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Physics Condensed Matter

cond-mat Watch Condensed Matter

Sunday 26 April

Soft Condensed Matter

Constitutive relations for colloidal gel

Saikat Roy and Yezaz Ahmed Gadi Man

The theoretical treatment of depletion gels with central interactions often involves expanding the free energy around a stress-free reference state to derive a constitutive relation between global stress and strain. The premise upon which the previous continuum theories are based, i.e., the stress-free reference state and the affine deformation, both of which do not hold in the context of amorphous gel materials. Gels never reach a true global minimum in the potential energy landscape and contain local regions of significant compressive and tensile stress, interspersed with zero-stress regions. Hence, expansion of free energy around a stressed reference state will produce scalar terms in harmonic expansion, the effects of which are qualitatively different from the terms appearing in the expansion around an unstressed reference state. In this study, we demonstrate the limitations of traditional continuum theories and propose simple constitutive relations that better capture the mechanical response of gel materials. The robustness of the proposed relations is established through large-scale numerical simulations of depletion and frictional gels across a vast parameter space.
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Strongly Correlated Electrons

Coherent spin waves in a maximal entropy phase

Arnau Romaguera, Eugenio Paris, Elizabeth Skoropata et al. (+24 more)

In solids, disorder is conventionally regarded as detrimental to coherence. It typically localizes and dampens collective excitations, as exemplified by Anderson localization or the broadening of magnetic modes in systems lacking long-range order. While high-entropy materials are specifically designed to harness disorder and stabilize homogeneous mixed-phase structures that can display unique properties, this same disorder is nonetheless expected to preclude the formation of coherent magnetic excitations. To test the limits of this picture, we selected the antiferromagnetic system YBaCuFeO5, as it features two distinct transition metal atoms with significantly different magnetic moments, rendering its spin dynamics exceptionally sensitive to local atomic ordering. Combining resonant inelastic x-ray scattering and linear spin wave theory, we reveal a surprising paradox: YBaCuFeO5 exhibits an unexpected, entropy-driven mixed phase, in which disorder, rather than reducing the lifetime of the collective excitations, favors coherence. In this mixed phase, the spin waves remain dispersive, markedly distinct from those expected for an ordered ground state, and exhibit well-defined acoustic and optical branches separated by a large optical gap. These results demonstrate that in entropy-stabilized magnets, disorder can favor coherent collective modes previously thought to be exclusive to low-entropy systems.
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All authors Arnau Romaguera, Eugenio Paris, Elizabeth Skoropata, Stefano Agrestini, Mirian Garcia-Fernandez, Marisa Medarde, Noah Schnitzer, Lopa Bhatt, Berit H. Goodge, Yun Yen, Matthias Krack, Michael Schüler, Romain Sibille, Tom Fennell, Daniel G. Mazzone, Jakob Lass, Ellen Fogh, Anirudha Ghosh, Marco Caputo, Carlos William Galdino, Zhijia Zhang, Thorsten Schmitt, Milan Radovic, Luc Patthey, Hiroki Ueda, Monica Ciomaga Hatnean, Elia Razzoli

Mesoscale and Nanoscale Physics

Van Hove Singularity-Driven Topological Magnetism in Twisted MoTe2

Heonjoon Park, Julian Stewart, Xiao-Wei Zhang et al. (+15 more)

Van Hove singularities (vHSs) strongly amplify electron interactions and can stabilize correlated phases in topological bands. Here we report signatures of topological magnetism in large-angle twisted bilayer MoTe2 driven by the interplay of vHSs, strong correlations, and valley topology. In a 4.8 degree device, electrostatic tuning to a vHS produces a spontaneous anomalous Hall hot spot near nu = -1. Combined transport and reflective magnetic circular dichroism measurements indicate that this regime is not governed by magnetization alone, but instead emerges from a correlated intervalley-coherent antiferromagnetic state that evolves with doping into a canted phase. With increasing magnetic field, the Hall response develops an additional finite-field component consistent with a topological Hall effect from a noncoplanar spin texture, before transitioning into a C = -1 Chern insulator. Our results establish tunable vHSs in moire topological bands as a route to chiral magnetism and engineering topological phase transitions.
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All authors Heonjoon Park, Julian Stewart, Xiao-Wei Zhang, Taige Wang, Canxun Zhang, Evgeny Redekop, Jiaqi Cai, Weijie Li, Eric Anderson, Takashi Taniguchi, Kenji Watanabe, Jiun-Haw Chu, David Cobden, Andrea Young, Liang Fu, Ting Cao, Di Xiao, Xiaodong Xu

Materials Science

Magnetic interactions and spin orders in Cr$_8$ and V$_8$ ring-shaped molecular magnets from non-collinear ab initio calculations

Maria Barbara Maccioni, Elia Stocco, Luca Binci et al. (+2 more)

