Strongly Correlated Electrons
Discovery of hidden order in the Shastry-Sutherland magnet Nd2Be2GeO7
Andi Liu, Samuel H. Moody, Thomas J. Hicken et al. (+9 more)
Hidden order typically manifests as a thermodynamic phase transition without a conventional order parameter, leaving its true nature concealed even at the lowest temperatures. In the frustrated Shastry-Sutherland magnet Nd$_2$Be$_2$GeO$_7$, we observe a related yet fundamentally distinct phenomenon. A sharp specific-heat anomaly appears at 250 mK, but zero-field
neutron diffraction and
muon spin relaxation detect no static magnetism down to 100 and 30 mK, respectively, pointing to a hidden-order state. Remarkably, this hidden order does not emerge under an applied
magnetic field, but instead reveals itself only after the field is applied and subsequently removed where
magnetic Bragg peaks appear, albeit with strongly suppressed moments. A glassy state is ruled out by ac susceptibility and specific heat measurements. Complementary $\mu$SR measurements reveal coherent
spin fluctuations at a rate on the order of gigahertz. Taken together, these results suggest that the system lies in close proximity to the
quantum spin liquid and long-range
magnetic order state such that a small perturbation can effectively drive the system towards distinct ground states. These findings also distinguish Nd$_2$Be$_2$GeO$_7$ from known frustrated systems, establishing it as a unique platform where the synergistic interplay among the
spin-orbit coupling,
crystal field, and
magnetic frustration leads to unexpected
quantum states.
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All authors Andi Liu, Samuel H. Moody, Thomas J. Hicken, Jonas A. Krieger, Hubertus Luetkens, George D. A. Wood, Helen C. Walker, Zhendong Fu, Jason S. Gardner, Jinkui Zhao, Zhaoming Tian, Hanjie Guo
Statistical Mechanics · Cross-listed from quant-ph
Non-equilibrium thermodynamics of collapse models in the strongly non-Gaussian regime
Pedro B. Melo, Pedro V. Paraguassú, Simone Artini et al. (+3 more)
Standard objective collapse models offer a unified approach to the
quantum measurement problem but predict an unphysical, indefinite increase in the energy of the system. The dissipative Diósi-Penrose (dDP) model resolves this heating issue by introducing a linear friction mechanism. However, this modification induces complex, non-Gaussian phase-space dynamics. We rigorously establish the thermodynamic consistency of this friction mechanism – extended to the CSL model – across both weakly and strongly non-Gaussian regimes. Using the Wigner phase-space formalism, we go significantly beyond the quadratic approximation and, to bypass the failure of perturbative methods under strong dissipation, introduce a novel exact pseudo-spectral simulation approach. Our analysis reveals that the system subjected to the dDP mechanism does not thermalize, but rather settles into a non-
equilibrium steady-state (NESS) where the asymptotic non-Gaussianity scales as the third power of the dissipation parameter $\beta$. By evaluating the Wigner
entropy production, we confirm the thermodynamic validity of the model and demonstrate that highly sensitive information-theoretic quantities require exact numerical methods to accurately capture the key non-Gaussian tails of the distribution.
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All authors Pedro B. Melo, Pedro V. Paraguassú, Simone Artini, Gabriele Lo Monaco, Sandro Donadi, Mauro Paternostro
Soft Condensed Matter
Investigating frictional instability due to pressurization in granular media: insights from coupled computational fluid dynamics discrete element method
Bimal Chhushyabaga and Behrooz Ferdowsi
Fluid pressurization can reactivate subcritically stressed granular layers in faults, slopes, and injection-perturbed reservoirs, but grain-scale feedbacks among pressure
diffusion, drainage, and contact-network degradation remain unresolved. Here, 3D coupled CFD-DEM simulations investigate pore-pressure-induced reactivation of confined, fluid-saturated granular shear layers under imposed shear stress. Strain-controlled tests define the Mohr-Coulomb strength envelope; stress-controlled simulations then impose subcritical shear stresses while basal pore pressure increases under drained and undrained conditions. Instability is governed not by pore pressure alone, but by its coupled
evolution with effective stress, drainage, dilation or compaction, hydraulic connectivity, and granular fabric. Undrained boundaries retain excess pore pressure, whereas drained boundaries maintain vertical gradients and suppress excess pressure. Internal fields reveal alternating dilation and compaction bands and reorganization of a porosity-derived permeability proxy, showing that hydraulic pathways evolve during deformation. Micromechanical diagnostics identify localized particle rotation, force-chain reorganization, porosity redistribution, and coordination-number variations controlled mainly by imposed shear-stress level rather than drainage. Second-order fabric metrics show that post-failure weakening coincides with loss of directional force-chain organization, especially at lower shear. Friction-velocity and friction-porosity trajectories indicate a transition from dilatancy-dominated strengthening to pore-pressure-driven weakening. Viscous-number scaling partially organizes the low-Iv creeping response, 10^-8 <= Iv <= 10^-5, but not onto a unique local rheology. These results clarify how drainage-controlled hydromechanical feedbacks and fabric degradation convert pore-pressure forcing into instability.
