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Physics Condensed Matter

cond-mat Watch Condensed Matter

Tuesday 3 February

Materials Science

Electron chirality and hydrodynamic helicity: Analysis in the atomic limit

Tatsuya Miki, Yuta Kakinuma, Masato Senami et al. (+4 more)

Electron chirality has been proposed as a microscopic quantity that characterizes electronic handedness, yet its underlying control parameter has not been clearly identified. Furthermore, its applicability is limited to systems with spin-orbit coupling, which motivates the need for alternative measures of chirality. In this work, we explore two complementary measures of chirality: electron chirality and hydrodynamic helicity. By analyzing a minimal atomic model under chiral crystal fields, we clarify how the interplay among crystal fields, spin-orbit coupling, and electron correlation gives rise to non-zero values of chirality measures. Although electron chirality increases with both spin-orbit coupling and chiral crystal field strength, the dependence on these two factors is highly non-trivial. Particularly, when the chiral crystal field is varied continuously and the energy levels approach quasidegenerate points, the electron chirality is insensitive to spin-orbit coupling, resulting in a remarkable enhancement of chirality. In contrast, the hydrodynamic helicity, defined as a two-body pseudoscalar quantity, remains non-zero even without spin-orbit coupling, originating from electron-electron interactions. Perturbative analysis reveals distinct symmetry selection rules governing the two quantities. Our results provide fundamental insight into the origin of chiralities in electronic systems.
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All authors Tatsuya Miki, Yuta Kakinuma, Masato Senami, Masahiro Fukuda, Michi-To Suzuki, Hiroaki Ikeda, Shintaro Hoshino

Statistical Mechanics · Cross-listed from math-ph

Equilibrium measures for higher dimensional rotationally symmetric Riesz gases

Sung-Soo Byun, Peter J. Forrester, Satya N. Majumdar et al. (+1 more)

We study equilibrium measures for Riesz gases in dimension $d$ with pairwise interaction kernel $|x-y|^{-s}$, subject to radially symmetric external fields. We characterise broad classes of confining potentials for which the equilibrium measure is supported on the unit ball and admits an explicit density. Our main contribution is a converse construction: starting from a prescribed radially symmetric equilibrium density given as a power series in the squared radius, we determine the associated external potential and establish the corresponding Euler-Lagrange variational conditions. A key ingredient in the proof is an identity between two ${}_3F_2$ hypergeometric functions evaluated at unit argument, which is of independent interest. As applications, we identify the external potentials corresponding to equilibrium densities proportional to $(1-|x|^2)^\alpha$, $\alpha>-1$, and show that these potentials can be expressed in terms of Gauss hypergeometric functions ${}_2F_1$, reducing to polynomials for special values of $\alpha$. We also determine the equilibrium measure associated with purely power-type external potentials, often referred to as Freud or Mittag–Leffler potentials in the context of log gases, for which the equilibrium density admits an explicit ${}_2F_1$ representation. Furthermore, we apply our framework to a Coulomb gas in dimension $d+1$ confined by a harmonic potential to the half-space. We derive a necessary condition under which the equilibrium measure is fully supported on the boundary hyperplane of dimension $d$, with the induced density corresponding to that of a Riesz gas with exponent $s=d-1$.
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All authors Sung-Soo Byun, Peter J. Forrester, Satya N. Majumdar, Gregory Schehr

Mesoscale and Nanoscale Physics · Cross-listed from physics.optics

Plasmonic Spin Meron Lattices with Height-Sensitive Topology Evolution

Anand Hegde, Komal Gupta and Chen-Bin Huang

We demonstrate height-controlled topological switching of plasmonic spin meron lattices above a metallic square coupling structure under circularly polarized illumination. Near the interface, an evanescent surface plasmon polariton (SPP) channel yields a Néel-type meron lattice with $\pm\frac{1}{2}$ like effective site charges. At larger heights, diffracted fields from the square edges dominate and convert the lattice into a Bloch-type configuration. Over a range of intermediate heights, crossover between the evanescent SPP and edge diffraction gives rise to rich rapid topology evolutions. The switching is accompanied by nucleation of off-boundary vortex-anti vortex pairs in the in-plane spin phase, producing height-dependent fractional site charges. Our findings are analytically formulated by linear superposition of SPPs in the plasmonic regime and Stratton-Chu model in diffraction regime and confirmed via full-wave finite-difference time-domain simulations.
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Disordered Systems and Neural Networks

Physics-inspired transformer quantum states via latent imaginary-time evolution

Kimihiro Yamazaki, Itsushi Sakata, Takuya Konishi et al. (+1 more)