We investigate the magnetic interactions in the octanuclear molecular rings Cr$_8$ and V$_8$ using density functional theory within a non-collinear framework. By probing magnetic configurations with fully unconstrained three-dimensional spin orientations, we systematically explore the rotational degrees of freedom of neighboring spins and go beyond the conventional low-energy Heisenberg description. This approach distinguishes in-plane from out-of-plane exchange interactions, quantifies biquadratic spin couplings, and reveals significant antisymmetric exchange of the Dzyaloshinskii-Moriya type. The comparison with linear chains of equal composition reveals a clear dependence of the Dzyaloshinskii-Moriya interactions on the curvature of the annular structure. Exact diagonalization of the spin Hamiltonian constructed from the computed effective couplings demonstrates the role of these unconventional interactions and identifies an exotic ground state in V$_8$, driven by the competition between nearest- and next-nearest-neighbor exchange couplings of opposite sign. The closest quantitative agreement with experiment, particularly for Cr$_8$, is achieved using extended Hubbard functionals that explicitly incorporate intersite hybridization couplings.
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All authors Maria Barbara Maccioni, Elia Stocco, Luca Binci, Andrea Floris, Matteo Cococcioni

Mesoscale and Nanoscale Physics

A Symmetric Unified Transport and Charge Model for Metal-Oxide-Semiconductor Field-Effect Transistor from Diffusive to Ballistic Regimes

Chien-Ting Tung

This paper presents a symmetric unified transport (UT) compact model for metal-oxide-semiconductor field-effect transistors (MOSFETs) that bridges drift-diffusion (DD) and ballistic transport (BT) regimes. The proposed model self consistently accounts for both current and charge across the DD-BT transition. Quantum capacitance and carrier transport are incorporated into the charge density formulation. Drain side velocity saturation and the source side thermal velocity limit are unified within a single framework using a physically motivated high field scattering length, enabling accurate modeling from DD square law behavior to the ballistic limit. In addition, a physical channel charge and capacitance model is developed to capture capacitance reduction in the quasi-ballistic regime, which is not considered in standard compact models. The model is verified using theoretical analysis and experimental data from MOSFETs with multiple channel lengths, achieving accurate fitting using only physically motivated model parameters. The formulation is continuous and symmetric, and it passes both DC and AC symmetry tests.
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Soft Condensed Matter

Cosolvency response in polymer brushes

Huaisong Yong and Binyu Zhao

We present the first analytic theory with elegant and closed-form analytical solutions to explore the cosolvency effect in polymer brushes, where polymer chains that are poorly soluble in two pure solvents become fully soluble in certain mixtures thereof. This effect is key to designing stimulus-responsive smart materials but has not previously been addressed by analytic theory for polymer brushes. Our theoretical framework reveals that preferential adsorption of cosolvent induces an effective repulsion between monomers solvated by cosolvent and those solvated by solvent. The equilibrium solvation of polymer chains by cosolvent gives rise to a concentration-dependent $\chi$-function, which captures the effective interactions within the brush and reproduces the reentrant behavior characteristic of the cosolvency effect. The model predicts a discontinuous soluble transition followed by a re-collapse transition at higher cosolvent concentrations. Analytical treatment within a minimal free-energy model for the case of two symmetric poor solvents shows that the swelling and re-collapse transitions share the same thermodynamic origin. For low-density brushes, we derive an analytical approximation and delineate the phase diagram of parameter space in which discontinuous transitions occur. For cosolvency to take place, the theory specifies a minimum strength for preferential solvation and the associated repulsive coupling. Furthermore, it demonstrates that, contrary to previous models, repulsive interactions between cosolvent and solvent in the bulk are not required. This work lays the groundwork for the rational design of smart stimulus-responsive materials based on the cosolvency effect in polymer brushes, a capability which was not previously established.
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Other Condensed Matter · Cross-listed from physics.optics

Ultrafast spectroscopy and role of interlayer coupling in high harmonic generation from layered solids

Eyal Uzner and Ofer Neufeld

High harmonic generation (HHG) in solids has recently emerged as a powerful all-optical approach for probing material properties and ultrafast electron dynamics in quantum systems. It has been widely applied for studying two-dimensional and layered solids of various kinds. In these studies, the laser is usually polarized within the layered planes, where most electron dynamics occurs, while out-of-plane hopping is commonly neglected. This is despite of interlayer hopping being ubiquitous in nano-systems. Here we develop theory for HHG in layered solids in presence of interlayer coupling and employ it for studying strong-field driven hexagonal BN, graphite, and the transition metal dichalcogenide WS$_2$. We show that sufficiently intense couplings can alter typical HHG emission characteristics such as angular or ellipticity dependence even when the driving laser is polarized in-plane. We develop an analytic perturbation theory for the laser-driven current expanded in the interlayer coupling parameter and explicitly show that HHG yields follow a 4'th order polynomial form, which is validated numerically. Our work should motivate experiments for probing interlayer coupling via HHG spectroscopy, as well as exploring its modulation as a control parameter for ultrafast dynamics and attopulse generation via laser driving and mechanical strain.
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Materials Science

