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Physics Condensed Matter

cond-mat Watch Condensed Matter

Sunday 9 August

Mesoscale and Nanoscale Physics

Thermodynamic evidence for interaction-driven first-order topological quantum phase transitions

Surajit Dutta, Nadav Auerbach, Chiho Yoon et al. (+12 more)

Topological quantum phase transitions in non interacting systems occur through continuous gap closing and reopening. In strongly interacting systems, however, competing ordered states have long been predicted to drive first order transitions, although this possibility has remained experimentally unresolved. Recent transport studies of correlated phases in charge neutral rhombohedral graphene were interpreted as evidence for continuous topological transitions. Here, using nanoSQUID on tip magnetometry, we directly image the local orbital magnetization of a spin orbit proximitized rhombohedral graphene quantum anomalous Hall (QAH) state. We provide the first real space visualization of a QAH phase with a record Chern number, reconstruct its local thermodynamic gap, and track the evolution of its magnetization across competing correlated states. Combined with self consistent Hartree Fock calculations, these measurements show that the sequential transitions between the layer antiferromagnetic, QAH, and layer polarized insulating states are first order, accompanied by discontinuous changes in orbital magnetization. Near the phase boundaries, we observe fluctuating magnetic domains, providing direct microscopic evidence of phase coexistence between nearly degenerate competing ordered states. Together, these observations provide the first direct thermodynamic evidence for first order topological quantum phase transitions and establish a microscopic framework for understanding interaction driven topological quantum phase transitions through phase competition and coexistence.
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All authors Surajit Dutta, Nadav Auerbach, Chiho Yoon, Tonghang Han, Matan Uzan, Zhengguang Lu, Niladri-Sekhar Kander, Yaozhang Zhou, Yuri Myasoedov, Martin E. Huber, Kenji Watanabe, Takashi Taniguchi, Long Ju, Fan Zhang, Eli Zeldov

Mesoscale and Nanoscale Physics

High second Chern number induced by long-range hopping in a four-dimensional Dirac model

Zheng-Rong Liu, Xiang Liu, Rui Chen et al. (+1 more)

Four-dimensional (4D) topological systems provide a promising platform for exploring topological phenomena beyond three dimensions. So far, extensive recent studies on 4D topological insulators have focused on the 4D Dirac model, while its second Chern number is restricted to a limited set of values. In this work, we demonstrate that introducing long-range hopping into the 4D Dirac model induces topological phases with high second Chern numbers. Furthermore, we show that the long-range hopping can transform a trivial insulator into a topological insulator with a nonzero second Chern number. Our work establishes long-range hopping as a powerful route for engineering 4D topological states and reveals new possibilities for realizing unconventional topological phases beyond minimal models.
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All authors Zheng-Rong Liu, Xiang Liu, Rui Chen, Bin Zhou

Statistical Mechanics

A Systematic Comparison of q-Gaussian Fitting Methods across Complex Systems

Facundo Máximo Abaca, Darío Javier Zamora, Bruno S. Zossi et al. (+1 more)

Nature exhibits a wide variety of nonlinear phenomena characterized by strong fluctuations that push the system away from equilibrium. Recent studies have shown that non-extensive statistics are more suitable for analyzing these phenomena, as they can capture long-range correlations and heavy-tailed distributions that conventional Boltzmann-Gibbs statistics cannot represent. However, estimating the non-extensiveness parameter ($q$) is not trivial, and methods vary significantly across disciplines. This paper presents a systematic and interdisciplinary comparison of three fitting methods: direct nonlinear fitting of the probability distribution function (PDF), linearization using the q-logarithm function, and numerical fitting of the cumulative distribution function (CDF). We applied this analysis to various phenomena, from areas such as geophysics, space weather and economics, analyzing the critical impact of discretization. The results show that $q$ estimates are highly sensitive to histogram binning, while the CDF-based method provides a bin-free alternative that yields more stable estimates for the datasets analyzed here. This contribution provides relevant information for the search for a method that allows the correct determination of the parameters of heavy-tailed distributions in various disciplines.
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All authors Facundo Máximo Abaca, Darío Javier Zamora, Bruno S. Zossi, Ana G. Elias

Materials Science

An Automated Magnetron Sputtering Chamber for Ferroelectric Thin Film Deposition

Stanislav A. Udovenko, Ian Mercer, Sarah Olandt et al. (+5 more)

Optimization of next-generation materials synthesis and manufacturing processes can be accelerated by effective use of digital datasets. However, a majority of existing custom research infrastructure, including that for thin film deposition, is primarily manually operated and not compatible with this new research paradigm. Here, a template is provided for upgrading existing manual deposition chambers to enable automated and autonomous experimentation. As an example, the upgrade of an existing magnetron sputtering chamber dedicated to synthesis of wurtzite ferroelectrics is presented. Focus is placed on automation of instrumentation; system and deposition control; and synchronized and automated data collection strategies. An example use case of the system for semi-autonomous determination of process-property relationships is presented, specifically minimization of coercive field in wurtzite Al$_{1-x-y}$Sc$_x$B$_y$N thin films.
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All authors Stanislav A. Udovenko, Ian Mercer, Sarah Olandt, Ric Wilburn, Kevin Dressler, Susan Trolier-McKinstry, Jon-Paul Maria, Darren C. Pagan

