Materials Science
Piezo-Hall effect in $\mathrm{RbCr_2Se_2O}$
San-Dong Guo, Shi-Hao Zhang and Feng-Ren Fan
The anomalous Hall effect (AHE) is of fundamental importance for understanding
spin-orbit coupling and
magnetic transport in
magnetic materials. Meanwhile, strain can significantly modify material properties, and the piezoelectric effect stands as a well-known representative. The piezo-Hall effect combines the two physical ingredients of AHE and strain. Specifically, the piezo-Hall effect describes a physical process in which the
magnetic space group (MSG) of a
magnetic material prohibits a net
magnetic moment and suppresses the AHE in the pristine state but transforms into a new MSG upon strain application that permits a net
magnetic moment, thereby enabling the emergence of the AHE. The underlying mechanisms cover strain-induced
magnetic electronic state transition and strain tuning of magnetization direction. We take the $d$-wave altermagnetic metal as an example to analyze the piezo-Hall effect in detail, with first-principles calculations carried out on the specific material $\mathrm{RbCr_2Se_2O}$ to verify this scenario. A notable result is that under identical uniaxial strain applied along the $x$ and $y$ directions, the anomalous Hall conductivity exhibits opposite signs but equal magnitudes. Similarly, the piezo-magneto-optical effect can also be proposed, and if strain tuning is replaced by electric-field modulation, the electro-Hall effect and electro-magneto-optical effect can be also put forward accordingly. Even temperature can serve as an external field to introduce the temperature Hall effect. Our work advances the understanding of the coupling between strain and physical properties.
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Statistical Mechanics
Periodically modulated traveling waves in integrate-and-fire networks: recursive speed law and propagation failure
Jie Nissel, Ricardo Erazo-Toscano, Rosahn Bhattarai et al. (+3 more)
Traveling waves of activity in neural tissue can be halted by spatial inhomogeneity in synaptic coupling. We study an integrate-and-fire network in which each
neuron fires once and the coupling decays exponentially. For this model the leading-edge firing map reduces exactly to a scalar equation for the wave speed in space, with a slow unstable and a fast stable homogeneous speed, $c_1$ and $c_2$, and a periodic modulation of the coupling enters this equation pointwise. Positive periodic waves terminate in folds. For slowly varying modulation the fold amplitude approaches a plateau set by the trough of the modulation, where the local bottleneck speed is $\sqrt{c_1 c_2}$; the approach to the plateau is set by the local geometry of the trough. For rapidly varying modulation the fold amplitude grows linearly with frequency, with a slope given by a full-amplitude average over the integrated modulation profile; a weak-ripple truncation of that average overestimates the slope about 2.5-fold at the default coupling. A perturbative recursion in the amplitude gives the speed profile explicitly. We distinguish loss of the periodic wave from failure of a particular launch, and the excitatory regime $\epsilon\le 1$ from its sign-changing extension. The results are checked against direct integration of the reduced equation and first-spike firing-map simulations of the network.
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All authors Jie Nissel, Ricardo Erazo-Toscano, Rosahn Bhattarai, Marius Osan, Mandeep Chauhan, Remus Osan
Other Condensed Matter
Physics-based logarithmic description of electrostatic field enhancement in hemisphere-on-cylindrical-post structures for high-field applications
Thiago A. de Assis, Fernando F. Dall-Agnol and Richard G. Forbes
Electrostatic (ES) field enhancement at sharp conducting structures plays a central role in lightning protection, corona discharge, electrical breakdown in
vacuum (for example in particle accelerators), and more generally in technological applications of field electron emitters. A canonical geometry for studying this ES effect is the hemisphere-on-cylindrical-post (HCP) model, in the regime where the structure stands on a planar conductor of large lateral extent, and a large gap exists between the structure and the counter-electrode. A parameter of major interest is the apex field enhancement factor (AFEF) (apex-ES-field/background-ES-field). For this (and other) structures, a formula for the AFEF can be written in the form AFEF = ASC x ASR, where the apex sharpness ratio (ASR) is given by the ratio (post-height/apex-radius-of-
curvature), and the apex sharpness coefficient (ASC) depends on the post shape. For the HCP model, no exact analytical formulas for the ASC or the AFEF are currently known. (Quite possibly none exist.) This paper develops a compact analytical approximation for the ASC and hence for the AFEF. This compact formula has a logarithmic-style structure, rather than the power-style structure used in previous approximations. When compared with precise finite-element analyses of the HCP model, over the computationally accessible range 1 to 1000 for the ASR, this logarithmic-style formula has a maximum error-magnitude of 0.15 percent, which is significantly better than older approximations.
