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Physics Condensed Matter

cond-mat Watch Condensed Matter

Tuesday 18 November

Materials Science

Photoemission tomography of excitons in 2D systems: momentum-space signatures of correlated electron-hole wave functions

Siegfried Kaidisch, Amir Kleiner, Sivan Refaely-Abramson et al. (+2 more)

The momentum-space signatures of excitons can be experimentally accessed through time-resolved (pump-probe) photoelectron spectroscopy. In this work, we develop a computational framework for exciton photoemission orbital tomography (exPOT) in periodic systems, enabling the simulation and interpretation of experimental observables within many-body perturbation theory. By connecting the GW +Bethe-Salpeter equation (BSE) approach to photoemission tomography, our formalism captures exciton photoemission in periodic systems, explicitly incorporating photoemission matrix element effects induced by the light-matter interaction via the probe pulse. The correlated nature of electrons and holes introduces distinct consequences for excitonic photoemission. Using the prototypical two-dimensional material hexagonal boron nitride, we demonstrate these effects, including a dependence of the photoemission angular distribution on the pump pulse polarization. Moreover, our framework extends to excitons with finite center-of-mass momentum, making it well-suited to studying momentum-dark excitons. This provides valuable insights into the microscopic nature of excitonic phenomena in quantum materials.
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All authors Siegfried Kaidisch, Amir Kleiner, Sivan Refaely-Abramson, Peter Puschnig, Christian S. Kern

Materials Science

Hydrothermal Synthesis of Ultra-high Aspect Ratio $\beta$-NaYF Disks via Methyliminodiacetic Acid (MIDA)

Lars Forberger, Jacob T. Baillie, Zhaojie Feng et al. (+6 more)

The hexagonal $\beta$-phase of sodium yttrium fluoride (NaYF) is a leading host material for lanthanide upconversion and anti-Stokes fluorescence laser refrigeration based on its low phonon energies and high upconversion efficiency. Recently experiments have been proposed to use this material as an optically-levitated sensor of high-frequency gravitational waves. In order to maximize signal-to-noise in this experiment, the NaYF sensor must have both a two-dimensional, disk-like morphology and also a large mass. Here we report a novel hydrothermal process based on the chelation ligand methylimidodiacetic acid (MIDA) to realize hexagonal $\beta$-NaYF prisms with corner-to-corner diameters up to 44 $\mathrm{\mu m}$ while keeping the height around 1 $\mathrm{\mu m}$. The surface quality is comparable to particles synthesized with EDTA based on atomic force microscopy (AFM) measurements. Unlike particles synthesized with EDTA the $\beta$-NaYF particles show no lensing based on curvature of the hexagonal basal plane. Single crystal X-ray diffraction data were refined to the P-62c (#190) space group which to the best of our knowledge has not been reported in the literature. One of six 44 $\mathrm{\mu m}$ $\beta$-NaYF disks doped with 10% ytterbium showed laser refrigeration of ($-4.9 \pm 1.0$) K suggesting future applications in both levitated optomechanics and microoptics.
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All authors Lars Forberger, Jacob T. Baillie, Zhaojie Feng, Rachel E. Gariepy, Sankhya Hirani, Daniel R. Gamelin, Shuai Zhang, Werner Kaminsky, Peter J. Pauzauskie

Materials Science

Automated laboratory x-ray diffractometer and fluorescence spectrometer for high-throughput materials characterization

Hyun Sang Park, Timothy Long, Michael Wall et al. (+8 more)

The increasing importance of artificial intelligence and machine learning in materials research has created demand for automated, high-throughput characterization techniques capable of rapidly generating large data sets. We describe here a new instrument for simultaneous X-ray diffraction and X-ray fluorescence spectroscopy, optimized for high-throughput studies of combinatorial specimens. A bright, focused, high-energy X-ray beam (24 keV) combined with a pixel array area detector allows spatially-resolved (~200 {\mu}m) transmission diffraction measurements through thick (100 {\mu}m) specimens of structural metals with exposure times as short as 1 s. Simultaneously, a silicon drift detector records X-ray fluorescence from the specimen for spatially-resolved measurement of composition. Specimen handling is fully automated, with a robot inside the X-ray enclosure manipulating the sample for measurements at different locations. Data orchestration is also automated, with data streamed off the instrument and processed autonomously. In this paper we assess the performance of the instrument in terms of throughput, resolution, and signal-to-noise ratio, and provide an example of its capabilities through a combinatorial study of Cu-Ti alloys to demonstrate rapid data set creation.
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All authors Hyun Sang Park, Timothy Long, Michael Wall, Alexander deJong, Ali Rachidi, Kacper Kowalik, Rohit Berlia, David Elbert, Timothy P. Weihs, Robert Drake, Todd C. Hufnagel

