Mesoscale and Nanoscale Physics
Flat and Topological Floquet Minibands from Patterned Light in Untwisted Bilayer Graphene
Muhammad Faisal and Michael Vogl
Two-dimensional superlattices in van der Waals materials host flat bands and nontrivial
topology, most famously at the so-called magic angles of twisted
bilayer graphene, where flat bands give rise to correlated and topological phases. Yet these superlattices are usually created by twisting the layers or applying strain, and once a sample is fabricated, their period is fixed and extremely difficult to tune. Here we propose an alternative route: imprint the superlattice optically, using patterned electromagnetic fields rather than a physical twist or strain. We show that patterned in-plane cir- cularly polarized light and a combined drive consisting of a patterned out-of-plane longitudinal field and a uniform circularly polarized field produce isolated bands in both AA- and AB-stacked
bilayer graphene. In AB stacking, the central bands additionally become nearly flat, capturing key features of a driven moire superlattice. In this approach, the superlattice period is set by the illumination and is straightforward to tune, and circularly polarized light breaks time-reversal
symmetry. Computing the valley Chern numbers of the central bands, we find a rich topological structure with several phase transitions in both stackings. Our results establish light-induced superlattices as a flexible and tunable platform for engineering flat bands and topological phases in
bilayer graphene.
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Materials Science
Ultrafast order-selective electron imaging and spectroscopy
Jan Philipp Bange, Till Domröse, Claus Ropers et al. (+2 more)
Ultrafast pump-probe spectroscopies in energy and momentum have become indispensable for probing non-
equilibrium dynamics in
quantum materials, revealing pathways in ultrafast energy conversion and light-induced phase transitions. Different modalities uniquely access the coupled
lattice, charge, and
spin degrees of freedom that underpin the functionality of these materials. However, investigating technologically relevant nanoscale devices also requires high spatial resolution. Although electron microscopy provides nanometer-scale imaging, it frequently lacks the simultaneous ultrafast temporal resolution and spectroscopic specificity needed for such studies. In this perspective, we review recent advances in dark-field electron and photoelectron microscopy, focusing on two complementary techniques: femtosecond photoelectron momentum microscopy and ultrafast transmission electron microscopy. Their application enables comprehensive insights into the ultrafast dynamics of the electron,
lattice, and
spin subsystems. These order-selective electron imaging and spectroscopy techniques open broad scientific opportunities for a microscopic understanding of non-
equilibrium phenomena in
quantum materials, nanostructures and devices.
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All authors Jan Philipp Bange, Till Domröse, Claus Ropers, Stefan Mathias, Marcel Reutzel.
Disordered Systems and Neural Networks · Cross-listed from cs.LG
On the Slow Convergence to Trivial Solutions of Algorithms for Hard Optimization Problems
Ali Hussaini Umar, Jean Barbier, Matthieu Jonckheere et al. (+1 more)
Hard combinatorial optimization problems, many of which are NP-hard, present fundamental algorithmic challenges. Average-case analysis on random instances has emerged as a powerful framework for understanding typical algorithmic performance beyond worst-case guarantees. A substantial body of work has established negative results: for sufficiently hard instances (often controlled by the underlying graph connectivity/constraints density), no known
polynomial-time
algorithm can significantly outperform naive heuristics in the double asymptotic limit where both problem size and constraints density tend to infinity. We revisit this picture by studying the finite-size behavior of some optimization
algorithms across easy, intermediate, and hard regimes. Through rigorous analysis of large-graph asymptotics combined with numerical experiments on canonical problems (maximum independent set and maximum $K$-SAT), we demonstrate that while
algorithms do eventually converge to theoretically predicted bounds, this convergence can be remarkably slow. In the intermediate regime where instances are already highly constrained, local
algorithms achieve solutions substantially better than their predicted performance in the high-constraint-density limit. This gap between finite-regime and asymptotic behavior has important practical implications: sophisticated algorithmic design remains crucial even when asymptotic theory predicts inevitable failure.