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Strongly Correlated Electrons
How Much Do We Understand Ce L3 XANES? Ab Initio Insights into Configuration-Specific Screening at the Multiplet–Continuum Frontier
Alessandro Mirone
How can local multiplet signatures, collective core-hole screening, and an extended photoelectron be reconciled in Ce $L_3$ x-ray absorption near-edge spectra? Motivated by the question raised by Kvashnina [Chem. Eur. J. 30, e202400755 (2024)], we address this challenge using XSpectruplet, which couples local multiplet dynamics to continuum photoelectron propagation in self-consistent Kohn–Sham screening potentials. For Ce$^{4+}$ in CeO$_2$, our heuristic search identifies primary and secondary stationary screening states whose calculated white-line maxima lie about 7.3~eV apart after including their independently calculated total-energy difference, yet both retain the same effective local spectroscopic $4f^0$ sector. Their squared overlaps with the unrelaxed sudden reference give approximate statistical weights of 63% and 30%, respectively, within an independent-channel description. Their contrasting screening responses arise from collective rehybridization of occupied orbitals, rather than occupation of an additional localized $4f$ spectator, and produce contributions in both the main white-line and satellite regions. Ce$^{3+}$ in monazite CePO$_4$ provides the contrasting case of a localized $4f^1$ spectator participating explicitly in the multiplet dynamics. Independent relaxation of neutral $4f$ and white-line states gives a CeO$_2$ pre-edge–white-line gap of 10.51~eV, close to the experimental separation of approximately 10.7~eV. Its excess over the fixed-background spectral gap defines a net correction, which we transfer heuristically to CePO$_4$ without changing its local multiplet structure. This gives an estimated CePO$_4$ gap of 6.38~eV, compared with approximately 8.1~eV experimentally. Distinct spectral contributions can thus arise from collective screening without a change in the effective local configuration.
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Materials Science
Impact of Phonon-Magnon Scattering on Thermal Transport across Antiferromagnetic and Ferromagnetic Oxides
Zeyu Xiang and Bolin Liao
Phonon–
magnon coupling has been extensively studied in ferromagnets, whereas its microscopic mechanisms in antiferromagnets remain less understood. Here, we combine first-principles calculations with
spin–
lattice dynamics to investigate
phonon–
magnon interactions and their impact on thermal transport in antiferromagnetic NiO and MnO, which are put in contrast to the ferromagnetic EuO with an identical
crystal structure. Green–Kubo and spectral energy density analyses show that
spin dynamics reduce
phonon thermal conductivity and lifetimes, with the strongest effects in MnO. Comparison of the excitation spectra indicates that
spin-induced
phonon scattering depends sensitively on the relative
phonon and
magnon energy scales, while the contrasting behavior of MnO and EuO further highlights the role of
magnetic order. A simplified phase-space model further illustrates that, for comparable
phonon and
magnon energy scales, the antiferromagnetic case can support a larger
phonon–
magnon scattering phase space than the ferromagnetic case, suggesting a greater number of kinematically allowed scattering channels. These results identify excitation energy scales and
magnetic order as key factors governing
phonon–
magnon scattering in
magnetic materials.
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Soft Condensed Matter
Domain growth in lipid membranes and the budding instability
Jean Wolff, Fabrice Thalmann, Carlos Marques et al. (+3 more)
We investigate the growth of a circular domain in a phase-separating lipid membrane and its relation to the budding instability. We describe the in-plane dynamics with a conserved, time-dependent Ginzburg-Landau model under radial
symmetry. Numerical solutions show a crossover of the domain size $D$ as a function of time $t$ from $D\sim t^{1/3}$ at early times to $D\sim t^{1/2}$ at later times, followed by saturation due to finite system size. The time-dependent line tension $\sigma(t)$, evaluated from the evolving concentration profile, rises rapidly before approaching the planar-interface value. This behavior distinguishes an early line-tension-dominated regime from a later domain-size-dominated regime. To estimate the budding instability, we combine $D(t)$ and $\sigma(t)$ with a quasistatic criterion for the loss of stability of a partially budded domain. The budding-instability time decreases with increasing membrane spontaneous
curvature and increases near the critical point. Thus, the
evolution of both domain size and line tension is essential for determining the time required for the budding instability.