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Statistical Mechanics · Cross-listed from cs.LG
Generative Criticality in Large Language Model Temperature Scaling
Huajian Ruan, Jinyang Li, Xingyu Guo et al. (+1 more)
We propose a statistical-field framework for text generated by large
language models (LLMs), treating
token embeddings as continuous
spin variables on a one-dimensional chain. Defining a susceptibility from the connected two-point correlator and an order parameter from the ensemble-averaged
embedding field, we vary the softmax temperature $T$ and observe a sharp susceptibility peak near a characteristic $T_c$ with power-law-like scaling, a concurrent rapid change in the order parameter, and a collapse onto a single semantic direction below $T_c$. The intrinsic dimension estimated by the two nearest neighbor (TwoNN) method independently corroborates these findings, reaching a minimum near $T_c$. Results are robust across model scales (Qwen3: 0.6B–32B) and
prompt categories. While the phenomenology closely resembles a continuous phase transition, the non-
equilibrium nature of autoregressive generation warrants further investigation. Our framework provides quantitative tools for probing the collective statistical structure of LLM outputs and suggests connections between decoding strategies and critical phenomena.
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All authors Huajian Ruan, Jinyang Li, Xingyu Guo, Lingxiao Wang
Soft Condensed Matter · Cross-listed from physics.flu-dyn
Flapping instability of elastic disks in Stokes flows
Yijiang Yu, Hugo Perrin, Michael D. Graham et al. (+1 more)
Fluid-structure interactions at low Reynolds number can lead to a much richer phenomenology than previously expected. Here, we study the dynamics of a freely suspended, thin elastic disk in a shear flow, where the plane of the disk is initially parallel to the flow plane. Using a combination of experiments and simulations, we demonstrate that beyond a critical flow strength the disk deforms, performing flapping dynamics, in which the disk curves up and down periodically relative to the horizontal shear plane. The bifurcation diagram obtained by simulation reveals several oscillatory solutions, including a wiggling motion that is predicted by a linear stability analysis. The flapping dynamics is shown to be a subcritical instability whose key ingredient is the finite extensibility of the disk. The behavior we observe has implications for emerging investigations on the flow dynamics of sheet-like particles, such as 2D
polymers and 2D crystalline materials immersed in viscous fluids.
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All authors Yijiang Yu, Hugo Perrin, Michael D. Graham, Lorenzo Botto
Mesoscale and Nanoscale Physics
Non-adiabatic Ehrenfest dynamics with norm-conserving and ultra-soft pseudo-potentials with nuclear velocity corrections on the atomic orbitals within the Projector Augmented Wave Method framework
Paolo Fachin, Francesco Macheda, Paolo Barone et al. (+1 more)
We derive the first-principles Ehrenfest molecular dynamics describing non-adiabatic processes with the inclusion of the
nuclear-velocity-dependent phases (also known as electron-translation factors) on the atomic-orbital basis. These phases, appearing when
nuclei are treated dynamically, affect effective
Hamiltonians constructed from localised orbitals. In this work, we focus on the effects in the first-principles pseudo-potential
Hamiltonian, both for the norm-conserving and ultra-soft cases, derived within the Projector-Augmented-Wave (PAW) method framework. Peierls-like phases depending on the
nuclear velocities appear in the non-local part of the potential, while additional
nuclear velocity and acceleration-dependent corrections appear in the ultra-soft pseudo-potential case. The use of velocity-including atomic orbital basis enables a Galilean-invariant description of the non-adiabatic Ehrenfest molecular dynamics, removing spurious non-adiabatic couplings that arise from neglecting the
nuclear velocity phases in the atomic orbitals.
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All authors Paolo Fachin, Francesco Macheda, Paolo Barone, Francesco Mauri
Statistical Mechanics
The mesoscopic foundations of non equilibrium thermodynamics and the arrow of time in the Dual Model of Liquids
Fabio Peluso
This manuscripts has two goals. The first goal is to show that the interaction in the Dual Model of
Liquids between the solidlike molecule aggregates and the
lattice excitations is appropriate to represent the link between the behaviour at macroscopic scale of normal
liquids and the physical processes characterizing those systems at mesoscopic scale. The second goal is to show that the duality allows identifying a time arrow on the mesoscopic scale in
liquids. The interaction of quanta of elastic energy with the molecular clusters introduces a privileged direction, which is relevant in time dependent and dissipative macroscopic processes, although the interaction remains temporally reversible
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Materials Science
Endowing variational phase-field fracture models with custom strength criteria
Roberto Alessi, Matteo Brunetti, Roshan Udaram Patil et al. (+1 more)
By now, several approaches have been proposed to endow phase-field fracture models with the ability to describe crack nucleation under multiaxial stress states. These include techniques for splitting the free energy, direct modifications of the phase-field driving or resisting forces that sacrifice the variational structure of the problem, and the introduction of additional internal variables, such as plastic strains or other nonlinear strains. In this paper, we propose a fundamentally different strategy for incorporating arbitrary elastic domains into phase-field fracture models, formulated within the variational framework of generalized standard materials. The proposed approach relies on letting the dissipation potential depend on the current state of the material. In this way, the variational structure of the problem is preserved, while elastic degradation and the strength criterion remain two distinct and independently controllable aspects of the material response. Simple yet representative models are presented and thoroughly discussed to demonstrate the effectiveness of the proposed methodology. The resulting
evolution of the elastic domain is investigated in both strain and stress spaces. Moreover, numerical simulations demonstrate a range of crack nucleation processes under multiaxial loading conditions for various analytical strength surfaces. This work paves the way for future developments and applications in several directions.
