Soft Condensed Matter
Active-learning mapping of the Vicsek model phase diagram
Grace T. Bai and Brandon B. Le
The Vicsek model is a minimal model of collective motion, capturing how local
alignment interactions can generate macroscopic nonequilibrium order in systems such as bird flocks. In this work, we use active learning to map the Vicsek phase diagram as a function of noise strength, density, and particle speed. A neural-network classifier is trained on global polar-order labels, and classifier
entropy is used to select new simulations near uncertain crossover regions. The resulting phase map resolves a high-noise disordered gas, a low-noise polar ordered regime, and an intermediate coexistence-candidate regime whose noise window shifts upward and broadens with increasing density. Independent density and local-order diagnostics indicate that the intermediate regime contains dense, locally ordered bands coexisting with a dilute, weakly ordered background. Comparison with the ordered regime shows that banded coexistence is identified by the joint enhancement of band contrast, local-order heterogeneity, and positive density-order correlation. Overall, these results establish a machine-learning-guided workflow for active matter, in which active learning constructs an operational phase map and independent spatial diagnostics convert classifier-defined regimes into physically interpretable nonequilibrium morphologies.
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Statistical Mechanics
Optimal current-based sensing of phonon temperature using a finite reservoir
Sindre Brattegard, Stephanie Matern, Mark T. Mitchison et al. (+1 more)
In realistic nanoscale transport set-ups, electron-
phonon coupling leads to the exchange of heat between
phonon baths and electronic reservoirs with finite heat capacities. Such exchange affects the finite reservoir's temperature. However, this sensitivity of the finite reservoir temperature to the exchange of heat with the finite reservoir has remained unexplored for thermometry. Here, we fill this gap by combining current metrology techniques with a thermodynamic framework encompassing finite reservoirs. We focus on an experimentally realizable set-up with a
quantum dot coupled to a finite reservoir and consider two distinct current-based strategies in the long time limit, namely monitoring quanta exchanged between the
quantum dot and finite reservoir and the measurement of the total current flowing from the
quantum dot into an infinite reservoir. A third strategy involves measurements of the
quantum dot occupation. For a large but finite reservoir, we show that the Fisher information for all three strategies captures the finite reservoir's contribution to sensitivity through common factors. We also demonstrate that monitoring quanta exchanged between the system and finite reservoir in the long time limit achieves optimal precision. Finally, we provide an optimization analysis that explores how maximal precision can be achieved within each of the current-based strategies by tuning the gate voltage.
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All authors Sindre Brattegard, Stephanie Matern, Mark T. Mitchison, Saulo V. Moreira
Statistical Mechanics
Domain-wall melting in all-to-all QSSEP from random-matrix theory
Denis Bernard, Lorenzo Piroli and Stefano Scopa
We study the melting of a domain wall in the
quantum simple exclusion process with all-to-all hoppings (a.k.a. the charged SYK$_2$ model). We show that the real-time dynamics of physical quantities of interest can be obtained exploiting spectral results in random
matrix theory. We first show that the
eigenvalues of the correlation
matrix corresponding to the initially charged subsystem evolve according to a Jacobi process, which is defined in terms of a closed system of
stochastic differential equations. In turn, this observation allows us to obtain the real-time dynamics of all the
eigenvalue moments. We present two physical applications. First, we study the dynamics of the averaged von Neumann
entanglement entropy, arriving at a fully explicit expression in the thermodynamic limit. Second, we compute analytically the full-counting statistics of the charge. Our formula allows us to perform a thorough comparison with the full-counting statistics of the classical simple exclusion process. Notably, we show that, in the thermodynamic limit, the
quantum and classical full-counting statistics coincide, with no finite-time corrections.
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Materials Science
Strong coupling between quantized magnon modes in a YIG microstucture and microwaves in a superconducting resonator
Seth W. Kurfman, Philipp Geyer, Anoop Kamalasanan et al. (+6 more)
Strong-coupling experiments based on
magnons enable the exploration into on-chip demonstrations involving numerous long-lived excitations. Yttrium iron garnet (YIG) has been considered for decades as a gold standard material for magnonics due to its low-loss magnonic properties. While YIG has successfully demonstrated strong-coupling in macroscopic device geometries, the strong coupling of
magnons in truly sub-10 micron YIG structures to date has not yet been realized. This obstacle is due to the difficulty producing large enough effective magnonic mode volume necessary primarily due to thickness limitations of YIG deposition and device fabrication techniques. Here, we demonstrate the use of a microplatelet of YIG, manufactured from a single
crystal of YIG via focused ion beam (FIB) techniques, placed on a constricted inductive line of an optimized superconducting lumped element LC resonator to achieve strong coupling between numerous
magnon modes and the LC resonator
photons. These experimental findings are qualitatively backed by micromagnetic simulations and quantitatively supported by analytical calculations to identify the
magnon modes corresponding to the experimentally observed anti-crossings in the microwave transmission signal. Further, we show that these anti-crossings remain even at incredibly low device input powers ($\leq 10$ fW). The fabrication techniques and device geometry enable the deterministic use of numerous confined
magnon modes in micron-scale YIG structures for various
magnetic field strengths and orientations at substantially reduced device powers. The results here establish a foundational path forward to achieving efficient
magnon-based strong-coupling experiments in micron-scale YIG
magnetic elements for effective on-chip studies.
