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Physics Condensed Matter

cond-mat Watch Condensed Matter

Thursday 2 October

Strongly Correlated Electrons

The Finite-Temperature Behavior of a Triangular Heisenberg Antiferromagnet

Cecilie Glittum and Olav F. Syljuåsen

We investigate the classical antiferromagnetic Heisenberg model on the triangular lattice with up to third-nearest neighbor exchange couplings using the Nematic Bond Theory. This approach allows us to compute the free energy and the neutron scattering static structure factor at finite temperatures. We map out the phase diagram with a particular emphasis on finite-temperature phase transitions that break lattice-rotational symmetries, spiral spin liquids and the broad specific heat hump that is ubiquitous in the antiferromagnetic 120 degree phase. We identify this specific heat hump as signaling the onset of an exponentially increasing correlation length. Further, we map out the temperature of the specific heat hump and the transition temperatures of the symmetry-breaking transitions throughout the exchange-coupling space. Along the line $J_3 = J_2/2$, the Fourier-transformed exchange coupling exhibits a degenerate ring-like minimum, giving rise to spiral spin liquid behavior at intermediate temperatures. We investigate the structure factor of the spiral spin liquid as function of $J_2$ and identify the corresponding low-temperature order, which coincides with the single-$\vec{q}$ spiral states of maximum spin-wave entropy along the degenerate ring.
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Statistical Mechanics · Cross-listed from quant-ph

Critical Quantum Sensing: a tutorial on parameter estimation near quantum phase transitions

George Mihailescu, Uesli Alushi, Roberto Di Candia et al. (+2 more)

Quantum phenomena offer the possibility of measuring physical quantities with precision beyond classical limits. However, current progress is constrained by scalability, environmental noise, and challenges in practical integration. This highlights the necessity for novel approaches. An emerging paradigm in this direction is critical quantum metrology – which harnesses the enhanced susceptibility and nonclassical correlations naturally occurring near quantum phase transitions as resources for quantum-enhanced precision. This tutorial provides a pedagogical introduction to key concepts and a detailed overview of prominent quantum sensing strategies that exploit critical phenomena in metrology. Through examples of increasing complexity, the reader is guided through various critical quantum sensing protocols applied to different critical systems. Special emphasis is placed on the optimal scaling of estimation precision with respect to fundamental resources. Finally, we discuss how critical quantum metrology extends from idealized models to realistic open-system, dissipative regimes, and strongly correlated fermionic systems, outlining both the challenges and opportunities for future quantum technologies.
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All authors George Mihailescu, Uesli Alushi, Roberto Di Candia, Simone Felicetti, Karol Gietka

Mesoscale and Nanoscale Physics

Pulsed-laser induced gold microparticle fragmentation by thermal strain

Yogesh Pokhrel, Meike Tack, Sven Reichenberger et al. (+2 more)

Laser fragmentation of suspended microparticles is an upcoming alternative to laser ablation in liquid (LAL) that allows to streamline the the delivery process and optimize the irradiation conditions for best efficiency. Yet, the structural basis of this process is not well understood to date. Herein we employed ultrafast x-ray scattering upon picosecond laser excitation of a gold microparticle suspension in order to understand the thermal kinetics as well as structure evolution after fragmentation. The experiments are complemented by simulations according to the two-temperature model to verify the spatiotemporal temperature distribution. It is found that above a fluence threshold of 750 J/m$^2$ the microparticles are fragmented within a nanosecond into several large pieces where the driving force is the strain due to a strongly inhomogenous heat distribution on the one hand and stress confinement due to the ultrafast heating compared to stress propagation on the other hand. The additional limited formation of small clusters is attributed to photothermal decomposition on the front side of the microparticles at the fluence of 2700 J/m$^2$.
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All authors Yogesh Pokhrel, Meike Tack, Sven Reichenberger, Matteo Levantino, Anton Plech

Statistical Mechanics · Cross-listed from quant-ph

Digital quantum simulation of many-body localization crossover in a disordered kicked Ising model

Tomoya Hayata, Kazuhiro Seki and Seiji Yunoki

Simulating nonequilibrium dynamics of quantum many-body systems is one of the most promising applications of quantum computers. However, a faithful digital quantum simulation of the Hamiltonian evolution is very challenging in the present noisy quantum devices. Instead, nonequilibrium dynamics under the Floquet evolution realized by the Trotter decomposition of the Hamiltonian evolution with a large Trotter step size is considered to be a suitable problem for simulating in the present or near-term quantum devices. In this work, we propose simulating the many-body localization crossover as such a nonequilibrium problem in the disordered Floquet many-body systems. As a demonstration, we simulate the many-body localization crossover in a disordered kicked Ising model on a heavy-hex lattice using $60$ qubits from $156$ qubits available in the IBM Heron r2 superconducting qubit device named ibm\_fez. We compute out-of-time-ordered correlators as an indicator of the many-body localization crossover. From the late-time behavior of out-of-time-ordered correlators, we locate the quantum chaotic and many-body localized regimes as a function of the disorder strength. The validity of the results is confirmed by comparing two independent error mitigation methods, that is, the operator renormalization method and zero-noise extrapolation.
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Quantum Gases