Neural quantum states (NQS) are powerful ansätze in the variational Monte Carlo framework, yet their architectures are often treated as black boxes. We propose a physically transparent framework in which NQS are treated as neural approximations to latent imaginary-time evolution. This viewpoint suggests that standard Transformer-based NQS (TQS) architectures correspond to physically unmotivated effective Hamiltonians dependent on imaginary time in a latent space. Building on this interpretation, we introduce physics-inspired transformer quantum states (PITQS), which enforce a static effective Hamiltonian by sharing weights across layers and improve propagation accuracy via Trotter-Suzuki decompositions without increasing the number of variational parameters. For the frustrated $J_1$-$J_2$ Heisenberg model, our ansätze achieve accuracies comparable to or exceeding state-of-the-art TQS while using substantially fewer variational parameters. This study demonstrates that reinterpreting the deep network structure as a latent cooling process enables a more physically grounded, systematic, and compact design, thereby bridging the gap between black-box expressivity and physically transparent construction.
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All authors Kimihiro Yamazaki, Itsushi Sakata, Takuya Konishi, Yoshinobu Kawahara

Mesoscale and Nanoscale Physics · Cross-listed from physics.optics

Ultralow radiative heat flux by Anderson localization in quasiperiodic plasmonic chains

Yizhi Hu, Kun Yan, Wei-Hua Xiao et al. (+1 more)

Anderson localization, arising from wave interference in disordered systems, profoundly hinders energy transport, yet its impact on radiative heat flux in many-body thermophotonic systems remains unclear. Here, we demonstrate a three-order-of-magnitude suppression of radiative heat transfer, resulting in ultralow radiative heat transfer, in a one-dimensional quasiperiodic chain of plasmonic nanoparticles. This suppression in radiative heat transfer is directly correlated with mode localization, as revealed by the mode decomposition of the transmission coefficient, which serves as evidence of Anderson localization. Furthermore, we elucidate the dependence of radiative thermal conductance reduction on interparticle spacing and material damping rates, uncovering the interplay between intrinsic Ohmic losses, mode localization, and long-range many-body interactions. Our findings advance the understanding of wave-mediated thermal transport in disordered photonic structures and suggest strategies for tailoring nanoscale heat management via engineered disorder.
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All authors Yizhi Hu, Kun Yan, Wei-Hua Xiao, Xiaobin Chen

Superconductivity

Tuning current flow in superconducting thin film strips by control wires. Applications to single photon detectors and diodes

Alex Gurevich

It is shown that integration of a thin film superconducting strip with current-carrying control wires enables one to engineer a profile of supercurrent density $J(x)$ with no current crowding at the edges of a strip wider than the magnetic Pearl length $\Lambda$. Moreover, $J(x)$ in a strip can be tuned by control wires to produce an inverted $J(x)$ profile with dips at the edges to mitigate current crowding at lithographic defects and block premature penetration of vortices. These conclusions are corroborated by calculations of $J(x)$ in a thin strip coupled inductively with side control wires or in bilayer strip structures by solving the London and Ginzburg Landau equations in the thin film Pearl limit. Thermally-activated penetration of vortices from the edges and unbinding of vortex-antivortex pairs in inverted $J(x)$ profiles are evaluated. It is shown that these structures can be used to develop single-photon strip detectors much wider than $\Lambda$. Such detectors can be tuned in situ by varying current in control wires to reach the ultimate photon sensitivity limited by unbinding of vortex-antivortex pairs. The structures considered here exhibit a non-reciprocal current response and behave as superconducting diodes. They can also be used to study the physics of vortex matter in thin films not masked by penetration of vortices from the edges.
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Materials Science

Ferroelectric dynamic-field-driven nucleation and growth model for predictive materials-to-circuit co-design

Yi Liang, Soohyeon Kim, Tony Chiang et al. (+8 more)

Real ferroelectric devices operate under mixed and distorted time-varying voltages, yet the standard nucleation-growth frameworks used to interpret ferroelectric switching - most notably the Kolmogorov-Avrami-Ishibashi (KAI) and nucleation-limited switching models (NLS) - are derived under the critically limiting assumption of a constant electric field. Thus, the prevailing interpretation of ferroelectric switching dynamics fails under real operating conditions. Here we introduce a compact dynamic-field-driven nucleation and growth (DFNG) model that enables quantitative fits to switching transients across multiple ferroelectric materials to extract time-varying domain wall velocity and growth dimensionality, even under arbitrary voltage waveform. This capability then motivates its use in device modeling under complex signals spanning disparate time and frequency scales. Coupling the compact model to application-related waveforms and circuit-level simulation platform facilitates a predictive materials-circuit co-design framework by linking nucleation and growth parameters to memory window, disturb error, speed, and energy dissipation for next-generation ferroelectric technologies.
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All authors Yi Liang, Soohyeon Kim, Tony Chiang, Megan K. Lenox, Ian Mercer, John J. Plombon, Jon-Paul Maria, Jon F. Ihlefeld, Wenhao Sun, Wei Lu, John T. Heron

Statistical Mechanics

Violation of local equilibrium thermodynamics in one-dimensional Hamiltonian-Potts model

