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Physics Condensed Matter

cond-mat Watch Condensed Matter

Friday 31 July

Strongly Correlated Electrons

Successive Electronic Topological Transitions in the Antiferromagnet UPd$_2$Al$_3$

Adrien Gourgout, Gaël Bastien, Dai Aoki et al. (+7 more)

We report successive anomalies at low temperature in the magnetic field dependence of the thermoelectric signal in the heavy fermion compound UPd$_{2}$Al$_{3}$ inside the antiferromagnetic state up to the metamagnetic transition at $H_\text{M} =18$~T. Based on renormalisation perturbation theory and the partitioning of the $f$ orbitals into localized and delocalized parts, our analysis attributes these anomalies to complex topological changes of the Fermi surface driven by Zeeman effect. The observation of a sudden change of sign both in the thermoelectric power and in the Hall coefficient at $H_\text{M}$ in addition to the appearance of large quantum oscillations in the thermoelectric power above $H_\text{M}$ indicate a strong Fermi surface reconstruction at the metamagnetic transition due to the unfolding of the electronic bands.
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All authors Adrien Gourgout, Gaël Bastien, Dai Aoki, Gabriel Seyfarth, Ilya Sheikin, Andrey Varlamov, Gertrud Zwicknagl, Jacques Flouquet, Georg Knebel, Alexandre Pourret

Mesoscale and Nanoscale Physics

The stability of electronic Poiseuille flow in two-dimensional materials

Marine Bastida, Alvaro Meseguer and Iacopo Torre

Motivated by the experimental observation of electronic Poiseuille flow in graphene [J.A. Sulpizio et al. Nature 576, 75 (2019)] we analyze the linear stability of plane-Poiseuille flow of electrons in a two-dimensional material using a modified Orr-Sommerfeld equation. We calculate the critical current needed to make the flow unstable as a function of the experimental parameters and characterize the most favorable situation, that is the one needing the lowest current density, to observe flow instability. We predict the streamwise wavenumber and frequency at which the instability occurs and discuss the difficulties of an experiment aimed at probing this phenomenon.
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Mesoscale and Nanoscale Physics

A low-temperature ultra-high-vacuum scanning probe microscope with in situ electronic transport capabilities: from macro to nano in 10 minutes

Diego Exposito, Oscar Custance and Ivan Brihuega

We have developed a low temperature (LT), ultra-high-vacuum (UHV) system that combines two complementary techniques, scanning probe microscopy (SPM) and electrical transport measurements, within a single platform. By providing simultaneous access to the atomic-scale surface landscape and the macroscopic device response of the same sample, the setup enables direct correlations between local structural/spectroscopic signatures and global electronic transport behavior in two-dimensional (2D) devices. The system allows experiments where atomic-scale modifications or controlled manipulations are performed while continuously monitoring their impact on device-scale performance. The setup consists of two interconnected UHV chambers: a dedicated preparation chamber and a separate measurement chamber that houses a liquid-helium cryostat and the SPM/transport stage. Base pressure is 1x10^-11 Torr. A key feature is direct optical access to the sample, enabling rapid and reliable tip positioning with an accuracy of 5 microns x 5 microns within 10 minutes. The SPM, operated using custom-built electronics, can track the exact same sample region across a temperature range from 2.9 K to 400 K, with mechanical stability below 1 pm. System performance is demonstrated on graphene devices, and bulk Pb is used to determine energy resolution. Using superconducting tips, scanning tunneling spectroscopy measures the superconducting gap with an energy resolution of 30 microV. Transport measurements track the temperature dependence of both resistivity and critical current across the superconducting transition of an in-situ prepared Pb nanowire.
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Mesoscale and Nanoscale Physics

Optomechanical tuning of nonlinearity in graphene resonators

Alberto Martín-Pérez, Peter G. Steeneken and Farbod Alijani

Graphene resonators exhibit unique mechanical and electrical properties, making them promising candidates for next-generation sensing applications. Here, we introduce an optomechanical approach that enables active, reversible, in situ tuning of the nonlinear nanomechanical response of graphene resonators. By exploiting photothermal heating and the resulting optomechanical interaction between the mechanical oscillations and the optical cavity formed by a graphene membrane suspended over a reflective substrate, we achieve dynamic control over both the linear restoring force and the nonlinear dynamic response of the resonator. We show that the strength and sign of the Duffing nonlinearity can be continuously tuned through the optical probe power and cavity depth, enabling transitions between hardening and softening behavior as well as near-complete suppression of geometric nonlinearities. Our results establish a versatile platform for on-demand engineering of nonlinear dynamics in graphene resonators and provide a general strategy for active control of the mechanical response of two-dimensional nanoelectromechanical systems.
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Materials Science

Charge-Density-Wave Phase Selection by Janus-Induced Intrinsic Strain in Monolayer NbSSiAs$_2$

Chun-Jie Zhang, Bing Zhang, Dongliang Mao et al. (+3 more)