Topological interactions in vortex-wave collisions in Bose-Einstein condensates

Vebjørn Øvereng, Andrew Baggaley and Luiza Angheluta

We study vortex-vortex and vortex-wave collisions in two-dimensional weakly interacting Bose-Einstein condensates, processes that play a central role in decaying quantum turbulence. Using numerical simulations of the Gross-Pitaevskii equation, we show that during collisions of vortex-antivortex dipoles, the kinetic energy is transferred from incompressible to compressible modes by two distinct mechanisms. Below the critical vortex separation for annihilation, the transfer is mediated by quantum energy released during annihilation events, while above the threshold it arises from vortex acceleration. In wave-vortex collisions, an incoming solitary wave splits into transient phase slips that interact with the vortex, one of the phase slips contributes to vortex annihilation, and the other phase slip acquires a stable core and forms a new vortex. By analyzing vortex trajectories and energy spectra, we provide new insights into energy transfer mechanisms in quantum turbulence and offer broader implications for topological interactions mediated by vortices.
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Mesoscale and Nanoscale Physics

Measuring the measurement problem: controlling decoherence with measurement duration in molecular MCB junctions

C.J. Muller

We investigate the influence of the measurement duration on quantum coherence in molecular mechanically controlled break junctions operating in a tetrahydrofuran (THF) partially wet phase. These systems represent a distinct class of enclosed open quantum systems with unusually long decoherence times at ambient conditions, on the order of 1-20 ms. By tuning the integration time of the current measurement in current-voltage (IV) characteristics, relative to the decoherence time, we observe a transition from quantum interference patterns, manifested as structured bands of data points, to classical behavior characterized by a single averaged response. This demonstrates that the duration of a measurement acts as a controllable parameter for probing quantum behavior in molecular junctions, offering new insights into decoherence dynamics in quantum mechanics.
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Disordered Systems and Neural Networks · Cross-listed from stat.ML

Precise Dynamics of Diagonal Linear Networks: A Unifying Analysis by Dynamical Mean-Field Theory

Sota Nishiyama and Masaaki Imaizumi

Diagonal linear networks (DLNs) are a tractable model that captures several nontrivial behaviors in neural network training, such as initialization-dependent solutions and incremental learning. These phenomena are typically studied in isolation, leaving the overall dynamics insufficiently understood. In this work, we present a unified analysis of various phenomena in the gradient flow dynamics of DLNs. Using Dynamical Mean-Field Theory (DMFT), we derive a low-dimensional effective process that captures the asymptotic gradient flow dynamics in high dimensions. Analyzing this effective process yields new insights into DLN dynamics, including loss convergence rates and their trade-off with generalization, and systematically reproduces many of the previously observed phenomena. These findings deepen our understanding of DLNs and demonstrate the effectiveness of the DMFT approach in analyzing high-dimensional learning dynamics of neural networks.
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Mesoscale and Nanoscale Physics

Electrically tunable ultrafast dynamics and interactions of hybrid excitons in a 2D semiconductor bilayer

Edoardo Lopriore, Charalambos Louca, Armando Genco et al. (+12 more)

Extended efforts have been devoted to the study of strongly-interacting excitons and their dynamics, towards macroscopic quantum states of matter such as Bose-Einstein condensates of excitons and polaritons. Momentum-direct layer-hybridized excitons in transition metal dichalcogenides have attracted considerable attention due to their high oscillator strength and dipolar nature. However, the tunability of their interactions and dynamics remains unexplored. Here, we achieve an unprecedented control over the nonlinear properties of dipolar layer-hybridized excitons in an electrically gated van der Waals homobilayer monitored by transient optical spectroscopy. By applying a vertical electric field, we reveal strong Coulomb interactions of dipolar hybrid excitons, leading to opposite density-dependent energy shifts of the two main hybrid species based on their dipolar orientation, together with a strongly enhanced optical saturation of their absorption. Furthermore, by electrically tuning the interlayer tunneling between the hybridized carriers, we significantly extend the formation time of hybrid excitons, while simultaneously increasing their decay times. Our findings have implications for the search on quantum blockade and condensation of excitons and dipolaritons in two-dimensional materials.
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All authors Edoardo Lopriore, Charalambos Louca, Armando Genco, Irantzu Landa, Daniel Erkensten, Charles J. Sayers, Samuel Brem, Raul Perea-Causin, Kenji Watanabe, Takashi Taniguchi, Christoph Gadermaier, Ermin Malic, Giulio Cerullo, Stefano Dal Conte, Andras Kis

Mesoscale and Nanoscale Physics

Band Gap Engineering of Nitrogen-Doped Monolayer WSe$_2$ Superlattice and its application to Field Effect Transistor