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All authors Ali Hussaini Umar, Jean Barbier, Matthieu Jonckheere, Manuel Sáenz
Materials Science
Detecting Magnetic Phase Transitions in Ion-Irradiated CrSBr Through Resonant Raman Scattering
Daria I. Markina, Alison Pfister, Priyanka Mondal et al. (+9 more)
Controlling
magnetic phases and accurately determining their transition temperatures are essential for the development of low-dimensional
magnetic materials. Here, we demonstrate that He$^+$ ion irradiation provides a versatile route for engineering
magnetic phases in layered CrSBr and establish temperature-dependent polarization-resolved Raman spectroscopy as a sensitive optical probe for identifying irradiation-induced
magnetic phase transitions. We reveal that the
magnetic response of the modified CrSBr is governed by both irradiation dose and
crystal thickness. The temperature
evolution of the Raman tensor elements resolves the
antiferromagnetic transition in pristine CrSBr at T$_N \approx 132$ K as well as irradiation-induced
magnetic transitions at T$_C \approx 105-110$ K and T$_D \approx 40$ K corresponding to
ferromagnetic and defect-related
magnetic phase transitions. Complementary magneto-optical measurements confirm the progressive suppression of
antiferromagnetic order and the emergence of new defect-engineered
magnetic phases, including pure
ferromagnetic behavior at high irradiation doses. These findings establish irradiated CrSBr as a platform for controllable
magnetic phase engineering while demonstrating polarization-resolved Raman spectroscopy as a rapid, non-destructive, and broadly applicable method for probing
magnetic phase transitions in van der Waals magnets.
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All authors Daria I. Markina, Alison Pfister, Priyanka Mondal, Lukas Krelle, Sai Shradha, Regine von Klitzing, Kseniia Mosina, Zdenek Sofer, Fangchao Long, Ulrich Kentsch, Shengqiang Zhou, Bernhard Urbaszek
Mesoscale and Nanoscale Physics · Cross-listed from physics.chem-ph
Molecular Insights into Gas Nanofilms Confined Between Bulk Liquid Phases
Yafan Yang, Zufeng Zuo, Xingyu Zhao et al. (+2 more)
Nanometer-thick fluid films play a critical role in confined multiphase processes, yet the thermodynamics and stability of free gas nanofilms remain poorly understood compared with their
liquid counterparts. Here, molecular dynamics (MD) simulations are employed to systematically investigate gas nanofilms confined between bulk
liquid phases using Lennard-Jones argon as a model system. The results show that the surface tension decreases exponentially with decreasing film thickness, accompanied by an increasing magnitude of the negative disjoining pressure. Upon thinning, the planar gas film undergoes a distinct morphological transition from a stable planar state to a transient or persistent spherical bubble through the formation and growth of a
liquid bridge. The film surface area strongly affects its thermodynamic properties, with larger areas producing stronger thickness dependence and larger deviations from classical density functional theory (cDFT) predictions. The closer agreement between MD and cDFT at smaller surface areas suggests that the discrepancy primarily arises from thermal capillary-wave fluctuations, which are included in MD but omitted in mean-field cDFT. Moreover, at small film thicknesses, the magnitude of the disjoining pressure increases with decreasing temperature, consistent with the enhanced sensitivity of the confined gas phase to thickness variations and contrasting with the trend generally reported for
liquid nanofilms. These findings provide molecular insights into the thermodynamics and stability of gas nanofilms, with implications for confined multiphase transport and droplet coalescence.
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All authors Yafan Yang, Zufeng Zuo, Xingyu Zhao, Shuyu Sun, Denvid Lau
Mesoscale and Nanoscale Physics
Spectral Fingerprints of Resonant Defect Scattering by Substitutional Mn in Graphene
Ahmed Samir Lotfy, Zviadi Zarkua, Renan Villarreal et al. (+10 more)
Using substitutional Mn in
graphene/Cu(111) as a model point defect, we combine scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) to test the predicted fingerprints of resonant scattering. As the Mn concentration increases to 0.44%, the Dirac point stretching reaches 0.52 eV, while momentum broadening remains energy independent. These spectral fingerprints classify substitutional Mn as a strong resonant scatterer and establish the combination of STM and ARPES as a powerful approach to identify and characterize resonant disorder in
graphene.