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All authors Jean Wolff, Fabrice Thalmann, Carlos Marques, David Andelman, Haim Diamant, Shigeyuki Komura
Superconductivity
Spin pumping reveals vortex-mediated angular-momentum dissipation at the superconducting transition
Maximilian Mangold, Alex Burg, Franz Weidenhiller et al. (+4 more)
Spin pumping into
superconductors is generally expected to become less efficient below the critical temperature $T_\textrm{c}$ because quasiparticle transport freezes out. Here we report a pronounced enhancement of the
spin pumping-induced
magnetic damping in Py/Nb
heterostructures, confined to a narrow temperature interval immediately below the superconducting transition. The damping exceeds its normal-state value and therefore cannot be explained within the conventional quasiparticle picture of
spin transport. The enhancement coincides with the finite-resistance
vortex-creep regime of the Nb layers and disappears upon cooling into the pinned
vortex state, indicating that mobile Abrikosov
vortices provide an additional channel for
spin-angular-momentum dissipation. These results establish ferromagnetic resonance as a sensitive probe of dissipative
vortex dynamics in
superconductors.
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All authors Maximilian Mangold, Alex Burg, Franz Weidenhiller, Thomas N.G. Meier, Hiroto Adachi, Lin Chen, Christian Back
Soft Condensed Matter
Active matter within flexible boundaries: a novel experimental approach with T. aceti nematodes
Christina M. Ceballos, Matthew S. Mizuhara, Mykhailo Potomkin et al. (+1 more)
We experimentally explore the collective behavior of the nematode T. Aceti inside flexible boundaries showing the emergence of previously unreported states. These nematodes have been previously shown to be able to synchronize their body oscillations in a favorable condition of confined space. In this collective state, they are able to exert a strong pushing force which we exploit to study how their collective motion could deform a pliable boundary. Using a novel experimental technique, we were able to simulate a non-rigid border at a
liquid-
liquid-gas triple point. We discovered a state with periodic boundary deformations which are due to the synchronized oscillations of the nematodes. More interestingly, we found a novel state where the nematodes are able to form multiple protrusions in the border which oscillate, rotate, merge, and split. We propose a numerical model that is able to reproduce some of the observed parameters of this state as well as offer an analytical insight into protrusions dynamics. Our research opens a pathway for exploring the interaction between active matter and flexible interfaces with a new kinds of active matter
agents forming dynamic non-chaotic border deformations.
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All authors Christina M. Ceballos, Matthew S. Mizuhara, Mykhailo Potomkin, Anton Peshkov
Materials Science
Parity-selected Bethe-Salpeter equation: symmetry-adapted regularization of the exciton-phonon self-energy in polar semiconductors
Michael O. Atambo
Finite-temperature Bethe-Salpeter calculations of
exciton-
phonon self-energies in polar materials are bottlenecked by the Fröhlich divergence of the intra-1s scattering vertex. We show that for a bound electron-hole pair this divergence is analytically eliminated by electron-hole interference: the intra-1s vertex admits the exact factorization $V_{\mathrm{el}}(q)=q\,\mathcal G(q)$ with $\mathcal G(0)\propto(m_h-m_e)/(m_h+m_e)$, vanishing identically for equal masses (parity protection). A multipole interference factor $\mathcal I_n=\beta_e^{\,n}-(-\beta_h)^{n}$ determines where the singularity survives-only in dipole-allowed inter-state channels. We formulate a parity-selected truncation of the
exciton-
phonon self-energy in which the intra-1s channel is evaluated exactly at negligible cost, converging with $10^2$-$10^3$ fewer $q$-points. We find that the regularized intra-1s channel, previously obscured by numerical cutoffs, controls the temperature-dependent
exciton shift in mass-asymmetric polar
semiconductors.
