonian, we trace the physical origins of the interference minima to the broadening of valence-band states induced by hydrogen-bond fluctuations, a mechanism expected to generalise to other hydrogen-bonded structures. Our work shows that nuclear
quantum effects are indispensable for capturing ultrafast electronic dynamics in
liquids, and demonstrates the power of
liquid HHG spectroscopy for attosecond probing of nuclear fluctuations.
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All authors George Trenins, Nicolas Tancogne-Dejean, Angana Mondal, Angel Rubio, Hans Jakob Wörner, Aaron Kelly, Ofer Neufeld, Mariana Rossi
Mesoscale and Nanoscale Physics
Observation of correlations beyond the single-particle picture in fermionic counting statistics
Johann Zöllner, Hendrik Mannel, Daniel Oppers et al. (+3 more)
The analysis of charge fluctuations in a mesoscopic device can shed light on the fundamental question of whether or not a single-particle picture of electron transport is appropriate, even in the presence of Coulomb interaction. It has been shown on general grounds that mapping the interacting system onto a fermionic single-particle description is subject to strict constraints on the charge-transfer statistics. These constraints can be formulated in terms of a sign rule for the factorial cumulants of the full counting statistics. We report here on the experimental observation of a sign-rule violation in a most elementary model system, i.e. a single-level
quantum dot, and provide theoretical modeling that can quantitatively reproduce the experimental data. This reveals the presence of correlations introduced by electron-electron interaction.
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All authors Johann Zöllner, Hendrik Mannel, Daniel Oppers, Martin Geller, Axel Lorke, Jürgen König
Strongly Correlated Electrons · Cross-listed from quant-ph
Majorana Edge Modes Reshape Kibble-Zurek Scaling in Kitaev Honeycomb Ribbons
Hyesung Choi, Antonio Mandarino and Mahn-Soo Choi
Recent progress in programmable
quantum simulators motivates exact
benchmarks for nonequilibrium dynamics in topological
quantum matter. We study the Kitaev honeycomb model driven through its critical regions by a linear ramp of the bond coupling $J_z$ over a ramp time $\tau$, comparing zigzag and armchair ribbons with fully periodic tubes. The production of excitations exhibits a geometry-dependent finite-size crossover controlled by $N_2/\sqrt{\tau}$, the ratio of the ribbon width to the Kibble–Zurek length. Periodic tubes obey the expected critical-line Kibble–Zurek scaling and serve as the boundary-free
benchmark. Zigzag ribbons follow the same scaling until defect production is exponentially suppressed by a finite-width algebraic gap floor associated with the formation of Majorana edge modes, whose occupation is pinned by inversion parity. Armchair ribbons instead support transient Majorana edge modes with exponentially small gaps, producing width-parity-dependent defect plateaus: exactly one defect per unit length for odd widths and an approximate, transient plateau near one half for even widths. The quench-generated
entanglement entropy and fermionic logarithmic negativity inherit the same geometry-dependent structure, and the negativity further distinguishes the coherence-borne
entanglement of the Kibble–Zurek regime from the population-borne
entanglement of the edge-mode plateaus. These results identify boundary Majorana modes as the mechanism controlling departures from ideal Kibble–Zurek dynamics and provide geometry-resolved
benchmarks for programmable simulations of topological matter.
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Materials Science · Cross-listed from cs.LG
Beyond DFT electronic band structures via dual-space orbital learning
Minhyuk Kang, Beom Seok Kang, Nikolai Argatoff et al. (+4 more)
Accurate electronic band structures are essential for predicting material properties, but high-fidelity calculations are computationally demanding. We introduce O-kRystal, a dual-space orbital-learning framework that uses a lower-cost Perdew-Burke-Ernzerhof (PBE) calculation as both an energy baseline and a source of structured orbital information. Combining PBE band energies with real- and reciprocal-space orbital
matrices, O-kRystal predicts higher-level corrections for individual electronic states as a function of
crystal momentum and
spin. For states within 5 eV of the band-edge midpoint, it predicts Heyd-Scuseria-Ernzerhof (HSE06) band energies with a mean absolute error of $0.171 \pm 0.009$ eV across nearly 6,000 three-dimensional materials spanning insulating, metallic,
magnetic, and nonmagnetic systems. On a separate
benchmark of six strongly correlated materials, it reduces the corresponding PBE band-energy MAE by 68.7% and resolves orbital-dependent changes in the relative placement of transition-metal d and ligand p states. Despite training only at high-
symmetry points, it maintains comparable accuracy at unseen points along band paths and reconstructs HSE06 densities of states from dense Brillouin-zone predictions. Finally, training O-kRystal on $G_0W_0$ quasiparticle energies for two-dimensional materials demonstrates its applicability across levels of theory and the potential of orbital learning for higher-fidelity, momentum-resolved electronic-structure prediction beyond density functional theory.