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All authors Ahmed Samir Lotfy, Zviadi Zarkua, Renan Villarreal, Rikkie Joris, Muhammad Saad, Koen van Stiphout, Karina Landivar, Steven Brems, Giovanni Di Santo, Aleksandr Seliverstov, Simona Achilli, Luca Petaccia, Lino M. C. Pereira
Soft Condensed Matter · Cross-listed from physics.flu-dyn
Hydrodynamic Brachistochrone: Conflicting Paths of Time and Energy Minima within Viscous Media
Ramin Gasimli, Lei Yi, Shrabin Bajracharya et al. (+2 more)
We experimentally and theoretically study the hydrodynamic analog of the classical brachistochrone problem: the time- and energy-minimizing paths for a spherical particle rolling down an incline within a viscous fluid. We show that in the presence of viscous dissipation, the paths of minima diverge from the classical cycloid, into curves of opposing
curvature for time and energy, and are characterized by an effective dimensionless parameter, $St_p$, representing the ratio of the particle's viscous response time scale to its gravitational time scale. Using a generalized variational framework, we show that the fastest path reduces to {nearly straight ramps}, however, beginning and terminating in localized cycloids of
curvature, $\kappa_c \sim St_p^{-2}$. Remarkably, the path of fastest descent on a given energy budget requires navigating a non-monotonic path ({\it“S-shaped”}) with an interior point of inflection. Our findings reveal a unification of temporal and energetic optimality for transport through dissipative media, and expand the celebrated brachistochrone solutions to the hydrodynamic regime.
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All authors Ramin Gasimli, Lei Yi, Shrabin Bajracharya, Anupam Pandey, Varghese Mathai
Superconductivity
Synthesis and stability of high-$T_c$ LaH$_{10\pm\delta}$ films at high pressures
Sam Cross, William Thomas, Lawrence Nobbs et al. (+12 more)
High-pressure hydrides hold the record for the highest superconducting critical temperatures across all classes of
superconductors. Currently lanthanum decahydride, LaH$_{10}$, exhibits the highest critical temperature among binaries, with $T_c \approx$ 250 K at pressures between 140-180 GPa. Here, we report the synthesis of LaH$_{10\pm\delta}$ films in two DACs at pressures of 168 GPa and 176 GPa via in situ
laser heating of elemental lanthanum films with ammonia borane (NH$_3$BH$_3$) as the hydrogen donor. The high-
symmetry fcc lanthanum sublattice (space group $Fm\bar3m$) is resolved using synchrotron X-ray diffraction, with unit
cell parameters in excellent agreement with previous studies on bulk samples. We provide confirmation of high-$T_c$ superconductivity in LaH$_{10\pm\delta}$ with highest $T_c$ of 247 K at 176 GPa evidenced in electrical measurements. The characteristic suppression of superconductivity is observed in
magnetic fields. Furthermore, combined diffraction and electrical measurements reveal remarkable temporal stability of both the
crystal structure and the high-$T_c$ superconductivity over the full measurement period of about 300 days post
laser heating. Our work establishes film precursors using physical vapour deposition (PVD) techniques as a practical route to hydride formation, opening a pathway toward the controlled synthesis of promising ternary hydrides and the integration of micro-fabricated device geometries in diamond anvil
cells.