Amorphous High Density Plutonium

J. K. Katz, A. Rollett and R. J. Hemley

Metastable aluminum-alloyed $\delta$-plutonium shrinks rapidly and pure $\alpha$-plutonium swells rapidly at 4 K. At ambient temperature alloyed $\delta$-plutonium swells about $10^{-3}$ as fast as it shrinks at 4 K, but its bulk density decreases more slowly than would be inferred from the increase in its lattice parameter determined by X-ray diffraction. These results might be explained as the result of ingrowth of the opposite phases, but they have not been found in X-ray diffraction. The cryogenic results may be explained by ingrowth of an amorphous phase with density intermediate between those of $\alpha$ and $\delta$. This phase, when formed from alloyed $\delta$-plutonium, rapidly but not instantaneously anneals to $\delta$-plutonium at temperatures $\gtrapprox 100\,$K; when formed from pure $\alpha$-plutonium it anneals to $\alpha$ at similar temperatures. The room temperature discrepancy between growth of lattice parameter and length of $\delta$-plutonium is also explained by ingrowth of a denser amorphous phase that is continuously formed and annealed out.
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Disordered Systems and Neural Networks

Linear equivalence of nonlinear recurrent neural networks

David G. Clark

Large nonlinear recurrent neural networks with random couplings generate rich, potentially chaotic activity and are of interest in neuroscience and other fields. A key object encoding the structure of activity is the $N \times N$ covariance matrix. Recent work proposed an ansatz in which, at large $N$ and for typical quenched couplings, this covariance matrix matches that of a linear network with the same couplings, driven by independent noise. We derive this ansatz using a two-site cavity method that gives access to the joint statistics of activities at a pair of sites without disorder averaging. Specifically, we decompose each unit's activity into a linear response to its local field and a nonlinear residual; using the cavity method, we show that cross covariances of residuals at distinct sites are strongly suppressed, so that the residuals act as independent noise driving a linear network. In an alternative derivation, we construct a self-consistent equation for the covariance matrix in which non-Gaussian contributions supply cross terms that, in a linear network, would correspond to an external drive. Higher-order cross-site moments admit a Wick decomposition into pairwise covariances at leading order, reducing them to the linear-equivalent ansatz. We confirm the results in simulations and discuss their neuroscience implications.
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Saturday 25 April

Statistical Mechanics

Conductance fluctuations in random resistor networks with hyperuniform disorder

Bikram Pal

We study conductance fluctuations in random resistor networks with hyperuniform bond disorder, where the fluctuations of the number of bonds present in a test volume $V$ scale as $V^{-a}$ with $a > 1/2$. Since small changes in the concentration of bonds present in a local region give rise to a proportionate increase in the locally averaged conductance, one may expect that in hyperuniform disorder, conductance fluctuations will also show suppressed fluctuations. We argue that this is not the case: conductance fluctuations scale as $L^{-d/2}$ for a sampling size $L$. We show numerical results for $d=2$.
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Mesoscale and Nanoscale Physics

Mesoscopic Josephson effect in graphene disk at magnetic field

Adam Rycerz

Unlike for tunneling Josephson junctions, for which the current-phase relation is given by the sine function, with the critical current ($I_c$) and normal-state resistance ($R_N$) following the relation $I_cR_N=(\pi/2)\,\Delta_0/e$ (where $\Delta_0$ is the superconducting gap and electron charge is $-e$), mesoscopic Josephson junctions show more complex current-phase relations, with the skewness $S>0$, what is related to the presence – in case the leads are in the normal state – of transmission probabilities taking the values comparable to $1$. Here, we show that these features also appear for a superconductor-graphene-superconductor (S-g-S) junction in the disk-shaped (Corbino) geometry, when the magnetic field is adjusted such that $I_c\rightarrow{}0$ and $R_N\rightarrow{}\infty$. In such a case, the product $I_cR_N\approx{}1.85\,\Delta_0/e$, and the skewness $S\approx{}0.14$. The results obtained from quantum-mechanical mode-matching analysis for the Dirac-Bogoliubov-De-Gennes equation are compared with simpler model assuming incoherent scattering between two circular interfaces separating the sample and the leads.
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Disordered Systems and Neural Networks

Physics-Informed Deep Image Prior Reconstruction of In-Plane Magnetization from Scanning NV Magnetometry

Zander Scholl, Justin Woods, Charudatta Phatak et al. (+1 more)