Statistical Mechanics

Retained hidden excess generates memory in price-limited markets

Debraj Das

The daily return of a stock is often restricted to an exchange-imposed band to curb extreme fluctuations. Any attempted price movement beyond this band is clipped, leaving an unobserved excess. We introduce a minimal stochastic latent-state model in which a fraction of this hidden excess is retained for the next day. This retention generates memory, even though the daily stochastic driving shocks are independent. For symmetric driving shocks with regularly varying tails, the stationary latent return preserves the tail index of the noise, but has an enhanced tail amplitude. In the wide-band limit, a close of the daily return at either limit of the band admits a single-dominant-shock description. We show that after such an event, the mean return on the following day has the same sign and grows proportionally to the band width, while the probability of reaching the same limit again approaches a finite value. Reaching the opposite band limit on the following day requires a second extreme shock of opposite sign and is power-law suppressed. Simulations support these analytical predictions. Empirical data from stocks subject to daily price limits are qualitatively consistent with the predicted same-sign response and its increase across wider price bands.
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Strongly Correlated Electrons

Elementary magnons and interacting multi-magnon quasiparticles in the effective spin-$\frac{1}{2}$ kagome-staircase magnet Co$_{3}$V$_{2}$O$_{8}$

Yuan Xiao, Jiajun Mo, Zhenmeng Jiang et al. (+6 more)

The excitation spectrum of an anisotropic magnet provides a direct link between its microscopic Hamiltonian and interaction-driven quasiparticles. Here we use high-resolution time-domain terahertz spectroscopy to map the magnetic excitations of the three-dimensional kagome-staircase compound Co$_{3}$V$_{2}$O$_{8}$ as functions of temperature and magnetic field. At low energies, polarization-resolved spectra identify magnetic-dipole-active one-magnon modes and track their evolution across the ferromagnetic and spin-density-wave phases. Combining their field dependence with previously reported inelastic-neutron-scattering dispersions, we determine an effective spin-$\frac{1}{2}$ Hamiltonian with strongly anisotropic exchange that quantitatively reproduces the one-magnon spectrum. This model provides a noninteracting benchmark for the high-energy response, where we observe sharp branches with field slopes that are two to four times those of the one-magnon modes, together with anticrossings between branches of different magnon numbers. Their sharpness, polarization dependence, and departure from the calculated multi-magnon continua identify them as interacting multi-magnon quasiparticles that can be stabilized by strong exchange anisotropy.
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All authors Yuan Xiao, Jiajun Mo, Zhenmeng Jiang, Haoyang Leng, Otkur Omar, Yanjun Li, Fengyi Song, Jianjun Ying, Shang Gao

Statistical Mechanics

Reliability-Safety Trade-off in AI Distillation: A Renormalization-Group Approach

Y.M.Du, Miao-Miao Yi, Tan-Ji Zhou et al. (+1 more)

Knowledge distillation transfers more than task competence: it also transmits response propensities, refusal policies, error boundaries, and latent safety biases. We formulate this behavioral inheritance as a coarse-graining model grounded in statistical mechanics, in which the student's answer and refusal decisions define two macrostates, while the teacher induces an effective field that reshapes the student's free-energy landscape. The model yields a reliability-safety trade-off relation controlled by a single parameter K, which we term the hazard discrimination capability. The predicted trade-off is consistent with refusal-token data [arXiv: 2412.06748]. In knowledge distillation, a teacher with strong hazard discrimination improves the student's attainable reliability and safety, whereas poor discrimination limits the attainable trade-off. Repeated distillation acts as an iterated renormalization-group-like transformation, under which K follows a flow across generations. The flow exhibits a tricritical structure separating regimes of K loss, stable transmission, and threshold-dependent inheritance, and yields testable scaling predictions for multigenerational distillation.
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All authors Y.M.Du, Miao-Miao Yi, Tan-Ji Zhou, C.P.Sun

Soft Condensed Matter

Heat and mass transport in a three-dimensional mesoscale odd fluid

Yuxing Jiao and Mingcheng Yang

Fluids with nonvanishing antisymmetric components of the transport coefficient tensor are named odd fluids. In our previous works, we proposed a mesoscale simulation model for isotropic two-dimensional odd fluids and then extended it to the three-dimensional case to model an anisotropic odd fluid with cylindrical symmetry. The Navier-Stokes equation and the viscosity tensor of this three-dimensional mesoscale odd fluid were derived previously via a kinetic theory. Herein, we derive the heat conduction and self-diffusion equations, along with the corresponding thermal conductivity and self-diffusivity tensors. These theoretical results are validated by simulations. We further investigate heat and mass transport behaviors of three-dimensional odd fluids in a confined geometry through our mesoscale model. In striking contrast to normal isotropic fluids, the steady-state temperature and density distributions of confined odd fluids are significantly deformed by the odd transport coefficients.
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Statistical Mechanics