Hitomi Endo and Michikazu Kobayashi

We investigate nonequilibrium phase coexistence associated with a first-order phase transition by numerically studying a one-dimensional Hamiltonian-Potts model with fractional spatial derivatives. The fractional derivative is introduced so as to reproduce the low-wave-number density of states of the standard two-dimensional model, allowing phase coexistence to occur in a minimal one-dimensional setting under steady heat conduction. By imposing a constant heat flux through boundary heat baths, we observe the stable coexistence of ordered and disordered phases separated by a stationary interface. We find that the temperature at the interface systematically deviates from the equilibrium transition temperature, demonstrating a clear violation of the local equilibrium description. This deviation indicates that equilibrium metastable states can be stabilized and controlled by a steady heat current. Furthermore, the interface temperature obtained in our simulations is in quantitative agreement with the prediction of global thermodynamics for nonequilibrium steady states. These results confirm that the breakdown of local equilibrium and the stabilization of metastable states are intrinsic features of nonequilibrium first-order phase transitions, independent of spatial dimensionality. Our study thus provides a minimal and controlled numerical model for exploring the fundamental limits of thermodynamic descriptions in nonequilibrium steady states.
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Strongly Correlated Electrons

Effect of magnetic field on whirling-anti-whirling order in icosahedral-quasicrystal approximant

Shinji Watanabe and Tatsuya Iwasaki

Recent neutron measurement in the icosahedral quasicrystal approximant Au-SM-Tb (SM=Al, Ga) has revealed unique noncollinear magnetic order “whirling-anti-whirling states”. Here, we report theoretical analysis on the magnetic-field-direction dependence on the whirling-anti-whirling order in the 1/1 approximant crystal. By performing exact-diagonalization calculation for the effective model taking into account the uniaxial magnetic anisotropy arising from the crystalline electric field, we show the metamagnetic transition takes place simultaneously with the topological transition under the magnetic field along the (111) direction. After the metamagnetic transition, the emergent fictious magnetic field induced by the chirality of noncoplanar magnetic moments appears, the analysis of which concludes that the topological Hall effect is expected to be observed in the electrical conductivity $\sigma_{xy}$ and $\sigma_{yz}$ for the applied field direction from (111) to (001).
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Materials Science

Highly controllable switching pathways in multiferroic GdMn$_2$O$_5$

M. Ryzhkov, A. Granero, J. Wettstein et al. (+7 more)

Controlling magnetic moments using electric fields remains a central challenge in spintronics. Multiferroics, where magnetic and electric orders coexist, may be a natural platform for such control, but progress has been limited because interactions between these orders are typically too weak to overcome the energy barriers between magnetic states. A recently demonstrated topologically protected switching circumvents this limitation by exploiting reduced barriers at a phase transition. Nevertheless, the conditions enabling this phenomenon remain elusive and electric field control is poorly understood. Here, we experimentally map the energy landscape by tracking transitions in GdMn$_2$O$_5$ under combined electric and magnetic fields. The experiments reveal that the switching pathways can be controlled by the electric field. A minimal theoretical model captures the observed behavior, identifies the parameter space where switching paths are sensitive to small perturbations and reveals design principles for implementing topological switching in other materials.
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All authors M. Ryzhkov, A. Granero, J. Wettstein, Anna Pimenov, X. Wang, L. Ponet, S.-W. Cheong, M. Mostovoy, Andrei Pimenov, S. Artyukhin

Monday 2 February

Materials Science · Cross-listed from physics.app-ph

The effect of graphene orientation on permeability and corrosion initiation under composite coatings

Majid Shaker, Erfan Salahinejad, Weiqi Cao et al. (+3 more)

The unique anisotropic properties of graphene, particularly impermeability, have made it a promising candidate for further advances in corrosion prevention applications. Despite the large number of experimental works divulging the use of graphene in anticorrosion coatings, there is no report on the numerical modelling and simulation of the relationships between the orientation of graphene sheets in composite coatings and the introduced corrosion protection efficiency, to our knowledge. Herein, it is tried to model the influence of the orientation of graphene sheets dispersed in organic coatings on the diffusivity and flux of corrosive substances besides the corrosion initiation time of the protected substrates. To discover the relationship between the graphene orientation and corrosion-related phenomena, this study introduces a novel model consisting of a trigonometric factor named unprotected projected surface area proportion, which calculates corrosion-related parameters based on the principal Fick's laws. The model reveals that the decrease in the angle between graphene sheets and the substrate is highly beneficial for postponing the corrosion onset. It is accordingly estimated that a mismatch angle of 10 degree can slow down the diffusion process significantly and delay the corrosion initiation by around 65 times in a 100 micron thick epoxy/graphene composite coating in comparison to the counterpart with the perpendicular alignment. The predicted corrosion parameters were in a good agreement with the experimental data, indicating the merit of the proposed model. Thus, this model can be further employed as the fundamental of future research on the optimum graphene orientation in anticorrosion composite coatings.
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All authors Majid Shaker, Erfan Salahinejad, Weiqi Cao, Xiaomin Meng, Vahdat Zahedi Asl, Qi Ge