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All authors Roberto Alessi, Matteo Brunetti, Roshan Udaram Patil, Jacinto Ulloa
Superconductivity
2.4 GHz Flip-flop Device within Nonequilibrium Superconducting Diode
Xiangyu Bi, Hongyi Li, Aoshen Yang et al. (+12 more)
Superconducting diode effect exhibits asymmetric critical supercurrent and has profound implications for condensed matter physics. The technical appeals of such superconducting diodes are their ultrahigh on-off ratio and diode efficiency for superconducting electronics owing to the dissipationless supercurrent therein. However, realizing superconducting diode operation at high working frequency, which is a key requirement for practical applications, remains elusive and challenging. Here, we demonstrate a polarity-controllable superconducting diode with non-
equilibrium Josephson junction and its edge-triggered flip-flop operation at a high frequency up to 2.4 GHz, within a van der Waals
superconductor 2M-WS$_2$. By simply tuning the thickness of superconducting 2M-WS$_2$ nanoflakes to engineer inversion asymmetry in the junction, we achieve a high diode efficiency of 67% and an on-off ratio exceeding 10$^5$. Importantly, the pulse width and duty cycle of output pulse signals in such superconducting diode flip-flop devices can be controlled in a broadband frequency range crossing 12 orders of magnitude. Theoretical analysis reveals that the non-
equilibrium dynamic nature of supercurrent in these Josephson junctions enables such a high diode operating frequency and the polarity control of supercurrent. The 2.4 GHz non-
equilibrium Josephson diode developed here provides a promising platform for advanced superconducting logic
circuits and broadband telecommunication applications.
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All authors Xiangyu Bi, Hongyi Li, Aoshen Yang, Yuqiang Fang, Ganyu Chen, Shichong Yang, Yicheng Shen, Qizheng Sun, Junwei Huang, Wei Jiang, Da Wang, Fuqiang Huang, Haijun Zhang, Qianghua Wang, Hongtao Yuan
Materials Science
Stacking-Dependent Magnetism and Tunable Half-Metallicity in Bilayer Janus 1T-MnSSe
Jakkapat Seeyangnok, Udomsilp Pinsook and Graeme J. Ackland
We investigate the structural, electronic, and
magnetic properties of
bilayer Janus 1T-MnSSe using first-principles calculations. Various AA- and AB-type stacking configurations are considered to examine the influence of interlayer registry on
magnetic ordering and exchange interactions. The nonmagnetic state is unstable for all stackings, confirming intrinsic magnetism. The AA2 stacking is identified as the ground state and exhibits A-type
antiferromagnetic ordering, indicating
antiferromagnetic interlayer coupling. Monte Carlo simulations based on an effective Ising model reveal enhanced
magnetic transition temperatures in the
bilayer relative to the monolayer, with Néel temperatures above 300~K for
antiferromagnetic stackings and Curie temperatures up to 250~K for
ferromagnetic phases. Several stacking configurations exhibit robust half-metallic ferromagnetism with nearly 100%
spin polarization at the
Fermi level. Moreover, the half-metallic state can be tuned and ultimately transformed into a metallic
ferromagnetic phase through carrier doping and biaxial strain. These findings establish
bilayer MnSSe as a promising platform for controllable interlayer magnetism and spintronic applications in two-dimensional materials.
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Strongly Correlated Electrons
Unveiling orbital landscapes in strongly correlated bulk nickelates with $s$-NIXS
Edgar Abarca Morales, Martin Sundermann, Brett Leedahl et al. (+10 more)
We leverage $s$-orbital non-resonant inelastic X-ray scattering ($s$-NIXS) to perform orbital imaging on three bulk rare-earth nickelates spanning a range of formal nickel valence (3$d$ electron filling) from Ni$^{3+}$ (3$d^7$) to Ni$^{1+}$ (3$d^9$). Our results directly reveal the ground states of these compounds all with minimal theoretical input. In particular, we demonstrate the low-
spin orbital configuration of trivalent LaNiO$_3$, the $d_{x^2-y^2}$ configuration of monovalent LaNiO$_2$, and resolve the effective $e_g$
crystal field splitting in the distorted octahedral environment of divalent La$_2$NiO$_4$. This work illustrates the potential of $s$-NIXS to study the ground state and excited states of strongly correlated materials without needing complex theoretical analysis of spectroscopic data.