Controlling phase selection among competing charge-density-wave (CDW) instabilities remains challenging in two-dimensional materials. Here, first-principles calculations show that Janus-induced intrinsic tensile strain redirects the off-M soft-mode tendency of NbS$_2$ to the M point in NbSSiAs$_2$, selecting a $2\times2$ CDW reconstruction. Electron-phonon coupling analysis identifies momentum-selective coupling between Nb-derived states and a longitudinal acoustic mode as the origin of the M-point instability. The reconstructed phase hosts two nearly degenerate Nb-trimerized configurations whose relative stability is tuned by biaxial strain. Both configurations retain phonon-mediated superconductivity on the 6-7 K scale, indicating the coexistence of CDW order and superconductivity. Compressive strain favors the 1+3-hollow configuration and induces a band-inverted, $Z_2$-nontrivial state while preserving superconductivity. Together, these results identify Janus-induced intrinsic strain as an internal structural route for CDW phase selection, whereas external strain provides access to a regime in which CDW order, topology, and superconductivity coexist.
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All authors Chun-Jie Zhang, Bing Zhang, Dongliang Mao, Yapeng Wu, Xiao-Ping Li, Lei Wang

Mesoscale and Nanoscale Physics

Emerging network model in a twisted monolayer-rhombohedral graphene

Juyoung Song, Jeyong Park and Jinhong Park

We investigate the coexistence of localized states and propagating one-dimensional (1D) modes in graphene moiré systems. We first show within a minimal model that a spatially varying scalar potential can confine localized states, while sign changes of a staggered potential generate 1D modes along the resulting domain walls. These two types of states can coexist within the same finite energy window and form a hybrid network. We then demonstrate a microscopic realization of this mechanism in twisted monolayer-rhombohedral N-layer graphene. Band structures, energy contours, and Bloch wave functions obtained in a realistic parameter regime reveal the coexistence of localized nearly flat-band states and propagating quasi-1D modes. Our results establish twisted monolayer-rhombohedral graphene as a promising platform for realizing hybrid electronic networks with coexisting states of distinct effective dimensionalities.
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Materials Science

First-principles carrier mobility and optical absorption of strained ZnO with self-consistent Hubbard interactions

Hong-Guk Min, Wooil Yang, Sabyasachi Tiwari et al. (+2 more)

Carrier mobility and optical absorption are key performance parameters of oxide semiconductors in transparent and flexible displays. We use a newly developed density-functional perturbation theory with a self-consistent Hubbard correction (DFPT+U) to study phonon-limited electron transport and phonon-assisted optical absorption in strained zinc oxide (ZnO). This parameter-free approach accounts for electron-phonon interactions and on-site correlation effects simultaneously. Electronic structures and phonon dispersions are computed under three distinct uniaxial strain directions. Uniaxial tensile strain up to 4.8% along [\bar110] is found to increase the room-temperature electron mobility by 19% while leaving visible-range optical absorption essentially unchanged. These results demonstrate that moderate strain can selectively enhance carrier transport without degrading optical transparency, and establish DFPT+U as an effective framework for predicting strain-dependent transport and optical properties in wide-band-gap oxides with implications for strain-engineered display and optoelectronic applications.
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All authors Hong-Guk Min, Wooil Yang, Sabyasachi Tiwari, Feliciano Giustino, Young-Woo Son

Materials Science

One-Step Epitaxial Access to Rhombohedral Graphene Flat-Band States on Step-Bunched SiC

Hao Zhong, Xingzhe Wang, Hanbin Deng et al. (+15 more)

Rhombohedral graphene multilayers provide a moiré-free platform for correlated and topological flat-band physics, but direct, transfer-free epitaxial access to thickness-tunable multilayers remains limited. Here we report a one-step graphitization route on 4$^\circ$ off-axis 4H-SiC, in which high-temperature flash annealing simultaneously drives self-organized step bunching and multilayer graphene formation. Atomic-resolution cross-sectional scanning transmission electron microscopy identify local ABC registry and distinguish rhombohedral from Bernal stacking. The thickness is tuned from bilayer to more than twenty layers by varying single parameter, the annealing temperature. Angle-resolved photoemission spectroscopy directly tracks the thickness-dependent evolution from interface-dominated low-energy states toward pronounced near-Fermi-level flat-band spectral weight in thick multilayers. Low-temperature scanning tunneling microscopy and spectroscopy on a 17-layer film further reveal a 13.4 meV low-energy spectral reconstruction and a $\sqrt{3} \times \sqrt{3}$ Kekulé-like modulation, providing microscopic signatures consistent with an intervalley-mixed electronic texture. This one-step, transfer-free approach establishes step-bunched SiC as an epitaxial platform that links stacking engineering with moiré-free correlated flat-band electronic states.
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All authors Hao Zhong, Xingzhe Wang, Hanbin Deng, Tianyu Yang, Haixuan Cao, Renzhe Li, Qiang Wan, Shangkun Mo, Keming Zhao, Shuming Yu, Dingkun Qin, Guang Zhu, Yifan Zhou, Jianping Shi, Shuangfeng Jia, He Zheng, Jia-Xin Yin, Nan Xu

Statistical Mechanics

Charge discreteness and the energy efficiency of information erasure in dynamic random-access memory cells

Takase Shimizu, Kouki Yamamoto, Kensaku Chida et al. (+2 more)