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All authors Minhyuk Kang, Beom Seok Kang, Nikolai Argatoff, Miguel Nouman, Danish Khan, William A. Goddard III, Anima Anandkumar
Statistical Mechanics · Cross-listed from quant-ph
Single and coupled quantum van der Pol oscillators: Wigner-function dynamics, exact qutrit solutions, and synchronization
Rohitashwa Chattopadhyay, Vinayak Pendse and Shamik Gupta
We study
quantum van der Pol oscillator in which single-
photon pumping competes with a nonlinear two-
photon dissipative channel to produce a
quantum limit cycle, and the question of how two such oscillators with non-identical intrinsic frequencies synchronize in the sense of phase locking when coupled dissipatively to one another. For a single oscillator, starting from a Lindblad description of the dynamics, we derive the exact Wigner-function
evolution equation, and show that its Fokker–Planck approximation admits in the appropriate regime an equivalent classical Itô
stochastic description that reproduces remarkably well the full Lindblad dynamics. Notably, the
stochastic approach provides an efficient means of capturing system dynamics in the semiclassical regime, where a large number of energy eigenstates contribute and direct numerical simulations become computationally demanding. Restricting to the three lowest Fock states and using a parity
symmetry, we obtain in closed form the stationary-state density
matrix and its Wigner-function representation that displays pronounced nonclassical features. For two non-identical oscillators, we study the full Lindblad dynamics numerically and characterize synchronization through the stationary-state order parameter as a function of frequency mismatch and dissipative coupling. We find that increasing the effective nonlinear dissipative strength suppresses the synchronized region. Restricting the two oscillators to the three-level qutrit subspace, a parity
symmetry reduces the stationary-state problem to an exact $41$-dimensional linear system, allowing to compute the synchronization order parameter. In this reduced description, results in the limit of large dissipation agree closely with those from the full dynamics, while exhibiting in the limit of small dissipation a double-peak synchronization structure accompanied by a phase reversal.
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Statistical Mechanics · Cross-listed from q-bio.QM
Maximum likelihood, neural networks and posterior mean for inferring fractional Brownian motion parameters
Daniel S. Han, Vaishnavi Ananthanarayanan and Matthew J. Simpson
Fractional Brownian motion (fBM) is a model of anomalous
diffusion parameterised by a Hurst exponent $H \in (0,1)$ and a generalised
diffusion coefficient $D_H > 0$. We jointly estimate $H$ and $D_H$ using maximum likelihood (MLE) from density measurements with Poisson counting noise and single-particle trajectory with Gaussian localisation error. We show that one density measurement in time cannot infer both $H$ and $D_H$ and that several time points are needed, which matters most for inference precision. For trajectories, we show that a neural network trained with a squared error loss approximates the posterior mean of $H$, which can be computed exactly at negligible cost. Paired with the profile-likelihood estimate of $D_H$, the posterior mean has the lowest root mean squared error in both $H$ and $D_H$, except near $H=0$ and $H=1$ where MLE estimates $H$ better. In vesicle trajectories from two
cell types, $H$ and $D_H$ form a continuum rather than distinct modes of motion. The posterior probability that each window within a track is subdiffusive or superdiffusive shows that $26\%$ of MRC5 fibroblast and $21\%$ of HeLa vesicle trajectories contain both subdiffusive and superdiffusive windows, against $2\%$ and $7\%$ expected from estimation uncertainty alone. Intracellular cargoes therefore switch between subdiffusion and superdiffusion over time rather than forming populations with fixed $H$.