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All authors Edgar Abarca Morales, Martin Sundermann, Brett Leedahl, Vignesh Sundaramurthy, Georg Poelchen, Ulrich Burkhardt, Raul Cardoso, Pascal Puphal, Alexander Komarek, Bernhard Keimer, Matthias Hepting, Liu Hao Tjeng, Berit H. Goodge
Quantum Gases
Charge-Conjugation Violation and Population Asymmetry in Bipartite Fermionic Lattices
Di Xiao, Xue-Ting Fang, Lushuai Cao et al. (+2 more)
Charge conjugation violation (CCV) is a central concept in particle physics and appears also for quasiparticles in
quantum many-body systems, which typically relies on an embedded external
symmetry breaking to the underlying system. An open question is how an intrinsic CCV mechanism could emerge and what its macroscopic consequences would be. We establish sublattice kinks in bipartite fermionic
lattices as a concrete setup showing intrinsic CCV. The intrinsic CCV of the sublattice kink is based on the graph-topological nature of the underlying
Hamiltonian, with no explicit
symmetry breaking taking place. It leads to a population asymmetry of different configurations and imprints a hidden leaf-like structure in the eigenenergy spectrum. The population asymmetry also leads to an imbalanced sublattice-kink production triggered by the
vacuum-instability in the quench dynamics. Our work demonstrates the graph
topology as the microscopic origin of intrinsic CCV, with the population asymmetry as the macroscopic consequence, of which the proposed setup is highly amenable to experimental implementation via cold-atom
quantum simulators.
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All authors Di Xiao, Xue-Ting Fang, Lushuai Cao, Zhong-Kun Hu, Peter Schmelcher
Soft Condensed Matter
Aging Time dependent Static Friction between Soft and Hard Solid Interfaces
Vinay A. Juvekar and Arun K. Singh
Understanding of friction between sliding surfaces is critical for variety of applications. We present a friction model between soft and hard solid interfaces for studying aging time dependent static friction. The model is based on strengthening of dangling chains with the substrate during aging period. The friction model is, in turn, validated with the experimental data from literature. Friction properties are also estimated in terms of gelatin concentration to justify the results.
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Materials Science
Ferroelectric brightening of spin forbidden dark excitons in a WSe2/hybrid perovskite heterostructure
Xinyun Wang, Magdalena Grzeszczyk, Maxim Trushin et al. (+14 more)
Long-lived dark
excitons in monolayer WSe2 present promising candidates for carrying
spin and valley information, but their optical access and
spin manipulation have conventionally required the use of strong external
magnetic fields. Here, using a ferroelectric hybrid perovskite
heterostructure, we leverage the ferroelectric proximity effect to break the WSe2's in-plane rotational
symmetry and brighten the
spin-forbidden dark
excitons under zero
magnetic field conditions. Furthermore, we show that the twist angle between the WSe2 and perovskite
crystals controls the ferroelectric coupling strength and valley-contrasting polarization. Our proposed mechanism, supported by a four-band tight-binding model, suggests that the ferroelectric proximity effect induces an asymmetric intersublattice interaction, generating an effective in-plane
spin-orbit coupling (SOC) field that rotates
spin/valley polarization and brightens dark
excitons. Our work establishes ferroelectric proximity coupling as an electrically reconfigurable,
magnetic-field-free strategy for
spin exciton control in two-dimensional
semiconductors.
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All authors Xinyun Wang, Magdalena Grzeszczyk, Maxim Trushin, Ivan Verzhbitskiy, Dmitrii Litvinov, Yi Wei Ho, Yuan Chen, Zhenyue Wu, Mykola Telychko, Chuanqi Zhang, Andres Granados del Aguila, Kuan Eng Johnson Goh, Xinwei Li, Goki Eda, Shaffique Adam, Maciej Koperski, Kian Ping Loh
Soft Condensed Matter
Geometry-Driven Polarization Control in Ferroelectric Nematic Liquid Crystals
Kazuma Nakajima, Hirokazu Kamifuji, Hirotsugu Kikuchi et al. (+2 more)
Ferroelectric nematic
liquid crystals (FNLCs) combine fluidity with spontaneous polarization, offering promising avenues for flexible electromechanical systems. Here, we demonstrate that mechano-electrical conversion in FNLCs can be enhanced by mechanically programming a robust macroscopic polarization
alignment. Using hybrid
liquid crystal cells composed of rigid glass and flexible substrates, we show that deformation in the ferroelectric nematic phase suppresses polarization domains and produces long-range ordered polarization
alignment over millimeter-scale areas. This geometry-driven
alignment originates from coupling between the FNLC's spontaneous splay deformation and the deformation-imposed
cell geometry, and we further find that the selected polarization direction exhibits clear material dependence. Leveraging this deformation-enabled
alignment, we develop an FNLC-based energy harvester that converts mechanical deformation into an output of approximately 1 V. These findings establish geometry-driven
alignment as a practical design strategy for boosting FNLC mechano-electrical conversion while providing polarization control for soft electronic devices.