Reconstructing magnetization in nanoscale magnetic thin films is essential for developing next-generation memory, sensors, and various spintronic technologies. However, this remains challenging due to the ill-posed nature of the stray field inverse problem, i.e., there are infinitely many magnetization solutions to a given stray field distribution. Here, we demonstrate that a physics-informed deep image prior (DIP) framework, using a simple convolutional autoencoder conditionally achieves a reasonable qualitative and quantitative reconstruction of complex in-plane magnetization patterns from scanning NV magnetometry. We find that the orientation of user-defined masks implemented to restrict the reconstruction solution space dramatically affects convergence. The optimal alignment of the mask improves the reconstruction signal-to-noise ratio by up to $\SI{3}{\decibel}$, thereby also serving as a diagnostic tool. The DIP approach requires no pre-trained datasets and is considered computationally less intensive as compared to supervised learning approaches. We analyze both Landau and dipole domain structures in lithographically patterned Permalloy nanostructures by incorporating experimentally-guided spatial constraints. Complementary magnetic force microscopy measurements were carried out to support the Scanning NV measurements.
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All authors Zander Scholl, Justin Woods, Charudatta Phatak, Hanu Arava

Materials Science

Multi-photon schemes for mid-infrared detection : Comparative study of bulk GaAs and Ge$_{1-x}$Sn$_x$

Alistair H. Duff and J. E. Sipe

We calculate the theoretical non-degenerate two photon absorption and three color injected current response tensors for bulk GaAs and Ge$_{1-x}$Sn$_x$ for a range of alloy compositions. In particular, by including a ”pump” beam we compare two ”schemes” that are sensitive to mid-infrared photons. In ”scheme I” we consider GaAs and a pump photon with energy greater than half the band gap, and in ”scheme II” we consider Ge$_{1-x}$Sn$_x$ with a pump photon with energy less than half the band gap. We find that for certain pump and alloy concentrations Ge$_{1-x}$Sn$_x$ has a substantially larger nonlinear response and three-color injected current than GaAs in the mid-infrared frequency window where both materials can absorb photons via non-degenerate two-photon absorption.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.optics

Stark-tunable O-band single-photon sources based on deterministically fabricated quantum dot–circular Bragg gratings on silicon

Sarthak Tripathi, Kartik Gaur, Priyabrata Mudi et al. (+5 more)

Semiconductor quantum dots (QDs) offer outstanding quantum-optical properties, making them highly attractive for quantum information technologies. However, combining wide-range electrical tunability, efficient photon extraction, elevated-temperature operation, monolithic silicon integration, and telecom-wavelength compatibility remains a major challenge. Here, we demonstrate electrically contacted circular Bragg grating (eCBG) resonators incorporating InGaAs QDs directly grown on silicon, enabling bright single-photon emission in the telecom O-band. Deterministic electron-beam lithography and a ridge-based vertical p–i–n diode architecture enable precise device integration and electrical control of individual emitters. The QD–eCBGs exhibit a quantum-confined Stark shift of approximately 16 nm (11 meV) at 4 K, representing a record for QDs embedded in nanophotonic structures at telecom wavelengths. This is achieved alongside a photon extraction efficiency of $(21.7 \pm 3.0)\%$ into the first lens, while maintaining excellent radiative properties and high single-photon purity, with $g^{(2)}(0)=0.0078 \pm 0.0012$ below saturation and $g^{(2)}(0)=0.0183 \pm 0.0021$ at saturation under pulsed excitation. Robust antibunching persists up to 77 K, with $g^{(2)}(0)=0.0663 \pm 0.0056$, enabling operation with liquid-nitrogen or compact Stirling cryocoolers. Furthermore, spatially separated QD–eCBGs can be electrically tuned into spectral resonance without degrading photon statistics. These results establish a silicon-compatible, electrically addressable telecom O-band quantum light platform combining wide spectral tunability, high single-photon purity, and elevated-temperature operation, providing a scalable route toward practical photonic quantum networks.
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All authors Sarthak Tripathi, Kartik Gaur, Priyabrata Mudi, Peter Ludewig, Alexander Kosarev, Kerstin Volz, Imad Limame, Stephan Reitzenstein

Mesoscale and Nanoscale Physics

Boundary-Robust Transmission Asymmetry as a Topological Signature in Open Floquet Lattices

Ren Zhang, Xiao-Yu Ouyang, Xu-Dong Dai et al. (+1 more)

We identify a boundary-robust topological signature of open Floquet lattices: although nonadiabatic boundaries strongly reshape the transmission lineshape, the integrated left–right transmission asymmetry saturates to a plateau set by the bulk Floquet winding number. Its origin is a deep-bulk branch-population principle: in the long-sample limit, each propagating Floquet–Bloch branch is generically populated with unit weight, since true Floquet bound states are nongeneric. The robust observable is therefore the cumulative transmission imbalance rather than the boundary-sensitive transmission profile. We propose direct detection by cold-atom transmission spectroscopy. For electronic transport, the same asymmetry admits contact-model-dependent electrical readouts: a coherent Floquet–Landauer–Büttiker interpretation predicts a near-\(2ef\) response in weak SAW devices, whereas a blocking-factor post-processing yields a qualitatively different signal.
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All authors Ren Zhang, Xiao-Yu Ouyang, Xu-Dong Dai, Xi Dai