Lee-Yang Theory Guided Force Field Refinement Based on Phase Diagrams

Bin Jin, Qijun Ye, Xinzheng Li et al. (+2 more)

We propose a general framework for automatic force field refinement guided by phase diagrams, grounded in Lee-Yang phase transition theory. The central idea is to directly use the partition function modulus as a phase-diagram-guided optimization target. Evaluating the modulus at points close to the real axis, where the Lee-Yang zeros are mostly associated with the phase transition, is more physically meaningful and avoids the numerical difficulty of explicitly solving for the zeros. This approach requires no system-specific order parameters or response properties for characterizing phase transition points, making it universal across various discontinuous phase transitions and material systems. We validate the method on refining parameters of a Lennard-Jones potential covering both gas-liquid and solid-liquid transitions, and a Cu embedded-atom method potential based on experimental melting curves. The refined force fields reproduce the target phase diagrams with significant improvement across all systems. For Cu, the refinement simultaneously improves predictions of enthalpy and heat capacity, which are observables beyond the optimization target. These results establish Lee-Yang theory as a practical tool for contemporary force field development.
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All authors Bin Jin, Qijun Ye, Xinzheng Li, Kuang Yu, Shenzhen Xu

Materials Science

High Thermal Conductivity of Back-End-of-Line Compatible Diamond Films

Jinwen Liu, Chufei Cheng, Feifei Tan et al. (+6 more)

Back-end-of-line (BEOL) thermal management requires electrically insulating heat-spreading dielectric that can be integrated within thermal budgets below 400 C. Here, we report polycrystalline diamond films grown directly on Si at a substrate temperature below 400C. Two films with average thickness of 760 and 1000 nm were characterized by Raman spectroscopy, scanning electron microscopy (SEM), and time-domain thermoreflectance (TDTR). Raman spectra show a sharp diamond peak with minor signatures of non-diamond carbon, while SEM reveals lateral growth and large grain size. Temperature dependent TDTR measurements were performed from room temperature to 100C. Sensitivity analysis indicates that the sensitivity of cross-plane thermal conductivity is comparative to the in-plane thermal conductivity. Accordingly, the films were analyzed using an isotropic thermal model by considering the nearly-isotropic grain structure, yielding room temperature effective thermal conductivity of 73 and 86 W m-1 K-1, respectively. These values are about two orders of magnitude higher than those of conventional dielectric materials and demonstrate the potential of diamond films grown at low temperatures as dielectric heat-spreading layers.
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All authors Jinwen Liu, Chufei Cheng, Feifei Tan, Jinquan Zhang, Di Lu, Bing Dai, Jiaqi Zhu, Runsheng Wang, Zhe Cheng

Mesoscale and Nanoscale Physics

Spin-Orbital Hall Nano-Oscillators using PtCr/NiFe

Utkarsh Shashank, Akash Kumar, Daegeun Jo et al. (+11 more)

The orbital Hall effect provides a promising route for generating angular-momentum currents beyond conventional spin Hall physics. PtCr alloys exhibit unusually large current-induced torques, but the contribution of orbital transport and the ability of these torques to sustain coherent nonlinear magnetization dynamics remain unresolved. Here we demonstrate spin-orbital Hall nano-oscillators by exploiting a homogeneous heavy-metal/light-metal alloy in which orbital Hall currents generated by Cr are converted by Pt into spin currents, producing giant spin-orbit torques. Using PtCr/NiFe heterostructures, the effective torque efficiency increases from ~0.14 in Pt/NiFe to ~0.40 in Pt0.38Cr0.62/NiFe despite substantial Pt dilution, enabling coherent auto-oscillations with the threshold current density reduced from ~ 1.07 x 10^12 to ~ 4.4 x 10^11 A m^-2. First-principles calculations show that Cr alloying suppresses the intrinsic spin Hall conductivity while enhancing the orbital Hall conductivity, and reproduce the observed torque enhancement only when orbital transport is included. Our combined experimental and first-principles results show that alloy engineering enables giant spin-orbit torques through an intrinsic orbital-mediated contribution, enabling coherent auto-oscillations without engineered multilayers and establishing a scalable materials platform for low-power nonlinear spintronic and orbitronic devices.
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All authors Utkarsh Shashank, Akash Kumar, Daegeun Jo, Thi Ngoc Anh Nguyen, Jong-Guk Choi, Sambit Ghosh, Michal Strach, Lunjie Zeng, Andrew B. Yankovich, Roman Khymyn, Ahmad A. Awad, Eva Olsson, Peter M. Oppeneer, Johan Åkerman

Mesoscale and Nanoscale Physics

Crystalline Germanium Josephson Junctions

Frederik H. Knudsen, Axel Leblanc, Yiliu Li et al. (+8 more)