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Materials Science
Composition-Driven Metal-to-Semiconductor Transition and Enhanced Phonon Transport in B-C substituted Clathrate
Ghulam Hussain, Dario Massa, Rajibul Islam et al. (+3 more)
Establishing chemical design rules that simultaneously control the electronic structure and thermal transport is a long-sought goal for heat-management and energy materials. Here, we demonstrate that a single B-to-C substitution changes the electron count and simultaneously reconstructs the bonding network and
crystal structure, driving a metal-to-
semiconductor transition while concurrently enhancing the
lattice thermal conductivity. Using density-functional theory (DFT) and machine-learned interatomic potentials (MLIPs), we investigate the electronic structure,
lattice dynamics, and
phonon thermal transport in cubic BaB3C3 and its B-to-C-substituted tetragonal BaB2C4 structure. The substitution donates one electron per formula unit to the B-C framework, thereby triggering the formation of strong C-C bonds and opening a bandgap of 0.33 eV. The Message Passing Atomic Cluster Expansion (MACE) model reproduces DFT energies and forces, yielding
phonon dispersion and
lattice thermal conductivity in excellent agreement with DFT
benchmarks. Thermodynamically stable cubic BaB3C3 possesses an isotropic
lattice thermal conductivity of 7.6 W/mK at 300 K. Substituting B with C hardens the
phonon dispersion (evidenced by a frequency upshift from ~800 to ~900 cm^-1) and simultaneously boosts both
phonon group velocities and lifetimes. Consequently, the
lattice thermal conductivity surges to 18.7 W/mK, representing a remarkable ~2.5-fold enhancement of the in-plane component and a transformation from isotropic to anisotropic behavior, characterized by kappa_x = kappa_y > kappa_z. Our results demonstrate that minimal B-to-C substitution provides a viable strategy to modulate the electronic structure, thereby transforming the
crystal from a metal to a
semiconductor and concomitantly enhancing
phonon thermal transport.
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All authors Ghulam Hussain, Dario Massa, Rajibul Islam, Magdalena Birowska, Carmine Autieri, Xiaoguang Li
Statistical Mechanics
Mpemba effect without a wall
Siddharth S. Sane and John Bechhoefer
Experimental demonstrations of the Mpemba effect in a colloidal-particle system have imposed an instantaneous temperature quench via an initial condition where individual particle trajectories are drawn from a high-temperature Boltzmann distribution and then evolve in a bath at lower temperature. The potential used for the high-temperature distribution has had two walls that impose a finite range of initial positions, even at effectively infinite temperatures. The potential for
evolution in the bath had no walls and matched that used for the initial condition over its support. For low bath temperatures, the difference between the dynamics with and without walls is negligible. Nonetheless, it has been speculated that the walls are perhaps more important than they seem and might even be required for the Mpemba effect to occur. Here, we introduce a new potential that lacks walls and use it for both the initial high temperature state and for subsequent
evolution. This new potential then allows us to investigate by simulation Mpemba effects at finite-quench rates. Our results help clarify intuitions concerning the existence of a Mpemba effect and show that the Mpemba effect appears only for bath-temperature quenches faster than a critical quench rate.
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Disordered Systems and Neural Networks
Improved Learning of Molecular Energetics Through an Electron-Wise Joint Charge Density and Energy Objective
Vadim Ionas, Jonas Elsborg, Felix Ærtebjerg et al. (+1 more)
We present a graph neural network-based learning framework trained to jointly predict electron densities and molecular energies. The model is trained to predict electron densities calculated with a Generalized Gradient Approximation (GGA) functional while simultaneously learning to predict molecular energies obtained with hybrid functional calculations at the B3LYP level of theory. Our results indicate that the rich spatial information in electron density distribution can be used to improve and accelerate the learning of accurate energies. By sharing an equivariant molecular representation across density and energy prediction heads, the model learns complementary molecular quantities within a unified framework. This provides a new promising route for practical use cases of many recent charge density learning frameworks for atomic scale simulations.
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All authors Vadim Ionas, Jonas Elsborg, Felix Ærtebjerg, Arghya Bhowmik
Materials Science
Digital Etching of Single-Crystalline Silicon Fin Barriers Down to Sub-10 nm
Yu Wu, Teun A. J. van Schijndel, Anthony P. McFadden et al. (+3 more)
This work presents a fabrication process to realize sub-10 nm-thick, high-aspect-ratio Si fins using high-resistivity float-zone single-
crystal Si wafers. Following anisotropic wet etching, a digital etching process based on repeated rapid thermal annealing (RTA) oxidation and vapor-phase hydrofluoric acid (VHF) etching of the resulting oxide was developed, enabling thinning of Si fins into the sub-10 nm regime. Cross-sectional scanning transmission electron microscopy confirms fin thicknesses as thin as 6 nm, although with a tapered profile resulting from the initial anisotropic wet etch. Tapering of the fin was reduced by using an N{_2}/O{_2} (5:1) ambient together with a reduced ramp rate during the temperature ramping for the RTA oxidation process. A thickness variation of 3 nm along a 1.2 {\mu}m tall fin was achieved on a fin with 12 nm near the top and 15 nm near the base. The developed process provides a pathway for fabricating Josephson junctions, setting a foundation toward reproducible and scalable superconducting
qubit fabrication with single-
crystal Si dielectric barriers using readily available Si processing technology.