Statistical Mechanics · Cross-listed from quant-ph

Principles of relativistic quantum statistical thermodynamics: a class of exactly solvable models

A. Yu. Zakharov

A system of interacting atoms is represented as an union of two subsystems, one of which is the system of atoms, and the other is an auxiliary scalar covariant field, which is equivalent to a given static interatomic potential of general form only in the non-relativistic approximation. It is shown that the auxiliary field is a superposition of Klein-Gordon fields, the parameters of which are related to singular points of the Fourier transform of the corresponding interatomic potential. The general form of the relativistic Hamiltonian system of interacting atoms is established. It is shown that the exact calculation of the relativistic partition function of a system of interacting atoms, taking into account the field degrees of freedom, reduces to renormalizing the parameters of the auxiliary field. It is established that the field degrees of freedom lead to a divergence in the total energy of a classical relativistic system - an analogue of the ultraviolet catastrophe. Quantization of the auxiliary field eliminates this divergence. The existence of a phase transition within the framework of relativistic quantum statistical thermodynamics has been proven.
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Materials Science

A Single Twist-Angle Selection Method for the Electronic Structure of Bilayer Materials

Ryan A. Baker, William Z. Van Benschoten and James J. Shepherd

Structure factor twist averaging (sfTA) is a newer method that has been shown to reproduce twist-averaged (TA) CCSD energies for bulk systems at a low computational cost. In this work, we extend this method for the treatment of low-dimensional materials in the form of two variants: paired sfTA and binding sfTA. These variants affect which twist angles are used in the sfTA protocol, as well as how the special twist angle is selected, namely by using the binding structure factor. These changes are meant to incorporate the binding interaction into the twist-angle selection algorithm within sfTA. Both variants are tested on a variety of bilayer systems, and the resulting binding correlation energies are compared to original sfTA results. We show that the variants are able to produce results approaching TA, with binding sfTA producing the most accurate energies. We also use contour plots of the test systems to show that these improvements are most likely caused by a cancellation of errors.
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Superconductivity

Unconventional mixed state in the nematic superconductor LiFeAs

G. Lamura, T. Winyard, P. Gentile et al. (+5 more)

In the mixed state of type-II bulk superconductors, the magnetic field penetrates in the form of vortices enclosing one magnetic flux quantum: this is the conventional Abrikosov vortex lattice. Here, by using transverse muon-spin spectroscopy, we demonstrate the presence of an unconventional vortex lattice in LiFeAs single crystals. We also show evidence that the new mixed phase consists of stripes of "coreless" vortices, which are bound states of two spatially separated half-quantum vortices.
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All authors G. Lamura, T. Winyard, P. Gentile, M. Speight, F. Anger, B. Buchner, S. Wurmehl, T. Shiroka

Materials Science

Electronic Spectroscopy of Atomic Defects in Molybdenum Disulfide under Ambient Conditions

Joshua R. Evans, Diego A. Garibay, Aiden N. Kuhls et al. (+1 more)

Transition metal dichalcogenides (TMDs) attract significant attention as potential building blocks in next-generation electronic devices. On the other hand, a comprehensive understanding of how various defects affect local electronic properties under realistic operational conditions is yet to be formed. Here, we present results of electronic spectroscopy experiments performed on individual defects in the prototypical TMD molybdenum disulfide (MoS2) under ambient conditions, by way of conductive atomic force microscopy (C-AFM). Data acquired in the form of consecutive, high-resolution current maps at various bias voltages allow the assessment of local conductivity and differential conductance as a function of bias voltage for individual defects, the effects of which range from single atomic sites to several nanometers in lateral size. Characteristic behavior in spectroscopy data allows the categorization of observed defects into distinct groups, and their chemical identification as n-type and p-type transition metal substitutions of molybdenum atoms, as well as oxygen substitutions of sulfur atoms.
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All authors Joshua R. Evans, Diego A. Garibay, Aiden N. Kuhls, Mehmet Z. Baykara

Superconductivity

Record magnetoresistance, enhanced superconductivity, and fermiology in WTe2

Gianluca Delgado, Elliott Runburg, Chaowei Hu et al. (+12 more)

The diverse electronic properties of transition metal chalcogenides can be very sensitive to crystal imperfections. A new crystal growth technique, known as horizontal flux transport, offers a route to improved crystal quality. By refining this technique and applying it to the topological semimetal WTe2, we achieved crystals with an order of magnitude less disorder as determined by electrical transport and scanning tunneling microscopy measurements. At low temperatures these crystals exhibit the largest magnetoresistance reported in a metal. Exfoliated monolayers show quantum oscillations for the first time in the electrostatically doped metallic states, enabling determination of band degeneracies and the valley splitting induced by an electric field. Moreover, they exhibit a gated superconducting dome with a greatly enhanced critical temperature approaching 1.8 K. This advance opens up new avenues for employing WTe2 in topological electronics and gated superconducting devices, and promises comparable breakthroughs with other chalcogenides.
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All authors Gianluca Delgado, Elliott Runburg, Chaowei Hu, Yuzhou Zhao, Jonathan M. DeStefano, Keng Tou Chu, Florie Mesple, Ellis Thompson, Kenji Watanabe, Takashi Taniguchi, Jihui Yang, Matthew Yankowitz, Xiaodong Xu, Jiun-Haw Chu, David H. Cobden