Conventional superconducting quantum electronics rely on well-established Josephson junctions made of Al/AlO$_{x}$ where the weak link AlO$_{x}$ is amorphous and is believed to host two-level systems that limit coherence. Crystalline Josephson junctions exhibit atomically ordered interface quality but remain constrained by complex fabrication and intrinsic asymmetry of epitaxial growth. Here, we demonstrate a fully epitaxial approach based on superconductivity in gallium-doped germanium, enabling the realization of Josephson junctions entirely grown in situ by molecular beam epitaxy. These devices feature atomically sharp interfaces and crystalline weak links, resulting in strong Josephson coupling in the ultra-short regime. We observe an unconventional enhancement of the switching current under applied magnetic field, which we attribute to quasiparticle-assisted thermalization processes from the Al contacts. This platform combines structural coherence, fabrication simplicity, and scalability, offering a promising route toward low-disorder, CMOS-compatible superconducting qubits in a merged element transmon architecture.
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All authors Frederik H. Knudsen, Axel Leblanc, Yiliu Li, Patrick J. Strohbeen, Jechiel van Dijk, Logan Kusher, Arunav Bordoloi, Alisa Danilenko, Xiangchao Ma, Salva Salmani-Rezaie, Javad Shabani

Superconductivity

Excitron-Induced Pair Fluctuations Reveal Superconductivity in the Electron Gas

Yasutami Takada

Understanding how superconductivity can emerge in dilute electronic systems remains a central challenge in condensed matter physics. By performing first-principles calculations of the electron self-energy \Sigma(k,iw_n) in the low-density three-dimensional electron gas, we identify a sharp divergence at r_s ~ 8 and T ~ 10^{-4}E_ F, signaling a second-order phase transition. This critical behavior originates from one-dimensional superconducting fluctuations mediated by virtual excitations of an excitron—a quasi-1D electronic composite formed by an electron and longitudinal electron-hole pairs. Although the superconducting mechanism itself is plasmon-mediated, the excitron channel provides a unique window into its fluctuation dynamics. Near the transition, we observe a pseudogap and a linear-in-T inverse electron lifetime, reminiscent of phenomena in high-T_c materials. These results reveal an unexpected route by which plasmon-driven superconductivity manifests in the dilute 3D electron gas through quasi-1D excitron dynamics.
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Statistical Mechanics

Nonisospectral Integrability and Exact Current Fluctuations in the Two-Dimensional SSEP

Tingfei Li

We study annealed current fluctuations in the two-dimensional symmetric simple exclusion process (SSEP) across a circular passive counting boundary. The initial average density is $\rho_1$ inside a disk of radius $R$ and $\rho_2$ outside, and the observable is the net decrease of the particle number in the disk over a finite time. Using convexity of the macroscopic fluctuation theory action and rotational averaging, we show that the minimizer of the full two-dimensional variational problem may be chosen radially symmetric. For the resulting radial problem, a suitable change of variables leads to a nonisospectral formulation on the half-line. Combining the associated scattering construction with a scalar factorization, we obtain a closed expression for the scaled cumulant generating function and hence the annealed large-deviation statistics of the current.
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Mesoscale and Nanoscale Physics

Quantum-Device Simulation of Optical Decoherence of Hole-Spin Qubits in Self-Assembled Quantum Dots

Jyun-Jie Jiang, Pericles Philippopoulos, Félix Beaudoin et al. (+1 more)

Spin-photon interfaces are essential for communications between distant spin qubits in quantum technologies, but the interband optical excitation can also damp electrically driven hole-spin Rabi oscillations in semiconductor self-assembled quantum dots (SAQDs). We report a device-level modeling workflow that integrates realistic SAQD geometry and multiband electronic-structure analysis with models of electrically driven spin control, interband optical transitions, and open-system dynamics. This workflow enables device-level estimation of Rabi-oscillation damping arising from repeated interband absorption-emission cycles. As an example, for a gated GaAs SAQD subjected to a uniform magnetic field $B_0$ along the growth direction of the SAQD, we predict the Rabi frequency of the hole spin qubit and its damping under external illumination. At $B_0=2$ T, the calculations yield a hole-spin Rabi frequency of 37.3 MHz. When the electrically driven SAQD is illuminated by a broadband LED centered at a wavelength of 790 nm, increasing the optical power from 0.3 to 1.5 mW shortens the Rabi-oscillation decay time from 90.3 to 17.5 ns. Increasing the SAQD height reduces the electron-hole overlap and thus the emission rate, but the resulting redshift moves the interband transitions into stronger spectral overlap with the LED spectrum, thereby increasing the rate of repeated absorption-emission cycles and enhancing photon-induced Rabi-oscillation damping. The results show that geometry, spin-control conditions, and illumination spectrum should be co-optimized in semiconductor spin-photon devices.
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All authors Jyun-Jie Jiang, Pericles Philippopoulos, Félix Beaudoin, Hong Guo