Strongly Correlated Electrons

5d-mediated indirect exchange and effective spin Hamiltonians in Ce triangular-lattice delafossites

Leonid V. Pourovskii

Anisotropic intersite exchange interactions in frustrated rare-earth magnets are difficult to assess both theoretically and experimentally. Here, we propose an ab initio force-theorem framework combining the quasi-atomic Hubbard-I approach to 4f correlations with a static mean-field treatment of the on-site intershell Coulomb interaction between rare-earth 4f and 5d states to simultaneously capture both 4f superexchange and 5d-mediated indirect exchange. Applying it to the triangular lattice Ce delafossites CsCeSe$_2$, KCeS$_2$, and RbCeO$_2$, we find that the indirect exchange dominates in the selenide, the superexchange in the oxide, while both mechanisms contribute almost equally in the sulfide. The magnetic exciation spectra of CsCeSe$_2$ and KCeS$_2$ evaluated from the calculated spin Hamiltonains are in good qualitative and quantitative agreement with experimental data.
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Strongly Correlated Electrons

Spin-lattice coupling induced chiral phonons and their signature in Raman Circular Dichroism

Eduard Koller, Swetlana Swarup, Johannes Knolle et al. (+1 more)

Recent Raman experiments on the Kitaev material $\alpha$-RuCl$_3$ have reported a finite Raman circular dichroism (RCD), revealing chiral phonon behaviour not expected from lattice symmetry alone. To explain this observation, we develop a diagrammatic framework for the spin-phonon coupled Kitaev model. We demonstrate that bare phonons contribute no RCD, but coupling to the chiral spin excitation continuum under an applied magnetic field renormalizes the phonon propagator, mixing real polarization eigenvectors into complex superpositions with finite angular momentum. This interaction-induced modification generates a nonzero RCD accompanied by characteristic Fano line shapes in the Raman response, reflecting interference between discrete phonons and the continuum. The resulting signal grows with magnetic field strength, consistent with experiment, and directly tracks the field-induced chirality of the spin sector. More broadly, our results establish RCD as a powerful probe of interaction-induced chiral phonons in correlated quantum materials.
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All authors Eduard Koller, Swetlana Swarup, Johannes Knolle, Natalia B. Perkins

Mesoscale and Nanoscale Physics

Quantum Transport Spectroscopy of Pseudomagnetic Field in Graphene

Divya Sahani, Sunit Das, Kenji Watanabe et al. (+3 more)

Nonuniform strain in graphene acts as a valley-dependent gauge field, generating pseudomagnetic fields (PMFs) that mimic real magnetic fields but preserve global time-reversal symmetry. While local probes have visualized such fields, their quantitative detection via macroscopic transport has remained elusive. Here, we demonstrate that high-mobility graphene exhibits distinct beating patterns in Shubnikov-de Haas oscillations, arising from valley-resolved Landau quantization under different effective magnetic fields. Systematic analysis of these beats reveals universal quadratic and linear scaling of the node carrier density and Landau level filling factor with the applied magnetic field, enabling the extraction of PMFs as small as a few millitesla. Our results establish quantum oscillation spectroscopy as a robust and broadly applicable probe of strain-induced gauge fields in Dirac materials, opening avenues for mechanically tunable valleytronic and straintronic devices.
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All authors Divya Sahani, Sunit Das, Kenji Watanabe, Takashi Taniguchi, Amit Agarwal, Aveek Bid

Strongly Correlated Electrons

Additional quantum many-body scars of the spin-$1$ $XY$ model with Fock-space cages and commutant algebras

Sashikanta Mohapatra, Sanjay Moudgalya and Ajit C. Balram

Quantum many-body scars (QMBS) represent a mechanism for weak ergodicity breaking, characterized by the coexistence of atypical non-thermal eigenstates within an otherwise thermalizing many-body spectrum. In this work, we revisit the spin-$1$ $XY$ model on a periodic chain and construct several new families of exact scar eigenstates embedded within its extensively degenerate manifolds that owe their origins to an interplay of $U(1)$ magnetization conservation and chiral symmetries. We go beyond previously studied towers of states and first identify a novel set of interference-protected eigenstates resembling Fock space cage states, where destructive interference confines the wave function to sparse subgraphs of the Fock space. These states exhibit subextensive entanglement entropy, and when subjected to a transverse magnetic field, form equally spaced ladders whose coherent superpositions display long-lived fidelity oscillations. We further reveal a simpler organizing principle behind these nonthermal states by using the commutant algebra framework, in particular by showing that they are simultaneous eigenstates of non-commuting local operators. Moreover, in doing so, we uncover two more novel families of exact scars: a tower of volume-entangled states, and a set of mirror-dimer states with some free local degrees of freedom. Our results illustrate the power and interplay of interference-based and algebraic mechanisms of non-ergodicity, offering systematic routes to identifying and classifying QMBS in generic many-body quantum systems.
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Mesoscale and Nanoscale Physics