Yi-Cheng Lo, Liao Jia Wang and Yu-Chang Chen

We systematically investigate the electronic structures of pristine monolayer WSe$_2$ and WSe$_2$ superlattices with periodic nitrogen substitution. Unlike random doping, which often introduces in-gap impurity states, periodic nitrogen doping primarily modulates the band gap, thereby facilitating effective band gap engineering for electronic and optoelectronic applications. The gap narrows monotonically with increasing dopant density (pristine $>$ 8-row $>$ 6-row $>$ 4-row), directly influencing device switching. We also evaluate the FET performance of nanojunctions created by these configurations by examining the contour plot of current density as a function of temperature and gate voltage, which quantifies how bandgap engineering affects switching characteristics. Our calculations clarify the classical-quantum crossover in sub-10 nm 2D FETs: as $T$ rises, $J$ approaches the thermionic current; as $T$ falls, quantum tunneling dominates, and the steep energy dependence of $\tau(E)$ may break the classical limit of subthreshold swing imposed by the Boltzmann tyranny. The optimal gating range ($V_g^\mathrm{ON}$, $V_g^\mathrm{OFF}$) is investigated for each temperature, insensitive to temperature in the high-temperature regime, confirming the good thermal stability of the FET devices. A comparison study demonstrates that the 4-row structure, with large $J_\mathrm{OFF}$ and restricted operation range, is inappropriate for realistic FET applications. The pristine structure has a high $V_g^\mathrm{OFF}$ ($\sim$1.1 V) makes it less practical, since such a large threshold voltage may promote time-dependent dielectric breakdown (TDDB) of the oxide layer, reducing device dependability. The 6-row and 8-row structures exhibit more favorable $V_g^\mathrm{OFF}$ values ($\sim$0.75 V), achieving compromise, making them more promising candidates for future FET integration.
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Strongly Correlated Electrons

Bad Metal Behavior and Lifshitz Transition of a Nagaoka Ferromagnet

Jonas Arnold, Peter Kopietz and Andreas Rückriegel

Using an extension of the fermionic functional renormalization group for systems where strong correlations give rise to projected Hilbert spaces we calculate the phase diagram and the electronic spectral function of the Hubbard model at infinite on-site repulsion. For a square lattice with nearest-neighbor hopping we find that the ground state evolves from a paramagnetic Fermi liquid at low densities via a state with antiferromagnetic stripe order at intermediate densities to an extended Nagaoka ferromagnet at high densities. The single-particle spectral function of the Nagaoka ferromagnet exhibits a flat but rather broad band characteristic for an incoherent non-Fermi liquid. We identify two distinct ferromagnetic regimes separated by a Lifshitz transition.
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Soft Condensed Matter · Cross-listed from quant-ph

Folding lattice proteins confined on minimal grids using a quantum-inspired encoding

Anders Irbäck, Lucas Knuthson and Sandipan Mohanty

Steric clashes pose a challenge when exploring dense protein systems using conventional explicit-chain methods. A minimal example is a single lattice protein confined on a minimal grid, with no free sites. Finding its minimum energy is a hard optimization problem, withsimilarities to scheduling problems. It can be recast as a quadratic unconstrained binary optimization (QUBO) problem amenable to classical and quantum approaches. We show that this problem in its QUBO form can be swiftly and consistently solved for chain length 48, using either classical simulated annealing or hybrid quantum-classical annealing on a D-Wave system. In fact, the latter computations required about 10 seconds. We also test linear and quadratic programming methods, which work well for a lattice gas but struggle with chain constraints. All methods are benchmarked against exact results obtained from exhaustive structure enumeration, at a high computational cost.
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Materials Science

Spin-phonon coupling and isotope-related pseudo-molecule vibrations in layered Cr$_2$Ge$_2$Te$_6$ ferromagnet

Grzegorz Krasucki, Katarzyna Olkowska-Pucko, Tomasz Woźniak et al. (+4 more)

The vibrational structure of chromium germanium telluride (Cr$_2$Ge$_2$Te$_6$, CGT) is investigated and a strong spin-phonon coupling is revealed. The measured high-resolution Raman scattering (RS) spectra are composed of the 10 Raman-active modes: 5A$_\textrm{g}$ and 5E$_\textrm{g}$, predicted by calculation using the density functional theory and identified using polarization-resolved RS measurements. We also studied the effect of temperature on the RS spectra of CGT from 5~K to 300~K. A strong magneto-phonon coupling in CGT is revealed at temperatures of about 150~K and 60~K, which are associated with the appearance of the local magnetic order in the material and the transition to the complete ferromagnetic phase, respectively. Moreover, a unique shape of the A$_g^5$ mode composed of a set of very narrow Raman peaks is simulated using a model that takes into account vibrations of Ge-Ge pseudo-molecules for various Ge isotopes.
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All authors Grzegorz Krasucki, Katarzyna Olkowska-Pucko, Tomasz Woźniak, Mihai I. Sturza, Holger Kohlmann, Adam Babiński, Maciej R. Molas