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All authors Sam Cross, William Thomas, Lawrence Nobbs, Rebecca Nicholls, Oliver Lord, Qian Zhang, Dominique Laniel, Max Gerin, Bjorn Wehinger, Mohamed Mezouar, Xiaojiao Liu, Egor Koemets, Annette Kleppe, Sven Friedemann, Jonathan Buhot
Statistical Mechanics · Cross-listed from math-ph
Cluster Representation of Renormalization Group Transformations and a Rigorous Proof for Convergence of the RG-Flow of the Ising Model to Trivial Fixed Points away from Criticality
Fabio Arz
Many rigorous results in the modern era of statistical mechanics have been obtained through geometrical representations. A much studied tool in this setting is the random cluster representation of
lattice spin models. Another area of statistical mechanics that is of great interest but lacking rigorous results is the theory of the renormalization group. This paper investigates the idea to find a common ground between these two concepts in order to obtain rigorous results on the renormalization group flow. A need for negative cluster-weights weakens the success of this approach. Nevertheless, we managed to establish a relation between the scaling limit of the renormalization group flow of the nearest-neighbour Ising model away from criticality with a simple one-dimensional dynamical system that follows the cluster connectivity of the renormalization group transformation. This allows for a rigorous proof of the convergence of this flow to the zero- and infinite-temperature fixed point respectively for a large family of renormalization group transformations. Explicit results will be established on $\mathbb{Z}^2$ followed by a discussion of the available generalizations to higher dimensions.
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Superconductivity
Predicting critical temperature in quantum simulators for high-$T_c$ superconductivity: the matrix product state plus mean field approach
Thomas Köhler and Adrian Kantian
Quantum simulation based on ultra cold atomic
lattice gases is one of the most promising platforms to investigate high-$T_c$ superconductivity beyond the limited capabilities of
quantum many body numerics on classical computers. Yet, despite enormous progress since the field's inception, realizing a high-$T_c$ superconducting state still remains out of reach. The present work lays the groundwork to purpose the recently proposed, and already partly realized, mixed-dimensional (mixD) models, towards this end. These systems offer the proven capability to realize very high pairing energies while retaining appreciable mobility of pairs. We specifically investigate the potential of 2D mixD-models with anisotropic tunneling, using the
matrix product state plus mean field theory (MPS+MF) for
fermions, and show that these models may enter a high-$T_c$ superconducting phase. These simulations in turn are based on a comprehensive characterization of the 1D mixD-systems, which are the sub-units of which the 2D system is comprized. In this, we cover the range of currently experimentally relevant system sizes, and establish practical heuristics to determine when finite\hyp size effects preclude the use of a 1D mixD-system to build the 2D ones.
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Mesoscale and Nanoscale Physics
Electromagnetic and Acoustic Fano Interference in Surface Acoustic Wave Resonators
P.K. Rath, J.M. Kitzman, M. Mesbah et al. (+2 more)
Surface acoustic wave-based resonators are sensitive probes of condensed matter systems, as well as surface-selective sensors for chemistry and biology. Surface acoustic wave devices have also been integrated into hybrid
quantum systems with
qubit platforms for applications in
quantum information processing and sensing. The sensitivity of piezoelectric surface wave-based resonators to investigate these various systems can be enhanced by optimizing the device architecture and electrical measurement techniques. Alternatively, tailoring the spectral
symmetry, arising from interference effects, offers a promising route to further improve sensitivity. In this work, we demonstrate the simultaneous introduction of both electromagnetic and acoustic Fano interference to shape the spectral response of GHz-frequency surface acoustic wave resonators. By systematically modifying the acoustic reflectivity of the resonators, we are able to isolate and analyze each interference mechanism independently. The broad range of temperature operation, from ambient to cryogenic temperatures highlights the potential for both classical and
quantum sensing applications.