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All authors Yu Wu, Teun A. J. van Schijndel, Anthony P. McFadden, Wilson J. Yánez-Parreño, Raymond W. Simmonds, Christopher J. Palmstrøm
Disordered Systems and Neural Networks
Trimers, pairs and localisation on disordered random graphs
Alexei Vazquez
Higher-order networks take group interactions as given. I ask what happens when a group interaction is made by pairwise dynamics on a graph, and follow it through the transition that decides whether it matters at large scale, Anderson localisation. An emergent hyperedge is a $k$-body bound state of a
Hamiltonian whose terms live on the vertices and edges of the graph, Borromean when none of its $(k-1)$-body faces is bound, detected on a finite graph by a $k$-fold coincidence that survives the thermodynamic limit while the $(k-1)$-fold ones vanish. Sparse random graphs produce one from the simplest pairwise dynamics: three distinguishable particles with on-site attraction $U$ and hopping $t$ bind into a trimer before any pair binds. The thresholds are obtained on the Bethe
lattice, in closed form for the pair, in both sectors that an expander graph has and a
lattice lacks, the one a condensate sees and the one a Fermi
liquid sees. On a disordered graph the emergent hyperedges nucleate on rare deep sites and localise before their parts: the compact trimer, with a narrow band and a threefold sensitivity to site energies, undergoes the Anderson transition at a disorder of $1.6t$, against $17t$ for a single particle and $4t$ for a pair, and the Fermi
liquid of trimers is a glass above a line between $1.5t$ and $1.7t$, where a
liquid of single particles at the same density needs $15t$ to $16t$. The transition is the single-particle cavity transition with the composite's hopping and effective disorder. A group interaction made by pairwise dynamics is an order of magnitude more fragile to heterogeneity than the network that makes it.
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Materials Science
Phase-selective growth of water-soluble strontium aluminate via sputtering deposition for freestanding oxide membranes
Evgenios Stylianidis, Marie Dallocchio, Bernat Mundet et al. (+6 more)
We report the epitaxial growth of high-quality water-soluble strontium aluminate films via sputtering deposition. We systematically investigate the dependence of the films structural quality on the growth conditions and we show that we can selectively grow, using the same target, both the cubic Sr3Al2O6 and the tetragonal Sr4Al2O7 phases by varying the atmosphere during growth. Using these water-soluble sacrificial layers we then synthesize ferromagnetic freestanding Sm2NiMnO6 membranes. By means of X-ray diffraction, synchrotron spectroscopic techniques and SQUID magnetometry, we demonstrate that our freestanding films exhibit properties comparable to epitaxial films.
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All authors Evgenios Stylianidis, Marie Dallocchio, Bernat Mundet, Lucia Varbaro, Clementine Thibault, Christo Guguschev, Mario Brützam, Javier Herrero, Marta Gibert
Disordered Systems and Neural Networks · Cross-listed from math.PR
Free energy of non-convex multi-species spherical spin glasses
Hong-Bin Chen and Jean-Christophe Mourrat
We identify the limit free energy of all mean-field multi-species
spin glasses defined on products of spheres. In particular, we do not make any convexity assumption on the covariance function of the
Hamiltonian, and we allow for the presence of external fields. We also obtain a one-species reduction of the formula in the special case of balanced models.