A dynamic random-access memory (DRAM) cell stores information as an integer number of electrons on a capacitor, and whether this discreteness is thermodynamically relevant depends on the competition between the charging energy and thermal fluctuations. This competition is quantified by the ratio $\kappa$ of the single-electron charging energy to the thermal energy, and here we investigate how $\kappa$ affects the energy efficiency of information erasure in a DRAM cell. Using a stochastic-thermodynamic model of a DRAM cell, we show that the nonquasistatic heat released during the discharge step is suppressed as $\kappa$ increases, whereas the quasistatic heat of the charge step approaches the Landauer cost. As a result, the energy efficiency increases monotonically with $\kappa$ and approaches the Landauer limit where the effect of charge discreteness is maximal and the cell is effectively reduced to two charge states. The parameter $\kappa$ thus connects two thermodynamic regimes: a multilevel single-well memory, whose nonequilibrium initial state prevents quasistatic erasure, and an effective two-level memory that can attain the Landauer limit. These results identify $\kappa$ as the parameter that controls the fundamental efficiency ceiling of transistor–capacitor memory circuits.
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All authors Takase Shimizu, Kouki Yamamoto, Kensaku Chida, Gento Yamahata, Katsuhiko Nishiguchi

Materials Science

Mechanism-Dependent Descriptors Enable Predictive Design of Oxygen Capacity in Perovskite Oxides

Shuiping Gong, Yi Li, Jie Su et al. (+9 more)

Perovskite oxides can reversibly accommodate substantial changes in oxygen stoichiometry, making them attractive for clean-energy technologies including chemical looping and oxygen storage. Despite extensive efforts to optimize their redox properties, predictive descriptors capable of assessing oxygen capacity across diverse compositions remain under development. Here, we combine experiments and first-principles calculations to establish composition and oxygen-capacity relationships in the model perovskite series LnxSr1-xCoO3. We confirm that increasing Sr2+ content promotes the formation of high-valence Co4+, expanding the cationic redox reservoir available during oxygen release and thereby enhancing oxygen capacity. In this regime, oxygen-vacancy formation energy captures the observed trend because oxygen release is primarily compensated by Co4+/Co3+/Co2+ redox. Across the rare-earth series, however, oxygen capacity decreases from La to Lu despite progressively lower oxygen-vacancy formation energies. We reveal that this counterintuitive behavior originates from an alternative charge-compensation pathway, in which lattice oxygen is partially oxidized to O1- -like species during oxygen removal. Heavy rare-earth compositions (Tb-Lu) preferentially stabilize these oxygen-hole species through distinct local bonding environments, with charge compensation involving both oxidized lattice oxygen and reduced rare-earth and cobalt cations, thereby suppressing net oxygen release despite favorable vacancy thermodynamics. We further identify average metal-oxygen bond strength, quantified by integrated crystal orbital Hamilton population, as a physically meaningful descriptor for oxygen capacity when anionic redox becomes dominant.
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All authors Shuiping Gong, Yi Li, Jie Su, Zhenhao Zhou, Xiaobo Liao, Jian Deng, Chris Wolverton, Tao Deng, Jiangang He, Runxia Cai, Chaochao Dun, Zhenpeng Yao

Strongly Correlated Electrons

Critical Phases of the extended isotropic $XY$ chain with four-spin interaction

Nika Kurdadze, Giorgi Gogaberishvili and G.I. Japaridze

Using the Jordan-Wigner transformation we calculate exactly the ground state and low-temperature thermodynamic properties of the spin $S=1/2$ isotropic $XX$ chain with four spin interaction. In terms of the equivalent spinless fermion (SF) representation the system is viewed as a lattice fermion gas with nearest-neighbor ($J$) and next-next-next-neighbor ($J^{\ast}/4$) hopping. It is shown that with the increase of four spin coupling, at $J^{\ast}_{c} = 4J/3$ the system experiences the Lifshitz type topological phase transition characterized by the tripling of Fermi points. The quantum phase transition (QPT) point marks transition from a gapless spin-liquid phase of standard $XX$ chain into again a gapless spin-liquid phase with different character of power-low decay of spin correlations. At the transition point the free fermion dispersion relation shows flattering at Fermi points, what determines singular character of density of states $\rho(\omega)\sim (\omega/J)^{-2/3}$ and as a consequence unconventional temperature dependence of heat capacity of the system $C\sim (T/J^{\ast})^{1/3}$, and singular magnetic susceptibility of the system $\chi(H)\sim (H/J^{\ast})^{-2/3}$. In the case of alternating magnetic field the system is characterized by the rich ground state phase diagram which contains fully polarized (ferromagnetic), gapped antiferromagnetic (AFM) and spin liquid phases. At the transition point from the gapped AFM phase into the gapless polarized spin liquid phase the system shows rapid increase of magnetization $m\sim(H-H_c)^{1/6}$ and magnetic susceptibility a singular behavior as $\chi(H)\sim (H-H_c)^{-5/6}$.
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Strongly Correlated Electrons

Generalized Holstein-Primakoff transformation with optimizable bosonic truncation

Yang Liu, Jia-Wei Mei, Rong Yu et al. (+3 more)

Spin-wave theory provides a quasiparticle description of excitation spectra in quantum spin systems. In this theory, the spin operators are usually bosonized by the Holstein-Primakoff transformation or the Dyson-Maleev transformation. In practical calculations, the bosonized Hamiltonian has to be truncated, and thus the resulting excitation spectra depend on the choice of bosonic representation. Here, we introduce a generalized Holstein-Primakoff transformation that continuously interpolates between the conventional Holstein-Primakoff and Dyson-Maleev transformations through a single parameter. The formulation naturally extends to the SU(N) algebra and provides a flexible framework for optimizing bosonic truncations beyond harmonic order. As an application, we investigate the spin-1 bilinear-biquadratic model on the square lattice. By combining the generalized Holstein-Primakoff transformation with continuous similarity transformations, we obtain excitation spectra in excellent agreement with tensor-network calculations. Our results demonstrate that optimizing the bosonic representation substantially reduces truncation errors and provides quantitatively reliable descriptions of both quasiparticle dispersions and multi-boson continua.
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All authors Yang Liu, Jia-Wei Mei, Rong Yu, Z. Y. Xie, Hong-Gang Luo, Jize Zhao

Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Parent Hamiltonian and intrinsic phase transition in non-Hermitian photonic systems

Yuntao Xiao, Yuchen Guo, Xiaojian Huang et al. (+6 more)

Non-Hermitian systems host phenomena absent in Hermitian physics, but realizing Hamiltonians with intrinsic non-Hermitian properties remains challenging. The theoretical method of non-Hermitian parent Hamiltonian (NH-PH) enables the construction of a non-Hermitian system from a pair of matrix product states (MPSs) with tailored properties. Here, we report the first experimental generation of NH-PHs. This generation starts from MPSs that represent asymmetric Affleck–Kennedy–Lieb–Tasaki (AKLT) states. The construction is validated with single photons via imaginary-time evolution of the generated NH-PH to obtain its left and right ground states. We then characterize the properties of the system by measuring four different order parameters that probe non-reciprocal correlations, chiral imbalance, and conventional antiferromagnetic correlations. Furthermore, extending the framework to a larger system with a different model, we observe an intrinsic non-Hermitian phase transition, manifested by abrupt jumps of an order parameter when the designated zero-energy modes cease to be the globally lowest-energy states. Our work provides the first experimental realization and characterization of non-Hermitian Hamiltonians with controllable and customizable properties, opening new avenues for exploring intrinsic non-Hermitian phenomena across diverse physical platforms.
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All authors Yuntao Xiao, Yuchen Guo, Xiaojian Huang, Huixia Gao, Dengke Qu, Lei Xiao, Kunkun Wang, Shuo Yang, Peng Xue

Soft Condensed Matter

Covariance-Driven Momentum Rectification at Liquid-Vapor Interfaces Near Wetting Transitions

Nelson Bolívar, Gabriel Abellán and Ivaylo Vasilev

Zero-mean forcing can generate directed transport when a medium responds in a spatially structured way and the relevant symmetries are broken. Liquid-vapor interfaces are a useful setting for this problem because surface-tension gradients, wetting dynamics, vapor exchange, capillary and acoustic waves, electro-ionic screening, thermal noise, and boundary compliance can all carry momentum. We develop a conservative continuum model in which the central object is the covariance between a local response field and a zero-mean tangential drive, $\langle Mf\rangle-\langle M\rangle\langle f\rangle$, embedded in an explicit momentum ledger. In a minimal diffuse-interface realization, this covariance gives the phase-selective drift law $U\propto\epsilon_h\Theta\sin\varphi$ for a Marangoni-driven liquid-vapor interface above a structured wall, with exact nulls when the symmetry is restored. Wetting susceptibility then acts as a bounded gain factor: first-order spinodal conditions provide the useful amplification regime, critical wetting saturates because interfacial unbinding removes the short-range drive, and bulk criticality suppresses the channel as the interface disappears. Additional reservoirs - phase change, waves, thermal transport, electro-ionic coupling, compliance, and fluctuations - are treated as compensating channels to be isolated by sign reversals, scaling laws, and budget closure. The result is neither a new microscopic force nor an apparatus-level claim, but a symmetry-constrained accounting scheme for rectified momentum transfer in water-based interfacial systems. Reduced numerical calculations illustrate the phase-selection rules, bounded spinodal gain, momentum-budget closure, transverse chirality, and grid convergence.
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Mesoscale and Nanoscale Physics

Symmetry selection rule for the band-edge shift current in two dimensions

Felipe Pérez Riffo, Matías Castro, Leonor Chico et al. (+2 more)

The shift current is the intrinsic bulk photovoltaic response of a crystal without an inversion center. In graphene multilayers, calculations report large band-edge shift currents that reverse sign under a gate, a behavior that neither the quantum metric nor the Berry curvature captures. We show that this follows from a symmetry principle. At the absorption edge of a gapped, inversion-broken two-dimensional Dirac-like system, an emergent rotational symmetry forbids the response, and the trigonal warping switches it on linearly in its strength, an angular selection rule that unifies multilayer graphene and the kagome lattice. The released current is governed by a signed, detuning-weighted three-point Bargmann invariant: the optical amplitude stays fixed while the sign alignment of the Bargmann triangles grows with the warping, invisible to any positive-definite figure of merit. In bilayer and trilayer graphene the gate alone reverses the sign, making the band-edge shift current a gate-switchable bulk photovoltaic response.
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All authors Felipe Pérez Riffo, Matías Castro, Leonor Chico, L.E.F. Foa Torres, Eric Suárez Morell

Materials Science

Molecular beam epitaxy synthesis of ternary nitride PrTaN$_2$ and its crystal structure determination

Kosuke Takiguchi, Yoshiharu Krockenberger, Eisuke Magome et al. (+2 more)