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Statistical Mechanics
Understanding the Mpemba Effect through a Rule of Thumb
Ran Yaacoby
In the Mpemba effect, a system prepared farther from
equilibrium relaxes faster than one prepared closer to it. Its persistence across diverse systems, and under different dynamics of the same system, suggests that it is largely rooted in
equilibrium properties, with the dynamics deciding only which mode relaxes slowest. Here we propose a simple rule of thumb for anticipating the Mpemba effect: the effect is expected when a single low-order
equilibrium moment is nonmonotonic in temperature. The bath singles out this moment as the slowest to relax, so that its displacement from its bath value sets the amplitude of the slowest relaxation mode. We establish this relation exactly for low-dimensional Ornstein–Uhlenbeck processes. We then extend the rule to mean-field models by approximating their relaxation near the final
equilibrium state by an Ornstein–Uhlenbeck process. For the mean-field Ising antiferromagnet, which we solve exactly in the thermodynamic limit, this approximation reproduces the main structure of the Mpemba phase diagram, including strong effects far from any phase transition. Finally, we give a general prescription for mean-field systems: identify the slowest
symmetry-allowed moment of the normal coordinates at the bath, and follow its
equilibrium value as a function of preparation temperature. This offers a practical route from
equilibrium observables to anomalous relaxation without diagonalizing the dynamics.
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Superconductivity
Andreev-reflection spectroscopy of SrPt3P single crystals
Valeriy A. Stepanov, Nikolai D. Zhigadlo, Erik Piatti et al. (+1 more)
The interplay between inversion
symmetry breaking and superconducting pairing mechanisms is a long-standing puzzle of solid-state physics. Ternary platinum phosphide SrPt$_3$P is an excellent case study in this regard, since it shares the distorted anti-perovskite
crystal structure of non-centrosymmetric
superconductor CePt$_3$Si but preserves the inversion
symmetry center, enabling a direct comparison between the two compounds. Despite intense scrutiny, the superconducting pairing strength, gap
symmetry, and magnitude of SrPt$_3$P remain an open question. Here, we study the gap structure of high-quality SrPt$_3$P single
crystals with $T_c = 8.6$ K by using point-contact Andreev-reflection spectroscopy. The spectra systematically showed symmetric conductance maxima and a zero-bias minimum, as is typical of fully gapped
superconductors, and dips at finite voltage $V_\mathrm{dip}$ due to the local transition of the contact region to the resistive state. In contacts with high resistance, where the dips fall sufficiently far from the gap features, the conductance curves can be well fitted with a single isotropic $s$-wave gap. The temperature dependence $\Delta(T)$ of the gap amplitude extracted from the fit is in good agreement with the one predicted by the BCS theory, with a zero-temperature mean value $\Delta(0)=1.88\pm0.08$ meV, and a mean gap ratio $2\Delta/k_\mathrm{B}T_c = 5.2\pm0.2$.
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All authors Valeriy A. Stepanov, Nikolai D. Zhigadlo, Erik Piatti, Dario Daghero
Materials Science
A practical route to amorphous solid-state electrolyte structures by hybrid reverse Monte Carlo with foundation potentials
Xintian Wang, Xinyu Zhang, Denys Butenko et al. (+3 more)
Amorphous alkali oxychlorides are fast ion conductors, but their atomic structure remains unresolved. Total scattering sums all partial pair correlations into one curve that has no unique inversion, so the structure must be modeled. Melt-quench molecular dynamics builds a model from the interatomic potential alone, and reverse Monte Carlo (RMC) builds one from the measurement alone. The two have rarely been compared with all else held fixed. We
benchmark both routes and two hybrid schemes on amorphous \ce{Li_{0.9}NbO_{0.9}Cl_{4.1}} against one
neutron pair distribution function with one foundation potential. The hybrid schemes put the energy either in the acceptance rule (inline) or in short relaxations between blocks of refinement (alternating). Each refinement starts from a melt-quench glass or from a random packing of isolated units. The glass has the lowest energy and places the first-shell peaks correctly, but carries almost none of the oscillation beyond 6~Å (goodness of fit $R^{2} = 0.29$). Pure RMC fits the measurement to within its counting noise ($R^{2} = 0.99$), but at more than $1.3$~eV/atom above the glass, and niobium loses about half of its first-shell chlorine. Started from the glass, both hybrid schemes reach $R^{2} = 0.92$ within $0.1$~eV/atom of its energy and keep its coordination chemistry. Structures from the two inputs fit almost equally well and differ in energy by a tenth of the RMC penalty, yet they differ in whether their polyhedra are connected. The input is therefore part of the method. We recommend quenching a glass with a foundation potential and refining it against the measurement with a hybrid scheme. The resulting structure carries both the local chemistry and the medium-range order, and it is the starting point for modeling ion transport in these conductors.