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Materials Science
Site-Selective Yttrium Substitution in Ti3AlC2 Enables Interlayer Engineering and Li Transport in Ti3C2Tx cathodes for High-Power Energy Storage
Tetiana Boichuk, Andrii Boichuk, Mahesh Eledath Changarath et al. (+5 more)
In this work, we introduce a precursor-level strategy by substitutionally doping the Ti3AlC2 MAX precursor with a small amount of Yttrium during top-down MAX preparation. This allows permanent structural changes that are retained in the resulting Ti3C2Tx MXene. Density functional theory (DFT) calculations identify the outer Ti(4f) site, facing the Al layer, as the preferred incorporation site for Y under the Al-excess synthesis conditions used here, rather than the inner Ti(2a) or Al(2b) sites. This site-specific substitution is consistent with the experimentally observed
lattice expansion and carbide-like Y bonding and provides a microscopic basis for the structural changes retained in the resulting Ti3C2Tx MXene. Y incorporation expands the MXene interlayer spacing due to an increase in the amount of hydroxyl surface terminations, creating more favorable pathways for lithium-ion transport. Comprehensive structural characterization (including XRD, XPS and TEM) confirms these modifications. As a result, the Y-doped Ti3C2Tx cathode shows
diffusion-controlled charge storage with a Li
diffusion coefficient of 10-9-10-11 cm2/s. It delivers a reversible capacity of about 130 mAh/g at 0.2 C and retains 74 mAh/g after 1000 cycles at 2 C with nearly 100% Coulombic efficiency. The electrode also achieves a high power density of up to 3970 W/kg, outperforming the undoped MXene and previously reported multilayered MXene cathodes. These results show that substitutional doping of the MAX precursor is a promising strategy for controlling the interlayer structure of MXenes and improving ion transport, providing a new route for the development of high-power energy-storage electrodes.
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All authors Tetiana Boichuk, Andrii Boichuk, Mahesh Eledath Changarath, Joao Fonseca, Said Agouram, Marie Finas, Alejandro Molina-Sanchez, Juan F. Sanchez-Royo
Statistical Mechanics
Backward Kolmogorov Transport: Sampling Invariant Laws of Reversible Diffusions from Trajectory Data
Yuanchao Xu and Isao Ishikawa
We consider a reversible
diffusion with unknown drift, observed through trajectories, and construct samplers of its invariant law. The trajectories contain samples of this law, but a sampler built from them by density or score estimation inherits the errors of these estimates, in particular on the barriers between metastable states. The trajectories also contain the dynamics, and for a reversible
diffusion the ratio between the law of the process and its invariant law solves the backward Kolmogorov equation, so that the eigenpairs of the generator propagate it in closed form. Backward Kolmogorov transport (BKT) evaluates this ratio with eigenpairs estimated from the trajectories and coefficients computed once from the initial particles, and moves the particles along the Wasserstein gradient flow of the Kullback-Leibler divergence. The particles do not interact, BKT defines a transport map, and no density, score or drift is estimated. With exact eigenpairs BKT transports the spectral projection of the initial law exactly when this projection is positive. For estimated eigenpairs we prove a transport identity in which truncation enters only through the initial law and estimation only through the residual of the eigenpairs, and a local stability bound in the 2-Wasserstein distance. With coefficients computed from the transported particles, the sampling error of the retained modes cancels, and a central limit
theorem identifies the asymptotic variance. Experiments on Ornstein-Uhlenbeck processes, multi-well potentials, separable products and alanine dipeptide, including comparisons with particle methods based on density estimates, agree with these results, and with an accurate spectrum the errors of BKT reach or fall below those of independent samples.
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Materials Science
Electro-Thermal Origins of Enhanced Performance and Scalability in Disc-Type Phase-Change Memory
Ghazi Sarwat Syed, Loris Coccia, Siddharth Gautam et al. (+2 more)
Disc-type phase-change
memory is a promising device architecture for analog in-
memory computing, enabled by the unique physical properties of ultra-thin phase-change materials and the device geometry. In this study, we develop a comparative analytical framework using compact electro-thermal models to examine the state dependent readout and programming behavior of these devices. The framework provides insight into the fundamental physical mechanisms responsible for their enhanced functionality and highlights promising directions for further device optimization and performance improvement.