Materials Science

Enhancing the Efficiency of Time-Dependent Density Functional Theory Calculations of Dynamic Response Properties

Zhandos A. Moldabekov, Sebastian Schwalbe, Uwe Hernandez Acosta et al. (+4 more)

X-ray Thomson scattering (XRTS) constitutes an essential technique for diagnosing material properties under extreme conditions, such as high pressures and intense laser heating. Time-dependent density functional theory (TDDFT) is one of the most accurate available ab initio methods for modeling XRTS spectra, as well as a host of other dynamic material properties. However, strong thermal excitations, along with the need to account for variations in temperature and density as well as the finite size of the detector significantly increase the computational cost of TDDFT simulations compared to ambient conditions. In this work, we present a broadly applicable method for optimizing and enhancing the efficiency of TDDFT calculations. Our approach is based on a one-to-one mapping between the dynamic structure factor and the imaginary time density–density correlation function, which naturally emerges in Feynman's path integral formulation of quantum many-body theory. Specifically, we combine rigorous convergence tests in the imaginary time domain with a constraints-based attenuation of narrow-band fluctuations to improve the efficiency of TDDFT modeling without the introduction of any significant bias. As a result, we can report a speed-up by up to an order of magnitude, thus substantially reducing the burden of computational cost required for XRTS analysis.
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All authors Zhandos A. Moldabekov, Sebastian Schwalbe, Uwe Hernandez Acosta, Thomas Gawne, Jan Vorberger, Michele Pavanello, Tobias Dornheim

Soft Condensed Matter · Cross-listed from physics.bio-ph

In Situ Observation of Proton-Induced DNA Fragmentation in the Bragg Peak: Evidence for a Protective Role of Water

R.Liénard, P.Barberet, K.Chatzipapas et al. (+5 more)

We report in situ single-molecule measurements of proton-induced double-strand breaks (DSBs) in DNA immersed in water, using real-time fluorescence tracking along the entire proton path, including the Bragg peak region. By chemically suppressing radical-mediated processes, we isolate direct DNA damage mechanisms and determine DSB cross sections as a function of depth. Near the Bragg peak, we observe a tenfold reduction in DSB cross sections in aqueous DNA compared to dry DNA, providing quantitative evidence for the protective role of water. These findings highlight the importance of intermolecular energy dissipation in mitigating radiation-induced damage in condensed biological matter, with implications for radiobiology and proton therapy modeling.
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All authors R.Liénard, P.Barberet, K.Chatzipapas, G.Devès, T.Dhôte, T.Guérin, H.Seznec, F.Gobet

Soft Condensed Matter

Non-Gaussian Rotational Diffusion and Swing Motion of Dumbbell Probes in Two Dimensional Colloids

Jeongmin Kim, Taejin Kwon and Bong June Sung

Two dimensional (2D) colloids exhibit intriguing phase behaviors distinct from those in three dimensions, as well as dynamic heterogeneity reminiscent of glass-forming liquids. Here, using discontinuous molecular dynamics simulations, we investigate the reporting dynamics of dicolloidal dumbbell probes in 2D colloids across the liquid-hexatic phase transition, where hexagonal bond-orientational order (HBOO) extends to quasi-long-ranged one. The rotational dynamics of dumbbell probes faithfully capture the structural and dynamical features of the host: Brownian in the isotropic liquid, and non-Gaussian in the hexatic and solid phases, reflecting both HBOO and dynamic heterogeneity of the medium. In the 2D hexatic and solid phases, probe rotation reflects heterogeneity as the dumbbells sample multiple dynamical domains of the host system: in mobile domains, they undergo rotational jumps of $\pi/3$ in accordance with HBOO, whereas in immobile domains they librate within cages formed by surrounding discs. Such non-Gaussianity disappears upon reentrant melting of the host medium driven by size polydispersity, highlighting a close connection between HBOO and probe dynamics. Furthermore, probe dynamics reveal both coupling (at a single particle level) and decoupling (at an ensemble-averaged level) between translation and rotation: swing motion emerges as their primary diffusion mode, while the Debye-Stokes-Einstein relation breaks down regardless of how the rotational diffusion coefficient is defined.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.optics

Purcell-enhanced single-photon generation from CsPbBr$_3$ quantum dots in in-situ selected Laguerre-Gaussian modes

Virginia Oddi, Darius Urbonas, Etsuki Kobiyama et al. (+9 more)