Mesoscale and Nanoscale Physics

Anomalous Non-Hermitian Topological Anderson Insulator

Mina Ren, Xi Shi, Haitao Jiang et al. (+3 more)

Strong disorder drives conventional Hermitian systems into Anderson insulating states, suppressing all topological phases. Here, we unveil symmetry-protected, anomalous topological phases in the strong disorder limit of a non-Hermitian system, characterized by a scale-invariant merging of zero-energy modes. Using the maximally symmetric Jx lattice as an ideal platform and introducing specifically engineered (ABBA-type) symmetry-preserving non-Hermitian disorder, we observe a sequence of disorder-induced phase transitions: from a trivial insulator into and through a non-Hermitian topological Anderson insulator (TAI) phase, culminating in a stable anomalous non-Hermitian TAI phase characterized by a quantized polarization P_x \approx 0.25. Within this anomalous phase protected by the mobility gap, the zero-energy modes exhibit a distinct (N/2)-mode coalescence that scales with system size. Our findings demonstrate that non-Hermitian disorder engineered to preserve symmetry can induce and protect novel topological order inaccessible to conventional Hermitian disorder, thereby advancing the fundamental understanding of topological phenomena mediated by the interplay of disorder and non-Hermiticity.
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All authors Mina Ren, Xi Shi, Haitao Jiang, Feng Liu, Hong Chen, Yong Sun

Mesoscale and Nanoscale Physics

Automated Spin Readout Signal Analysis Using U-Net with Variable-Length Traces and Experimental Noise

Yui Muto, Motoya Shinozaki, Hideaki Yuta et al. (+5 more)

Single-shot spin-state discrimination is essential for semiconductor spin qubits, but conventional threshold-based analysis of spin readout traces becomes unreliable under noisy conditions. Although recent neural-network-based methods improve robustness against experimental noise, they are sensitive to training conditions, restricted to fixed-length inputs, and limited to trace-level outputs without explicit temporal localization of transition events. In this work, we apply a U-Net architecture to spin readout signal analysis by formulating transition-event detection as a point-wise segmentation task in one-dimensional time-series data. The fully convolutional structure enables direct processing of variable-length traces. Point-wise and sample-wise evaluations demonstrate low readout error rates and high classification accuracy without retraining. The proposed method generalizes well to previously-unseen trace lengths and experimental non-Gaussian noise, outperforming a conventional threshold-based approach and providing a robust and practical solution for automated spin readout signal analysis.
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All authors Yui Muto, Motoya Shinozaki, Hideaki Yuta, Tatsuo Tsuzuki, Kotaro Taga, Akira Oiwa, Takafumi Fujita, Tomohiro Otsuka

Statistical Mechanics

Quantum phase transition in transverse-field Ising model on Sierpiński gasket lattice

Tymoteusz Braciszewski, Oliwier Urbański and Piotr Tomczak

We investigate the quantum phase transition in the transverse-field Ising model on the Sierpiński gasket using finite-size scaling (FSS) and numerical renormalization group (NRG). Since next generations of the fractal lattice contain exponentially more spins, which in turn increase exponentially the Hilbert space dimension, we challenge and prove usefulness of small systems in FSS. We identified a quantum critical point at $\lambda_c \approx 2.63 - 2.93$, with critical exponents $ \nu \approx 0.64 - 0.71, \beta \approx 0.30, \gamma \approx 1.67$ and $z \approx 1.33$. The numerical renormalization group method produced results consistent with finite-size scaling approach ($\lambda_c = 2.766$$, \beta = 0.316$), supporting our findings. Compared to the values reported so far in the literature, critical field is in a strong disagreement, while exponents are generally similar excluding $\beta$ and $\gamma$. However, it should be noted, that the lattice investigated in previous works is different from ours, while the latter is in our opinion the standard Sierpiński gasket.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.optics

Time-Resolved dynamics of semiconductor nanolaser via four-wave mixing gating

Federico Monti, Guilhem Madiot, Giuseppe Modica et al. (+5 more)

We experimentally demonstrate the direct time-domain characterization of photonic-crystal nanolasers at telecom wavelengths using a nonlinear optical gating technique based on four-wave mixing. This approach enables the temporal characterization of the ultrafast emission dynamics under short-pulse excitation with picosecond time resolution. When a weak continuous-wave component is added to the pulsed pump, the emission becomes deterministic and the build-up time is considerably reduced. The difference between purely pulsed and hybrid excitation regimes points to the influence of pulse-to-pulse timing fluctuations. To elucidate this effect, we perform Langevin-based simulations that reproduce the experimentally observed broadening and confirm that time jitter, originating from spontaneous-emission noise near threshold, dominates the temporal dispersion. These results establish four-wave-mixing gating as a powerful method to probe nanolaser dynamics with picosecond precision.
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All authors Federico Monti, Guilhem Madiot, Giuseppe Modica, Grégoire Beaudoin, Konstantinos Pantzas, Isabelle Sagnes, Alejandro M. Yacomotti, Fabrice Raineri