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All authors Ghazi Sarwat Syed, Loris Coccia, Siddharth Gautam, Vara Prasad Jonnalagadda, Abu Sebastian
Mesoscale and Nanoscale Physics
Finite-temperature phase transitions across spin symmetries in two-dimensional magnets
Chen Chen, Moritz A. Goerzen, Megha Arya et al. (+3 more)
Recent advances in intrinsic two-dimensional (2D) magnets have created a need for a unified understanding of finite-temperature phase transitions across different
spin-
symmetry classes. Here, we investigate finite-temperature magnetism in the classical 2D Heisenberg model using large-scale Monte Carlo simulations, continuously spanning the easy-plane, isotropic, and easy-axis regimes. We find that characteristic temperatures are strongly affected by the interplay of finite-size effects and
spin symmetry, induced by magnetocrystalline anisotropy, being most pronounced in the isotropic Heisenberg regime and substantially reduced toward the easy-plane and easy-axis limits. Examining Berezinskii–Kosterlitz–Thouless (BKT) physics in the easy-plane regime, we find that the characteristic temperatures independently estimated from the magnetization and heat capacity deviate significantly from those proposed by BKT theory, but match a previously reported anomaly in the XY-model. We report that characteristic temperatures instead are correlated with the proliferation of
vortex-antivortex pairs, suggesting the gain in
entropy by their accelerated nucleation as the origin of the anomaly. Establishing this connection both theoretically and numerically, we argue that the thermodynamic characteristic temperature in finite easy-plane Heisenberg magnets, including the deviation from BKT theory, is subject to the interplay between finite-size
spin correlations and
vortex proliferation. These results clarify how experimental and technologically relevant temperature scales evolve with
magnetic anisotropy,
spin symmetry, and system size in finite 2D magnets.
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All authors Chen Chen, Moritz A. Goerzen, Megha Arya, Lionel Calmels, Yongping Du, Dongzhe Li
Strongly Correlated Electrons
GEM: An implementation of the ghost-Gutzwiller approximation for simulating interacting quantum systems
Samuele Giuli, Tsung-Han Lee, Yong-Xin Yao et al. (+6 more)
We present GEM (Ghost
Embedding Method), an open-source software package written in Python for computing
equilibrium properties of strongly correlated electronic systems within the ghost-Gutzwiller approximation method. GEM provides a computationally efficient framework for studying multi-orbital
lattice models. It supports zero- and finite-temperature calculations and
symmetry broken phases. It is integrated with the TRIQS ecosystem, providing tools for model construction, self-consistent solution, and evaluation of physical observables. We first detail the method's theoretical formulation, then we present the software architecture, and finally we introduce some practical workflow, which also validates the implementation against established results. In particular, we illustrate the capabilities of GEM through multiorbital and finite-temperature applications and discuss its computational cost relative to more demanding
quantum embedding approaches.
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All authors Samuele Giuli, Tsung-Han Lee, Yong-Xin Yao, Ina Park, Harrison LaBollita, Ivan Pasqua, Nils Wentzell, Nicola Lanatà, Olivier Gingras
Disordered Systems and Neural Networks
Dense auto-hetero associative memories applied to noisy communication channels
Elena Agliari, Andrea Alessandrelli, Adriano Barra et al. (+1 more)
Networks of interacting Hebbian networks have recently been shown to perform a task beyond associative
memory, namely pattern disentanglement: when fed with a spurious mixture of stored patterns, the different modules spontaneously specialize on, and retrieve, the different constituents of the mixture. So far, this capability has only been established for pairwise interactions, which limits the number of patterns that can be handled. Here we introduce a dense extension of these modular networks, in which both the auto-associative couplings within each module and the hetero-associative couplings among modules are promoted to higher-order Hebbian interactions. We show that, with a suitable choice of the interaction orders, the network disentangles mixtures while storing a number of patterns that scales linearly with the module size, a regime where its pairwise counterpart fails. Through a statistical-mechanical analysis based on Guerra's interpolation, we derive the self-consistency equations for the order parameters and draw the phase diagrams identifying the region where disentanglement is achieved; these predictions are confirmed by Monte Carlo simulations. Finally, we show that disentanglement provides a natural decoding primitive, and we illustrate it with two applications: the explicit reconstruction of all the hidden patterns from the Hebbian tensors and a stream of unlabeled mixtures, and a proof-of-concept communication protocol in which each message
token is transmitted as a masked mixture of hidden patterns and decoded by the network dynamics. Owing to its attractor-based decoding, the protocol degrades gracefully under strong channel corruption, where conventional secure-transmission pipelines fail abruptly.