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All authors P.K. Rath, J.M. Kitzman, M. Mesbah, C. Undershute, J. Pollanen
Materials Science · Cross-listed from math.NA
A shifted energy barrier approach for phase-field modeling of tensile-dominated brittle fracture
Yaode Yin, Luigi Greco, Hongjun Yu et al. (+1 more)
The classical AT1 phase-field model contains an intrinsic energy barrier for crack nucle ation, which makes the predicted strength depend on the fracture toughness and the regularization length. For tensile-dominated brittle fracture, this barrier is shifted by mapping the Rankine criterion, evaluated on the effective stress, onto a state-dependent active-energy threshold. The prescribed tensile strength then controls crack nucleation, while the AT1 crack-density functional, stiffness degradation, and degraded stress response remain unchanged. Since the threshold depends on the current stress state, the field equations are derived from a restricted variational principle. A microforce formulation identifies the barrier shift as a dissipative resistance and provides the corresponding lower bound on the regularization length. In one-dimensional tension, closed-form solutions recover the prescribed peak strength and give a cosine-type localization profile that ap proaches the classical AT1 profile as the shift vanishes. Numerical examples show that, within the admissible range, the nucleation load is nearly insensitive to the regularization length and the predicted multiaxial nucleation states follow the Rankine envelope. Under overall compression, crack nucleation remains associated with local tensile stress concen trations. The formulation also captures the transition from strength-controlled failure for small flaws to the LEFM limit for large cracks.
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All authors Yaode Yin, Luigi Greco, Hongjun Yu, Simone Morganti
Materials Science
On the temperature dependence of the optical band gap in the material system of lithium niobate and lithium tantalate
Maximilian Henneke, Michael Ruesing, Nina A. Lange et al. (+13 more)
Lithium niobate and lithium tantalate see widespread use in optics and electronics, and are increasingly used for cryogenic applications. Despite their broad deployment, their optical band gap and its relation to the
crystal stoichiometry are not well characterised as a function of temperature. In this work, we study the optical absorption properties of congruent, stoichiometric, MgO-doped and Er-doped lithium niobate as well as congruent lithium tantalate across the temperature range between 7~K and 1000~K by means of optical transmission spectroscopy. Our results demonstrate that the difference of the optical band gap typically observed at room temperature between different stoichiometries is not primarily attributable to the intrinsic electronic structure, but rather to different electron-
phonon couplings and the average
phonon energies. Additionally, we exemplarily study the temperature shift of the 523 nm absorption line in Er-doped lithium niobate due to the increased interest in optically active dopants. To facilitate future analyses, we present the open-source software suite PhoQS-Treat (Tauc Regression Edge Analysis Tool), which enables automated Tauc regressions alongside additional analytical capabilities. This work advances the development of high-performance lithium niobate-based devices.
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All authors Maximilian Henneke, Michael Ruesing, Nina A. Lange, Timon Schapeler, Noah Spiegelberg, Ernst-Lukas Kuhlmann, Elke Beyreuther, Philipp Mues, Ludmila Eisner, Lukas M. Eng, Laura Padberg, Donat J. As, Klaus-Dieter Becker, Tim J. Bartley, Christine Silberhorn, Christof Eigner
Mesoscale and Nanoscale Physics
A miniature evaporator for in-operando deposition of isolated atoms in a low-temperature scanning tunneling microscope
Jeongmin Oh, Hermann Osterhage, Vasily Cherepanov et al. (+5 more)
Depositing dilute atomic ensembles onto cold samples is challenging in low-temperature scanning tunneling microscopes (STM) because radiation shields and restricted internal geometries often preclude a direct deposition path, particularly in instruments designed for millikelvin operation. We present a compact, milliwatt-range evaporation source fabricated from a commercial miniature incandescent lamp and integrated directly into a millikelvin STM head. The exposed tungsten filament is coated with a micrometre-thick Fe film and positioned about 1 cm from the sample. We deposit isolated Fe atoms onto MgO/Ag(100) while operating the microscope near 5 K. Evaporation increases the STM-body temperature by only about 2 K, and the same nanoscopic surface region can be readily scanned after deposition with a lateral displacement of less than 5 nm. Differential-conductance spectra displaying symmetric inelastic steps near $\pm$14 mV identify the deposited atoms on MgO as Fe. From STM images, we estimate a local deposition flux of $1.5\times10^{-5}~\mathrm{nm^{-2}\,s^{-1}}$, corresponding to a nominal evaporator lifetime of ~150 h. The fixed evaporator enables repeated low-flux deposition without a room-temperature line of sight, the need for movable radiation shields, or mechanical evaporator access after cooldown, while preserving access to the same atomic-scale surface region before and after deposition.