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All authors Xintian Wang, Xinyu Zhang, Denys Butenko, Bolong Hong, Jinlong Zhu, Peichen Zhong
Materials Science
Nonadiabatic polaron dynamics from machine-learned hopping trajectories
Daniela Mangano, Luca Leoni, Zhongfei Xu et al. (+2 more)
The formation and motion of small polarons are ubiquitous in energy materials and influence photoinduced processes. Polaron transport is governed by thermally activated
lattice dynamics and rare hopping events, requiring spatial and temporal scales only attainable with machine-learning molecular dynamics, which enables extensive sampling of atomic trajectories and polaron configurations over nanoseconds. Moreover, the Born-Oppenheimer approximation underlying standard ab initio molecular dynamics cannot describe the nonadiabatic coupling between electronic states mediated by nuclear motion, which controls energy exchange and excited-state relaxation. Here, we develop a simulation framework that combines machine-learning-assisted polaron dynamics with nonadiabatic molecular dynamics to bridge these complementary aspects of polaron physics, and determine how different polaron-hopping mechanisms affect excited-state dynamics. The enlarged trajectory space allows us to provide the first direct dynamical confirmation of the adiabatic or nonadiabatic classification of polaron hopping regimes originally proposed on the basis of static Marcus-theory arguments, and to connect individual hopping events directly to nonadiabatic electronic transitions. In bulk TiO$_2$, nonadiabatic hoppings occur preferentially when the energy of the polaron-induced gap state approaches the conduction-band
manifold, promoting both electronic de-excitation and, via a Landau-Zener mechanism, excitation from the ground state. In MgO, the predominantly adiabatic character of hole-polaron hopping suppresses these effects, leading to longer charge-carrier lifetimes and an absence of thermally activated excitations. At the rutile TiO$_2$(110) surface, hopping events are rarer than in bulk because the excess electron is strongly stabilized in the subsurface region, leading to correspondingly longer de-excitation times.
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All authors Daniela Mangano, Luca Leoni, Zhongfei Xu, Li-Min Liu, Cesare Franchini
Materials Science
Error-controlled switching between discrete and continuum dislocations in a unified defect framework
Albert Linda
Simulating defects of every dimension together is expensive, and the savings of adaptive methods are rarely tested at equal accuracy. We present a variational framework in which fields, dislocation lines and defect densities share one free energy, one dissipation law and exact ledgers of mass and Burgers vector, so that a region can change its representation without gaining or losing defects. On this basis, an allocator switches the dislocations themselves between discrete particles, where they are sparse, and a continuous density, where they are dense. The switch is made block by block from local counts, conserves the Burgers vector exactly and is controlled by a single threshold. For $5\times10^5$ dislocations, the allocator reproduces the discrete model to within 1% in the sparse region at 14-37 times lower cost, depending on the machine; the advantage grows to 81 times at $10^6$ dislocations, and a threshold calibrated on one ensemble meets its error target on an independent one. For an idealized array of dislocation walls in copper, the allocator is 37.5 times cheaper than particle dislocation dynamics and, at almost the same cost, six times more accurate than continuum dislocation dynamics between the walls. In a three-dimensional copper model coupling grain-boundary cavitation with damage, ligaments left at nominal coalescence delay creep rupture by 25-48% (post hoc sweep). By contrast, grids refined at interfaces gain less than twofold once the active region exceeds about 30% of the domain, so representation switching, rather than grid refinement, is the effective form of adaptivity for defect-rich microstructures.