Tuning of exciton type by environmental screening

Igor L. C. Lima, M. V. Milošević, F. M. Peeters et al. (+1 more)

We theoretically investigate the binding energy and electron-hole (e-h) overlap of excitonic states confined at the interface between two-dimensional materials with type-II band alignment, i.e., with lowest conduction and highest valence band edges placed in different materials, arranged in a side-by-side planar heterostructure. We propose a variational procedure within the effective mass approximation to calculate the exciton ground state and apply our model to a monolayer MoS$_2$/WS$_2$ heterostructure. The role of nonabrupt interfaces between the materials is accounted for in our model by assuming a W$_x$Mo$_{1-x}$S$_2$ alloy around the interfacial region. Our results demonstrate that (i) interface-bound excitons are energetically favorable only for small interface thickness and/or for systems under high dielectric screening by the materials surrounding the monolayer, and that (ii) the interface exciton binding energy and its e-h overlap are controllable by the interface width and dielectric environment.
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All authors Igor L. C. Lima, M. V. Milošević, F. M. Peeters, Andrey Chaves

Quantum Gases

Phonon scattering from spatial relaxation of one-dimensional Bose gases

Bilal Alilou, Clément Duval, Frederick Del Pozo et al. (+1 more)

We theoretically investigate the nonequilibrium relaxation of a spatial density modulation in a one-dimensional, weakly interacting Bose gas, and its connection to the equilibrium scattering rate $\smash{\gamma_k\propto k^{3/2}}$ of the system's phononic excitations. We show that the relaxation is generally governed by a nonequilibrium scattering rate $\gamma_{k,t}$ coupled to quantum fluctuations, which approaches its equilibrium value $\gamma_k$ only at long times. Numerical simulations of quantum kinetic equations reveal an algebraic convergence, $\smash{\gamma_{k,t} - \gamma_k \sim t^{-2/3}}$, confirmed by analytical predictions. More broadly, our results establish a theoretical framework for experimentally probing phonon dynamics through the temporal evolution of local perturbations in quantum gases.
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All authors Bilal Alilou, Clément Duval, Frederick Del Pozo, Nicolas Cherroret

Soft Condensed Matter

Rheology of dense vibrated granular flows: non-monotonic response controlled by granular temperature

A. Plati, G. Petrillo, L. de Arcangelis et al. (+4 more)

We study the rheology of dense granular materials subjected to vertical vibration {by} using numerical simulations of a stress-imposed vane rheometer. The effective viscosity increases with confining pressure, decreases with vibration amplitude, and exhibits a non-monotonic dependence on frequency: weakening is observed at intermediate frequencies but is lost at high frequencies. We show that the rheological response is governed by the balance between grain-scale agitation energy and the stabilizing effect of confinement. This framework reconciles previously observed trends in viscosity and friction weakening and emphasizes the central role of energy injection and dissipation in determining granular flow properties under vibration.
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All authors A. Plati, G. Petrillo, L. de Arcangelis, A. Gnoli, A. Puglisi, A. Sarracino, E. Lippiello

Strongly Correlated Electrons

Breakdown of Quantum Chaos in the Staggered-Field XXZ Chain: Confinement and Meson Formation

Julia Wildeboer, Marton Lajer and Robert M. Konik

Confinement produces mesonic bound states, but their existence alone does not determine the statistical organization of the many-body spectrum. We investigate this connection in the spin-\(\tfrac12\) XXZ chain subject to a longitudinal staggered field, using symmetry-resolved exact diagonalization for spin chains of up to \(N=22\) sites. As the exchange anisotropy is increased into the antiferromagnetic Ising regime, the finite-size spectra cross over from Gaussian-orthogonal-ensemble-like statistics to reduced level repulsion. This evolution occurs in both the zero-momentum and generic nonzero-momentum sectors examined. Direct measurements of the domain-wall-number operator and its eigenstate variance reveal increasingly sharp approximate domain-wall sectors, accompanied by banding of spin correlations and bipartite entanglement. Statistics within a selected domain-wall band further characterize the spectral organization beyond the full-sector gap-ratio analysis. We identify the one-meson branch and compare its energies and level spacings with established Airy and semiclassical descriptions and a finite-chain strong-anisotropy treatment, quantifying their accuracy and systematic deviations. These results connect the known confined two-spinon spectrum to the broader finite-anisotropy organization of many-body eigenstates and provide quantitative evidence for emergent domain-wall constraints in the full microscopic Hamiltonian.
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Strongly Correlated Electrons · Cross-listed from hep-th