Materials Science

Unified open-boundary electrostatics in real-space density functional theory

Rajat Kumar, David Codony, Phanish Suryanarayana et al. (+1 more)

We present an electrostatic formulation in real-space density functional theory that provides a systematic and unified treatment of the open-boundary electrostatics of isolated and partially periodic systems, including in the presence of an applied uniform electric field along the open (finite) directions. Specifically, we formulate a local electrostatic energy functional whose stationarity yields the Poisson equation for the electrostatic potential, subject to periodic and Dirichlet boundary conditions along the periodic and open directions, respectively. Using a Green's function approach, we derive analytical expressions for the Dirichlet values arising from the total charge density of the system. We also derive the expressions for the energy, atomic forces, and stress tensor. We implement the resulting expressions within the large-scale parallel real-space SPARC electronic structure code. Using representative examples, we verify the accuracy and efficiency of the framework, demonstrating exponential convergence of the computed quantities with vacuum size and excellent agreement with established plane-wave codes while requiring significantly less vacuum at comparable accuracy. Since no existing implementation provides the stresses for such systems, we instead verify them against numerical derivatives of the energy, finding close agreement. Finally, we apply the framework to compute static polarizabilities and piezoelectric coefficients, obtaining very good agreement with values reported in the literature.
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All authors Rajat Kumar, David Codony, Phanish Suryanarayana, Abhiraj Sharma

Statistical Mechanics

Truly Solving the Gibbs Paradox by Local Free Space and Collision Potential

Chuang Li

This paper argues that the Gibbs paradox can be resolved without using the concept of identical particles in quantum mechanics. The molecules in different regions of the gas can be distinguished, so there is no need to introduce the N! factor. For each molecule, the volume of its free movement space is local at every instant. Moreover, collisions are the primary way of interaction between gas molecules and the fundamental driving force for reaching equilibrium. The potential energy during collisions cannot be ignored. Based on the local free space assumption and collision potential energy, this paper uses the canonical ensemble method to rederive the entropy increment formula for gas mixing. It includes parameters such as molecular mass, effective radius, and collision characteristic time, which vary with the type of gas molecules. This solves the problem that the entropy increment of gas mixing is independent of gas properties, that is, it truly resolves the Gibbs paradox instead of providing a new conceptual explanation.
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Statistical Mechanics · Cross-listed from hep-th

Organizing transitions and their cascades: Generalized symmetry enforcement in massless flows or Higgs transitions

Yoshiki Fukusumi and Yuma Furuta

We study the role of generalized symmetry in massless renormalization group flows or Higgs transitions. In particular, we revisit the massless renormalization group flows in unitary minimal models, $\mathbf{M}(p,p+1) \rightarrow \overline{\mathbf{M}(p-1,p)}$ preserving the fusion ring symmetry $\text{FR} (SU(2)_{p-2})\subset \mathbf{M}(p,p+1)$ where $p$ is an integer satisfying $p>3$. In this series of flows, we demonstrate that the unbroken fusion ring symmetry $\text{FR} (SU(2)_{p-2})$ eliminates all relevant perturbations in the $\overline{\mathbf{M}(p-1,p)}$ model. Hence, the infrared theory $\overline{\mathbf{M}(p-1,p)}$ is stable at the level of the scaling analysis and can be interpreted as a (weak-)symmetry-enforced gapless phase in contemporary theoretical physics. Phenomenologically, by the folding trick, the unbroken fusion ring symmetry corresponds to a (half-)integer spin nonsimple current, a variant of the Cooper pair involving nonabelian anyons generated from the coset or level-rank duality structures. Moreover, we demonstrate that the structure of (half-)integer spin nonsimple current plays a fundamental role in causing the resonance effect of relevant and dangerously irrelevant perturbations. This resonance effect may result in the cascade of phase transitions (or the system flows to unexplored fixed points), and the symmetry can be a stopper of such unconventional flows.
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Materials Science

Thermo-mechanical Characterization of 2D hexagonal Boron Phosphide (h-BP)

Zahabul Islam

This study explores the thermo-mechanical properties of a two-dimensional (2D) monolayer hexagonal boron phosphide (h-BP). h-BP is predicted to possess a moderate band gap, high thermal stability, and excellent carrier mobility, making it suitable for advanced electronic, sensing, and energy applications. A classical molecular dynamics (MD) potential for h-BP was developed using density functional theory (DFT) calculations. The derived parameters were implemented in MD simulations to evaluate mechanical behavior under tensile loading along both zigzag and armchair directions at varying temperatures (300 K to 900 K). The results reveal significant anisotropy in mechanical performance, with higher tensile strength and elastic modulus in the zigzag direction across all temperatures. Increasing temperature reduces both tensile strength and stiffness due to thermal softening and increased atomic vibrations. The influence of structural defects was also investigated, revealing that Stone-Wales and vacancy defects reduce the tensile strength and failure strain of h-BP, with the two-atom vacancy producing the most pronounced mechanical degradation due to localized stress concentration and premature crack initiation. These findings provide a foundation for future research on the mechanical stability of h-BP in extreme environments.
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Soft Condensed Matter