Mesoscale and Nanoscale Physics

Switching Characteristics of Electrically Connected Stochastically Actuated Magnetic Tunnel Junction Nanopillars

Dairong Chen, Ahmed Sidi El Valli, Jonathan Z. Sun et al. (+3 more)

We investigate the stochastic dynamics of nanoscale perpendicular magnetic tunnel junctions (pMTJs) and the correlations that arise when they are electrically coupled. Individual junctions exhibit thermally activated spin-transfer torque switching with transition probabilities that are well described by a Poisson process. When two junctions are connected in parallel, circuit-mediated redistribution of voltages that occurs in real time as the junction resistances change leads to correlated switching behavior. A minimal stochastic model based on single-junction statistical switching properties and Kirchhoff's laws captures the coupled switching probabilities, while a Markov-chain formalism describes nonequilibrium steady states under multi-pulse driving. Further, these circuit-mediated interactions can be mapped onto the parameters of an Ising Hamiltonian, providing an interpretation in terms of effective spin-spin interactions. Our results demonstrate how simple electrical connections can generate Ising-like couplings and tunable stochastic dynamics in nanoscale magnets.
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All authors Dairong Chen, Ahmed Sidi El Valli, Jonathan Z. Sun, Flaviano Morone, Dries Sels, Andrew D. Kent

Materials Science

Origin of donor compensation in monoclinic (Al$_x$Ga$_{1{\rm -}x})_2$O$_3$ alloys

Sierra Seacat and Hartwin Peelaers

(Al$_x$Ga$_{1{\rm -}x})_2$O$_3$ alloys are frequently used in heterostructures with monoclinic Ga$_2$O$_3$, resulting in a large conduction-band offset, which leads to charge carrier confinement, a property that is desirable for device applications. However, when (Al$_x$Ga$_{1{\rm -}x})_2$O$_3$ alloys are $n$-type doped with Si, the most efficient shallow donor, there is a significant reduction in the number of charge carriers when the Al content of the alloys is greater than 26%, rendering intentional doping ineffective. Here we show that this compensation is due to cation vacancies forming in response to donor doping. We use density functional theory with the HSE06 hybrid functional to study cation vacancies in monoclinic AlGaO$_3$ and monoclinic Al$_2$O$_3$. We find that vacancies prefer to occupy split-vacancy configurations, similar to vacancies in Ga$_2$O$_3$. Furthermore, by comparing the formation energy of the vacancy with the formation energy of Si donors, we show that vacancies are lower in energy than Si donors, independent of the Fermi level, as soon as the alloys contain more than 16% Al. Therefore, cation vacancies will compensate the donor doping, explaining experimental observations.
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Strongly Correlated Electrons

Dynamical Effective Hamiltonian Approach to Second-Harmonic Generation in Quantum Magnets: Application to NiI$_2$

Banasree S. Mou and Stephen M. Winter

Although second harmonic generation (SHG) is a promising and widely used method recently for studying 2D magnetic materials, the quantitative analysis of the full SHG tensor is currently challenging. In this letter, we describe a first-principles-based approach towards quantitative analysis of SHG in insulating magnets through formulation in terms of dynamical effective operators. These operators are computed by solving local many-body cluster models. We benchmark this method on NiI$_2$, a multiferroic 2D van der Waals antiferromagnet, demonstrating quantitative analysis of reported Rotational Anisotropy (RA)-SHG data. SHG is demonstrated to probe local ring-current susceptibilities, which provide sensitivity to short-range chiral spin-spin correlations. The described methods may be easily extended to other non-linear optical responses and materials.
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Materials Science

CVD Grown Hybrid MoSe$_2$-WSe$_2$ Lateral/Vertical Heterostructures with Strong Interlayer Exciton Emission

Md Tarik Hossain, Sai Shradha, Axel Printschler et al. (+14 more)

Lateral heterostructures of 2D transition metal dichalcogenide offer a powerful platform to investigate photonic and electronic phenomena at atomically sharp interfaces. However, their controlled engineering, including tuning lateral domain size and integration into vertical van der Waals heterostructures with other 2D materials, remains challenging. Here, we present a facile route for the synthesis of two types of heterostructures consisting of monolayers of MoSe$_2$ and WSe$_2$ - purely lateral (HS I) and hybrid lateral/vertical (HS II) - using liquid precursors of transition metal salts and chemical vapor deposition (CVD). Depending on the growth parameters, the heterostructure type and their lateral dimensions can be adjusted. We characterized properties of the HS I and HS II by complementary spectroscopic and microscopic techniques including Raman and photoluminescence spectroscopy, and optical and atomic force microscopy, and scanning electron and transmission electron microscopy. The photoluminescence measurements reveal strong interlayer exciton emission in the MoSe$_2$/WSe$_2$ region of HS II, which dominates the spectrum at 4 K and persisting up to room temperature. These results demonstrate high optical quality of the grown heterostructures which in combination with scalability of the developed approach paves the way for fundamental studies and device applications based on these unique 2D quantum materials.
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All authors Md Tarik Hossain, Sai Shradha, Axel Printschler, Julian Picker, Luc F. Oswald, Julian Fuehrer, Nicole Engel, Honey J. Shah, Christof Neumann, Daria I. Markina, Moritz Quincke, Johannes Biskupek, Kenji Watanabe, Takashi Taniguchi, Ute Kaiser, Bernhard Urbaszek, Andrey Turchanin