Exploring the Infrared Landscape of the SYK Model

Weam Abou Hamdan and Damián A. Galante

We analyse a class of SYK models whose Hamiltonian is the sum of two SYK Hamiltonians with different numbers of fermions $q, \tilde q$ in each interaction. We consider both Euclidean and Lorentzian probes of the quantum system in the large $N$ limit. In the strong coupling phase, the entropy provides a diagnostic of the thermal renormalisation group flow. Under certain conditions, two parametrically separated regimes of near-conformal behaviour emerge. The first reproduces the standard linear-in-temperature scaling characteristic of the single SYK model. The system then flows to another near-fixed point whose entropy scaling depends on the ratio $n = q/\tilde q$. For $n<3/2$, the entropy exhibits anomalous, stronger-than-linear scaling in temperature. At $n=3/2$, there is an additional logarithmic enhancement. Using conformal perturbation theory, we argue that in the infrared regime of the SYK model, there may exist disordered conformal operators with dimensions $1 < \Delta \leq 3/2$. In Lorentzian signature, we study the out-of-time-ordered correlator and show that these deformed theories exhibit near-maximal chaos in both regimes (when they exist). We comment on the relation between the anomalous scalings found here and those observed in certain near-extremal black holes in two and higher dimensions.
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Quantum Gases

Observation of critical scaling in the Bose gas universality class

Leon Kleebank, Frank Vewinger, Arturo Camacho-Guardian et al. (+4 more)

Critical exponents characterize the divergent scaling of thermodynamic quantities near phase transitions and allow for the classification of physical systems into universality classes. While quantum gases thermalizing by interparticle interactions fall into the XY model universality class, the ideal Bose gas has been predicted to form a distinct universality class whose signatures have not yet been revealed experimentally. Here, we report the observation of critical scaling in a two-dimensional trapped quantum gas of essentially noninteracting photons, which thermalize by radiative contact to a reservoir of molecules inside a microcavity. By measuring the spatial correlations near the condensation transition, we determine the critical exponent for the correlation length to be $\nu = 0.52(4)$. Our results constitute a first experimental test of the long-standing scaling predictions for the Bose gas universality class.
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All authors Leon Kleebank, Frank Vewinger, Arturo Camacho-Guardian, Victor Romero-Rochín, Rosario Paredes, Martin Weitz, Julian Schmitt

Soft Condensed Matter

From Equilibrium Multistability to Spatiotemporal Chaos in Channel Flows of Nematic Fluids

Rahil N. Valani, Sumesh Thampi and Julia M. Yeomans

We investigate channel-confined, nematic liquid crystals using the Beris-Edwards model of nematohydrodynamics. Using strong homeotropic anchoring at the walls, we find multistability i.e. multiple coexisting states where the uniform nematic state coexists with states having spatially varying scalar nematic order and director fields. When a pressure gradient is applied, flows develop, and the inherent multistability of the system organizes a variety of complex dynamics. For low pressure gradients, steady flows are established, and the director fields that emerge from the multistable states at equilibrium correspond to Bowser and Dowser configurations similar to those reported in experiments. An increasing pressure-gradient destabilizes steady Bowser and Dowser flow states sequentially, leading to unsteady periodic and chaotic regimes featuring cyclical topological transitions, pulsating flows, advecting defects and spatiotemporal chaos. These findings demonstrate that modest variations in the scalar nematic order, as captured by the Beris-Edwards model, can qualitatively modify equilibrium structures and give rise to complex nonequilibrium behaviour in confined nematics-contrasting with the Ericksen-Leslie model, which assumes a constant scalar order parameter. Our key model predictions - multistability, periodically oscillating states and advecting defect-mediated turbulence can be experimentally investigated in pressure-driven channel flows of nematic fluids.
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Strongly Correlated Electrons · Cross-listed from quant-ph

From Random Determinants to the Ground State

Hao Zhang and Matthew Otten

Accurate quantum many-body calculations often depend on reliable reference states or good human-designed ansätze, yet these sources of knowledge can become unreliable in hard problems like strongly correlated systems. We introduce the Trimmed Configuration Interaction (TrimCI) method, a prior-knowledge-free algorithm that builds accurate ground states directly from random Slater determinants. TrimCI iteratively expands the variational space and trims away unimportant states, allowing a random initial core to self-refine into an accurate approximation of exact ground state. Across challenging benchmarks, TrimCI achieves state-of-the-art accuracy with strikingly efficiency gains of several orders of magnitude. For [4Fe-4S] cluster, it matches recent quantum computing results with $10^6$-fold fewer determinants and CPU-hours. For the nitrogenase P-cluster, it matches selected-CI accuracy using $10^5$-fold fewer determinants. For $8\times8$ Hubbard model, it recovers over $99\%$ of the ground-state energy using only $10^{-28}$ of the Hilbert space. In some regimes, TrimCI attains orders-of-magnitude higher accuracy than AFQMC method. These results demonstrate that high-accuracy many-body ground states can be discovered directly from random determinants, establishing TrimCI as a prior-knowledge-free, accurate and highly efficient framework for quantum many-body systems. The compact explicit wavefunctions it produces further enable direct and rapid evaluation of observables.
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Soft Condensed Matter