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All authors Kazuma Nakajima, Hirokazu Kamifuji, Hirotsugu Kikuchi, Kenjiro Fukuda, Masanori Ozaki
Strongly Correlated Electrons
A microscopic design rule for spin supersolids in triangular-lattice magnets
Ryota Ono, Jun'ichi Ieda, Michiyasu Mori et al. (+1 more)
Spin supersolids emerge as a central topic in frustrated magnetism, motivating the search for realization in
quantum materials. To this end, we study the origin of exchange anisotropy, $\Delta$, in triangular-
lattice cobaltate families $X_2$$Y$Co(PO$_4$)$_2$ and $X_2$Co(SeO$_3$)$_2$ ($X$ = Na, K, Rb, Cs; $Y$ = Mg, Ca, Sr, Ba) by tailoring realistic
spin models. We show that $\Delta$ is determined by the ratio of trigonal
crystal field to
spin-orbit coupling strength. This framework explains contrasting anisotropies in these families, predicts systematic trends in $\Delta$ across $X/Y$-substitutions, and identifies candidate materials for
spin supersolids. Our results establish trigonal field engineering as a microscopic route toward the design of
spin supersolids.
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All authors Ryota Ono, Jun'ichi Ieda, Michiyasu Mori, Sadamichi Maekawa
Statistical Mechanics
Hidden periodicities allow the prediction of locked particle motions on quasicrystalline surfaces
Seemant Mishra, Artem Ryabov and Philipp Maass
Motion of particles across quasicrystalline surfaces exhibits peculiar features due to the presence of long-range order without translational periodicity. Under time-periodic forcing, this motion can become locked in directions thatn deviate strongly from the mean driving direction. We show that for surface potentials with a quasicrystalline pattern of minima generated by a superposition of plane waves, particle trajectories are nonperiodic, yet their mean direction and speed are determined by hidden periodic potentials. The
lattice vectors of these underlying potentials define characteristic velocities that dictate both directional and speed locking. The particle motion does not synchronize with the driving, and it is possible for the mean speed to remain nonlocked even in directionally locked states. These findings are demonstrated using a model directly amenable to experimental realization.
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Superconductivity
Superconductivity beyond band geometry: emergence of pair quantum geometry
Mehmet Akif Keskiner and M. Iskin
Quantum geometry is known to influence the effective mass of single particles in multiband systems through the
geometric properties of Bloch states. Here, we show that this connection extends beyond the one-body level. Starting from a multiband Hubbard model with on-site attractive interaction, we derive an exact effective-mass
theorem for two-body bound states in
vacuum and establish the corresponding
geometric framework for Cooper pairs near the superconducting critical temperature within Gaussian fluctuation theory. We demonstrate that pair effective masses contain not only “conventional” contributions arising from band dispersion and band geometry, but also distinct
geometric terms governed by
quantum metrics defined on the
manifolds of paired states, provided pairing is non-uniform across sublattices. This reveals a hierarchy linking band geometry, pair geometry, and Cooper-pair geometry. In the many-body problem, analytic continuation naturally leads to a non-Hermitian fluctuation kernel and a biorthogonal
quantum geometry, yielding a generally complex Cooper-pair effective mass when collective modes overlap with the fermionic continuum. Exact numerical calculations for the sawtooth, Su-Schrieffer-Heeger, Hofstadter, and fluorite-like
lattice models show that pair geometry can provide a quantitatively significant contribution to the effective mass of bound pairs. Our results identify pair
quantum geometry as an essential ingredient governing bound-pair dynamics and multiband superconductivity beyond band geometry.