Single photons in Laguerre-Gaussian (LG) beams, which carry orbital angular momentum (OAM), could enable more robust and efficient photonic quantum communication and information processing, as well as enhanced sensitivity in quantum metrology and imaging. However, as most implementations are indirect or require additional mode-shaping elements, direct generation of single photons with OAM has received growing interest. Colloidal lead halide perovskite quantum dots (QDs) have recently emerged as a versatile material that can produce indistinguishable single photons quasi-deterministically at high rate. Here, we integrate single CsPbBr$_3$ QDs into an open Fabry-Perot microcavity with a nanofabricated Gaussian-shaped deformation, demonstrating Purcell-enhanced single-photon generation into individual cavity modes with up to 18.1 $\pm$ 0.2 times accelerated decay, down to tens of picoseconds. By in-situ tuning of the cavity resonance, we can selectively couple a single QD to different LG modes carrying OAM and observe the spatial patterns of the generated single-photon beams emitted from the cavity. Our findings open the door to high photon rate sources that directly generate single-photon LG beams for advanced quantum photonic applications.
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All authors Virginia Oddi, Darius Urbonas, Etsuki Kobiyama, Ioannis Georgakilas, Ihor Cherniukh, Kseniia Shcherbak, Chenglian Zhu, Maryna I. Bodnarchuk, Maksym V. Kovalenko, Rainer F. Mahrt, Gabriele Rainò, Thilo Stöferle

Materials Science

A General-Purpose and Efficient Machine-Learned Potential for SiC from Ambient to Extreme Environments

Jintong Wu, Zhuang Shao, Junlei Zhao et al. (+5 more)

Silicon carbide (SiC) polymorphs are widely employed as nuclear materials, mechanical components, and wide-bandgap semiconductors. The rapid advancement of SiC-based applications has been complemented by computational modeling studies, including both ab initio and classical atomistic approaches. In this work, we develop a computationally efficient, general-purpose machine-learned interatomic potential (ML-IAP) capable of multimillion-atom molecular dynamics simulations over microsecond timescales. Using the ML-IAP, we map a broad pressure-temperature phase diagram and the threshold displacement energy distributions for the 2H and 3C polymorphs. Across a benchmark covering conditions from ambient to extreme, including high-pressure/high-temperature states and high-energy cascade damage, tabGAP provides a favorable balance of accuracy, robustness, transferability, and computational cost among the tested empirical and ML-IAPs.
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All authors Jintong Wu, Zhuang Shao, Junlei Zhao, Flyura Djurabekova, Kai Nordlund, Fredric Granberg, Qingmin Zhang, and Jesper Byggmästar

Strongly Correlated Electrons

Ultrafast giant enhancement of second harmonic generation in a strongly correlated cobaltite

Yuchen Cui, Qiaomei Liu, Qiong Wu et al. (+15 more)

In quantum materials, nonlinear optical responses are highly sensitive to electronic structure and many-body interactions. Probing and manipulating such nonlinear processes is a complex and subtle endeavor, yet it offers deep insights into emerging physics and functionalities. Here, we report an anomalous ultrafast enhancement of second harmonic generation (SHG) in a strongly correlated cobaltite YbBaCo$_4$O$_7$. Above-bandgap femtosecond pumping increases SHG intensity by up to 60 % within 200 fs, with the enhancement persisting for tens of picoseconds. The enhancement is strongly anisotropic, with substantial amplification of in-plane susceptibility tensors, whereas the out-of-plane counterpart shows negligible change. We attribute these anomalies to ultrafast photodoping-induced modulation of the on-site Coulomb repulsion, which dynamically renormalizes the band structure and selectively amplifies specific $\chi^{(2)}$ components. These nonlinear dynamics encode rich information about the orbital symmetries and energies of the states involved, opening new avenues for all-optical probing of electronic structure in strongly correlated materials.
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All authors Yuchen Cui, Qiaomei Liu, Qiong Wu, Shuxiang Xu, Junhan Huang, Hao Wang, Rongsheng Li, Shanshan Han, Wei Xu, Li Du, Ming Lu, Chunmei Zhang, Shangfei Wu, Xinbo Wang, Tao Dong, Li Yue, Dong Wu, Nanlin Wang

Statistical Mechanics

Optimal Control of Engineered Swift Equilibration of Nanomechanical Oscillators

Julia Sanders and Paolo Muratore-Ginanneschi

We propose a reformulation of the problem of optimally controlled transitions in stochastic thermodynamics. We impose that any terminal cost specified by a thermodynamic functional should depend only on state variables and not on control protocols, according to the canonical Bolza form. In this way, we can unambiguously discriminate between transitions at minimum dissipation between genuine equilibrium states, and transitions at minimum work driving a system from a genuine equilibrium to a non-equilibrium state. For underdamped dynamics subject to a mechanical force, genuine equilibrium means a Maxwell-Boltzmann probability distribution defining a vanishing current velocity. Transitions at minimum dissipation between equilibria are a model of optimal swift engineered equilibration. Remarkably, we show that transitions at minimum work do not directly imply explicit boundary conditions on terminal values of parameters of the mechanical force and on control protocols. Thus, the problem often discussed in the literature, that optimal protocols need terminal jumps to satisfy boundary conditions, completely disappears. The quantitative properties of optimal controls are entirely determined by the form of the penalty modelling an experimental setup. More generally, we use centre manifold theory to analytically account for the tendency of optimal controls to exhibit a turnpike property: optimal protocols in the bulk of the control horizon tend to converge to a universal centre manifold determined only by the running cost. Exponential deviations from the centre manifold occur at the ends of the control horizon in order to satisfy the boundary conditions. Our findings are supported numerically.
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Mesoscale and Nanoscale Physics