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All authors Jeongmin Oh, Hermann Osterhage, Vasily Cherepanov, Sven Just, Denis Krylov, F. Stefan Tautz, Taner Esat, Ruslan Temirov
Materials Science
Revisiting the topological properties of XMg2Bi2 (X = Ca, Sr, Ba, Yb and Eu)
Antoni Facca, Xujia Gong, Amar Fakhredine et al. (+2 more)
Density functional theory is known to underestimate band gaps in
semiconductors and to over- estimate inverted band gaps, frequently exaggerating the predicted size of the topological phase diagram of materials. Employing hybrid functionals and calculating the topological invariants, we revisit the topological properties of compounds crystallizing in the CaAl2Si2-type and demonstrate that the overestimation of the inverted band gaps is particularly pronounced in compounds with this
crystal structure. Among these, the class of XMg2Bi2 materials (X = Ca, Sr, Ba, Yb, and Eu) is topologically trivial for all considered cations. Our calculations show that these materi- als are narrow-gap
semiconductors with direct band gaps of 0.24-0.34 eV, slightly decreasing with increasing the atomic weight of the element X. We confirm this by applying uniaxial strain and hydrostatic pressure, confirming these results. We emphasize that the experimental observation of surface states alone is insufficient to establish nontrivial
topology, as trivial
semiconductors may host surface states without a surface Dirac point. Consequently, since these materials are intrinsi- cally topologically trivial, any experimentally observed topological signatures should be attributed to extrinsic effects such as doping or surface reconstruction. Our results underscore the importance of accurately treating electronic correlations when assessing topological character, even in materials containing heavy elements with strong
spin-orbit coupling, such as bismuth.
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All authors Antoni Facca, Xujia Gong, Amar Fakhredine, Carmine Autieri, Asiyeh Shokri
Materials Science
Linear and nonlinear optical responses in the chiral multifold semimetal BeAu: A quantum-geometric perspective
Babu Baijnath Prasad and Taisuke Ozaki
Chiral topological semimetals provide a natural platform for exploring how multifold band
topology and
quantum geometry manifest in optical and photovoltaic responses. BeAu is a
chiral multifold semimetal hosting band crossings at $\Gamma$, $M$, and $R$ with Chern numbers $C_\Gamma=-4$, $C_M=-2$, and $C_R=+4$, respectively. In this work, we study the linear optical conductivity and second-order dc photocurrent responses of BeAu using fully relativistic first-principles calculations. The calculated interband linear optical conductivity, Re $\sigma_{xx}(\omega)$, is quantitatively reproduced by $(e^2/\hbar)\omega g_{xx}(\omega)$, showing that its spectral features are governed by the
photon energy factor and the variation of the
photon energy-resolved
quantum-metric spectral weight. The linear shift current conductivity is closely related to the
symplectic connection, whereas the circular injection current susceptibility is governed by the transition-resolved product of Berry
curvature and the interband group velocity difference. At the
Fermi level, the linear shift current conductivity reaches approximately -810 $\mu$A/V$^2$ at a
photon energy of 0.05 eV. Aligning the chemical potential with the multifold crossings strongly reshapes both responses, producing the largest linear shift current conductivity peak for $\mu=\mu_R$ and pronounced changes in the magnitude and sign of the circular injection current susceptibility. The circular photogalvanic trace is strongly
photon-energy and chemical-potential dependent and does not exhibit a broad quantized plateau, indicating competing multiband transitions. Our results establish a unified
quantum-
geometric description of the linear and nonlinear optical responses of BeAu and identify it as a promising platform for optoelectronic phenomena governed by multifold band
topology and
quantum geometry.