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

When pre-training hurts LoRA fine-tuning: a dynamical analysis via single-index models

Gibbs Nwemadji, Bruno Loureiro and Jean Barbier

Pre-training on a source task is usually expected to facilitate fine-tuning on similar downstream problems. In this work, we mathematically show that this naive intuition is not always true: excessive pre-training can computationally slow down fine-tuning optimization. We study this phenomenon for low-rank adaptation (LoRA) fine-tuning on single-index models trained under one-pass SGD. Leveraging a summary statistics description of the fine-tuning dynamics, we precisely characterize how the convergence rate depends on the initial fine-tuning alignment and the degree of non-linearity of the target task. The key take away is that even when the pre-training and downstream tasks are well aligned, strong pre-training can induce a prolonged search phase and hinder convergence. Our theory thus provides a unified picture of how pre-training strength and task difficulty jointly shape the dynamics and limitations of LoRA fine-tuning in a nontrivial tractable model. On the practical side, we empirically show that our theoretical findings extend beyond our toy model and remain relevant in the context of a vision-transformer model trained on real data.
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Materials Science

Revealing Short- and Long-range Li-ion diffusion in Li$_2$MnO$_3$ from finite-temperature dynamical mean field theory

Alex Taekyung Lee, Kristin A. Persson and Anh T. Ngo

Li$_2$MnO$_3$ is a key component of Li-excess layered cathodes of the form $(1-x),\mathrm{LiMO_2} + x,\mathrm{Li_2MnO_3}$ ($M$ = Mn, Ni, Co, \dots), yet its role in setting Li-ion transport limitations remains under debate. Here we combine DFT+$U$, finite-temperature DFT+DMFT with a continuous-time quantum Monte Carlo impurity solver, and nudged-elastic-band (NEB) calculations to study Li$^{+}$ migration in paramagnetic Li$_2$MnO$_3$ in the presence of a single Li vacancy. Evaluating DMFT total energies along the DFT+$U$ NEB geometries reveals that dynamical correlations strongly renormalize the lowest-barrier processes, reducing the activation energies to $E_a = 0.18$ eV for the shortest-range hop and $E_a = 0.50$ eV for the next-lowest (transport-controlling) step. The 0.18 eV barrier quantitatively reproduces the short-range activation energy from $\mu^{+}$SR, while the 0.50 eV barrier is consistent with the long-range transport scale extracted from ac-impedance measurements. This single-vacancy, paramagnetic DMFT description thus provides a unified interpretation of local and macroscopic probes without invoking clustered vacancy configurations or strong extrinsic disorder, consistent with nearly stoichiometric Li$_2$MnO$_3$ powders. More broadly, our results highlight finite-temperature dynamical correlations as an essential ingredient for predicting ionic migration energetics in correlated oxide electrodes.
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Materials Science

Phonon assisted absorption in Transition Metal Dichalcogenide heterostructures

Yifan Liu, Robert Dawson, Nathaniel Gabor et al. (+1 more)

The coupling of atomic vibrations to electronic excitations - traditionally understood to be a source of energy loss in semiconductors - has recently been explored in photosynthetic light harvesting as a means to circumvent dissipation by harnessing quantum vibronic coherence. Motivated by recent photocurrent measurements of vibronic sidebands in WSe$_2$/MoSe$_2$ optoelectronic devices, we present a nonperturbative theoretical framework for phonon-assisted absorption in van der Waals heterostructures. Using a polaron transformation, a closed-form expression for the optical absorption spectrum at arbitrary temperatures is presented. Our model includes both intraband and interband electron-phonon coupling. Detailed analysis shows that the observed periodic sidebands originate from the strong coupling between interlayer excitons and nearly dispersionless optical phonon modes. Comparing two limiting cases - one involving only intraband couplings, and another incorporating coherent interband processes - we show that interband phonon-assisted transitions are needed to account for the observed data. Beyond enabling the direct estimation of vibronic coupling strengths from spectroscopic data, these findings have profound consequences for our understanding of optical and optoelectronic responses: coherent interband coupling of atomic vibrations to excitons is essential to quantifying photoresponse in transition metal dichalcogenide heterostructures.
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All authors Yifan Liu, Robert Dawson, Nathaniel Gabor, Vivek Aji

Soft Condensed Matter

Flow-induced bending response rheometer to measure viscoelastic bending of microrods

Barrett T Smith, Michal Czerepaniak, Maciej Lisicki et al. (+1 more)