Transient fluid removal at soft interfaces: Contact-time-controlled squeeze-out in a cylinder-on-flat contact

R. Xu and B.N.J. Persson

We study transient fluid removal in cylinder-on-flat contacts between stiff PMMA cylinders and a soft PDMS substrate. The cylinder geometry eliminates the edge-scraping mechanism that can occur for deformable rubber blocks, allowing the influence of sliding on fluid squeeze-out to be examined more directly. Experiments were performed mainly in glycerol at the low sliding speed $v=3 \ {\rm \mu m/s}$ using cylinders with different surface roughness. After the initial elastic-loading stage, we find that the friction during sliding depends primarily on the total time elapsed since application of the normal load rather than on the preceding sliding distance. The subsequent friction evolution follows approximately the same dependence on total contact time. Stationary squeeze-out calculations predict the evolution of the mean surface separation and real contact area, in reasonable agreement with that inferred from the measured friction. These results show that essentially the same squeeze-out process governs fluid removal during stationary contact and low-speed sliding, and that sliding-induced elastohydrodynamic effects have only a minor influence under these conditions.
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Saturday 8 August

Statistical Mechanics

A Shared Observation Shields Collective Fluctuations while Preserving Local Independence

Hu Cang

As a liquid approaches its glass transition, its dynamics turns heterogeneous: mobile and immobile regions coexist, and the four-point susceptibility $\chi_4$ that quantifies this heterogeneity grows sharply. Interpreting that growth is subtle, because the collective signals experiments record, such as a tagged particle's trajectory, an overlap function, or a mean field, are generated by the same particles they describe. Here we compute exactly what conditioning on such a shared record does to the population that produced it, for a broad class of stochastically observed systems; the guiding example is a tagged particle and the cage of $z$ neighbors that drives its force history. Using a Girsanov path transformation, we prove that the conditioning multiplies the independent joint law of the $z$ trajectories by exactly one term: a centered-square penalty along the single collective direction the record can see. Any fixed pair of particles stays nearly independent, with covariance falling as $O(z^{-1})$ and mutual information as $O(z^{-2})$, the property known as propagation of chaos, yet the $z(z-1)$ weak pair correlations add coherently into a finite suppression of collective fluctuations, the Schur shield $D - C = -C^2(aI + C)^{-1} \preceq 0$. An exactly solvable Brownian model calibrates the construction. The physical consequence is a calculable baseline for dynamical heterogeneity: conditioning itself contributes a computable, nonpositive amount to the susceptibility of a conditioned ensemble, so the genuine cooperative signal is the excess of the measured $\chi_4$ over this baseline rather than over zero, a comparison that existing simulation data can already perform.
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Soft Condensed Matter · Cross-listed from physics.chem-ph

Chemical potentials from structure factors: I. Neutral multi-component mixtures

Roya Savoj, Xiaoyu Wang, Musahid Ahmed et al. (+1 more)

The chemical potentials of multi-component mixtures underlie many physical and chemical phenomena, but remain challenging to compute. The S0 method enables the computation of chemical potentials from equilibrium molecular dynamics simulations, by leveraging the thermodynamic relationship between particle number fluctuations and derivatives of chemical potentials, followed by numerical integration along different compositions. Here we generalize the S0 method from two-component mixtures to neutral multi-component mixtures. We first extend the statistical mechanical formalism to high-dimensional compositional space, and then introduce a Gaussian process integration scheme combined with active learning to efficiently integrate chemical potentials and sample diverse compositions. We use this method to compute the mixing free energies of a molten metal alloy, and the solubilities of two paracetamol polymorphs in water-ethanol solvents. The extended S0 method provides a practical and scalable route for computing chemical potentials in neutral bulk multi-component mixtures from atomistic simulations.
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All authors Roya Savoj, Xiaoyu Wang, Musahid Ahmed, Bingqing Cheng

Mesoscale and Nanoscale Physics

Magnetoelastic coupling in stripe-domain states of yttrium iron garnet

Nimisha Arora, Daniel Prestwood, Takashi Kikkawa et al. (+4 more)

We study magnetoelastic coupling in stripe-domain magnetic states of $3\,\mathrm{\mu m}$-thick YIG thin films grown on a GGG substrate. Broadband ferromagnetic resonance reveals low-frequency stripe-domain magnon branches modulated by a field-independent phonon comb with a frequency spacing of $3.5\,\mathrm{MHz}$, matching the value predicted for confined thickness-shear modes of the GGG substrate. Analytical fitting yields coupling rates that vary between $0.33$–$0.54\,\mathrm{MHz}$ and cooperativities of order $10^{-1}$, indicating that the system is in the weak-coupling regime without resolvable avoided-crossing gaps. Magnon-phonon mode-overlap calculations using finite-element simulations show that the weak coupling arises from phase and domain-sign cancellation: the local magnetoelastic coupling is sizable, but more than $99\%$ of the coherent overlap cancels across the stripe texture. Fully coupled simulations further demonstrate phonon-mediated excitation of a remote YIG layer and show that efficient propagating-phonon generation requires spatially asymmetric magnon modes, establishing magnetic texture as a control parameter for magnon–phonon coupling.
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All authors Nimisha Arora, Daniel Prestwood, Takashi Kikkawa, Eiji Saitoh, Jack Gartside, Will Branford, Hidekazu Kurebayashi