We report the discovery of a novel ternary nitride PrTaN$_2$ synthesized as a thin film using molecular beam epitaxy. The combination of e-beam evaporation for refractory elements and a radio-frequency nitrogen radical source enables growth under a highly nitriding environment, providing access to phases not readily obtained in bulk synthesis. Structural characterization by X-ray diffraction and high-angle annular dark-field scanning transmission electron microscopy reveals that the compound crystallizes in an orthorhombic structure and grows with a well-defined orientation on YAlO$_3$ substrates, while remaining essentially strain-free. To determine the crystal structure from limited thin-film diffraction data, we developed a fitting procedure based on structure factors. By combining extinction rules with constraints from Wyckoff positions, the number of fitting parameters is significantly reduced, enabling reliable structure determination. Systematic exclusion of alternative candidate phases in the Pr-Ta-N system, together with structure factor fitting, identifies the space group as $P222$ and determines the atomic coordinates. The present results demonstrate that thin-film growth with molecular beam epitaxy can stabilize previously unexplored ternary nitrides, and establish a practical approach for structural determination in such systems. This work provides a pathway for the exploration of new complex nitride materials.
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All authors Kosuke Takiguchi, Yoshiharu Krockenberger, Eisuke Magome, Yoji Kunihashi, Hideki Yamamoto

Materials Science

Knudsen-Controlled Switching of Thermal Conductivity Response by Targeted Phonon Excitation

Shixian Liu, Tianhao Li, Fei Yin et al. (+4 more)

Targeted phonon excitation offers a route to dynamically control heat conduction, yet no general principle predicts whether a spectrally selective nonequilibrium phonon population will enhance or suppress thermal transport. A Knudsen-controlled competition between the increased contribution of long-mean-free-path phonons and excitation-enhanced intrinsic scattering governs the sign of the thermal-conductivity response. First-principles three-phonon scattering rates combined with phonon-tracking Monte Carlo simulations are used to examine Ge, Si, and 3C–SiC from bulk crystals to confined nanofilms. In bulk systems, excitation-enhanced scattering dominates and thermal conductivity is predominantly suppressed. In nanofilms, by contrast, low-frequency excitation can increase the contribution of quasi-ballistic heat-carrying channels and enhance thermal conductivity, whereas higher-frequency excitation is predominantly suppressive. At fixed background temperature and excitation strength, these opposite responses are organized in a frequency–Knudsen map based on the normalized target frequency, $\omega_{\mathrm t}/\omega_{\mathrm D}$, and the Knudsen number, $\mathrm{Kn}$. The resulting framework provides a general physical basis for controlling nonequilibrium heat transport beyond static phonon engineering.
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All authors Shixian Liu, Tianhao Li, Fei Yin, Yu He, Alexander A. Barinov, Han Meng, Nuo Yang

Thursday 30 July

Soft Condensed Matter

Untrainable elements determine what physical learning remembers

Bijaya Dangol

Physical learning rules such as equilibrium propagation (EP), coupled learning (CL), and adjoint coupled learning (AL) train resistive networks through local measurements. The learned function is decided by where on the solution manifold training lands. Two properties could decide it, and they have not been separated: the circuit's invariance under rescaling every conductance, and the rule's conservation of the mass K = (1/2) sum_e kappa_e^2. We separate them. When every element is trainable, all three vector fields are homogeneous in the conductances, so the initialization scale is provably inert. An element the rule does not adjust breaks that homogeneity whatever its constitutive law. Across twenty topologies the learned function moves with the initialization scale by a median of twelve percent with fixed rectifiers and eight with fixed linear resistors, against 3e-8 when every element is trainable; a single fixed rectifier produces the whole effect. The conservation law is not what protects the function: AL, which we prove dissipates the mass at exactly twice its own loss, remembers its initialization as strongly as the rules that conserve it, and the memory survives in runs where K is conserved to 1e-4. Raising the fixed-element count from one to eight multiplies the conservation drift by five thousand and leaves the memory unchanged, while the all-trainable circuit under AL drifts comparably and remembers nothing. What the rule's conservation structure does control is solution quality: at matched training loss AL is worse than EP and CL in four of six small circuits, by a median of three to seven percent, though the ordering is not stable across checkpoints and does not reproduce at fifty nodes. Physical learning therefore carries two independent inductive biases, one belonging to the circuit and one to the rule, and only the first is a memory of how the device was built.
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Materials Science · Cross-listed from physics.app-ph

SH-SAW Acousto-Electric Amplifier in Epitaxial InGaAs on Lithium Niobate on Insulator

Chuan Tian, Christopher Heidelberger and Siddhartha Ghosh

This work demonstrates shear-horizontal surface acoustic wave (SH-SAW) acoustoelectric (AE) amplification on an epitaxial InGaAs / X-cut lithium niobate on insulator (LNOI) heterostructure formed by Al2O3-mediated wafer bonding. Deployable passivated devices show a stable fundamental-mode non-reciprocity of 32 dB/mm at 1.11 GHz (30 V bias, 64 mW consumed), while unpassivated devices reach 174 dB/mm across 1.1-2.8 GHz, reported as upper bounds. Device characterization establishes the role of mode-dependent K^2 in determining the achievable gain. Hall-effect measurements of the transferred InGaAs serve as a quantitative diagnostic: the extracted carrier density, elevated by unintentional silicon doping during epitaxy, accounts for the absolute AE gain when inserted into the analytical model and identifies epitaxial process control as a clear lever for further enhancement. We further identify ambient oxidation of the bare InGaAs surface as a distinct aging mechanism that extinguishes the AE response within weeks, and show that an InP or ALD Al2O3 passivation layer suppresses it, at the cost of redistributing the piezoelectric field away from the channel. These results establish InGaAs-on-LNOI as a compact, low-power platform for non-reciprocal RF components and acoustoelectric delay lines, with strong relevance to in-band full-duplex (IBFD) transceivers and spectrum-efficient wireless front ends.
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Materials Science