Soft, microscale hydrogel fibers and rods play important roles in tissue engineering, flexible electronics, soft robotics, drug delivery, sensors, and other applications. Their viscoelastic mechanical properties, while critical for their function, can be challenging to characterize. We present a flow-induced bending response (FIBR) rheometer that quantifies the bending modulus and viscoelastic properties of small, hydrated fibers and rods using flow through a glass capillary. The fiber is positioned across the capillary entrance, and pressure-driven, controlled inflow of water exerts a quantifiable force on the sample. Fiber deflection is determined by video microscopy obtained simultaneously with measurements of flow rate. We develop an analytical model to resolve the hydrodynamic forces applied to the rod, and use Euler-Bernoulli beam theory to determine its material properties. Using a constant volume flow rate of water enables measurement of steady rod deflection, and thus the bending modulus. Application of viscous forces to the rod in a stepwise, cyclic or oscillatory manner enables measurement of time-dependent responses, creep recovery, viscoelastic moduli, and other properties. We demonstrate the versatility of this technique on natural and synthetic materials spanning diameters from 5 to 300 microns and elastic moduli ranging from 100 Pa to >100 MPa. Because the technique uses water to exert forces on the fiber, it works particularly well for hydrated materials, such as hydrogels and biological fibers, providing a versatile platform to characterize microscale mechanical properties of elongated structures.
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All authors Barrett T Smith, Michal Czerepaniak, Maciej Lisicki, Sara M Hashmi

Mesoscale and Nanoscale Physics

Fractal Topology of Majorana Bound States in Superconducting Quasicrystals

William Caiger, Felix Flicker and Miguel-Ángel Sánchez-Martínez

Quasicrystalline order induces a fractal energy spectrum, yet its impact on topological protection remains an open fundamental question. Here, we demonstrate that the topological phase transitions characterised by the appearance of Majorana Bound States themselves have a fractal character. By extending this analysis to the full family of Sturmian words, we uncover Kitaev's Butterfly $-$ a spectral fractal analogous to Hofstadter's butterfly, but fundamentally distinguished by a central superconducting gap. Within this framework, we identify Majorana's Butterfly as a fractal topological phase diagram governed by the competition between quasicrystallinity and superconducting pairing. We show that this competition dictates a hierarchy of Majorana stability, where the survival of the topological phase against fractal fragmentation is determined by the relative strength of these competing energy scales.
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Mesoscale and Nanoscale Physics

Single-Emitter Spectra from an Ensemble

Jonah R. Horowitz, Oliver J. Tye, Oliver M. Nix et al. (+5 more)

The heterogeneity in nanoscale emitters hinders efforts to understand their basic photophysics and limits their use in practical applications. Existing methods have difficulty accurately characterizing single-emitter spectra and optical heterogeneity on a statistical scale. Here, we introduce SPICEE (SPectrally Imbalanced Correlations from Ensemble Emission), a spectrally filtered photon-correlation technique that recovers single-particle emission lineshapes from an ensemble sample. Analytical derivations, numerical modeling, and experiments on a solution ensemble of emitters validate the technique. We apply SPICEE to blue-emitting ZnSeTe semiconductor nanocrystals relevant to display applications and find that the low color purity in the ensemble spectrum is primarily caused by a small subpopulation of nanocrystals with a distinct emission mechanism. This work demonstrates that SPICEE is a powerful high-throughput tool for accurately characterizing the single-emitter properties of nanoscale systems.
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All authors Jonah R. Horowitz, Oliver J. Tye, Oliver M. Nix, Shaun Tan, Hogeun Chang, Ji Hyun Min, Taehyung Kim, Moungi G. Bawendi

Materials Science · Cross-listed from physics.optics

Gear-based 3D-printed Micromachines Actuated by Optical Tweezers

Alaa M. Ali, Gwenn Ulliac, Edison Gerena et al. (+4 more)

The miniaturization of mechanical mechanisms is crucial to enable the development of compact, high-performance micromachines. However, the downscaling actuation of conventional gears and micromotors has remained limited by the inherent challenges of implementing mechanical/electrical powering. Here, we present the design, fabrication, and characterization of an optomechanical, gear-driven micromachine realized through two-photon polymerization 3D printing. The actuation is achieved using optical tweezers. The device integrates a microgear transmission system with an optically actuated part, enabling light-controlled micromachines. When illuminated by a highly focused laser source, the first gear generates rotational torque within the gear assembly, converting optical energy into directional mechanical work that can be transmitted to the coupled gear. We demonstrate the fabrication of micromachines using two-photon polymerization (2PP) laser writing, enabling the fabrication of spur gear trains and bevel gears that can produce out-of-plane rotations, which is not achievable with traditional micromachining fabrication techniques. The micromachines are composed of a single gear or a train of two or three gears without any unwanted adhesion between the components, leading to functioning systems. Experimentally, the fabricated micromachines were actuated using optical tweezers, demonstrating continuous gear rotation, effective motion transmission in gear trains, out-of-plane rotations, and the ability to amplify velocity or torque. Optical-tweezer actuation broadens the potential applications of these micromachines, particularly in biomedical and lab-on-a-chip systems, where precise, minimally invasive control at the microscale is essential.
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All authors Alaa M. Ali, Gwenn Ulliac, Edison Gerena, Abdenbi Mohand-Ousaid, Sinan Haliyo, Aude Bolopion, Muamer Kadic

