arXane › Physics › Condensed Matter
Wednesday, 4 November 2026: 2 new papers, 6 cross-lists and 8 replacements in Disordered Systems and Neural Networks (cond-mat.dis-nn), in arXiv's order, with full abstracts. The same papers as arXiv's own listing, set as a newspaper; every title links to arXiv.
We present a closed theory, exact in the $N\to\infty$ limit, of the equilibrium spin correlations of the Sherrington--Kirkpatrick spin-glass model evolving with Glauber dynamics, above and at the transition temperature $T_c$. The derivation uses a dynamic cavity approach: in the evolution equation for the selected spin the other spins are represented by a fluctuating field. The influence of the selected spin on this field is included at the linear response level; it is given as a sum of the spin's past flips weighted by the correlations of the unperturbed fluctuating field. The self-consistency condition relates the time-dependent correlations of the selected spin and those of the fluctuating field. The theory predicts a $(T-T_c)^{-2}$ divergence of the relaxation time upon approaching the transition and an $At^{-1/2}$ decay of spin correlations at $T_c$. A numerical solution of the theory gives the amplitude, $A=0.964\pm 0.007$. The theoretical predictions are in excellent agreement with Glauber dynamics computer simulations.
Polaron hopping in disordered molecular semiconductors such as eumelanin is set by energetic disorder $\sigma$ and reorganization energy $\lambda$. We show that the reduced polaron absorption band encodes this transport problem. Its value at zero energy fixes the mean hop rate, which bounds the mobility in any dimension. For Gaussian disorder and classical vibrations its centroid is $E_\mathrm{op}=\lambda+(1-c)\sigma^2/k_BT$, its variance obeys the Hush relation, and the bound is the Marcus rate at $\lambda\to E_\mathrm{op}$; a 1D lower bound vanishes at $k_BT^*=(1-c)\sigma^2/\lambda$, read from optics.
The coherent elastic wave in untextured polycrystals, central to ultrasonic and seismic characterization, was recently observed to have two modes per polarization at short wavelengths. However, their physical origin and amplitude partition have remained open. We address both by theory, grain-scale numerical simulations and a grain velocity estimate. The first-order smoothing approximation, simplified by the far-field approximation, predicts three waves, which are the classical effective wave and one for each type of scattered wave (longitudinal and transverse). At long wavelengths, only the effective wave is weakly attenuated. At short wavelengths, one of the other two waves joins it with comparable attenuation, whereas the third stays far more attenuated. A new residue method, which needs no fitting, gives the same pattern for the amplitude partition, with the two waves sharing the coherent field and the third carrying little. The simulations reveal exactly these two modes, for transverse waves in iron and longitudinal waves in strongly anisotropic SrMgSn, and never the third. The wavefields reveal the mechanism, in which the incident wave is extinguished within one grain correlation length and the coherent field is rebuilt by forward multiple scattering. Strikingly, a grain velocity estimate from single-crystal constants provides independent evidence, tracing the two modes to plane waves of the individual grains and closely reproducing their velocities. Observability of multiple waves is thus set by attenuation and amplitude partition together, and short-wavelength measurements in random media must allow for more than one coherent mode.
Tantalum pentoxide (Ta2O5) thin films deposited by ion beam sputtering (IBS) are essential components in high-power laser systems, where post-deposition annealing is routinely used to optimize film quality. In this work, we systematically investigate the structural and optical evolution of IBS-deposited Ta2O5 single layers subjected to post-deposition annealing at temperatures from 400 to 800 $^{\circ}\mathrm{C}$. The films undergo an amorphous-to-polycrystalline transition between 600 $^{\circ}\mathrm{C}$ and 700 $^{\circ}\mathrm{C}$, accompanied by a marked increase in surface roughness. Argon bubbles inherently incorporated during IBS serve as structural markers: their size correlates positively with the surface roughness from the as-deposited to 700 $^{\circ}\mathrm{C}$ but decouples at 800 $^{\circ}\mathrm{C}$, revealing a transition from matrix-relaxation-dominated to surface-diffusion-dominated behavior. Transmittance spectra show a red shift at 400 and 600 $^{\circ}\mathrm{C}$ which can be explained by physical thickness increase. Weak absorption decreases from ~6 ppm in as-deposited films to ~2 ppm at 400-600 $^{\circ}\mathrm{C}$ due to annihilation of oxygen vacancies, rises markedly at 700 $^{\circ}\mathrm{C}$ due to grain-boundary defects and then drops again at 800 $^{\circ}\mathrm{C}$ as grain boundaries relax. We identify 400 $^{\circ}\mathrm{C}$ as the optimal annealing window for IBS-deposited Ta2O5 single layers, with the lowest surface roughness and effectively reducing weak absorption while preserving the amorphous phase.