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Materials Science
Resolving room temperature microscale fracture and plasticity of iron oxides along the cascade of iron ore reduction via nanoindentation and microcantilever bending
Shreehard Sahu, James P. Best, Gerhard Dehm et al. (+1 more)
Understanding the fundamental mechanical behaviour of iron
oxide phases is essential for controlling attrition and fracture during iron ore reduction process, particularly in hydrogen-based direct reduction systems. This study investigates the room temperature plasticity and fracture behaviour of single-
crystal hematite, magnetite, and Wustite using nanoindentation and micro-cantilever fracture testing. Hematite exhibited the highest hardness, H and elastic modulus, E (H=18.5 GPa, E=281 GPa), followed by magnetite (H=8.7 GPa, E=165 GPa) and Wustite (H=7.5 GPa, E=145 GPa), reflecting differences in slip activity along the iron
oxide reduction sequence. Furthermore, fracture toughness was measured using notched microcantilevers for all three iron
oxide phases, aligned along low index and high index crystallographic planes, respectively. For the low index-oriented case hematite showed increased fracture toughness owing to crack deviation and faceting while magnetite and Wustite exhibited single plane cleavage fracture. Distinct changes in the deformation behavior in terms of plasticity and cracking of the three iron
oxides were evident from both methods. Further investigation of a magnetite-gangue interface, particularly relevant to low-concentration ores, revealed significantly reduced fracture toughness compared to the magnetite phase. Overall, these results provide a comprehensive set of mechanical properties of iron
oxides with potential application in material models for predicting fracture and attrition during hydrogen-based direct reduction.
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All authors Shreehard Sahu, James P. Best, Gerhard Dehm, Anwesha Kanjilal
Materials Science
The KNN rollercoaster: from bulk ceramics to phase engineered wafer-scale thin films
Giulia Pavese, Federico Orlando, Fabio Melzi et al. (+12 more)
Since the initial disclosure of the extraordinary piezoelectric coefficients of Potassium sodium niobate (KNN) in near-equimolar bulk ceramics, its development trajectory has resembled a rollercoaster, with its integration into microelectronics severely lagging due to thermodynamic stability issues and poor planar process compatibility. In this work, we revisit the bulk-derived phase diagram for the specific case of thin films integrated on silicon. By systematically investigating Mn-doped K1-xNaxNbO3 films grown on 8-inch wafers, we demonstrate that the optimal stoichiometry for thin films fundamentally diverges from the bulk equimolar standard. A Na-rich composition (> 70 at.%) is required to overcome substrate-induced constraints, effectively suppressing pyrochlore formation and chemical phase segregation while promoting dense columnar growth with a complete (001) out-of-plane polar orientation. Consequently, Na-rich films deliver outstanding functional properties, reaching remanent polarizations up to 14 uC cm-2, with piezoelectric coefficients of d33f= 79 pm/V and e31f = 10 C/m2. Supported by Density Functional Theory simulations, we correlate this enhancement with improved stability and a strain-driven structural reorientation toward a lower-
symmetry monoclinic phase with tilted polarization. By redefining the phase engineering rules for wafer-scale thin films, our results establish a clear route toward KNN integration in microsystems.
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All authors Giulia Pavese, Federico Orlando, Fabio Melzi, Walter Piazzi, Andrea Pescarolo, Federico Maspero, Marco Asa, Riccardo Gianola, Andrea Picco, Andrea Serafini, Kui Yao, Silvia Picozzi, Laura Castoldi, Miguel-Ángel Badillo-Ávila, Riccardo Bertacco
Statistical Mechanics
Thermodynamics of bouncing grains
Olivier Devauchelle, Predrag Popović, Piotr Szymczak et al. (+2 more)
When a horizontal plate vibrates strongly enough, it causes small particles such as sand grains to continually bounce on it and, over time, to diffuse across its surface. This phenomenon is the cause of the well-known Chladni figure, which is drawn by a higher density of grains gathering along the nodal lines of a resonating elastic plate. Using a heterogeneous, non-resonating plate, we investigate experimentally this type of
diffusion. We find that, for the most part, is it comparable to classical molecular
diffusion. We can define a temperature for the bouncing grains, and the system then obeys the fluctuation-dissipation
theorem. We also recover Maxwell-Boltzmann statistics at
equilibrium, when temperature is uniform. However, when temperature varies across the vibrating plate, the microscopic details of the grains' dynamics affect their macroscopic behavior: Fick's law, for instance, no longer applies. Instead, our experiments support a new transport relation that was recently proposed to represent
diffusion in Chladni's experiment. Finally, we propose an expression for the heat flux associated to the non-
equilibrium steady state predicted by this new relation, and test it against observations.
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All authors Olivier Devauchelle, Predrag Popović, Piotr Szymczak, Anaïs Abramian, Arnaud Lazarus
Soft Condensed Matter
Run and tumble dynamics of a soft robotic cell
Siddhant Mohapatra, Fanny Wéry, Filip Novkoski et al. (+3 more)
The continuous regulation of transport properties through softness remains a longstanding challenge in active matter. Here, we show that encasing a programmable active particle within a deformable membrane naturally gives rise to intermittent stop-and-go dynamics, with ballistic motion at short times crossing over to
diffusion at long times. Crucially, membrane softness acts as a single control parameter that continuously tunes persistence, intermittency, and long-time transport, linking the internal driving to the emergent locomotion of the synthetic
cell. Combining experiments, simulations, and a run-and-tumble theoretical framework, we identify the minimal physical ingredients underlying this behavior and establish design principles for programmable soft active transport, opening new avenues at the interface of active matter physics and synthetic robotics.