Automating the analysis of micron-scale synchrotron diffraction data on inhomogeneous polycrystalline samples: a solid oxide electrolysis cell case study

Liam A. V. Nagle-Cocco, Christopher A. Crain, Michael J. Dzara et al. (+6 more)

A novel approach to post mortem characterisation of electrochemical and photovoltaic devices is spatially-resolved diffraction using a hyper-focused, micron-width x-ray beam to examine the distribution of degradation products and strain, a technique called $\mu$-XRD. Aside from the experimental difficulties associated with beam focusing and sample preparation, which are themselves non-trivial, the analysis of resulting data is complex and challenging, with full Rietveld analysis rarely attempted in literature. The difficulty lies in the size of the data, which may consist of hundreds or even thousands of diffraction patterns with very different crystallographic phase compositions depending on position within the device, and the difficulty in fitting the data due to the presence of many phases at the same position, including possible degradation products which may be difficult to index and assign to known phases. In this paper, we present a fully-automated open access Python routine for performing phase identification and Rietveld analysis on 2D datasets of diffraction pattern taken at micron-scale positions, measured over the cross-section of a chemically inhomogeneous device with polycrystalline phases. Solid oxide electrolyser cells are a promising technology for green hydrogen production which can utilise waste heat to split water at higher efficiencies than low-temperature electrolysis techniques such as polymer electrolyte membranes, but exhibit many degradation modes due to the high operating temperatures. We present a case study using our analysis protocol on an SOEC fragment encompassing the air electrode, cation diffusion barrier, electrolyte, and fuel electrode. With modification, this protocol could be applied to other devices such as all-solid-state batteries, wet-electrolyte battery electrodes, solid oxide fuel cells, photovoltaic devices, and metal-oxide pseudocapacitors.
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All authors Liam A. V. Nagle-Cocco, Christopher A. Crain, Michael J. Dzara, Mathias A. Kiefer, Madeline G. Port, Tolga Han Ulucan, Madeline Van Winkle, Oscar Hathaway, Nicholas A. Strange

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

Transcorrelated Random-Phase Approximation

Abdallah Ammar, Enzo Monino, Anthony Scemama et al. (+2 more)

We extend the random-phase approximation (RPA) to the non-Hermitian transcorrelated (TC) Hamiltonian, which explicitly includes three-body interactions generated by a Jastrow correlation factor. We consider both the direct RPA (dRPA) and RPA with exchange (RPAx). We apply the resulting TC-dRPA and TC-RPAx methods to calculate ground-state correlation energies and vertical excitation energies for atoms (\ce{He} and \ce{Ne}) and small molecules (\ce{H2O}, \ce{NH3}, \ce{CH4}, and \ce{H2CO}). For ground-state correlation energies, the TC treatment substantially improves accuracy and accelerates basis set convergence, reducing errors by nearly an order of magnitude relative to conventional RPA calculations. By contrast, it yields only marginal improvements in vertical excitation energies. We attribute this limited effect to the ground-state optimization of the Jastrow factor, which does not adequately capture the distinct electronic character of excited states. These results establish TC-RPA as an accurate and computationally efficient approach to ground-state energetics, while highlighting the need for state-specific Jastrow optimization to achieve reliable descriptions of excited states.
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All authors Abdallah Ammar, Enzo Monino, Anthony Scemama, Emmanuel Giner, Pierre-François Loos

Materials Science

An End-to-End Differentiable Forward Model for High-Energy Diffraction Microscopy

Hemant Sharma, Nina Andrejevic, Simon Zhang et al. (+1 more)

High-Energy Diffraction Microscopy (HEDM) recovers crystallographic orientation, strain, and grain position from rotating-crystal X-ray diffraction patterns. Existing forward models in far-field (FF), near-field (NF), and point-focused (pf) HEDM are not differentiable, which forecloses gradient-based joint parameter refinement, physics-informed regularisation, and Bayesian uncertainty quantification. We present the first end-to-end differentiable HEDM forward model covering all three geometries, implemented in PyTorch with pixel-exact agreement against the established MIDAS reference simulators (162/162 FF, 2304/2304 NF including a non-zero detector-tilt sweep, and 1088/1096 pf-HEDM spots matched). Three demonstrations validate the framework: joint orientation-strain-position recovery in NF-HEDM at ~6 nm precision; round-trip refinement on a real 214-grain alpha-Ti FF-HEDM dataset reaching 100% grain recovery from a 1.5 degree initial perturbation with residuals matching the production fit to 0.3%; and joint refinement of all per-detector geometry parameters and per-grain state on a synthetic four-panel FF-HEDM setup, recovering panel rotations about the beam axis to ~10 mu-rad and a global rotation-axis wedge to ~26 mu-rad. The framework is released as the open-source midas-diffract package (pip install midas-diffract).
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All authors Hemant Sharma, Nina Andrejevic, Simon Zhang, Mathew Cherukara