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Materials Science
The parity gap in crystal tensor prediction
Can Polat, Mustafa Kurban, Erchin Serpedin et al. (+1 more)
Crystal symmetry dictates whether a physical response tensor must vanish, establishing a direct test for machine learning predictions independent of property calculations. We derive the parity gap, a group-theoretic metric quantifying the piezoelectric tensor freedom permitted by a
crystal's proper rotation subgroup $SO(3)$ but eliminated by inversion
symmetry in $O(3)$. Across state-of-the-art equivariant neural network architectures, unconstrained $SO(3)$ models systematically predict forbidden non-zero responses matching the parity gap of each centrosymmetric
crystal class, while polar distortion paths dynamically map output responses to the loss of inversion
symmetry. Regression controls confirm that enforcing full $O(3)$ parity incurs no consistent accuracy cost across predictive tasks. Crucially, while training interventions using explicit zero labels reduce violation magnitudes, they leave residual forbidden outputs. Exact physical compliance instead requires structural enforcement through $O(3)$ representation design or explicit output antisymmetrization. The parity gap thus provides a unified framework to distinguish empirical error reduction from exact structural compliance with physical law.
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All authors Can Polat, Mustafa Kurban, Erchin Serpedin, Hasan Kurban
Strongly Correlated Electrons
Transitions and Critical Divergences in Periodically Hopping Modulated Su-Schrieffer-Heeger Chains
Surajit Mandal and Satyaki Kar
We use a
curvature renormalization group (CRG) approach to study the topological phase transitions in a Su-Schrieffer-Heeger chain and its extensions coming from periodic hopping modulations. A
curvature function is defined in terms of system parameters near high-
symmetry points where the divergence of this function at critical points, in analogy to usual phase transitions, signals a topological phase transition. According to this theory, the phase transition line for the two-site Su-Schrieffer-Heeger (SSH) model is visible at the critical line \Delta = 0 where the
curvature function diverges. Our study involves this model and also the modulated one with periodicity of four
lattice spacing where the
curvature function not only diverges at the topological phase transition point (Dirac-like) |\Delta/t| = \sqrt(2) but also shows faster divergence at the non-topological gapless point \Delta = 0. We further notice faster divergence of correlation length for \Delta -> 0 as compared to that for the |\Delta/t| -> \sqrt(2) resulting in two different sets of critical exponents making them lie in different universality classes. The edge state exhibits very slow decay into the bulk near the \Delta = 0 point while a much quicker decay from edge into bulk is discernible around the |\Delta/t| = \sqrt(2) point. We also continue similar analysis for a SSH model with hopping periodicity of eight
lattice spacing.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
An ultracompact dilution refrigerator for fast quantum device characterization
Clḿent Geffroy, Dorian Nicolas, Eric Eyraud et al. (+10 more)
Rapid thermal cycling is a central bottleneck in the development of superconducting
quantum devices: conventional dilution refrigerators require cooldowns of a day or more and substantial cryogenic infrastructure, which throttles the fabricate-measure-redesign loop. We present an ultracompact dilution refrigerator (3 kg in mass and 100 mm in diameter) that completes a full cooldown-warm-up cycle to a base temperature of 70 mK in 1.2 hours when unloaded, and in 2.1 hours when fully equipped with the microwave wiring required for
qubit measurements, while delivering 20 microW of cooling power at 100 mK. We validate the platform through a complete characterization of a two-fluxonium device: we extract the full
circuit Hamiltonian by two-tone spectroscopy, measure energy-relaxation and coherence times, and
benchmark single-
qubit control. Although the relaxation time is limited by the base temperature of the system, we reach a single-
qubit gate fidelity of up to 99%, at the coherence limit set by our operating temperature. These results demonstrate that compact, fast-cycling dilution refrigeration can support state-of-the-art
quantum-device characterization without sacrificing measurement quality, offering a practical route to high-throughput
quantum-hardware development.