Intermediate diffusive-ballistic electron conduction around mesoscopic defects in graphene

Toni Markovic, Wei Huang, William S. Huxter et al. (+2 more)

Non-diffusive effects in charge transport become relevant as device sizes and features become comparable to the electronic mean free path. As a model system, we investigate the electric transport around mesoscopic defects in graphene with scanning tunneling potentiometry. Diffusive and ballistic contributions to the scattering dipole are probed by simultaneously resolving the nanoscale topography of pits in the graphene layer and measuring the local electrochemical potential in the surrounding area. We find evidence of transport in the intermediate regime between the diffusive and ballistic limits, such that the magnitude of the electrochemical potential around the defects is substantially underestimated by diffusive models. Our experiments and modeling are supported by lattice Boltzmann simulations, which highlight the importance of the ratio between defect size and mean free path in the intermediate transport regime. The magnitude of the scattering dipole depends on the shape of the pits in both the ballistic and diffusive transport modes. Remarkably, ballistic contributions to the electron transport are found at feature sizes larger than the mean free path and rapidly increase at lower sizes, having a noticeable impact already at mesoscopic length scales.
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All authors Toni Markovic, Wei Huang, William S. Huxter, Pietro Gambardella, Sebastian Stepanow

Superconductivity · Cross-listed from quant-ph

Three-Dimensional Niobium Coaxial Cavity with $\sim0.1\,$second Lifetime

Takaaki Takenaka, Takayuki Kubo, Imran Mahboob et al. (+4 more)

We report on the internal quality factor of a three-dimensional niobium quarter-wave coaxial cavity, with mid-temperature annealing, exhibiting $Q_{\rm int} \gtrsim 3\times10^9$ at the single-photon level below 20\,mK, which corresponds to an internal photon lifetime of $\tau_{\rm int}\sim90\,\mathrm{ms}$. Moreover, $Q_{\rm int}$ of the mid-temperature annealed cavities remains almost unchanged even after several cooldown cycles and air exposure. These results suggest that stable low-loss niobium oxides might be formed by mid-temperature annealing on the surface of three-dimensional niobium cavity. This surface treatment could be applicable to the fabrication of 2D superconducting circuits and help improve the lifetime of Nb-based superconducting qubits.
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All authors Takaaki Takenaka, Takayuki Kubo, Imran Mahboob, Kosuke Mizuno, Hitoshi Inoue, Takayuki Saeki, Shiro Saito

Statistical Mechanics · Cross-listed from q-fin.TR

Mean-field theory of the Santa Fe model revisited: a systematic derivation from an exact BBGKY hierarchy for the zero-intelligence limit-order book model

Taiki Wakatsuki and Kiyoshi Kanazawa

The Santa Fe model is an established econophysics model for describing stochastic dynamics of the limit order book from the viewpoint of the zero-intelligence approach. While its foundation was studied by combining a dimensional analysis and a mean-field theory by E. Smith et al. in Quantitative Finance 2003, their arguments are rather heuristic and lack solid mathematical foundation; indeed, their mean-field equations were derived with heuristic arguments and their solutions were not explicitly obtained. In this work, we revisit the mean-field theory of the Santa Fe model from the viewpoint of kinetic theory – a traditional mathematical program in statistical physics. We study the exact master equation for the Santa Fe model and systematically derive the Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchical equation. By applying the mean-field approximation, we derive the mean-field equation for the order-book density profile, parallel to the Boltzmann equation in conventional statistical physics. Furthermore, we obtain explicit and closed expression of the mean-field solutions. Our solutions have several implications: (1)Our scaling formulas are available for both $\mu\to 0$ and $\mu\to \infty$ asymptotics, where $\mu$ is the market-order submission intensity. Particularly, the mean-field theory works very well for small $\mu$, while its validity is partially limited for large $\mu$. (2)The “method of image” solution, heuristically derived by Bouchaud-Mézard-Potters in Quantitative Finance 2002, is obtained for large $\mu$, serving as a mathematical foundation for their heuristic arguments. (3)Finally, we point out an error in E. Smith et al. 2003 in the scaling law for the diffusion constant due to a misspecification in their dimensional analysis.
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Materials Science

Machine-learning-enabled methodology for the ab-initio simulations of sub-$\mu$m-wide nanoribbons

Guan-Hao Peng, Chin-Jui Huang, Wen-Teng Yang et al. (+1 more)