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Mesoscale and Nanoscale Physics
Magnetic field induced modulation of heat transport in CrPS$_4$ probed by TaIrTe$_4$ transverse Seebeck thermometry
Krishnaraajan Sundararajan, Sergio Alvarruiz, Cédric A. Cordero-Silis et al. (+5 more)
Understanding and controlling heat flow in low-dimensional
magnetic materials is central to thermoelectrics and
spin caloritronics. One of the challenges is the absence of sensitive probes of small, field-dependent modulations in thermal transport. Here we demonstrate transverse Seebeck thermometry using exfoliated TaIrTe$_4$ as a probe of thermal conductivity modulated by an external
magnetic field on a van der Waals magnet, CrPS$_4$. With a temperature gradient oriented at $45^\circ$ to its principal crystallographic axes, TaIrTe$_4$ exhibits a transverse thermopower exceeding $200~\mathrm{\mu V\,K^{-1}}$ for 900 nm wide and 30 $\mu$m long electrodes, enabling precise detection of variations in the local temperature gradients. Exploiting this with CrPS$_4$ in a nonlocal geometry, we resolve distinct field dependent thermal conductivity changes associated with both the
spin-flip and
spin-flop
magnetic transitions which we attribute to variations in the
magnon$-$
phonon coupling in CrPS$_4$. At 5 K, the transverse Seebeck response is modulated by more than 8$\%$ by an external
magnetic field, demonstrating strong
magnetic control of heat transport in CrPS$_4$. This all van der Waals platform offers high sensitivity to modulations of thermal conductivity and a broad adaptability to a wide range of exfoliated
magnetic systems.
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All authors Krishnaraajan Sundararajan, Sergio Alvarruiz, Cédric A. Cordero-Silis, Majid Ahmadi, Marcos H.D. Guimarães, Jagoda S\lawińska, Joseph Barker, Bart J. van Wees
Mesoscale and Nanoscale Physics
Nonadiabatic dynamics under slow parametric modulation in nonlinear graphene resonators
Chris F. D. Wattjes, Haemin Lim, Pierpaolo Belardinelli et al. (+3 more)
Resonantly driven nonlinear systems, from superconducting
circuits to nanomechanical resonators, are often described in a rotating frame, where amplitude and phase evolve slowly within a fixed landscape of stable and unstable states. How such systems behave when this landscape moves on their intrinsic timescale remains to be understood. Here, we exploit the strong tension sensitivity of a
graphene drum's resonance frequency to reshape the rotating-frame phase space of a bistable mechanical mode in real time, combining an MHz-frequency resonant drive with kHz-frequency tension modulation. When the modulation rate becomes comparable to the mechanical relaxation rate, adiabatic tracking of the fixed points breaks down. Instead, parametric instabilities emerge in the slow dynamics at frequencies set by libration around these points rather than by the mechanical resonance; stronger modulation drives period doubling and chaos. A slow-variable theory predicts where the instabilities occur, and Melnikov analysis captures the onset of chaos. These results show that the rotating frame itself can be parametrically driven, offering a general route to controlling nonadiabatic dynamics in systems ranging from Josephson bifurcation amplifiers to cold atoms.
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All authors Chris F. D. Wattjes, Haemin Lim, Pierpaolo Belardinelli, John E. Sader, Peter G. Steeneken, Farbod Alijani
Strongly Correlated Electrons
Interplay of electric field and electronic correlations in pairing competition in bilayer nickelates
Yanmei Cai, Yicheng Xiong, Ying Liang et al. (+1 more)
Layer polarization provides a means of tuning the competition between interlayer and intralayer pairing in bilayer nickelates. Using ground-state constrained-path
quantum Monte Carlo, we study interlayer $s^{\pm}$ and intralayer $d$ pairing in a bilayer two-orbital Hubbard model with a layer potential-energy difference $\varepsilon$ representing an effective perpendicular electric field. Within the parameter range studied, increasing $\varepsilon$ generally enhances the finite-size pairing vertex contribution $V_d$ and reduces $V_{s^{\pm}}$, shifting the relative pairing preference toward the $d$-wave channel. Increasing the onsite repulsion enhances both vertex contributions, more strongly in the $d$-wave channel, whereas increasing the
crystal-field splitting reduces both and makes $s^{\pm}$ pairing relatively more competitive. Hund's coupling increases the relative competitiveness of $s^{\pm}$ pairing at weak bias, while enhancing the relative strength of $d$-wave pairing at large bias. The bias primarily redistributes charge between layers within each orbital sector, accompanied by an increase in the antiferromagnetic
spin structure factor. Hund's coupling also enhances local interorbital parallel-
spin correlations. These observations suggest possible connections between charge redistribution,
magnetic correlations, and pairing competition, offering insight into how layer polarization and multiorbital interactions influence the relative pairing tendencies in the bilayer nickelate model.