Materials Science

Pressure-Induced Stacking Disorder and Suppression of Long-Range Sm-type Order in Medium-Entropy Rare-Earth Alloys

Raimundas Sereika, Matthew P. Clay, Kallol Chakrabarty et al. (+1 more)

Rare-earth medium-entropy alloys provide a platform for investigating how chemical disorder modifies the well-established pressure-induced structural evolution of close-packed $4f$ lanthanides. Here, we study TbHoEr and TbHoDy using synchrotron X-ray diffraction in diamond anvil cells. Both alloys transform from the ambient hexagonal close-packed (hcp) structure to a double hexagonal close-packed (dhcp) phase, while no well-resolved bulk Sm-type intermediate phase is observed. For TbHoEr, compression to 70 GPa further reveals a high-pressure rhombohedral hR24 phase. Unlike the constituent heavy lanthanides, however, both alloys bypass the intermediate Sm-type phase. Two-dimensional diffraction images further reveal streak-like diffuse scattering in the transition region, indicating stacking disorder and limited stacking coherence along the close-packed direction. These observations indicate that the transformation proceeds through a stacking-disordered close-packed state rather than through a well-ordered bulk Sm-type phase. We propose that configurational disorder, local lattice distortion, stacking-fault energetics, and transformation kinetics collectively suppress the development of long-range Sm-type order. The results demonstrate that medium-entropy alloying can fundamentally modify pressure-induced stacking pathways in rare-earth materials under extreme conditions.
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All authors Raimundas Sereika, Matthew P. Clay, Kallol Chakrabarty, Yogesh K. Vohra

Disordered Systems and Neural Networks · Cross-listed from cs.LG

Correlation flow governs learning at criticality

Andrea Combette, Nelly Pustelnik and Antoine Venaille

The initialization of deep neural networks determines whether information and gradients can propagate across depth, yet a unified theory connecting these properties to learning dynamics remains elusive. Combining mean-field theory and random matrix theory, we establish a direct link between correlation propagation and the Neural Tangent Kernel (NTK) that governs learning in the sequential limit of infinitely wide, infinitely deep networks. Correlation propagation to infinite depth is possible only at a single, critical point in the weight-bias variance plane. At this point, we leverage the algebraic decay of the end-to-end Jacobian with depth to prove that the NTK becomes exactly proportional to the output correlation at infinite depth, tying together information propagation and learning dynamics. We further show that orthogonal initialization suppresses the leading finite-size corrections present under Gaussian initialization, clarifying the respective roles of the two initialization ensembles in this limit. These theoretical predictions are validated quantitatively on finite-width, finite-depth networks. Together, these results demonstrate that orthogonal initialization and criticality are required to control the asymptotic dynamics of deep learning.
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Statistical Mechanics

Persistence probability based dynamics and phase diagrams in biased q-voter models

Amit Pradhan, Pratik Mullick and Parongama Sen

Persistence probability in opinion dynamics models estimates the tendency of the agents not to change their initial opinion till the present time. Here we consider two nonlinear q-voter models with binary opinions, where the dynamics are governed by a biased choice when the q panel is not unanimous. The models are studied for different parameter ranges corresponding to the known stationary states. Mean field theory and numerical simulations are used to compute the persistence probability for the two types of opinion separately. The long time behavior in general is either a saturation or a decay that can be approximated by an exponential form, depending on the chosen parameters. Based on this, phase diagrams in the parameter space are presented for both the models. The regions in the phase diagrams indicate a strong correlation with the behavior of fixed points in the corresponding models, which is non-trivial as far as the persistence probability is concerned.
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Materials Science

An Interfacial Balance Rule Governs Binder-Electrolyte Coupling in Lead-Free Perovskite Energy Storage

Arun Kumar, Ayush Kumar Pandey, Ankur Yadav et al. (+4 more)