Ionic conduction in solid-state electrolytes is conventionally described by mobile cations migrating through a static host lattice. Yet the fastest conductors are often the most disordered, and how disorder enhances transport remains unresolved at the mesoscale, inaccessible to ensemble-averaged probes. We use Bragg X-ray photon correlation spectroscopy to measure nanoscale structural dynamics of a single ball-milled Na$_3$PS$_4$ particle. We find that the cubic phase behaves as a strain glass: a frustrated configuration of three degenerate tetragonal variants with short-range order. Its domain-wall motion is stress-driven, crossing over irreversibly to thermally activated dynamics on annealing as residual milling stress is depleted. Rearrangements follow scale-free avalanche statistics, matching ferroelastic martensites but without external driving. The tetragonal polymorph shows none of these features and relaxes fifty times more slowly. These dynamics occur on timescales comparable to estimated ionic diffusion across the particle, indicating the host structure cannot be assumed static on transport-relevant timescales.
We study the one- and two-boson energy subspaces of finite open Bose--Hubbard chains with Fibonacci-modulated hopping and zero on-site potential. Their spectra exhibit a hierarchy of bands and gaps, and a critical zero-energy mode for odd lengths, in agreement with prior research. For the singe-excitation subspace we find two families of length-invariant finite-energy states localized at the left cluster boundary. For the two-boson subspace, we report both doublon and separated-pair excitations in the form of Tamm/Shockley states localized near either end. In particular, the right-edge doublon states occur across all Fibonacci generations whereas the left-side states appear every second generation. We check numerically that the second-order perturbation theory accounts well for the structure of the spectrum and the edge states in the doublon subspace in the strong-coupling regime. The results are useful for future explorations of many-body physics beyond numerical tractability, specifically using near-term Josephson quantum simulators.
The spin-1/2 kagome Heisenberg antiferromagnet is a central model in the study of quantum spin liquids, yet its ground-state properties remain unresolved. We perform large-scale variational Monte-Carlo calculations using neural-network quantum states (NQSs) with kagome-geometry-aware message-passing convolution on two-dimensional tori containing up to 300 spins, and achieve exceptionally low variational energies for the kagome model. The spin and dimer structure factors show no evidence of magnetic or valence-bond long-range order. Moreover, the dominant symmetry-resolved spin and dimer correlations exhibit a common scaling behavior consistent with the emergent $SO(6)$ monopole multiplet of the Dirac spin liquid. Finally, the fidelity of the optimized NQSs with Gutzwiller-projected mean-field states increasingly favors the Dirac state over the competing $Z_{2}$ candidates as the system size grows. Together, these complementary results provide evidence for a $U(1)$ Dirac spin-liquid ground state in the kagome Heisenberg antiferromagnet.
Maximum-entropy models of neural recordings can be treated as Boltzmann distributions at a fictitious temperature $T$, with the fitted model at $T=1$. Their specific-heat peak grows with population size and approaches that temperature, which has been read as a sign of criticality. The case rests on pairwise models, yet retinal codes require higher-order interactions. The thermodynamics of a model with such interactions fitted cell by cell has not been computed. Here I compute it exactly for pairwise and third-order models of the same populations of up to $20$ retinal ganglion cells, in salamander and marmoset. The triplet terms, required by the held-out likelihood, shift the peak by more than the finite-sample bias, with opposite sign in the two retinas. The peak is lower and grows more slowly in salamander, higher and grows faster in marmoset, and lies closer to $T=1$ in both. An exact decomposition traces most of the shift to the energy variance across population spike counts. A constraint on the spike-count distribution alone reproduces all of it in salamander and over half in marmoset. A second recording of each species, re-binning, and a common-input surrogate show that the sign is set by the spike-count structure of a recording at its temporal resolution, not by species. The thermodynamic signature of a neural code is therefore set not by the data alone but by the interaction order of the model used to read it.