Materials Science

From Data-Driven Models to Physical Insight: Vibrational Entropy Governed by Atomic Volume

Shivam Tripathi, Jatin Kawatra, Varun Malviya et al. (+1 more)

Vibrational entropy plays a central role in determining phase stability and temperature dependent behavior in materials, yet its calculation from first-principles phonon methods remains computationally demanding. In this work, we combine data-driven modeling with physically motivated analysis to develop an efficient and interpretable framework for predicting vibrational entropy. Using a dataset derived from PhononDB, a feedforward neural network trained on Materials Project and composition based descriptors achieves high predictive accuracy, while SHAP analysis identifies atomic volume as the dominant factor governing vibrational entropy. Guided by this insight, simplified analytical models are constructed, revealing a logarithmic dependence of vibrational entropy on atomic volume consistent with lattice dynamical considerations. A logarithmic linear model is shown to provide an accurate and physically interpretable description across the full range of materials. To extend the analysis to finite temperatures, a temperature dependent formulation is introduced that incorporates T3 scaling at low temperatures and logarithmic dependence at higher temperatures, consistent with Debye and Einstein type behavior. This unified model captures both structural and thermal contributions to vibrational entropy with good accuracy. Overall, the proposed framework demonstrates that vibrational entropy can be predicted using simple, physically meaningful relationships, offering a computationally efficient alternative to full phonon calculations and enabling entropy informed materials screening.
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All authors Shivam Tripathi, Jatin Kawatra, Varun Malviya, Krishna Mehta

Statistical Mechanics

Bayesian phase transition for the critical Ising model: Enlarged replica symmetry in the epsilon expansion and in 2D

Kay Joerg Wiese, Alapan Das and Adam Nahum

A process that images or measures bond energies in the critical Ising model can be in distinct measurement 'phases', depending on the precision of measurement. We study the transition into the strong-measurement phase using replica field theory (an epsilon expansion around six dimensions), a symmetry analysis, and numerical simulations in two dimensions. The results show that certain inference problems possess unexpected structure associated with an enlarged symmetry of the replica description. The symmetry imposes constraints on correlation functions, and is an analog of the Nishimori phenomenon in the Ising spin glass, but in a distinct replica limit. It determines the exact critical exponent for an Edwards-Anderson-like correlator in the measured Ising model, both in 2D and near the upper critical dimension. The symmetry is present microscopically for certain measurement protocols, and it emerges in the infrared for generic protocols at the critical measurement strength. We also examine the epsilon expansion for models with power-law interactions and/or long-range measurement. Finally, we describe an analogous replica symmetry enlargement for measurement in Wegner's lattice gauge theory.
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Statistical Mechanics · Cross-listed from physics.soc-ph

Nesting Controls Phase Transitions in Higher-Order Contagion

Hugo P. Maia, Guilherme Ferraz de Arruda, Silvio C. Ferreira et al. (+1 more)

The organization of higher-order interactions plays a central role in shaping collective dynamics, yet a general structural principle governing contagion on hypergraphs remains lacking. Here we introduce a nesting coefficient that quantifies how lower-order interactions are embedded within higher-order ones, defining a continuum between simplicial complexes and random hypergraphs. Using a higher-order susceptible-infected-susceptible model, we show that increasing nesting lowers the activation threshold and suppresses discontinuous transitions, while weak embedding favors explosive behavior. We further demonstrate that correlations between nesting and interaction order modulate the onset of activity while only weakly affecting transition discontinuity. Analysis of synthetic and empirical networks reveals that nesting strongly predicts hysteresis, establishing it as a key structural determinant of phase transitions in higher-order systems.
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All authors Hugo P. Maia, Guilherme Ferraz de Arruda, Silvio C. Ferreira, Yamir Moreno

Materials Science

Weak Polar Optical Phonon Scattering Decouples Electron and Phonon Transport in Layered Thermoelectric Materials

Zhonghao Xia, Michele Reticcioli, Yateng Wang et al. (+3 more)