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All authors Seth W. Kurfman, Philipp Geyer, Anoop Kamalasanan, Karl Heimrich, Kwangyul Hu, Tharnier O. Puel, Frank Heyroth, Michael Flatté, Georg Schmidt
Materials Science
Multi-scale calculation of light-induced structural changes in low-angle twisted bilayer WSe$_2$
Rafael R. Del Grande and David A. Strubbe
Exciton-
phonon interactions in transition metal dichalcogenides (TMD) are strong and lead to phenomena such as coherent
phonon generation. When stacked and twisted, their properties can be tuned by the twisting angle. In experiments with 1.1$^\circ$ twisted 2L WSe$_2$, a change of 0.1 Å in the interlayer distance was observed when light was shone on this material, and here we explain the microscopic mechanism behind this. Theoretical works to study such systems are limited because the Moiré unit
cell is too large. To overcome this, we combined classical force field relaxations with our implementation of ab initio GW/Bethe-Salpeter excited state forces (ESF). From the relaxations we found that the low-angle twisting induced an in-plane strain field, the AB regions are large enough to be simulated as periodic AB stacked 2L WSe2, and the interlayer force constant becomes softer in relation to the perfect AB stacking. From the ab initio ESF we obtained that the in-plane strain increases the out of plane ESFs. Those two effects combined, the weakening of the interlayer force constant and strain dependence of the ESF, make light-induced changes in the interlayer distance of twisted 2L WSe2 stronger than in the perfectly stacked case, in agreement with experimental observations. Therefore, our results show that the
exciton-
phonon interactions can be tuned in twisted 2L TMDs and can be observed experimentally, which makes those materials excellent platforms to study light-induced changes in materials.
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Materials Science
Unveiling the potential of NdPO4 magnetocaloric phases in cryogenic refrigeration
M. Balli, L. Attou, S-E. Bouzarmine et al. (+4 more)
The RPO4 orthophosphates (R = rare earth element) have recently attracted a wide interest due to the strong coupling between their electronic, orbital and structural ordering parameters resulting in a variety of functional properties. Herein, we demonstrate that NdPO4 phases synthesized via a facile precipitation growth process unveil promise in low-temperature
magnetic cooling. The analysis of their structural properties reveals nanorod forms with diameters of 10 to 20 nm and lengths ranging from 200 to 500 nm while the heat treatment transforms their hexagonal rhabdophane-type structure to a monoclinic anhydrous monazite-type
symmetry. Magnetization measurements and DFT calculations show strong
antiferromagnetic couplings and the absence of any
magnetic ordering in the 2-300 K range. On the other hand, the monoclinic phase of NdPO4 exhibits a large magnetocaloric effect of about 19 J/kg K under 5 T near 3 K, outperforming some reference materials containing more expensive rare-earth elements with high
magnetic moments.
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All authors M. Balli, L. Attou, S-E. Bouzarmine, S. Oubad, K. El Maalam, P. Fournier, S. Mangin
Materials Science
Polar Topologies in a Ferroelastic Metal Membrane
Rahil Haria, Noah Schnitzer, T. Ben Britton et al. (+10 more)
Polar metals, materials in which electric polarisation and
metallicity coexist, are exceptionally rare because itinerant electrons screen long-range dipoles and favour centrosymmetric structures. Engineering polar textures in a conducting magnet holds promise for reconfigurable
spin orbit coupling and magnetoelectric functionality. Here we show that releasing epitaxial SrRuO3 films from their substrates drives a hierarchy of ferroelastic domain refinement from micrometre to nanometre length scales, and that this structural reorganisation spontaneously generates two distinct classes of emergent polar texture that are ubiquitous across the freestanding membrane. Using correlative microscopy from mesoscale electron channelling contrast imaging (ECCI) to atomic resolution scanning transmission electron microscopy (STEM), we demonstrate that electric polarisation emerges selectively at translation-inequivalent antiphase boundaries (APBs). At these boundaries multicomponent aac tilt field undergoes Neel-like interpolation that preserves the in-phase tilt component and amplifies roto flexoelectric coupling, while translation-equivalent boundaries remain nonpolar. The Neel like interpolation at hard APBs and Ising like collapse of all tilt components at easy APBs is corroborated with ab initio calculations. While embedded 90 ferroelastic walls provide an additional mechanistically distinct source of electric polarisation resulting in polar nanoclusters (4 nm). These distinct nanotextures at 90 walls from via elastic accommodation of strain mismatch between variants and rotostriction as the tilt field interpolates across the boundaries. These findings show that, in a membrane form, metal
oxides provide a robust platform for hosting nanoscale ferroelastic domains that generate polar textures.