Mesoscale and Nanoscale Physics

Universal reconstructive polarimetry with graphene-metal infrared photodetectors

Valentin Semkin, Kirill Kapralov, Ilya Mazurenko et al. (+8 more)

Recent advent of smart photodetectors, where in-situ tuning of responsivity enables the reconstruction of light intensity, polarization and spectrum by a single device, has revolutionized the field of optoelectronics. So far, most such reconstructive detectors were realized with non-scalable technology of van der Waals stacking. Here, we demonstrate the infrared reconstructive polarimetry with photodetectors based on conventional gated graphene-metal junctions. The reconstruction exploits the gate tuning of polarization contrast, which enables the determination of both infrared power and polarization angle from photovoltage measurements at two different gate voltages. The physics enabling the polarimetry lies in polarization-dependent shift of the electron hot spot near the contact, and the gate tuning of photosensitive barrier width. We further show the universality of polarization reconstruction, i.e. its feasibility with different geometries of the junction, and with graphene of different quality, from boron-nitride encapsulated flakes to the scalable chemical vapor deposited films.
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All authors Valentin Semkin, Kirill Kapralov, Ilya Mazurenko, Mikhail Kashchenko, Alexander Morozov, Yakov Matyushkin, Dmitry Mylnikov, Denis Bandurin, Li Lin, Alexey Bocharov, Dmitry Svintsov

Strongly Correlated Electrons

Dynamic Simulations of Strongly Coupled Spin Ensembles for Inferring Nature of Electronic Correlations from Nuclear Magnetic Resonance

Charles Snider, Stephen Carr, D. E. Feldman et al. (+2 more)

We develop an efficient package for the simulation of nuclear magnetic resonance spin echo experiments to study the effects of strong electronic spin correlations on the dynamics of the nuclear spin ensemble. A mean-field model is used to study correlated electronic phases through their hyperfine interaction with nuclear spins. We explore the dynamics of the interacting nuclear ensemble and discuss the key behaviors of the system. In particular, we classify the types of temporal asymmetry that the interaction induces in the system as well as a pulse-dependent shift in the spectral domain. Using these results, we discuss how careful measurement of the pulse-dependent shift can be used to extract information about the anisotropy of the electronic interaction and how these results represent a novel tool for the examination of exotic NMR signatures in strongly correlated materials. Finally, we review specific aspects of the simulation package developed for our exploration and give explicit examples where package can be used to infer range and anisotropy of electronic correlations. In particular, we discuss its structure, accuracy, and the technical merits of the various approximations used to model the nuclear spin ensemble.
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All authors Charles Snider, Stephen Carr, D. E. Feldman, Chandrasekhar Ramanathan, V. F. Mitrović

Mesoscale and Nanoscale Physics

Straintronics and twistronics in bilayer graphene

Federico Escudero, Dong Wang, Pierre A. Pantaleón et al. (+3 more)

The interplay of twist and strain in bilayer graphene enables the formation of moiré patterns and narrow bands that host correlated and topological phases. While magic-angle twisted bilayer graphene has been widely studied, strain provides an additional and realistic control knob for band engineering. In this work, we first generate a global method to construct commensurate supercells for arbitrary twist and heterostrain. Then, using atomistic tight-binding and strain-extended continuum models to study the commensurate structures, we identify configurations that minimize the bandwidth beyond the magic angle. The results reveal a strong dependence of band narrowing and topology on strain type, magnitude, direction and lattice relaxation. Particularly, shear strain produces a stronger distortion than uniaxial strain. Including electron-electron interactions through a self-consistent Hartree potential shows that strain broadens the bare bands while reducing electrostatic renormalization. Strain also drives topological transitions as the narrow and remote bands hybridize, establishing twisted and strained bilayer graphene as a tunable platform for flat-band and topological phenomena.
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All authors Federico Escudero, Dong Wang, Pierre A. Pantaleón, Shengjun Yuan, Francisco Guinea, Zhen Zhan

Statistical Mechanics

Thermalization in classical systems with discrete phase space

Pavel Orlov and Enej Ilievski

We study the emergence of statistical mechanics in isolated classical systems with local interactions and discrete phase spaces. We establish that thermalization in such systems does not require global ergodicity; instead, it arises from effective local ergodicity, where dynamics in a subsystem may appear pseudorandom. To corroborate that, we analyze the spectrum of the unitary evolution operator and propose an ansatz to describe statistical properties of local observables expanded in the eigenfunction basis - the classical counterpart of the Eigenstate Thermalization Hypothesis. Our framework provides a unified perspective on thermalization in classical and quantum systems with discrete spectra.
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