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All authors Yanmei Cai, Yicheng Xiong, Ying Liang, Tianxing Ma
Mesoscale and Nanoscale Physics
Orientation-Dependent Universality of Spin-Transport Fluctuations in Two-Dimensional Altermagnets
Yihan Zhang, Zheng Qin, Donghui Xu et al. (+3 more)
Universal conductance fluctuations are conventionally organized by
symmetry class, but crystalline anisotropy introduces additional structure in
spin transport. Here we demonstrate orientation-dependent universal
spin-transport fluctuations in disordered two-dimensional $d$-wave altermagnets without
spin-orbit coupling. Rotating the altermagnetic orientation changes both longitudinal
spin-conductance and transverse
spin-Hall conductance fluctuations, while each conserved-
spin sector remains in class AI. Altermagnetic orientation controls both the fluctuation amplitudes and the development of disorder-insensitive plateaus. These plateaus become flatter and less sensitive to the altermagnetic strength over the studied parameter ranges as the orientation approaches $45^\circ$, with representative fluctuation amplitudes of approximately $0.60e/(4\pi)$ for the longitudinal
spin conductance and $0.40\,e/(4\pi)$ for the
spin-Hall conductance. The longitudinal fluctuation amplitude correlates strongly with anisotropy of the clean-band Fermi-velocity tensor.
Spin-sector covariances explain how the conserved channels combine into total fluctuations. Our results extend anisotropy-sensitive universal-fluctuation physics to conserved-
spin altermagnetic transport and establish crystalline orientation as a control parameter within a fixed
symmetry class.
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All authors Yihan Zhang, Zheng Qin, Donghui Xu, Junjie Zeng, Zhen Ning, Rui Wang
Strongly Correlated Electrons
Beyond Kitaev exchange: $\Gamma'$-driven magnetism in Ag$_3$LiRh$_2$O$_6$
Efrain Insuasti Pazmino, Shi Feng, Ramesh Dhakal et al. (+4 more)
In Kitaev magnets, frustration arises from bond-dependent
spin anisotropic interactions, which can potentially realize sought-after
quantum spin liquid phases. However, material realizations with dominating Kitaev exchange are scarce and the broad parameter space of extended Kitaev models remains largely unexplored experimentally. Here, we use density-functional theory, cluster many-body,
spin-wave, and density-
matrix renormalization calculations to show that the recently discovered honeycomb rhodate Ag$_3$LiRh$_2$O$_6$ hosts an off-diagonal exchange $\Gamma_1'$ that is second in magnitude only to the ferromagnetic Kitaev exchange $K_1$. This combination of $K_1$ and $\Gamma_1'$ explains key experimental observations, an unusually high
magnetic ordering temperature of 94 K at ambient pressure and its rapid decrease under hydrostatic pressure as Ag$_3$LiRh$_2$O$_6$ evolves from ferromagnetic toward zigzag order within the honeycomb planes. Our
spin-wave calculations further predict an unusual
topology of the
magnon bands arising from the $\Gamma_1'$ term in the
spin Hamiltonian of the honeycomb layer. We show that the large trigonal splitting of $t_{2g}$ states enhances $\Gamma_1'$, the hitherto underappreciated term in Kitaev magnets, and opens the way to new interesting regimes of the extended Kitaev model.