Materials Science

Ultrafast Nonthermal Lattice Destabilization and Suppression of Polar Optical Scattering in Electronically Excited $\alpha$-SiO$_2$ from First-Principles and Deep Neural Network Potential Modeling

Iyyappa Rajan Panneerselvam, Mark Yeung, Charlotte Palmer et al. (+2 more)

We present a multiscale first-principles-to-machine-learning approach to investigate ultrafast lattice dynamics in electronically excited $\alpha$-SiO$_2$. Ab initio molecular dynamics (AIMD) based on electronic-temperature-dependent density functional theory (DFT) are used to train electronic-temperature-dependent deep neural network potentials (DNNPs). The use of DNNPs enables atomistic modeling at near-DFT accuracy of large $\alpha$-SiO$_2$ cells with thousands of atoms. In particular, DNNPs allowed us to obtain accurate phonon band structures and molecular dynamics (MD) of $\alpha$-SiO$_2$ excited by a sudden increase in electronic temperature. With increasing electronic temperature, $T_e$, pronounced lattice destabilization of $\alpha$-SiO$_2$ is found, as evidenced by violations of elastic stability criteria, substantial volumetric expansion, a sharp reduction of the bulk modulus, and progressive weakening of Si-O bonding due to antibonding-state occupation. From the electronic and phonon band structures, we estimated the Frohlich coupling constant, which decreases as $T_e$ increases, suggesting a crossover to a nonpolar phase of $\alpha$-SiO$_2$ at elevated electronic temperature. This is corroborated by the Bader charge analysis. We also suggest that polar optical phonon scattering should be strongly suppressed at $T_e > 2$ eV. From large-cell DNNP-MD simulations, we show that a well-defined thermal equilibrium, as defined by the Maxwell-Boltzmann distribution, is not achieved over the first few hundred femtoseconds. This behavior explains the non-monotonic equilibration of the kinetic temperature after a sudden rise of $T_e$. After $T_e$ is raised to 2.6 eV, Si and O atoms first equilibrate separately at two different temperatures, suggesting an atomic fluid phase, in agreement with recent experimental and theoretical findings.
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All authors Iyyappa Rajan Panneerselvam, Mark Yeung, Charlotte Palmer, Brendan Dromey, Lorenzo Stella

Materials Science

Giant Exfoliation Induced Magnetic Coercivity in Fe$_3$GaTe$_2$

Lingrui Mei, PeiYu Cai, Sang-Eon Lee et al. (+6 more)

Permanent magnets with strong anisotropy and high coercivity underpin modern information and energy technologies, yet rare-earth-free alternatives remain limited. Here, we show that thickness engineering via mechanical exfoliation induces hard magnetic behavior in the van der Waals ferromagnet Fe$_3$GaTe$_2$. Bulk crystals exhibit Curie temperatures above 350 K but negligible room-temperature coercivity. When thinned below 100 nm, the coercive field is dramatically enhanced, reaching nearly 1 T at room temperature for in-plane fields which is comparable to values of conventional hard magnets. Micromagnetic analysis reveals a crossover in magnetization reversal from domain-mediated processes in bulk samples to quasi-coherent rotation in thin flakes, driven by increased effective anisotropy and suppressed domain formation. This thickness-dependent transition enables tuning of magnetic hardness without chemical modification. Combined with high saturation magnetization and robust room-temperature performance, Fe$_3$GaTe$_2$ emerges as a promising rare-earth-free material for spintronic applications. Its layered structure further allows integration into van der Waals heterostructures, where large in-plane coercivity can stabilize magnetic states against perturbations and interlayer coupling, offering potential for high-density nonvolatile memory and domain-wall-based devices.
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All authors Lingrui Mei, PeiYu Cai, Sang-Eon Lee, Yue Li, Shyam Raj Karullithodi, Vadym Kulichenko, Charudatta Pathak, Elton J. G. Santos, Luis Balicas

Mesoscale and Nanoscale Physics

Engineering micro-disorder for macro-performance in magnetic nanoparticles

Jonathan Leliaert and Elizabeth M Jefremovas

Spin disorder, inherent to magnetic nanoparticles, has traditionally been regarded as a detrimental feature, with materials-engineering efforts largely focused on producing ”perfect particles” containing as few defects as possible. Alongside this pursuit of perfection, however, an alternative framework has emerged in recent years that reframes intra-particle disorder as an ”ugly duckling” whose functional potential remains to be unlocked. In this Perspective, we review the emerging concept of disorder engineering in magnetic nanoparticles, identify its current challenges, and outline promising future directions. From a theoretical standpoint, progress requires moving beyond the widely used macrospin approximation, which severely restricts the description of intra-particle degrees of freedom. Micromagnetic modelling, in contrast, treats magnetisation as a continuous vector field and thereby enables (i) the explicit representation of intra-particle degrees of freedom, linking microstructural features to internal magnetisation textures, and (ii) direct correspondence with polarized small-angle neutron scattering, an experimental technique that provides quantitative access to ensemble-averaged magnetic correlations on nanometre length scales. The field must now advance towards falsifiable and uncertainty-aware models with structurally motivated parameters and predictions that can be tested against independent experimental observables. The overarching goal is to establish quantitative relationships between particle structure, intra-particle magnetisation textures, and macroscopic functionality, thereby transforming spin disorder from an elusive hidden variable into an engineerable design parameter.
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Strongly Correlated Electrons · Cross-listed from math-ph