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All authors Rahil Haria, Noah Schnitzer, T. Ben Britton, Yaqi Li, Tom J. P. Irons, Sophia Linssen Pitsaros, Ella Banyas, Geri Topore, Annabel Hoyes, Mariana Palos, Sinead M. Griffin, Katherine Inzani, Michele Shelly Conroy
Statistical Mechanics
Strong Mpemba Effect Through a Reentrant Phase Transition
Kristian Blom, Doron Benyamin, Uwe Thiele et al. (+2 more)
We investigate temperature quenches across the reentrant phase transition of the
antiferromagnetic Ising model in a
magnetic field and show that it provides a natural mechanism for strong direct and inverse Mpemba effects. For quenches terminating in the paramagnetic phase, the slowest relaxation mode is purely staggered. Initial states in the paramagnetic phase therefore have exactly zero overlap with this mode and exhibit a strong Mpemba effect, whereas
antiferromagnetic initial states excite it and develop a slow-relaxation tail. Moreover, reentrance makes the
equilibrium staggered magnetization nonmonotonic, producing conventional direct and inverse Mpemba effects when both initial states lie in the
antiferromagnetic phase and the quench terminates in the paramagnetic phase. By varying the
lattice coordination number, we show that this mechanism disappears in the absence of reentrance. Our results provide the first demonstration of (strong) Mpemba effects in the
antiferromagnetic Ising model within the pair approximation and establish a direct link between anomalous relaxation and
equilibrium phase behavior.
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All authors Kristian Blom, Doron Benyamin, Uwe Thiele, Oren Raz, Aljaz Godec
Materials Science
Machine Learning and Molecular Simulations Reveal Mechanisms of ZIFs Polymorph Selection
Emilio Méndez and Rocio Semino
Zn(imidazolate)$_2$ metal-organic frameworks (MOFs) exhibit a remarkable degree of polymorphism. Because of their promising industrial applications, many research groups have investigated phase transitions, phase diagram and relative stability of these polymorphs. There is now wide consensus in the research community that these MOFs are solvothermally formed via non-classical nucleation mechanisms, in which pre-nucleation clusters are first formed, followed by an intermediate amorphous structure that subsequently reorganizes to yield the final crystalline MOF. However, no study up to date has uncovered which part of the synthesis process determines the final polymorph obtained. In this work, path collective variable metadynamics simulations performed with a partially reactive force field give insights into mechanistic and thermodynamic aspects of the self-assembly of these MOFs. Databases of transient and intermediate synthesis structures are built from the simulations. By developing and applying neural network classifiers over these databases, it is found that both pre-nucleation clusters and the amorphous intermediate structures are polymorph-dependent. These results suggest that polymorph selection happens as early as the pre-nucleation cluster stage.
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Materials Science
From Narrow-gap Semiconductor to Metallic Altermagnet: Optical Fingerprints of Co-Doped FeSb2
R. Mathew Roy, M. Povolotskiy, J. Kirschke et al. (+11 more)
The realization of bulk metallic altermagnetism has remained elusive despite the growing number of candidate materials. Here, we present evidence that moderate cobalt substitution ($\sim$15%) drives the correlated narrow-gap
semiconductor FeSb$_2$ into a metallic altermagnetic state persisting up to room temperature. The infrared optical conductivity reveals low-energy interband transitions near 0.1 eV that emerge upon doping and grow with Co concentration. Density functional theory calculations show that these transitions originate exclusively from altermagnetic
spin ordering, with
spin split bands ($\sim$0.2 eV) of non-relativistic origin, together with
spin-orbit coupling induced band splitting of the order of $\sim$5 meV near the
Fermi level. Co substitution further leads to Fano lineshapes and mode mixing in the infrared-active
phonons, reflecting enhanced electron-
phonon coupling and local inversion
symmetry breaking, while leaving the altermagnetic
spin symmetry intact. Our results establish carrier-tuned FeSb$_2$ as a platform for exploring metallic $d$-wave altermagnetism and its coupling to
lattice degrees of freedom.
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All authors R. Mathew Roy, M. Povolotskiy, J. Kirschke, C. Prange, Y. Xia, V. Sundaramurthy, P. Puphal, M. Pinteric, M. van de Loo, A. Kreyssig, T. Zhang, A. E. Böhmer, M. Dressel, M. Wenzel
Materials Science
Radio Frequency Field-Induced Enhancement of Detection Sensitivity in Silicon Nanowire Sensors
Ang Liu, Jingsong Shang, Jiangang J. Du et al. (+2 more)
Sensitive biomarker detection in physiological fluids is often limited by Debye screening, which suppresses electrostatic signals at sensor surfaces. Here we report a sensing approach based on flexoelectric resonance in silicon nanowire field-effect transistors. An applied radiofrequency field induces strain gradients in the nanowires, generating flexoelectric polarization that is amplified at resonant frequencies. This effect enhances the sensitivity of conductance measurements to small surface charge variations associated with biomolecular binding. Using C-reactive
protein as a model biomarker, we observe an order-of-magnitude improvement in detection sensitivity compared to conventional operation, with a 62% conductance increase versus 30% without radiofrequency modulation. The high-frequency field also perturbs the electrical double layer, reducing Debye screening in high-ionic-strength environments. These combined effects enable direct biomarker detection without sample dilution. This work establishes flexoelectric resonance as a general strategy for improving nanoscale biosensing performance in physiologically relevant conditions.