Electrode binders are conventionally regarded as inert structural components. Here, we show that in lead-free perovskite supercapacitors, the binder defines the optimal electrolyte composition. Across a factorial matrix of poly(vinylidene fluoride) (PVDF) loadings and LiTFSI concentrations in CsSnCl$_3$ electrodes, the capacitance optimum shifts systematically with binder content along a single linear relationship, described by the Interfacial Balance Rule ($\lambda+\theta=1$), where $\lambda$ and $\theta$ are the normalized lithium-supply and polymer contributions at the optimized interfacial state. The same relationship holds for hybrid MASnCl$_3$, showing that the optimum is governed by the polymer-electrolyte interface rather than the perovskite lattice chemistry. Simulations using a pre-trained MACE machine-learned interatomic potential show that PVDF adopts a planar configuration on CsSnCl$_3$ and simultaneously interacts with cationic and anionic sites. This configuration homogenizes lithium adsorption energetics, introduces fluorine-mediated coordination, and confines lithium to a two-dimensional interfacial region while preserving lateral mobility. Tuning polymer coverage through surface density and chain length reveals a finite interfacial lithium accommodation capacity that marks the onset of out-of-plane aggregation. The Interfacial Balance Rule provides a macroscopic descriptor of this finite interfacial resource, balancing polymer-mediated lithium stabilization against limited accommodation space. Binder loading is therefore an active design parameter for polymer-regulated energy-storage interfaces.
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All authors Arun Kumar, Ayush Kumar Pandey, Ankur Yadav, Vishnu Saraswat, Shiladitya Sengupta, Abhishek Tewari, Monojit Bag

Strongly Correlated Electrons

Quantum Formulation of Chiral Vortical Effect in Weyl Semi-metals

B. Q. Song

The chiral vortical effect (CVE) is the generation of an axial current in a rotating Weyl fermion; its description is presently based on semiclassical frameworks. In this work, we develop a fully quantum formulation for CVE, solving the exact evolution of microscopic spinful wavefunctions, which enables a bottom-up quantitative test of semi-classical theories and postulated distributions $f_{\text{CVE}}$ in different reference frames. Notably, it shows that $f_{\text{CVE}}$ is over a ground-state-free Floquet spectrum, qualitatively distinct from a thermal equilibrium distribution (i.e., fermi form $f_F$), underscoring CVE as a non-equilibrium phenomenon, distinguished from other chiral transports. The $f_F$ only approximately holds when three conditions are simultaneously fulfilled: (1) slow rotation $\omega R/v_F\ll 1$, (2) high chemical potential $\mu/(\hbar v_F R)\gg 1$, (3) isotropic symmetry, where $R$ is the size, $v_F$ is fermi velocity. In these conditions, the theory recovers established semiclassical results, including the current-response coefficients and the magnetization contribution; otherwise, it uncovers quantum phenomena such as “void states", deviation from the semiclassical formula $j_{\text{CVE}} \sim \mu^2$, a $v_F$-independent charge pumping. The theory is based on semimetals, providing more experimentally accessible detection than fundamental Weyl particles.
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Statistical Mechanics

High-Capacity Generalized Hopfield Networks

Victor Galitski

Generalized Hopfield networks are introduced where memories and neurons are continuous variables that lie on a Riemannian manifold. We explicitly focus on symmetric spaces associated with the special unitary groups SU(d), and use both numerical and analytical (replica) techniques to demonstrate an almost order of magnitude enhancement in critical capacity over the vector networks starting with d=3 and further rapidly growing with d. To circumvent the non-linear geometric constraints, we use a Lie algebraic method [following V. Galitski, Phys. Rev. A 84, 012118 (2011)] to exactly describe the classical neural network in terms of linear algebra in an auxiliary Hilbert space. It is shown that in contrast to the traditional Hopfield networks, memory recall in SU(d) Hopfields corresponds to neuron alignment along a top eigenvector of a spiked matrix, which is less susceptible to random matrix crosstalk than other models with continuous neuron variables. Physical platforms to realize SU(d) Hopfields are briefly discussed and physical (in addition to algorithmic) recall mechanism is demonstrated, where memory recovery occurs naturally through generalized Landau-Lifshitz-Gilbert dynamics. To illustrate SU(3) memory recall, we introduce a color (RGB) image encoding/decoding protocol and explicitly run image recovery on corrupted cues. Finally, we quantize the generalized Hopfields which are shown to reduce to Sachdev-Ye glassy type of models. Their many-body spectra generally feature two types of dark and memory bands, where the latter exhibits chaotic Wigner-Dyson level statistics that hides Hebbian data.
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Statistical Mechanics

Out-of-equilibrium inhomogeneous XX chains: Exact results and the hydrodynamic limit

Vincenzo Alba and Federico Rottoli

We study the out-of-equilibrium dynamics in the XX chain with step-like magnetic field, which maps to an inhomogeneous tight-binding chain after Jordan-Wigner transformation. We obtain exact analytic expressions for the fermionic two-point correlation function after a quantum quench from several initial product states, both homogeneous and inhomogeneous ones. This is achieved by using a combination of Fourier and Laplace transforms, which al low us to map the problem to a standard Riemann-Hilbert problem on the unit circle. For arbitrary positions and times the correlators are not expressed in terms of elementary functions. However, in the hydrodynamic limit $x,y,t\to\infty$ with fixed ratios, we provide explicit formulas that depend only on the effective transmission coefficient across the origin. We benchmark our analytic predictions against exact numerical simulations and find excellent agreement in the hydrodynamic limit, apart from finite-time corrections.
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