Free to Interacting Map for Crystalline SPT Phases: Equivariance vs Crystalline Equivalence

Daniel Sheinbaum and Omar Antolín-Camarena

Freed and Hopkins developed an ansatz for classifying interacting SPT phases using invertible field theories with a natural Free to Interacting (FTI) map from free fermion phases. This ansatz has been generalized to include crystalline phases and a crystalline equivalence principle (CEP). However, motivated by failure of the CEP for weak free fermions and the FTI, here we generalize the original Freed and Hopkins ansatz to a fully equivariant version for symmorphic crystallographic symmetries and show there is a natural equivariant FTI map from symmorphic crystalline weak free fermions. We further discuss why this equivariant ansatz is both mathematically and physically more natural than the spatial symmetry extension by Freed and Hopkins and why full equivariance should hold over the CEP.
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Statistical Mechanics

Exact Results for the Symmetric Dyson Exclusion Process

Ali Zahra, Jerome Dubail and Gunter M. Schutz

The symmetric Dyson exclusion process (SDEP) is an exclusion process on the lattice with a long-range logarithmic Coulomb-type interaction. It appears in several equivalent forms: as symmetric random walkers conditioned, in the Doob-transform sense, not to collide; as the maximal-activity limit of a conditioned SSEP; and as a ground-state transform of the spin- 1/2 XX chain. In this work, we exploit this latter representation to obtain exact evolution formulas from deterministic initial configurations. The resulting determinantal kernel is expressed through a finite interpolation expression in terms of Lagrange polynomials, leading to explicit density evolution in finite and infinite lattices. For the melting of a densely packed block, we show that the full hierarchy of density moments is governed by a finite-dimensional polynomial algebra, and we identify Catalan numbers in the leading time coefficients. We also derive the Euler-scale density profile and the arctic curve separating frozen and liquid regions, and show that they coincide with conjectured hydrodynamic results obtained in a previous work.
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Mesoscale and Nanoscale Physics

Symmetry-assisted computation of the magnetic field at the site of a point dipole

D. Pescia

In this paper, we revisit this concept of the magnetic field at the site of a magnetic dipole using symmetry arguments that are mathematically rigorous yet intuitive enough to be well-suited for upper-level undergraduate courses in electrodynamics and quantum mechanics. Furthermore, we present symmetry-based results that go beyond those discussed in the standard literature, which are directly relevant to fundamental problems in two-dimensional physics.
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Materials Science

Open-Shell Molecules as Sensitive Probes of Spin-Orbit-Controlled Surface Electronic Structure

Solange Mariel Di Napoli, Alexei Nefedov, María Andrea Barral et al. (+5 more)

Open-shell molecules are highly sensitive probes of correlated oxide surfaces because their partially occupied frontier orbitals strongly amplify subtle changes in adsorbate-substrate hybridization. Combining infrared reflection-absorption spectroscopy with hybrid density functional theory including non-collinear magnetism and spin-orbit coupling, we show that inclusion of spin-orbit coupling is essential to reproduce the vibrational spectrum of NO adsorbed on UO$_2$(111) by suppressing an otherwise artificial overhybridization between the NO frontier orbitals and uranium 5f states. While spin-orbit coupling determines the position of the N-O stretching and, dispersion-driven intermolecular interactions account for its asymmetric broadening at monolayer coverage. These results establish open-shell molecules as sensitive probes of spin-orbit-controlled surface electronic structure and identify NO/UO$_2$(111) as a stringent benchmark for electronic-structure methods describing open-shell molecule-surface interactions.
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All authors Solange Mariel Di Napoli, Alexei Nefedov, María Andrea Barral, Gustavo E. Murgida, Ana María Llois, Christof Wöll, M. Verónica Ganduglia-Pirovano, Verónica Laura Vildosola

Materials Science

Electrical Control over Volatile Mott Switching through Non-Volatile Memory Effects

Amihai Kronman, Gil Levi and Yoav Kalcheim

Vanadium Dioxide (VO2) and Vanadium Sesquioxide (V2O3) are Mott insulators that undergo an Insulator-to-Metal Transition (IMT) at ~340K and ~160K, respectively, manifested as an orders-of-magnitude reduction in their electrical resistance. These transitions provide the physical basis for their Volatile Resistive Switching (VRS) behavior, making them promising candidates for threshold-switching devices. Here, we show that the volatile switching in these materials can be strongly affected by non-volatile processes associated with the creation and annihilation of the conducting filament. These processes give rise to pronounced memory effects in which the initial switching voltage substantially exceeds that of subsequent cycles. We identify two distinct mechanisms underlying this behavior in different thermal regimes. In the phase-coexistence regime, a memory effect is observed in V2O3, arising from the spatial redistribution of metallic and insulating domains. Well below the hysteresis regime, an additional electroforming-like memory effect is observed in both VO2 and V2O3 which is attributed to the formation and/or migration of defects under the influence of a high electric field and a current surge associated with the IMT-driven switching. Using a protective internal resistor and a multi-step writing protocol, this normally destructive process can be harnessed to tune the switching voltage and power over a wide temperature range. These results demonstrate a route toward controlled programming of switching parameters in IMT-based resistive switching devices.
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