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All authors Ang Liu, Jingsong Shang, Jiangang J. Du, Shyamsunder Erramilli, Pritiraj Mohanty
Materials Science
Yukawa screening derivation of the bond-valence rule
Michael L. Whittaker, Pan Wang, Chunhui Li et al. (+2 more)
The bond-valence model is a standard way to estimate bond strengths in
crystals, but its exponential dependence on bond length has lacked a derivation from a specific physical interaction. We show that this form emerges as the leading-order limit of screened Coulomb electrostatics and that the fitted bond-valence softness can be interpreted in terms of an electronic screening length. This turns bond valence from an empirical fitting rule into a transferable descriptor of local screened charge response across coordination environments. The resulting theory predicts how the bond-valence parameters should vary with ionic charge and coordination number, and that prediction agrees with 150 fitted valences from 94 cation-oxygen species, including 68 in fourfold coordination and 82 in sixfold coordination, at an abundance-weighted coefficient of determination of 0.986. A comparison with first-principles charge densities shows that the bond-valence shell radius tracks the electronic screening cloud with coefficients of determination of 0.9998 for ten alkali and alkaline-earth
oxides and 0.967 for 21 other binary
oxides whose nearest-neighbor environments match the theory's assumptions. The widely used bond-valence model is thus the leading-order expression of screened electrostatics in ionic solids.
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All authors Michael L. Whittaker, Pan Wang, Chunhui Li, Naman Katyal, Piotr Zarzycki
Materials Science
Quantifying Thermal, Photovoltage, and Defect Contributions to Transient Absorption of Ta$_{3}$N$_{5}$ Photoanodes
Johannes Dittloff, Lukas M. Wolz, Matthias U. Quintern et al. (+4 more)
Ta$_{3}$N$_{5}$ is among the most intensively studied photoanode materials for
solar-driven water oxidation, yet its performance often remains limited by short carrier lifetimes and defect mediated recombination. Although transient absorption spectroscopy is widely used to probe carrier dynamics in photoelectrodes, spectral assignments are frequently ambiguous due to overlapping contributions. Here, microsecond-to-second transient absorption of Ta$_{3}$N$_{5}$ thin films is combined with complementary optical spectroscopies to disentangle contributions from
lattice heating, electrostatics, and defect states. Photoreflectance reveals three critical points in the Ta$_{3}$N$_{5}$ band structure, including two anisotropic near-edge transitions at 2.14 eV and 2.27 eV and a higher-lying transition near 2.80 eV, all closely aligned with dominant transient absorption features. A previously unreported photo-induced absorption at 2.80 eV is attributed to pump-induced
lattice heating, while potential-dependent measurements reveal that near-edge bleach features arise from pump-induced band flattening and subsequent surface photovoltage relaxation. Fitting transient absorption spectra with independently measured thermal and electrostatic components enables quantification of both thermal and photovoltage dynamics, while the sub-bandgap response provides insight into the redistribution of defect charge states. Thus, this approach to quantifying thermal, electrostatic, and defect-mediated contributions to microsecond-to-second transient absorption provides broadly applicable insights into photoexcitation and relaxation mechanisms in functional
semiconductor photoelectrodes.
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All authors Johannes Dittloff, Lukas M. Wolz, Matthias U. Quintern, Laura I. Wagner, Matthias Kuhl, Johanna Eichhorn, Ian D. Sharp
Mesoscale and Nanoscale Physics
Observation of the Magnus Nonlinear Hall effect from Chiral Weyl Monopoles
Heda Zhang, Nikolai Peshcherenko, Ning Mao et al. (+4 more)
The nonlinear
Hall effect (NLHE) connects crystalline
symmetry to
quantum geometry, offering a probe of band
topology beyond linear transport. While most studies have focused on the Berry
curvature dipole in low-
symmetry crystals, mechanisms that directly probe Berry monopoles in higher-
symmetry chiral lattices remain unexplored. Here, we report the observations of the NLHE in the
chiral Weyl semimetal CoSi, a platform where the Berry
curvature dipole is
symmetry-forbidden. By employing focused ion beam-fabricated crossbar devices, we detect a robust second-harmonic
Hall voltage under zero
magnetic field, hosting all key signatures of the NLHE. Theoretical analysis attributes the nonlinear
Hall conductivity to skew scattering of self-rotating electron wave packets, whose chirality is dictated by the underlying band
topology, a process reminiscent of the classical Magnus effect. Furthermore, the NLHE signal exhibits a temperature-dependent sign reversal, and a strong, linearly field-dependent modulation that grows with carrier mobility, directly reflecting the topological Weyl nodes distribution near the
Fermi level. These findings establish CoSi as a platform for Berry monopole-driven nonlinear transport, demonstrating a skew-scattering route to topological nonlinear
Hall responses that bypasses conventional
symmetry constraints.