Cell Shape Emerges from Motion

Gautham Gopinath, Emmanuel Y. Mintah, Aashrith Saraswathibhatla et al. (+6 more)

We perform cell segmentation on images from experimental studies of confluent, mobile cells in epithelial monolayers and show that these systems possess a broad, positively-skewed shape parameter distribution $P(\mathcal{A})$, where $\mathcal{A}=p^2/4\pi a$, $p$ is the perimeter, and $a$ is area of each cell. $P(\mathcal{A})$ is peaked at a value higher than the typical shape parameter $\mathcal{A}^* \sim 1.15$ that occurs for randomly packed, static confluent cell monolayers. The distribution does not arise from a heterogeneous population of cells with different fixed $\mathcal{A}$, nor can it arise from cell shape fluctuations from strains below the elastic limit. Instead, we find that all cells in each monolayer sample $\mathcal{A}$ values that span the full shape parameter distribution. We develop a deformable particle model that allows cell perimeter to adapt to local forces during cell motion, and this model recovers $P(\mathcal{A})$ to within $5\%$ for both MDCK and HaCaT epithelial cell monolayers. These results emphasize that confluent epithelial monolayers of mobile cells generate a well-defined broad shape parameter distribution that is independent of the initial cell shapes.
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All authors Gautham Gopinath, Emmanuel Y. Mintah, Aashrith Saraswathibhatla, Jonah J. Spencer, Shahar Nahum, Lior Atia, Jacob Notbohm, Mark D. Shattuck, Corey S. O'Hern

Statistical Mechanics

On the Modified Eguchi-Oki-Matsumura System

P.O. Mchedlov-Petrosyan and L.N. Davydov

To describe the simultaneous order-disorder transformation and phase separation Eguchi, Oki and Matsumura [\doi{10.1557/proc-21-589}] introduced the system of two equations: one equation, governing the evolution of a conserved order parameter, and the second equation for the non-conserved order parameter. The key feature of their model is the free energy functional, which contains the square gradient terms of the both order parameters and a fourth power polynomial depending on both order parameters. According to the general Hohenberg-Halperin classification it is the type C model. We show that if the dynamics of the conserved order parameter is governed by the convective-viscous Cahn-Hilliard equation, this system allows exact traveling wave solution.
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Statistical Mechanics

Compensating random transition-detection blackouts in Markov networks

Alexander M. Maier, Benjamin Häsler and Udo Seifert

In Markov networks, measurement blackouts with unknown frequency compromise observations such that thermodynamic quantities can no longer be inferred reliably. In particular, the observed currents neither discern equilibrium from non-equilibrium nor can they be used in extant estimators of entropy production. Our strategy to eliminate these effects is based on formally attributing the blackouts to a second channel connecting states. The unknown frequency of blackouts and the true underlying transition rates can be determined from the short-time limit of observed waiting-time distributions. A post-modification of observed trajectory data yields a virtual effective dynamics from which the lower bound on entropy production based on thermodynamic uncertainty relations can be recovered fully. Moreover, the post-processed data can be used in waiting-time based estimators. Crucially, our strategy does not require the blackouts to occur homogeneously or symmetrically under time-reversal.
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Materials Science

Giant enhancement of attosecond tunnel ionization competes with disorder-driven decoherence in silicon

D. N. Purschke, D. Vick, A. Cárdenas et al. (+16 more)

High-harmonic generation (HHG) is a strong-field phenomenon that is sensitive to the attosecond dynamics of tunnel ionization and coherent transport of electron-hole pairs in solids. While the foundations of solid HHG have been established, a deep understanding into the nature of decoherence on sub-cycle timescales remains elusive. Furthermore, there is a growing need for tools to control ionization at the nanoscale. Here, we study HHG in silicon along a crystalline-to-amorphous (c-Si to a-Si) structural phase transition and observe a dramatic reshaping of the spectrum, with enhanced lower-order harmonic yield accompanied by quenching of the higher-order harmonics. Modelling the real-space quantum dynamics links our observations to a giant enhancement (>250 times) of tunnel ionization yield in the amorphous phase and a disorder-induced decoherence that damps the electron-hole polarization over approximately six lattice sites. HHG spectroscopy also reveals remnant order that was not apparent with conventional probes. Finally, we observe a rapid and targeted non-resonant laser annealing of amorphous silicon islands. Our results offer a unique insight into attosecond decoherence in strong-field phenomena, establish HHG spectroscopy as a probe of structural disorder, and pave the way for new opportunities in lightwave nanoelectronics.
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All authors D. N. Purschke, D. Vick, A. Cárdenas, N. Haram, P. Bastani, S. Gholam-Mirzaei, S. Mokhtari, V. Jelic, J. Chen, J. Canlas, J. Tordiff, Md. W. Rahman, A. Yu. Naumov, D. M. Villeneuve, A. Staudte, M. Salomons, R. E. F. Silva, Á. Jiménez-Galán, G. Vampa