High-performance thermoelectric (TE) materials are crucial for efficient waste-heat recovery and solid-state cooling technologies. A persistent challenge in TE materials design arises from the strong interdependence among the electrical conductivity ($\sigma$), Seebeck coefficient ($S$), and lattice thermal conductivity ($\kappa_{\mathrm{L}}$). Layered compounds can effectively suppress $\kappa_{\mathrm{L}}$ along the cross-plane direction owing to weak interlayer interactions; however, they often suffer from low carrier mobility ($\mu$) caused by limited band dispersion and strong polar optical phonon (POP) scattering. Here, we perform high-throughput density functional theory calculations to screen 236 layered semiconductors and identify candidates with low effective mass ($m^{*}$) and weak POP scattering. We identify 23 compounds with high cross-plane $\mu$, among which 14 exhibit large power factors ($S^{2}\sigma$). Notably, GaGe$_{2}$Te stands out with exceptionally high cross-plane $\sigma$ and power factor, enabled by a favorable combination of small $m^{*}$ and a small ionic dielectric constant. Simultaneously, GaGe$_{2}$Te exhibits an ultralow cross-plane $\kappa_{\mathrm{L}}$ of 0.57~W~m$^{-1}$~K$^{-1}$ at 300~K, originating from weak interlayer bonding and pronounced phonon anharmonicity. These results demonstrate an effective strategy to decouple electron and phonon transport in layered materials by mitigating POP scattering, thereby providing a promising pathway toward high-performance thermoelectric materials.
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All authors Zhonghao Xia, Michele Reticcioli, Yateng Wang, Yali Yang, Alessandro Stroppa, Jiangang He

Strongly Correlated Electrons

Grassmann time-evolving matrix product operators for fermionic impurities coupled to a superconducting bath

Chu Guo, Wei Wu, Xiansong Xu et al. (+4 more)

The Grassmann time-evolving matrix product operator (GTEMPO) method, which represents the Feynman-Vernon influence functional as a temporal matrix product state, has been shown to be a flexible and potentially scalable solution for fermionic quantum impurity problems. In this work, we extend GTEMPO to solve fermionic impurity problems in the Nambu formalism, in which the impurity is coupled to a superconducting bath. A key insight is that by employing the Bogoliubov transformation for the superconducting bath, one could obtain the analytic expression of the Feynman-Vernon influence functional in a similar form to the case of a normal bath, after which the core algorithms of GTEMPO can be straightforwardly adapted. We demonstrate the accuracy of our method by benchmarking it against exact diagonalization in several exactly solvable cases, and against the continuous-time quantum Monte Carlo method using converged dynamical mean field theory (DMFT) iterations on the imaginary contour in the non-integrable case. In all cases, we perform both imaginary- and real-time calculations to illustrate the flexibility of our method. These results illustrate that our method could be potentially useful as an impurity solver in DMFT as well as its non-equilibrium extension for fermionic impurity problems in the Nambu formalism.
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All authors Chu Guo, Wei Wu, Xiansong Xu, Ping-Xing Chen, Changming Yue, Tian Jiang, Ruofan Chen

Mesoscale and Nanoscale Physics

Noise spectroscopy of insulating and itinerant altermagnets

Lucas V. Pupim and Mathias S. Scheurer

One of the central goals in the emergent field of altermagnetism is the unambiguous experimental identification and characterization of altermagnetic order across a variety of compounds. This motivates exploring tools that can clearly distinguish altermagnets from antiferromagnets, based on symmetry signatures, and offer access to the dominant orbital character (e.g., $d$-wave vs. $g$-wave) of the magnetic order parameter. In this work, we theoretically explore the potential of noise magnetometry for this task, studying contributions from both magnons and itinerant electrons in different regimes and scenarios. While altermagnetism and antiferromagnetism also lead to different noise spectra for magnons, we find the most striking and symmetry-sensitive signatures in the charge fluctuations of itinerant altermagnets. Both for the homogeneous bulk case and in the presence of strain and/or around domain walls, we identify noise contributions that are only permitted by symmetry in the altermagnet and, thus, provide a unique signature of altermagnetism. Furthermore, the angular dependence of noise around domain walls also offers access to the orbital character of the altermagnet. On a more technical note, we discuss the role and relevance of lattice effects related to the dipole tensor. We hope that our work will help pave the way towards the clear experimental identification of altermagnetism across a wide range of candidate materials.
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Materials Science · Cross-listed from physics.optics

Role of ultrafast electron-optical-phonon interactions in high harmonic generation from graphene

Adam Herling and Ofer Neufeld

High harmonic generation (HHG) is a widely explored process in solids, where intense lasers drive attosecond-to-femtosecond electron dynamics within bands, causing high-energy emission. While electrons and photons are considered the main players in HHG, solids also host ubiquitous phonons that are typically assumed negligible in HHG due to their longer timescales. We theoretically study HHG in graphene with a formalism including optical phonons in the static limit, where the lattice is frozen on the electronic timescale and HHG is computed by sampling thermally-occupied phonons and ensemble-averaging. We show that in graphene: (i) Optical phonons strongly suppress HHG yields by coupling to interband currents and causing harmonic phase scrambling (destructive interference), explaining the lack of experimental HHG above ~3 eV. (ii) HHG yields become temperature-dependent due to phonon occupations, though in graphene this dependence is weak since phonon energy scales are dominated by zero-point motion. (iii) Optical phonons dephase interband coherences at a rate equivalent to T2~5.7 fs, substantially faster than e-e scattering, suggesting thermal phonons dominate electronic decoherence in strong fields. (iv) Phonons smoothen HHG ellipticity-dependent curves, yielding better agreement with experiments. Remarkably, all effects are timescale-independent, arising in the static picture of electron-phonon interactions, making results transferable to attosecond phenomena. Our results shed light on the dephasing time problem in HHG and the role of phonons on attosecond timescales, with implications for other systems and processes such as Floquet gaps and photocurrents in graphene.
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Mesoscale and Nanoscale Physics