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All authors Heda Zhang, Nikolai Peshcherenko, Ning Mao, Nianlong Zou, Jiaqiang Yan, Claudia Felser, Yang Zhang
Mesoscale and Nanoscale Physics
Giant Spin Magnetization from Quantum Geometry in Altermagnets
Neelanjan Chakraborti, Sudeep Kumar Ghosh and Snehasish Nandy
Altermagnets host
spin-split band structures while exhibiting vanishing
equilibrium spin magnetization, making field-induced responses a direct probe of their
quantum geometry. A central question, in this regard, is which
quantum-
geometric mechanism can generate a linear
spin magnetization in centrosymmetric systems. Here we develop a unified framework based on a generalized
quantum geometric tensor that incorporates both momentum translations and
spin rotations of Bloch states, and decompose
spin magnetization into
equilibrium, electric-field-driven, and
magnetic-field-driven contributions. We show that inversion
symmetry forbids the linear electric-field response in centrosymmetric systems, while $C_n T$
symmetry further suppresses the
equilibrium contribution in altermagnets. Consequently, centrosymmetric altermagnets provide a particularly clean realization in which the
magnetic-field-induced
spin magnetization emerges as the only
symmetry-allowed linear
quantum-
geometric response. We demonstrate that this contribution originates entirely from the
spin-rotation
quantum metric, establishing it as the sole linear
quantum-
geometric mechanism in such systems. Using representative centrosymmetric altermagnets, including the $d$-wave compound $\mathrm{FeSb}_2$ and the $g$-wave compound $\mathrm{CrSb}$, we show that the
spin-rotation
quantum metric directly controls this response. Crucially, we predict a
giant linear
spin magnetization of order $10^{-2}\mu_B\,\mathrm{nm}^{-3}$ at
magnetic fields of $\sim 10\,\mathrm{mT}$, exceeding typical experimental values for conventional magnets by several orders of magnitude. Our results identify a universal
quantum geometric mechanism of
spin magnetization operative in centrosymmetric systems in general, and establish centrosymmetric altermagnets as an ideal platform for its experimental detection with potential applications in spintronics.
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Mesoscale and Nanoscale Physics
g-tensor Optimization in Ge/SiGe Quantum Dots
Aram Shojaei, Edmondo Valvo, Maximilian Rimbach-Russ et al. (+2 more)
Planar germanium
heterostructures hosting hole-
spin qubits are among the leading platforms for scalable
semiconductor-based
quantum computing. Yet, device performance is hindered by significant
quantum dot variability, which leads to uncertainty in
qubit energy levels and random orientations of the
spin quantization axis. Tailored control of the g-tensor offers a strategy to overcome these limitations and achieve more reliable
qubit operations. Here, we introduce a flexible optimization framework for engineering g-tensor properties. As a
benchmark, we numerically obtain the optimal reshaping of the out-of-plane potential in a SiGe-Ge-SiGe
quantum well to suppress the in-plane g-tensor components and realize the recently proposed gapless single-
spin qubit encoding. This reshaping is achieved through
heterostructure engineering, specifically by adjusting the silicon concentration within the
quantum well, though the framework remains readily adaptable to alternative design objectives. Our results provide practical design principles for improving the tunability of the
spin response, paving the way towards large-scale germanium-based
quantum computers.
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All authors Aram Shojaei, Edmondo Valvo, Maximilian Rimbach-Russ, Eliska Greplova, Ana Silva
Superconductivity
Local probing of superconductivity at oxide interfaces with atomic force microscopy
Dilek Yildiz, Sungmin Kim, Dengyu Yang et al. (+10 more)
Superconductivity in strontium titanate has remained enigmatic for more than 50 years. The LaAlO$_3$/SrTiO$_3$ (LAO/STO) heterointerface enables systematic dimensional confinement, from a two-dimensional electron gas to quasi-one-dimensional nanostructures, providing access to this
quantum state. Transport measurements in patterned devices reveal puzzling phenomena, including width-independent critical currents and anomalous pairing suggestive of one-dimensional behavior, but direct local probes of the patterned interface and its superconducting response have been lacking. Here we use ultralow-temperature non-contact atomic force microscopy, dissipation spectroscopy, and Kelvin probe force microscopy to locally probe signatures of superconductivity in patterned LAO/STO devices. Spatially resolved energy-dissipation measurements reveal superconducting signatures, with features confined in some devices to edge channels approximately 200 nm wide. Dissipation spectra exhibit a characteristic nonlinear bias dependence that provides a local diagnostic of superconductivity, consistent with the intermediate carrier-density regime near the superconducting dome, and persisting up to the critical field. These results establish atomic force microscopy as a local probe of superconductivity in patterned LAO/STO structures and provide a route to addressing longstanding questions about
quantum confinement and transport anomalies in correlated
oxide nanostructures.
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All authors Dilek Yildiz, Sungmin Kim, Dengyu Yang, Muqing Yu, Kyoungjun Lee, Ruiqi Sun, En-Min Shih, Steven R. Blankenship, Patrick Irvin, Franz J. Giessibl, Chang-Beom Eom, Jeremy Levy, Joseph A. Stroscio
Materials Science
Anharmonic phonon coupling enabled by local inversion symmetry breaking at domain walls in ferroelastics
Seyyed Jabbar Mousavi, Vivek Unikandanunni, Niccolo Sellati et al. (+7 more)
In ferroelastic materials, spontaneous
symmetry breaking leads to the formation of twin domains. Although the bulk
crystal typically remains centrosymmetric, inversion
symmetry can be locally broken at the domain walls, potentially changing
phonon selection rules and enabling local anharmonic
phonon coupling. Here we report direct evidence of such anharmonic coupling in ferroelastic LaAlO$_3$ using two-dimensional Raman-terahertz spectroscopy. We attribute the cross-peaks observed in the two-dimensional spectra to both mechanical and electrical anharmonicity between the $A_{1g}$ Raman-active
phonon and the $E_g$
phonon, which acquires finite infrared activity through local inversion
symmetry breaking at ferroelastic domain walls. These findings provide new insight into the complex
lattice dynamics of ferroelastic materials and highlight the potential of two-dimensional Raman-terahertz spectroscopy to uncover subtle
symmetry breaking through the detection of intrinsically weak anharmonic signals.