Materials Science

Automated Prediction of Thermodynamic Properties via Bayesian Free-Energy Reconstruction from Molecular Dynamics

Ekaterina Spirande, Timofei Miryashkin, Andrei Kolmakov et al. (+1 more)

Accurate free-energy calculations are essential for predicting thermodynamic properties and phase stability, but existing methods are limited: phonon-based approaches neglect anharmonicity and liquids, while molecular dynamics (MD) is computationally demanding, neglects low-temperature quantum effects, and often requires manual planning and post-processing of simulations. We present a unified workflow that reconstructs the Helmholtz free-energy surface from MD data using Gaussian Process Regression (GPR), augmented with zero-point energy corrections from harmonic/quasi-harmonic theory. The framework propagates statistical uncertainties, mitigates finite-size effects, and employs active learning to optimize sampling in the volume-temperature space. It applies seamlessly to both crystalline and liquid phases. We demonstrate the methodology by computing heat capacities, thermal expansion, isothermal and adiabatic bulk moduli, and melting properties for nine elemental FCC and BCC metals using 20 classical and machine-learned interatomic potentials, with all predictions accompanied by quantified confidence intervals. Automated, general, and uncertainty-aware, the workflow advances high-throughput thermodynamics and provides a systematic benchmark for interatomic potentials.
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All authors Ekaterina Spirande, Timofei Miryashkin, Andrei Kolmakov, Alexander Shapeev

Statistical Mechanics

Search by Return: Stochastic Resetting in Fluctuating Harmonic Potentials

Derek Frydel

We study a class of stochastic resetting (SR) processes in which a diffusing particle alternates between free motion and confinement by an externally controlled potential. When the particle is recaptured, it undergoes a return trajectory that drives it toward a designated reset point. In standard SR, such returns are treated as instantaneous, but in realistic setups they have finite duration and introduce imprecision in the starting points of subsequent search attempts. We analyze a fluctuating harmonic potential in which return trajectories are forcibly terminated the moment the particle reaches the origin, ensuring that all outward (diffusive) trajectories begin from the same point. This is implemented through instantaneous positional information: a feedback signal that shortens the return phase without incurring additional mechanical energetic cost. We examine several search protocols built on this controlled return mechanism and determine their mean first-passage times (MFPTs). Of particular interest is a protocol in which outward diffusion is eliminated entirely and the return motion itself becomes the search mechanism. This "search by return" perspective reverses the conventional logic of SR and yields a closed-form MFPT that highlights the efficiency of using return dynamics as the primary search strategy.
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Materials Science · Cross-listed from physics.plasm-ph

Real-time time-dependent density functional theory for high-energy density physics

Alina Kononov, Minh Nguyen and Andrew D. Baczewski

Electronic response properties of high-energy density (HED) systems influence planetary structure, drive evolution of fusion targets, and underpin diagnostics in laboratory astrophysics. Real-time time-dependent density functional theory (TDDFT) offers a versatile modeling framework capable of accurately predicting the dynamic response of HED materials – including free-free, bound-free, and bound-bound contributions without requiring ad hoc state partitioning; capturing both collective and non-collective behavior; and applicable within the linear-response regime and beyond. We review the theoretical formalism of real-time TDDFT as applied to HED systems, provide a practical tutorial for computing relevant response properties (dynamic structure factors, conductivity, and stopping power), and comment on avenues for further development of this powerful computational method in service of HED science.
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Mesoscale and Nanoscale Physics

Interplay of Electron Phonon Coupling Dissipative Phonon Bath and Electron Electron Interaction in a Triangular Quantum-Dot Trimer

Hemant Kumar Sharma

Nonequilibrium charge transport through a trimer molecular transistor composed of three quantum dots arranged in a triangular geometry, which is placed on a substrate, has been studied in the presence of electron electron and electron phonon interactions. The entire system is described by an extended Anderson Holstein Caldeira Leggett Hamiltonian, in which the Caldeira Leggett term accounts for phonon damping arising from the coupling between the molecular vibrations and the substrate phonon bath. The electron phonon interaction is treated nonperturbatively using the Lang Firsov canonical transformation, while the electron electron interaction is incorporated at the mean field level. Keldysh nonequilibrium Greens function framework is used to study the transport properties, allowing us to calculate the spectral function, tunneling current, ad differential conductance of the trimer molecular transistor. The formalism enables systematic evaluation of the effects of Coulomb interaction, electron phonon coupling, and dissipation on the devices electronic transport characteristics.
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Soft Condensed Matter