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All authors Elena Agliari, Andrea Alessandrelli, Adriano Barra, Alberto Fachechi
Other Condensed Matter
Phonon angular momentum transfer torque
Verena Brehm, Daniel A. Bustamante Lopez, Shu Zhang et al. (+1 more)
Angular momentum in solids is carried by (quasi)particles such as
magnons, plasmons, and
phonons, and coupling between these reservoirs enables diverse hybrid phenomena. Here, we introduce the transfer of angular momentum from
lattice vibrations to
magnetic order through the
phonon angular momentum transfer torque (PAMTT), generated by a polarized
phonon bath. We identify thermal routes for generating
phonon angular momentum: a
phonon Edelstein accumulation in noncentrosymmetric
crystals and temperature-gradient-induced PAM currents with longitudinal Seebeck-like and transverse Hall components. Using realistic order-of-magnitude estimates, we find that PAMTT could produce measurable ferromagnetic-resonance frequency shifts and, for favorable interfacial coupling, drive magnetization reversal on nanosecond timescales.
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All authors Verena Brehm, Daniel A. Bustamante Lopez, Shu Zhang, Dominik Juraschek
Mesoscale and Nanoscale Physics · Cross-listed from hep-ph
Wide-Mass-Scanning-Range Setup and Phase-Resolved Protocol for Axion Dark Matter Detection
A. Cottet and T. Kontos
We propose a paradigm for
quantum enhanced
axion dark matter search, which does not rely on power measurements. We propose to measure directly the
axion amplitude and phase in an interferometric protocol at the
quantum limit, using a non-linear cavity. In addition, we introduce gyromagnetic modes as wide mass range transducers for
axion signals compatible with standard haloscope designs. We expect this scheme to offer an improvement of at least 4 orders of magnitude in figure of merit and at least 2 orders of magnitude in mass window with respect to standard haloscopes. Owing to its generality, our proposed protocol has the potential to speed up
axion search but also the search for dark
photons or other cosmological objects, such as galactic masers.
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Mesoscale and Nanoscale Physics · Cross-listed from math-ph
Absolute and relative invariants for low-dimensional topological insulators
Federico Manzoni, Domenico Monaco and Gabriele Peluso
[An extended abstract is provided in-text] Motivated by topological
insulators, this paper analyzes classification schemes for projection-valued maps on $d$-dimensional
tori, $d \le 2$, subject to Altland-Zirnbauer-Cartan (AZC)
symmetries (time-reversal, particle-hole, and chiral). The main objective is to compare three notions of equivalence for these maps: Murray-von Neumann,
unitary, and homotopy equivalence. The analysis relies on constructing symmetric bases for the projection ranges under a pseudo-periodicity constraint, where select basis vectors pick up a phase when looped around the
torus. If $d=1$, fully periodic symmetric bases exist across all AZC classes. If $d=2$, topological obstructions appear as non-trivial phases characterized by $\mathbb{Z}$ (Chern number) or $\mathbb{Z}_2$ (Fu-Kane-Mele) invariants. These serve as absolute invariants that classify both Murray-von Neumann and
unitary equivalences. For homotopy equivalence, the behavior depends on the ambient
Hilbert space: if this has non-minimal dimension, homotopy equivalence reduces directly to
unitary equivalence, while in minimal ambient spaces additional relative homotopy invariants emerge. Their precise values change under
unitary conjugation, leaving only relative topological phases between maps well-defined. Ultimately, this provides a unified
geometric framework for gapped
quantum systems in low dimensions, demonstrating how distinct equivalence relations generate specific absolute and relative topological invariants.
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Mesoscale and Nanoscale Physics
Electrically switchable one-dimensional quadrupolar excitons in lateral double heterojunctions
Ryo Kitaura
Two neighboring lateral interfaces provide a spatial degree of freedom for controlling one-dimensional charge-transfer
excitons within a single
semiconductor monolayer. We investigate a type-II WS2-MoS2-WS2 double heterojunction using an effective-mass two-particle
Hamiltonian with a screened Coulomb interaction. For equivalent left and right interfaces at zero electric field,inter-interface coupling produces energetically split even- and odd-parity
exciton states, each with zero permanent dipole. An electric field perpendicular to the interfaces continuously converts the lower state from a quadrupolar superposition with a quadratic Stark shift into a predominantly single-interface dipolar
exciton with an approximately linear shift. The spatially resolved calculation gives binding energies of approximately 104 and 100 meV, a doublet splitting of 4.6 meV, and a crossover field of 0.87 V/um for a representative 1.5-nm MoS2 strip. Projection tests show that the lowest doublet controls the response near this crossover. Strip width tunes the coupling much more strongly than the binding energy, providing
geometric control of the low-field Stark sensitivity. These results establish a continuum-model route to electrically reconfigurable one-dimensional quadrupolar
excitons.
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