Simulation of mesoscopic nanostructures is a central challenge in condensed matter physics and device applications. First-principles methods provide accurate electronic structures but are computationally prohibitive for large systems, while empirical band theories are efficient yet limited by parameter fitting that neglects wavefunction information and often yields non-transferable parameters. We propose a methodology that bridges these approaches, achieving first-principles-level reliability with computational efficiency through a machine-learning-enabled tight-binding framework. Our approach starts with Wannier tight-binding (WTB) parameters from small nanostructures, which serve as training data for machine learning (ML). To remove the gauge freedom of Wannier functions that obscures size- and geometry-dependent parameter trends, we construct gauge-independent (GI) bases and transform the WTB model into a gauge-independent WTB (GI-WTB) model. This enables robust parameter fitting and ML prediction of parameter variations, yielding the machine-learning GI-WTB (ML-GI-WTB) model. Applied to MoS2 armchair-edge nanoribbons, the ML-GI-WTB model shows excellent agreement with first-principles results and enables reliable simulations of sub-$\mu$m-wide nanoribbons. This framework provides a scalable tool for predicting electronic properties of realistic nanostructures beyond the reach of conventional first-principles methods.
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All authors Guan-Hao Peng, Chin-Jui Huang, Wen-Teng Yang, Shun-Jen Cheng

Statistical Mechanics

Variational formulation of stochastic thermodynamics: Finite-dimensional systems

Héctor Vaquero del Pino, François Gay-Balmaz, Hiroaki Yoshimura et al. (+1 more)

In this paper, we develop a variational foundation for stochastic thermodynamics of finite-dimensional, continuous-time systems. Requiring the second law (non-negative average total entropy production) systematically yields a consistent thermodynamic structure, from which novel generalized fluctuation-dissipation relations emerge naturally, ensuring local detailed balance. This principle extends key results of stochastic thermodynamics including an individual trajectory level description of both configurational and thermal variables and fluctuation theorems in an extended thermodynamic phase space. It applies to both closed and open systems, while accommodating state-dependent parameters, nonlinear couplings between configurational and thermal degrees of freedom, and cross-correlated noise consistent with Onsager symmetry. This is achieved by establishing a unified geometric framework in which stochastic thermodynamics emerges from a generalized Lagrange-d'Alembert principle, building on the variational structure introduced by Gay-Balmaz and Yoshimura [Phil. Trans. R. Soc. A 381, 2256 (2023)]. Irreversible and stochastic forces are incorporated through nonlinear nonholonomic constraints, with entropy treated as an independent dynamical variable. This work provides a novel approach for thermodynamically consistent modeling of stochastic systems, and paves the way to applications in continuum systems such as active and complex fluids.
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All authors Héctor Vaquero del Pino, François Gay-Balmaz, Hiroaki Yoshimura, Lock Yue Chew

Mesoscale and Nanoscale Physics

Tunable Wigner Molecules in a Germanium Quantum Dot

Chenggang Yang, Jun Lu, Hongzhang Wang et al. (+7 more)

The interplay between Coulomb interactions and kinetic energy underlies many exotic phases in condensed matter physics. In a two-dimensional electronic system, If Coulomb interaction dominates over kinetic energy, electrons condense into a crystalline phase which is referred as Wigner crystal. This ordered state manifests as Wigner molecule for few electrons at the microscopic scale. Observation of Wigner molecules has been reported in quantum dot and moire superlattice systems. Here we demonstrate hole Wigner molecules can be formed in a gate-defined germanium quantum dot with high tunability. By varying voltages applied to the quantum dot device, we can precisely tune the hole density by either changing the hole occupancy or the quantum dot size. For densities smaller than a certain critical value, Coulomb interaction localizes individual holes into ordered lattice sites, forming a Wigner molecule. By increasing the densities, melting process from a Wigner molecule to Fermi liquid-like particles is observed. An intermediate configuration which indicates the coexistence of ordered structure and disordered structure can be formed within a narrow effective density range. Our results provide a new platform for further exploration of the microscopic feature of strong correlated physics and open an avenue to exploit the application of Wigner molecules for quantum information in a very promising spin qubit platform.
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All authors Chenggang Yang, Jun Lu, Hongzhang Wang, Jian Zeng, Wendong Bian, Zhengshan Guo, Jiankun Li, Yulei Zhang, Junwei Luo, Tian Pei

Materials Science

Metallurgy at the nanoscale: domain walls in nanoalloys

Grégoire Breyton, Hakim Amara, Jaysen Nelayah et al. (+8 more)

In binary alloys, domain walls play a central role not only on the phase transitions but also on their physical properties and were at the heart of the 70's metallurgy research. Whereas it can be predicted, with simple physics arguments, that such domain walls cannot exist at the nanometer scale due to the typical lengths of the statistical fluctuations of the order parameter, here we show, with both experimental and numerical approaches how orientational domain walls are formed in CuAu nanoparticles binary model systems. We demonstrate that the formation of domains in larger NPs is driven by elastic strain relaxation which is not needed in smaller NPs where surface effects dominate. Finally, we show how the multivariants NPs tend to form an isotropic material through a continuous model of elasticity.
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All authors Grégoire Breyton, Hakim Amara, Jaysen Nelayah, Christine Mottet, Riccardo Gatti, Jérôme Creuze, Adrien Moncomble, Damien Alloyeau, Nathaly Ortiz Peña, Guillaume Wang, Christian Ricolleau