Active Matter as a framework for living systems-inspired Robophysics

Giulia Janzen, Gaia Maselli, Juan F. Jimenez et al. (+3 more)

Robophysics investigates the physical principles that govern living-like robots operating in complex, realworld environments. Despite remarkable technological advances, robots continue to face fundamental efficiency limitations. At the level of individual units, locomotion remains a challenge, while at the collective level, robot swarms struggle to achieve shared purpose, coordination, communication, and cost efficiency. This perspective article examines the key challenges faced by bio-inspired robotic collectives and highlights recent research efforts that incorporate principles from active-matter physics and biology into the modeling and design of robot swarms.
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All authors Giulia Janzen, Gaia Maselli, Juan F. Jimenez, Lia Garcia-Perez, D A Matoz Fernandez, Chantal Valeriani

Superconductivity · Cross-listed from quant-ph

Measuring Reactive-Load Impedance with Transmission-Line Resonators Beyond the Perturbative Limit

Xuanjing Chu, Jinho Park, Jesse Balgley et al. (+8 more)

We develop an analytic framework to extract circuit parameters and loss tangent from superconducting transmission-line resonators terminated by reactive loads, extending analysis beyond the perturbative regime. The formulation yields closed-form relations between resonant frequency, participation ratio, and internal quality factor, removing the need for full-wave simulations. We validate the framework through circuit simulations, finite-element modeling, and experimental measurements of van der Waals parallel-plate capacitors, using it to extract the dielectric constant and loss tangent of hexagonal boron nitride. Statistical analysis across multiple reference resonators, together with multimode self-calibration, demonstrates consistent and reproducible extraction of both capacitance and loss tangent in close agreement with literature values. In addition to parameter extraction, the analytic relations provide practical design guidelines for maximizing energy participation ratio in the load and improving the precision of resonator-based material metrology.
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All authors Xuanjing Chu, Jinho Park, Jesse Balgley, Sean Clemons, Ted S. Chung, Kenji Watanabe, Takashi Taniguchi, Leonardo Ranzani, Martin V. Gustafsson, Kin Chung Fong, James Hone

Strongly Correlated Electrons

Non-Fermi-liquid behaviour and Fermi-surface expansion induced by van Hove-driven ferromagnetic fluctuations: the D-TRILEX analysis

Ilia S. Dedov, Andrey A. Katanin and Evgeny A. Stepanov

We consider the electronic and magnetic properties of the Hubbard model on a square lattice with the Fermi level near van Hove singularity and the ratio of the next-nearest-neighbor and nearest-neighbor hoppings $t'/t=-0.45$, which favours the ferromagnetic instability. We find, that a self-consistent consideration of the ferromagnetic fluctuations within the D-TRILEX approach results in the splitting of the electronic spectral function at low temperatures. This splitting exhibits only a weak momentum dependence, and only one of the split bands crosses the Fermi level. As a result, the Fermi surface itself remains unsplit, but its area increases, reflecting the presence of non-Fermi-liquid electronic excitations. We show that both the self-consistent account of the non-local contributions to the electronic self-energy and the proper treatment of electron interaction vertices in D-TRILEX are important to obtain this behaviour.
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Strongly Correlated Electrons

Collective modes, Yb-valence instability and metal-insulator transition in the cage-cluster borides RB12 (R- Ho, Er, Tm, Yb, Lu, Zr)

Gennady Komandin, Elena Zhukova, Boris Gorshunov et al. (+6 more)

A thorough study of the wide-range (40-35000 cm-1) dynamic conductivity spectra of the rare-earth (RE) dodecaborides RB12 (R- Ho, Er and Tm) and Tm1-xYbxB12 substitutional solid solutions was carried out at room temperature. Both the Drude-type components and overdamped excitations have been separated and analyzed. An additional absorption band observed above 200 cm-1 in these RB12 with magnetic RE ions is attributed to the cooperative Jahn-Teller dynamics of the B12 complexes, which depends crucially on the RE-ion cage space and is compared with the same effect found in the non-magnetic LuB12 and ZrB12. It was shown that non-equilibrium (hot) electrons participating in the formation of the collective JT modes dominate in charge transport, and portion of Drude-type carriers changes by 20-40% in these compounds with unstable boron lattice. Strong renormalization of the infrared response is observed in Tm1-xYbxB12 solid solutions with metal-insulator transition (MIT) and is discussed in terms of localized collective modes caused by Yb-ion valence instability. We demonstrate that even at room temperature the MIT is accompanied with simultaneous decrease in concentration of Drude-type electrons and redistribution of carriers to localized JT collective modes.
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All authors Gennady Komandin, Elena Zhukova, Boris Gorshunov, Alexander Melentyev, Ludmila Alyabyeva, Andrey Azarevich, Andrey Muratov, Yurii Aleshchenko, Nikolay Sluchanko