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All authors Efrain Insuasti Pazmino, Shi Feng, Ramesh Dhakal, Ioannis Rousochatzakis, Stephen M. Winter, Johannes Knolle, Alexander A. Tsirlin
Disordered Systems and Neural Networks
Resolving Structural Uncertainty in Stochastic Dynamical Systems via Ensemble-Based Bayesian Inference
Francesco Zambelli, Valeria D'Andrea and Manlio De Domenico
Recovering the interaction structure of complex
stochastic dynamical systems from sparse time-series observations is an inherently ill-posed inverse problem. The posterior over network configurations is high-dimensional, highly degenerate, and shaped by the non-
equilibrium nature of the available data. To address this, we propose a fully
Bayesian, ensemble-based inference framework. Our approach grounds a continuous-time
Stochastic Differential Equation (SDE) likelihood formulation—acting mathematically as an Onsager-Machlup quasi-potential for non-
equilibrium paths—within a non-parametric Minimum Description Length (MDL) structural prior. This framework rigorously penalizes model complexity and avoids kinetic overfitting by forcing continuous parameters into a finite vocabulary of discrete categories. The joint posterior is optimized using a block-coordinate scheme combining sub-quadratic Markov Chain Monte Carlo (MCMC) topological sampling with deterministic Nelder-Mead parameter optimization. Using highly non-linear macroscopic population dynamics as a
benchmark, we demonstrate that inference requires non-stationary transient data to resolve causal information. Furthermore, we prove that propagating structural uncertainty via ensemble-based posterior analysis produces significantly more robust and reliable predictions for downstream perturbation outcomes than traditional single-point (MAP) estimates. The framework is highly generalizable, capturing fundamental topological constraints even under severe generative model mismatches, offering a robust tool for uncertainty-aware inference in complex
stochastic systems.
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Strongly Correlated Electrons
Role of Gd 5d-Au 5d hybridization for the magnetic properties of Au-Al-Gd quasicrystal approximants
Goro Nozue, Hidenori Fujiwara, Satoru Hamamoto et al. (+18 more)
We have studied the electronic states of
magnetic Au-Al-Gd quasicrystal approximants, which exhibit different
magnetic ground states at low temperatures, by orbital-selective soft and hard x-ray electron spectroscopies in the paramagnetic phase. Our results reveal that the valence electronic states vary among the Au-Al-Gd approximants with different
magnetic ground states, whereas the Gd 4f electrons are localized in all of them. In particular, we find that the Gd 5d states are hybridized with the Au 5d states around the Fermi level, providing information about a local pathway for the intersite exchange interaction between the localized Gd 4f electrons. This hybridization strength varies among these samples. Our findings bring an orbital-resolved microscopic picture to the magnetism of rare-earth-based quasicrystals and approximants.
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All authors Goro Nozue, Hidenori Fujiwara, Satoru Hamamoto, Yuto Torii, Mai Sakaguchi, Sho Nakajima, Takuya D. Nakamura, Nonoka U. Sakamoto, Takayuki Kiss, Masaki Oura, Akira Yasui, Kotaro Higashi, Naomi Kawamura, Kojiro Mimura, Arata Tanaka, Asuka Ishikawa, Farid Labib, Shintaro Suzuki, Takafumi D. Yamamoto, Ryuji Tamura, Akira Sekiyama
Strongly Correlated Electrons
Room-Temperature Metallic 2D Ferromagnetism Stabilized by Quantum Metric
Teng-Fei Ying, Jianqi Zhong, Songyuan Geng et al. (+3 more)
The recent discovery of long-sought dice-
lattice bands in layered electride YCl opens up unprecedented opportunities for studying correlated electrons in real dice
lattice systems. While the weakly dispersive band at the Fermi level is partially filled and density-wave states are expected due to Fermi surface nesting based on conventional theories, the ARPES measurement nevertheless revealed a uniform flat band splitting of 0.5 eV, suggesting unusual ferromagnetism (FM) borne out of the metallic 2D dice
lattice. In this Letter, we propose that the uniform metallic FM order is stabilized by the prominent
quantum metric near topological band crossing points at K and K', which reconstructs the
magnetic instability in the dice metal and tips the density-wave instability toward a uniform FM order. Importantly, our analysis based on random phase approximation (RPA) and Hartree-Fock theory predicts a remarkably high Curie temperature above room temperature for the 2D FM order. These findings establish the first example of a room-temperature 2D metallic magnet stabilized by
quantum metric, which not only exemplifies the fundamental significance of
quantum geometry in solids but also bears far-reaching applications in energy-efficient high-density
magnetic memories.
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All authors Teng-Fei Ying, Jianqi Zhong, Songyuan Geng, Fangjie Chen, Haoxiang Li, Benjamin T. Zhou