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All authors Siddhant Mohapatra, Fanny Wéry, Filip Novkoski, Piotr Nowakowski, Ana-Suncana Smith, Nicolas Vandewalle
Statistical Mechanics
A granular Büttiker-Landauer motor
Olivier Devauchelle, Predrag Popović, Piotr Szymczak et al. (+2 more)
Random walkers usually diffuse according to Fick's law. On average, they move down the gradient of their concentration and, in the absence of external force, tend to distribute themselves uniformly. In some experiments, however, this familiar notion is at odds with observation. Sand grains, for instance, gather along the nodal lines of a vibrated elastic plate to form a Chladni figure, thus accumulating where fluctuations are weak – a fact that escapes the reach of Fick's law. On theoretical grounds, Büttiker [Zeitschrift für Physik B, 68, 1987] and Landauer [J Stat Phys, 53, 1988] proposed that particles submitted to a non-uniform temperature field would indeed gather where the temperature is low. They also predicted that, in the presence of a potential force, a non-uniform temperature could drive a steady current of particles, powered only by noise. Here, we present an experimental realization of these phenomena in a macroscopic system, which confirms the quantitative predictions of Büttiker.
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All authors Olivier Devauchelle, Predrag Popović, Piotr Szymczak, Anaïs Abramian, Arnaud Lazarus
Disordered Systems and Neural Networks · Cross-listed from cs.LG
High-Dimensional Theory of LoRA Fine-Tuning in a Solvable Attention Model
O. Duranthon, F. Boncoraglio and L. Zdeborová
We develop a high-dimensional statistical theory of low-rank adaptation (LoRA) in
attention models, capturing the interplay between pre-training and fine-tuning. We introduce a solvable framework in which a single-head
attention layer is first pre-trained on a data-abundant task and subsequently adapted via a rank-one LoRA update on limited data. In the high-dimensional limit, both stages admit a sharp asymptotic characterization in terms of a finite set of order parameters, yielding explicit predictions for test errors and representation
alignment. Our analysis shows that the impact of pre-training on LoRA is summarized by an effective noise term, from which we derive prescriptions for the optimal pre-training procedure. We also demonstrate a regime with a mismatch between the value of the test error and representation quality, and propose an application of our theory to active fine-tuning.
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Materials Science · Cross-listed from physics.plasm-ph
Coupled simulation of plasma-surface interactions during early stages of vacuum arcing
Roni Koitermaa, Andreas Kyritsakis, Tauno Tiirats et al. (+2 more)
We describe fully coupled simulations that bridge atomistic cathode dynamics and
plasma formation during the earliest stages of
vacuum arcing. The model combines molecular dynamics, finite element electrothermal calculations, electron emission and particle-in-
cell plasma simulations via dynamic transfer of particles between the surface and
plasma domains. Simulations of Cu nanoprotrusions reveal two routes to thermal runaway: direct Joule heating-driven instability and a novel nanoparticle-assisted mechanism, where detached nanoparticles generate neutral vapor that becomes ionized.
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All authors Roni Koitermaa, Andreas Kyritsakis, Tauno Tiirats, Flyura Djurabekova, Veronika Zadin
Statistical Mechanics
Polylogarithmic Structure of Bragg Diffraction in Finite-Coherence Lattices
Evangelos G. Filothodoros
We develop a polylogarithmic structure for Bragg diffraction based on a weighted multi-plane interference model. Within this kind of construction, the scattering amplitude is expressed as a polylogarithmic generating function. By introducing extra contributions with power-law and the usual exponential decay, it takes the form $F(\theta) = \mathrm{Li}_m\left(e^{i\theta_{\mathrm{eff}} - \epsilon}\right)$, where $\epsilon$ is a finite coherence length. In the limit where $\epsilon \rightarrow 0$, the argument of the polylogarithm approaches the unit circle and the classical Bragg condition corresponds to the approach of the polylogarithm argument toward its branch point $z=1$. This formulation provides a compact analytical framework for describing diffraction line shapes within a generalized correlation model in which peak positions, widths, and line shapes arise from a single analytic structure. Although we are able to recover the standard Bragg law for ideal
crystals, the polylogarithm model captures deviations due to finite correlation length, disorder and non-uniform
lattice coherence. We show that if Bragg peaks correspond to boundary singularities of the polylogarithm, a connection between diffraction theory and complex analysis arise. The proposed theoretical model may be particularly relevant for disordered or partially coherent materials, where conventional diffraction models often require additional phenomenological broadening assumptions.