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All authors Seyyed Jabbar Mousavi, Vivek Unikandanunni, Niccolo Sellati, Paolo Barone, Martina Basini, Steven L. Johnson, Andrey Shalit, Peter Hamm, Mattia Udina, Thomas Feurer
Strongly Correlated Electrons
Dimensionality-Driven Electronic and Orbital Transitions Mediating Interfacial Magnetism in LaNiO3/CaMnO3 Observed In Situ
B-A. Courchene, A. Hampel, S. Beck et al. (+16 more)
Emergent
magnetic states at
oxide interfaces arise from the interplay of charge transfer, orbital reconstruction, and dimensional confinement, offering a route to engineered correlated-electron behavior in nanoscale spintronic materials. Here, we combine in situ synthesis, polarization-dependent angle-resolved photoelectron spectroscopy, X-ray
magnetic circular dichroism, and first-principles electronic-structure calculations to investigate LaNiO3/CaMnO3 superlattices. We show that reducing the LaNiO3 thickness drives a metal-
insulator transition accompanied by loss of electronic coherence and an orbital-polarization crossover in the ultrathin limit. These changes weaken charge transfer across the interface and suppress the interfacial Mn
magnetic moment in CaMnO3, revealing that the emergent
ferromagnetic state is directly governed by electronic confinement in LaNiO3. The insulating state and orbital reconstruction are reproduced by density functional theory combined with dynamical mean-field theory. Together, these results establish a direct and tunable coupling among electronic, orbital, and
magnetic degrees of freedom in
oxide heterostructures.
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All authors B-A. Courchene, A. Hampel, S. Beck, J. R. Paudel, J. D. Grassi, L. A. Lapinski, A. M. Derrico, M. Terilli, M. Kareev, C. Klewe, A. Gloskovskii, C. Schlueter, S. K. Chaluvadi, F. Mazzola, I. Vobornik, P. Orgiani, J. Chakhalian, A. J. Millis, A. X. Gray
Mesoscale and Nanoscale Physics
Deep Strong light-matter Coupling in 3D Kane Fermions
Dmitriy Yavorskiy, David Hagenmuller, Noureddine Charrouj et al. (+5 more)
Deep strong light-matter coupling represents an extreme non-perturbative regime of
quantum electrodynamics, in which the interaction strength exceeds the bare frequencies of the uncoupled systems. The ground state features strong
quantum correlations between
photons and matter excitations, and new cavity-driven phase transitions are expected to occur. Whether a superradiant
quantum phase transition, marked by spontaneous dipole ordering and
photon condensation, is possible has remained a long-standing and controversial question. Such phenomena have been proposed to arise in exotic electronic systems hosting Dirac and Kane
fermions, owing to the formal absence of an $A^2$ term in their low-energy
Hamiltonian. Here we exploit the ultralow effective mass of Kane
fermions to realise Landau polaritons in a bulk mercury cadmium telluride layer coupled to a Fabry-Perot resonator. Using thermally tunable carrier density, we continuously tune the coupling from the weak to the deep-strong regime, achieving a record normalised coupling ratio exceeding 1.6 above room temperature. The measured polariton spectra are in excellent agreement with a rigorous, gauge-invariant microscopic theory. Despite the nonlinear Landau level structure of relativistic Kane
fermions, we show that a diamagnetic $A^2$ term naturally emerges and precludes a superradiant phase transition. These results resolve the long-standing controversy surrounding cavity
quantum electrodynamics of relativistic-like matter systems, extend deep-strong-coupling physics to Kane
fermions, and open new opportunities for polaritonic
semiconductor devices operating in extreme light-matter coupling regimes.
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All authors Dmitriy Yavorskiy, David Hagenmuller, Noureddine Charrouj, Yurii Ivonyak, Alexander Kazakov, Yanko Todorov, Wojciech Knap, Marcin Bialek
Other Condensed Matter
A nanoionic diode: Equilibrium rectifying junction enabling large and stable resistance variations
Chuanlian Xiao and Joachim Maier
We report on a new type of rectifier which is in full contact
equilibrium and thus, if down-sized to the nanoscale, shows no drift even if exposed to elevated temperatures and/or extreme waiting times. This is in contrast to existing diodes which rely on frozen doping profiles and are hence non-
equilibrium devices. Our rectifiers are related to Schottky diodes but employ "dopants" whose mobilities are high enough to follow the electrical field quickly but low enough to not compete with the electrons in terms of conductivities. In order to realize such a device based on mixed conductors, we use nanosized TiO2 films on Ru as a substrate which can store Li at the interface according to a job-sharing mechanism (Li-ions on the TiO2 side, electrons on the Ru side). The excellent functionality of this nanoionic device is demonstrated (e.g., current on-off ratio can exceed 6-7 orders of magnitude) and the additional advantages stressed (such as ease of preparation and tuning the characteristics electrochemically).