Materials Science

Giant enhancement of terahertz high-harmonic generation by cavity engineering of a three-dimensional Dirac semimetal

Siyu Duan, Lili Shi, Patrick Pilch et al. (+7 more)

We report on time-resolved ultrafast terahertz high-harmonic generation of strong field driven-dynamics of many-body Dirac fermions. We demonstrate an experimental realization of near saturation regime of the high-order nonlinear responses with a giant enhancement of terahertz third- and fifth-order harmonic yields by cavity-engineering a three-dimensional Dirac semimetal Cd3As2. By fabricating a designed structure of metasurface microcavities on a nanometer Cd3As2 thin film, we significantly enhance the near-field intensity of a picosecond terahertz excitation pulse in resonance with the microcavity eigenmode. The strong terahertz field drives the far-from-equilibrium Dirac fermions deeply into a nonperturbative regime, leading to the observation of near-saturation high-harmonic emission. Our experimental results confirm the predictions by Boltzmann transport theory, and substantiate a field-driven kinetic description of the strong nonthermal nonlinearity at the terahertz frequencies.
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All authors Siyu Duan, Lili Shi, Patrick Pilch, Anneke Reinold, Sergey Kovalev, Renato M. A. Dantas, Yunkun Yang, Faxian Xiu, Miriam Serena Vitiello, Zhe Wang

Strongly Correlated Electrons

Hopf symmetry protected topological phases in the vicinity of spin orders

Grgur Palle

Hopf terms are topological theta terms that are associated with a host of interesting physics, including anyons, statistical transmutation, chiral edge states, and the spin quantum Hall effect. Here, we show that Hopf terms can appear in two-dimensional metals without spin-orbit coupling in the vicinity of spin-ordered phases. In their vicinity, their spin-like order parameters have a finite amplitude, but fluctuating orientation. When both a magnetic and a spin loop-current order parameter fluctuate in the system, we show that the phase is governed by the Hopf term and realizes a Hopf symmetry protected topological phase. This phase is protected by the unbroken $\mathrm{SU}(2)$ spin rotation symmetry, is gapped in the bulk, has chiral gapless edge states, and its spin-Hall conductance is quantized. Lattice models that realize this phase are introduced. In addition, we provide an elementary proof that the $\theta$ angle of the Hopf term must be quantized to multiples of $\pi$ in non-relativistic systems, thereby precluding anyonic skyrmions in condensed matter systems.
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Mesoscale and Nanoscale Physics

Orbital Magnetization of Correlated States in Twisted Bilayer Transition Metal Dichalcogenides

Xiaoyu Liu, Chong Wang, Haoran Chen et al. (+3 more)

Recent observations of quantum anomalous Hall effects in moiré systems have revealed the emergence of interaction-driven ferromagnetism with significant orbital contributions. To capture this physics, we extend the modern theory of orbital magnetization to Hartree-Fock states and show that the standard expression remains valid with Hartree-Fock orbitals and Hamiltonians. We then benchmark our theory against the Kane-Mele-Hubbard model in a weak field, which yields excellent agreement with direct numerical calculations. Applying our theory to twisted MoTe$_2$ bilayers, we find orbital magnetization of order one Bohr magneton per moiré cell with a non-monotonic twist-angle dependence. Our work establishes a general theory of orbital magnetization in interacting moiré systems and provides quantitative guidance for interpreting recent experiments.
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All authors Xiaoyu Liu, Chong Wang, Haoran Chen, Xiao-Wei Zhang, Ting Cao, Di Xiao

Mesoscale and Nanoscale Physics

Electric spin and valley Hall effects

W. Zeng

The electric Hall effect (EHE) is a newly identified Hall effect characterized by a perpendicular electric field inducing a transverse charge current in two-dimensional (2D) systems. Here, we propose a spin and valley version of EHE. We demonstrate that the transverse spin and valley currents can be generated in an all-in-one tunnel junction based on a buckled 2D hexagonal material in response to a perpendicular electric field, referred to as the electric spin Hall effect and electric valley Hall effect, respectively. These effects arise from the perpendicular-electric-field-induced backreflection phase of electrons in the junction spacer, independent of Berry curvature. The valley Hall conductance exhibits an odd response to the perpendicular electric field, whereas the spin Hall conductance shows an even one. The predicted effects can further enable the transverse separation of a pair of pure spin-valley-locked states with full spin-valley polarization while preserving time-reversal symmetry, as manifested by equal spin and valley Hall angles. Our findings present a new mechanism for realizing the spin and valley Hall effects and provide a novel route to the full electric-field manipulation of spin and valley degrees of freedom, with significant potential for future applications in spintronics and valleytronics.
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