Strongly Correlated Electrons

Interlayer Coupling Driven Correlated and Charge-Ordered Electronic States in a Transition Metal Dichalcogenide Superlattice

Yiwei Li, Lixuan Xu, Shihao Zhang et al. (+11 more)

4Hb-TaS_2, a van der Waals superlattice comprising alternate stacked Ising superconducting 1H-TaS_2 and cluster Mott insulating 1T-TaS_2, exhibits emergent properties beyond those of its constituent layers. Notable phenomena include time-reversal-symmetry-breaking superconductivity and spontaneous vortex phases, which are driven by nontrivial interlayer interactions that remain debated. Using area-selective angle-resolved photoemission spectroscopy, we provide direct spectroscopic evidence of such interaction by systematically probing the electronic structures of 1T- and 1H-terminted surfaces of 4Hb-TaS_2. The metallic states of subsurface 1H-layers are folded to the Brillouin zone center by the sqrt(13) by sqrt(13) modulation of the surface 1T-layer, forming chiral "windmill" Fermi surfaces via Umklapp scattering. These conducting states further hybridize with the incipient flat band of the surface 1T-layer, producing a Kondo-like peak at the Fermi level. Interlayer charge transfer induces distinct 3 by 3 and 2 by 2 charge orders on the surface and subsurface 1H-layers, respectively, which result in characteristic segmented Fermi surfaces and dichotomously shift the van Hove singularities. These findings reconcile the competing Kondo and Mott-Hubbard models in this material and emphasize the interplay of flat bands, van hove singularities, charge orders, and unconventional superconductivity in correlated superlattices.
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All authors Yiwei Li, Lixuan Xu, Shihao Zhang, Lanxin Liu, Yifan Zhou, Qiang Wan, Shiwei Chen, Shiheng Liang, Yulin Chen, Yi-feng Yang, Xuan Luo, Yuping Sun, Nan Xu, Zhongkai Liu

Statistical Mechanics

Nonlinearity-induced transition in heat conduction through a topological metamaterial of rotors

T. R. Vishnu and Dibyendu Roy

We investigate heat conduction in a one-dimensional chain of rigid rotors. The rotors are constrained to rotate in a plane about fixed pivot points and coupled by springs, such that in equilibrium, the neighboring rotors lie on opposite sides of the chain axis. The linearized limit of this model valid for small angular displacements, was first introduced by Kane and Lubensky (KL) as a topological mechanical insulator hosting zero-energy vibrational edge modes. We show that the linearized KL chain behaves as a thermal insulator at low temperature in both the topological phases with a finite band gap, and the heat current falls exponentially with the chain length. When the gap vanishes at the topological phase transition, the KL chain becomes a good thermal conductor and conducts heat ballistically. The chain of rotors for arbitrary angular displacements hosts nonlinear solitary waves and distinct topological mechanical phases. Our numerical analysis shows normal (diffusive) heat conduction in all topological phases of the nonlinear chain. Nevertheless, a finite thermal conductivity is achieved for different system sizes in different topological phases of this nonlinear chain.
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Materials Science

Precise, efficient and flexible modeling of crystallizing elastomers based on physics-augmented neural networks

Konrad Friedrichs, Franz Dammaß, Karl A. Kalina et al. (+1 more)

We propose a precise and efficient physics-augmented neural network (PANN) to model strain-induced crystallization in rubbery polymers. We demonstrate that the model can be flexibly employed for both unfilled and filled natural rubber (NR). The approach is based on a two potential framework, similar to the concept of generalized standard materials (GSMs). To describe the material behavior, neural network-based free energy and dissipation potentials are employed. The evolution of crystallinity is derived from the two potentials. To ensure boundedness of the crystallinity, a novel constrained GSM-type evolution problem is proposed. To this end, two additional Lagrange multipliers together with the corresponding Karush-Kuhn-Tucker conditions are introduced. As a result, it is guaranteed that crystallinity can be interpreted as a variable of concentration type. The neural network-based potentials ensure all physically desirable properties by construction. Most importantly, objectivity, material symmetry and thermodynamic consistency are automatically fulfilled. In addition, an alternative derivation of the governing model equations in time-discrete form is presented based on an incremental variational framework, which also serves as the basis for a finite element implementation. We demonstrate the predictive capability of the PANN using three different experimental data sets from literature, considering both stress and crystallinity evolution at material point level as well as the corresponding field distributions in a notched specimen. Moreover, we show that model parameterization is also possible when experimental crystallinity data is not available, still enabling suitable stress predictions.
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All authors Konrad Friedrichs, Franz Dammaß, Karl A. Kalina, Markus Kästner