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Statistical Mechanics
Order by disorder up to arbitrarily high temperature
Ravish Mehta
We prove that a class of classical
lattice models on $\mathbb{Z}^d$ ($d \geq 2$) with on-site space $\mathbb{N}_0$ and nearest neighbour interaction, exhibits long-range checkerboard order at sufficiently high temperature. The ordering mechanism is purely entropic. The class of models contains the recently introduced model of Han–Huang–Komargodski–Lucas–Popov (arXiv:2503.22789), by which our work is inspired. The proof uses Pirogov–Sinai theory and the key input is a Peierls bound.
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Mesoscale and Nanoscale Physics
Compressibility of micromagnetic solutions in tensor train format
Thierry Valet and Nicolas Vukadinovic
For three-dimensional (3D)
magnetic objects with linear size $L$ exceeding a few exchange lengths, the micromagnetic state exhibits pronounced informational sparsity: low-dimensional, high-gradient regions (e.g., domain walls) coexist with near-uniformly magnetized volumetric domains. Because standard micromagnetic simulation methods discretize the magnetization on near-uniform 3D grids with linear
cell size $a$, they cannot take advantage of this sparsity. The computational problem scales as $\sim L^3$ and $\sim (1/a)^3$. In this Letter, we establish that direct tensor-train (TT) representations overcome these poor scalings by exploiting the spatial sparsity optimally, while preserving accuracy in a controlled way. Focusing on representative flux-closure configurations in soft-
magnetic rectangular prisms, in the near-micrometer regime, we demonstrate that the parameter count of TT-compressed micromagnetic data scales approximately as $L^{1.8}$ and $(1/a)^{1.2}$. Hence the relative advantage over dense discretizations rapidly grows with the problem size and refinement level. These first results provide a strong motivation for future developments of micromagnetic solvers in TT format which could transcend the limitations of traditional simulators, with far reaching potential impacts on fundamental research and technology development.
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Disordered Systems and Neural Networks
The Synergistic Route to Stretched Criticality
Lorenzo Lucarini, Sandro Meloni and Pablo Villegas
Griffiths phases are typically associated with quenched disorder, while frustration gives rise to multistability and
spin-glass behavior. Whether extended criticality can arise in other contexts remains an open question. Here, we show that synergistic interactions provide a distinct route to non-conventional critical phenomena. By combining spreading mechanisms that reinforce activity through complementary pathways, we uncover a broad distribution of relaxation rates, leading to Griffiths-like slow dynamics and extended criticality. We demonstrate that this mechanism is robust across networks and emerges both in systems with explicit higher-order interactions and in purely pairwise systems with nonlinear dynamical rules.
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Disordered Systems and Neural Networks
Discontinuous BBP transitions
Dario Bocchi, Giulio Biroli, Chiara Cammarota et al. (+1 more)
The Baik-Ben Arous-Peche (BBP) transition sets fundamental limits for detecting low-rank structure in noisy high-dimensional data and underlies a wide range of spectral methods in many fields from physics to statistics and data sciences. In standard settings, this transition is continuous, implying that signal recovery emerges gradually above a sharp threshold. We show that BBP transitions can instead be discontinuous in very general settings and provide a full theory of this phenomenon. When the
eigenvalue density vanishes faster than linearly at the spectral edge, the overlap between the leading eigenvector and the signal jumps discontinuously at the critical point. We study this mechanism in deformed Gaussian and reweighted Wishart ensembles. We analyze in detail the finite-size effects, which play a central and qualitatively new role in the discontinuous BBP transition. Unlike the continuous BBP transition, we establish the existence of an extended pre-critical region where informative eigenvectors emerge well before the asymptotic threshold. The main consequence-and difference from the continuous BBP transition-is that signal recovery can occur at significantly lower signal-to-noise ratio and it is accompanied by strong sample-to-sample variability. Our results show the relevance and the novelty of the discontinuous BBP transition, and highlight the practical implications for signal detection.
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All authors Dario Bocchi, Giulio Biroli, Chiara Cammarota, Federico Ricci-Tersenghi
Soft Condensed Matter
Propelling catalytic structures using active biomolecular condensates
Benjamin Sorkin and Ned S. Wingreen
Living systems routinely consume energy to achieve motility, often using intricate biomolecular machinery. In this work, we demonstrate that active biomolecular condensates enable the self-propulsion of micron-sized objects in otherwise homogeneous, isotropic, and autonomous environments. Our proposed minimal mechanism consists of phase-separating
proteins, the enzymes that passivate them, and complementary enzymes isotropically anchored to a spherical-colloid surface which reactivate the
proteins. This passivation-activation cycle gives rise to a
symmetry breaking wherein a single biocondensate is stabilized near the colloid surface, which in turn exerts a repulsive force on the colloid. We numerically demonstrate that this mechanism can propel micron-sized colloids at speeds of up to a hundred microns per second. This propulsion mode is strongly resistant to Brownian fluctuations and external forces. Propulsion mechanisms based on biomolecular condensates may thus offer a complementary, motor-free route to biological transport.