Analytical Treatment of Noise-Suppressed Klein Tunneling in Graphene with Possible Implications for Quantum-Dot Qubits

Kamal Azaidaoui, Ahmed Jellal, Hocine Bahlouli et al. (+2 more)

We study quantum tunneling through a potential barrier whose height fluctuates in time and is modeled by Gaussian white noise. We map the stochastic dynamics onto an equivalent time-independent Lindblad equation for the density matrix, allowing fully analytical solutions. For Schrödinger particles, noise introduces dissipation that suppresses Fabry-Pérot oscillations and yields an exponentially decaying transmission. Applying the same formalism to graphene, we demonstrate that noise induces a complex longitudinal wavevector within the barrier, leading to a strong suppression of transmission and Klein tunneling, even at normal incidence. Our approach promises improved control over Klein tunneling. These results demonstrate that noisy barriers can act as tunable dissipative elements, offering a pathway to enhanced control of electron transport in graphene-based devices. We also briefly discuss how our results could guide the design of graphene quantum dots for potential use in spin qubit devices.
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All authors Kamal Azaidaoui, Ahmed Jellal, Hocine Bahlouli, A. Al Luhaibi, Michael Vogl

Materials Science

Wafer-scale hybrid molecular beam epitaxy of BaTiO3 and SrTiO3 on silicon

Xiaodong Tian, Yan Lin, Hanbin Gao et al. (+8 more)

The integration of epitaxial barium titanate (BTO) on silicon represents a highly promising pathway for next-generation, energy-efficient photonic integrated circuits due to BTO's exceptionally high Pockels coefficients. However, the scalable epitaxy of BTO on Si remains hindered by complex stoichiometric control and slow growth rates. In this work, we demonstrate the continuous, uniform wafer-scale growth of high-quality BTO films on SrTiO3 (STO)-buffered 4-inch Si(001) wafers using a fully hybrid molecular beam epitaxy (hMBE) approach. By utilizing titanium tetraisopropoxide as a titanium precursor, we achieve a self-regulating, adsorption-controlled layer-by-layer growth at rates exceeding 75 nm/h, while maintaining an atomically sharp and structurally coherent BTO/STO interface. We systematically compare the structural, ferroelectric, and electro-optic (EO) properties of fully hMBE-grown BTO with those deposited via pulsed laser deposition (PLD) on identical STO/Si templates. While both techniques yield high-quality c-domain dominated films, the optimized hMBE-grown BTO exhibits superior crystallinity and a larger effective EO coefficient of 248 pm/V, surpassing that of the PLD-grown films (220 pm/V). These results highlight the advantages of the fully hMBE approach as a scalable, deterministic, and high-performance materials platform for wafer-scale integrated ferroelectric photonics.
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All authors Xiaodong Tian, Yan Lin, Hanbin Gao, Han Yu, Yunpeng Ma, Ruiqi Liang, Changfu Chen, Wei Li, Chenguang Deng, Qiang Zheng, Qian Li

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

Pulsed Vertical Electric Dipole Over a Lossy Halfspace: On the Time-Domain Zenneck Wave

Giampiero Lovat

We investigate the transient electromagnetic field radiated by a pulsed vertical electric dipole above a lossy half-space and identify its time-domain signatures associated with the Zenneck wave. Starting from the classical Sommerfeld representation, we derive a causal time-domain formulation based on the double-deformation technique, with successive contour deformations in the transverse-wavenumber and frequency planes. This yields an explicit decomposition of the field into source-pole, loss-pole, modal-pole, and residual steepest-descent contributions. The resulting expressions exactly satisfy causality and are validated against a reference solution obtained through a standard double inverse transform. The analysis shows that one modal contribution, generated by the frequency-plane deformation and related to the frequency-domain Zenneck pole, exhibits reduced-time invariance and a spatial attenuation consistent with a surface-wave component. Under suitable source and observation conditions, this term can dominate the field over a broad and physically relevant finite late-time interval. At the same time, for the considered damped-sinusoidal excitation, the strict asymptotic tail at fixed distance remains algebraic of order $t^{-5/2}$, with contributions from both the residual continuous spectrum and the modal-pole family. These results provide a rigorous and physically interpretable time-domain manifestation of the frequency-domain Zenneck wave in the pulsed half-space problem.
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