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Strongly Correlated Electrons
Symmetry-Selective Stabilization of Charge-Density Wave in ScV$_6$Sn$_6$
A. Korshunov, C.-Y. Lim, J. Corral-Sertal et al. (+10 more)
Charge-density-wave (CDW) order in kagome metals is highly sensitive to external tuning parameters such as chemical substitution and hydrostatic pressure, which generally suppress long-range order. Here, using high-resolution X-ray diffraction under controlled uniaxial strain, we show that anisotropic
lattice deformation instead stabilizes and enhances the CDW state in ScV$_6$Sn$_6$. Compression along the [H00] and [HH0] directions lowers the
crystal symmetry from hexagonal to orthorhombic, lifts the degeneracy between
symmetry-equivalent in-plane CDW domains, and promotes long-range order while preserving the underlying trimer instability.
Phonon calculations indicate only a moderate stabilization of the imaginary flat
phonon mode, demonstrating that the increase in T$_\mathrm{CDW}$ is primarily driven by the in-plane ordering of the Sn$^\mathrm{T}$–Sc–Sn$^\mathrm{T}$ rattling chains within the frustrated kagome
lattice. A phenomenological model incorporating strain-dependent Ising couplings within a three-state Potts framework successfully reproduces the
evolution of T$_\mathrm{CDW}$ under compression and captures the continuous nature of the transition. Our results establish uniaxial strain as a powerful
symmetry-selective tuning parameter for order-disorder transformations in frustrated
lattices.
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All authors A. Korshunov, C.-Y. Lim, J. Corral-Sertal, G. Garbarino, D. Chernyshov, A. Rajapitamahuni, C. Yi, S. Roychowdhury, C. Shekhar, C. Felser, V. Pardo, Ella M. Schmidt, S. Blanco-Canosa
Soft Condensed Matter
Aqueous-alcohol mixtures in dimension two: miscibility and micro-segregation
Camille de la Vaissiere, Ayse Butuner and Aurélien Perera
Two dimensional site interaction models of water and alcohols are mixed in various proportions and studied by Monte Carlo simulations, with the purpose to clarify problems related to simulation of real micro-heterogeneous systems. Three alcohols are considered, methanol, pentanol and octanol. The main finding is that, while real alcohols demix with water from butanol onward, their 2D analogs are always fully miscible, while developing increasingly pronounced micro-segregation as the alcohol tail length increases. This is not a consequence of the intrinsically higher fluctuations in 2D, but rather a reorganization of these fluctuations under the charge ordering mechanism. The second finding is that water drives the micro-segregation through strong self-aggregation, but this is not enough to achieve full phase separation because of the water-alcohol contact at the outer rim of the water domains. In this work we examine how this local heterogeneity develops with increasing alcohol alkyl tails, monitored with the study of pair correlation functions, structure factors and Kirkwood-Buff integrals. The absence of clear local self-averaging of the latter provides an illustration of the tension between energy driven maintaining of local structures and
entropy driven global homogeneity. In that, the 2D modelisation of real hydrogen bonding mixtures allows to better capture and reveal the physics behind the chemistry of these
liquids.
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Strongly Correlated Electrons
Understanding deconfined quantum critical points from crystalline categorical Landau paradigm
Hiromi Ebisu, Bo Han and Weiguang Cao
Deconfined
quantum critical points (DQCPs) involving
lattice symmetries evade the conventional Landau paradigm because the competing orders break incompatible internal and crystalline
symmetries. We show that a class of DQCPs can nevertheless be understood as Landau-type transitions after gauging anomalous onsite
symmetries. For
spin chains with Lieb-Schultz-Mattis (LSM) anomalies, gauging produces a noninvertible
lattice translation whose fusion closes only up to ordinary translations, giving rise to a crystalline categorical
symmetry. In the gauged description, the original DQCP becomes a transition between different
symmetry breaking patterns of this categorical
symmetry. We demonstrate this mechanism in microscopic
lattice models; the
magnetic-valence-bond-solid (VBS) DQCP realizes a Rep($D_8$)-type crystalline categorical Landau transition, whereas a y-
antiferromagnetic-VBS DQCP realizes a Rep($H_8$)-type one. Although Rep($D_8$) and Rep($H_8$) share the same fusion rules, they have inequivalent $F$-symbols and therefore define distinct categorical descriptions. Our results show that the universal categorical structure underlying these DQCPs is encoded in the full fusion category, rather than in the fusion ring alone.
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Mesoscale and Nanoscale Physics
Breakdown of Fluctuational Electrodynamics in the Extreme Near Field
Philippe Ben-Abdallah
Fluctuational electrodynamics relies on the assumption that thermal fluctuations in distinct bodies are statistically independent. It is shown that this approximation breaks down in the extreme near-field regime, where hybridization of surface
phonon-polaritons across nanometric
vacuum gaps generates finite fluctuating-current cross correlations between opposite interfaces. Using a microscopic coupled-oscillator model combined with a Green-tensor formulation of the Poynting vector, the resulting correlation-induced correction to radiative heat transfer is derived. For polar materials, these correlations become significant when the hybridization energy approaches the intrinsic damping rate and can substantially modify conventional fluctuational-electrodynamics predictions at subnanometric separations.
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