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Mesoscale and Nanoscale Physics
Theory of quantum decoherence in macroscopic topological insulators
Xian-Peng Zhang, Yan-Qing Feng, Wanxiang Feng et al. (+1 more)
Quantum decoherence-the loss of
quantum coherence due to interactions with an environment-plays a central role in
quantum transport, and controlling this ubiquitous yet inevitable phenomenon is essential for practical
quantum technologies. Despite its importance, the microscopic mechanisms of decoherence in infinite-size topological
insulators remain poorly understood. Here, we develop a comprehensive theory that quantitatively investigates how
quantum decoherence shapes the
quantum spin Hall effect in macroscopic topological
insulators, and reveal that decoherence-induced corrections scale quadratically with
impurity density. Besides, we uncover a previously unidentified mechanism of the extrinsic
spin Hall effect: a second-order skew-scattering process intrinsically tied to
quantum decoherence-fundamentally distinct from, yet substantially stronger than, the conventional third-order skew-scattering mechanism. Furthermore, we predict a new scaling law in which the decoherence-induced
spin Hall conductivity scales quadratically with the longitudinal conductivity, providing a clear experimental signature of decoherence effects. Our results establish the essential role of decoherence in
quantum transport of topological
insulators and reveal that macroscopic topological
insulators offer a promising platform for next-generation spintronic applications.
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All authors Xian-Peng Zhang, Yan-Qing Feng, Wanxiang Feng, Yugui Yao
Soft Condensed Matter · Cross-listed from q-bio.BM
Complex Effects of Salt on Small-Angle X-ray Scattering of BSA Originate From the Interplay of Ions and Hydration Water
Anshika Dhiman, Sanbo Qin and Huan-Xiang Zhou
Salts are an integral part of the environment for living systems and, therefore, understanding their effects on
proteins and other biomolecules is of fundamental interest. Small-angle X-ray scattering (SAXS) of
protein solutions can provide valuable information on salt effects, but extracting this information has been a significant challenge. For example, SAXS data of bovine serum albumin (BSA) at various salt concentrations were fit to three different spherical models. Here we combined the newly developed FMAPIq approach with explicit-solvent all-atom molecular dynamics simulations to show that the complex effects of salt on the SAXS of BSA originate from the interplay of ions and hydration water, leading to a general picture of
protein-ion-water interactions.
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Materials Science
Fragment-Constrained Charge Equilibration for Charge-Aware Machine Learning Potentials at Electrochemical Interfaces
Akhil Reddy Peeketi, Blas P Uberuaga and Travis E Jones
Predictive simulation of electrochemical interfaces requires atomistic models that capture reactive bond rearrangements, long-range electrostatics, and charge distributions reflecting the electronic distinctness of electrode and electrolyte. Existing charge-aware machine-learned interatomic potentials (MLIPs) built on global charge equilibration (QEq) settle electrode and electrolyte at a common electrochemical potential, leaving no room for the interfacial gradient that the double layer requires and admitting spurious charge transfer between electronically disconnected regions. Per-fragment charge equilibration is the established remedy in classical molecular dynamics, but reliance on predefined molecular
topology has confined it to non-reactive systems. We lift this restriction by making fragment identification itself a differentiable function of atomic geometry, yielding soft fragment-constrained charge equilibration (Soft-FQEq) – a solver layer that restores fragment-resolved charge conservation in reactive MLIPs. The layer consumes four scalar MLP readouts from a shared atomic-feature network – per-atom electronegativity, source charge, short-range energy, and a soft bond connectivity – and returns equilibrated charges together with per-fragment chemical potentials. We implement Soft-FQEq as an extension of the hippynn framework on a HIP-NN feature network and train it on DFT energies, forces, and DDEC6 charges for IrO2/H2O/Na+/ClO4- interfaces. The trained model recovers a clear electrode-to-electrolyte gradient in the per-atom electrochemical potential. With the same trained weights but the fragment-constrained solver replaced by global QEq at inference, this gradient collapses to an essentially uniform profile, directly showing that the gradient cannot be sustained within global QEq while the fragment formulation recovers it.
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Statistical Mechanics
Discrete Lattice Models for Interface Growth on a Complete Graph
J. M. Marcos, J. J. Meléndez, R. Cuerno et al. (+1 more)
We investigate the behavior of discrete interface growth models belonging to the Edwards–Wilkinson (EW) and Kardar–Parisi–Zhang (KPZ) universality classes, when defined on a complete graph, a
topology commonly used to probe the infinite-dimensional limit of statistical mechanical systems. Our aim is to assess to what extent discrete
lattice models reproduce the behavior of their corresponding continuum equations in this highly connected setting. After assessing the trivial behavior shown by some well known cases (like random deposition with surface relaxation or the etching model), we focus on two paradigmatic models associated with the KPZ universality class, the Restricted Solid-on-Solid (RSOS) and Ballistic Deposition (BD) models, and assess non-trivial behavior through several observables including the roughness, height fluctuations, power spectra, and two-time autocorrelation functions. Still, despite similarities with continuum equations, important differences arise in the fluctuations and long-time dynamics. In both discrete models the rescaled height fluctuations display a pronounced left tail, indicating the presence of lagging nodes. While the RSOS model exhibits a saturation roughness that decreases with system size, similarly to the EW and KPZ equations, the BD model exhibits a saturation roughness that increases with system size and an additional ultrafast growth regime, placing it outside the KPZ universality class on a complete graph.
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All authors J. M. Marcos, J. J. Meléndez, R. Cuerno, J. J. Ruiz-Lorenzo