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All authors Clḿent Geffroy, Dorian Nicolas, Eric Eyraud, Shelender Kumar, Supriya Mandal, Julien Jarreau, Laura Kowalski, Laurent Del-Rey, Didier Dufeu, Nicolas Roch, Wolfgang Wernsdorfer, Quentin Ficheux, Matias Urdampilleta
Statistical Mechanics · Cross-listed from quant-ph
Exact Matching-Polynomial Solution of the Periodic Baxter-Fendley $Z_N$ Clock Chain
Yuguan Li, D. C. Liu and Murray T. Batchelor
The periodic non-Hermitian Baxter-Fendley $Z_N$ clock chain has lacked a complete finite-size spectral solution, whereas its open-chain counterpart admits a solution in terms of independent quasienergies. For the periodic model we show that the operator-valued matching
polynomial associated with its cyclic Weyl
algebra simultaneously generates a set of conserved quantities, including the
Hamiltonian, and realizes a cyclic $\tau^{(2)}$ Yang-Baxter transfer
matrix. Root-of-unity closure yields a finite system of
polynomial spectral equations in each charge sector, which reproduces the complete finite-size energy spectrum counted with algebraic multiplicity. As a first application of this result, we show that Newton continuation of these equations provides a practical numerical route to the periodic ground-state energy without enumerating the full spectrum. For homogeneous chains the thermodynamic seam response yields a criterion for boundary-induced criticality; for $N=3$ it predicts two reciprocal critical couplings with singular ground-state
curvature, in contrast to the single self-dual open boundary critical point.
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Materials Science
Thickness-dependent degradation and optical access in epitaxial 2H-MoTe2 protected by metallic capping layers
Wojciech Ryś, Iaroslav Lutsyk, Michał Piskorski et al. (+13 more)
We investigate degradation and surface protection of epitaxial 2H-MoTe2 films grown by molecular beam epitaxy on GaAs(111)B substrates. Using X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy, atomic force microscopy (AFM), Kelvin probe microscopy (KPM), Raman spectroscopy, and density functional theory (DFT), we examine the structural, chemical, and electronic
evolution of MoTe2 protected by Co and Ni capping layers. XPS shows that the metallic caps effectively suppress oxidation during short-term air transfer, while revealing a pronounced Te-rich near-surface composition. With time, the caps become increasingly difficult to remove, suggesting gradual interfacial bonding promoted by excess tellurium and defect-rich MoTe2 interfaces. AFM and KPM reveal pronounced thickness-dependent ageing, with ultrathin regions showing markedly different contact-potential
evolution from thicker films. DFT calculations support the sensitivity of work function and density of states to thickness and surface chemistry. Raman measurements through approximately 20 nm thick metallic caps demonstrate partial optical access to the protected material. Additional AFM and Raman observations suggest local formation of Te-rich nanostructures under
laser illumination or near mechanically damaged regions. These results provide practical guidelines for protecting, transferring, delaminating, and optically characterizing air-sensitive MoTe2 and related van der Waals materials.
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All authors Wojciech Ryś, Iaroslav Lutsyk, Michał Piskorski, Maxime Le Ster, Maciej Rogala, Paweł Dąbrowski, Paweł Krukowski, Katarzyna Ranoszek-Soliwoda, Jarosław Grobelny, Zuzanna Ogorzałek-Sory, Bartłomiej Seredyński, Wojciech Pacuski, Janusz Sadowski, Marta Gryglas-Borysiewicz, Karol Szałowski, Paweł J. Kowalczyk
Materials Science
Néel-order-dependent transverse transport in noncoplanar antiferromagnet $\text{MnTe}_{2}$
Qi Feng, Yilin Han, Yongkai Li et al. (+9 more)
Antiferromagnets hold appealing potential in next-generation spintronic devices with higher frequency and scalability, thanks to their alternating
spin orientations that cancel out net magnetization. However, the lack of a nonzero magnetization makes the detection of the
magnetic configuration of antiferromagnet difficult, hampering the applications of antiferromagnets. Here, we report a new transverse transport effect in noncoplanar antiferromagnet $\text{MnTe}_{2}$. This effect is antisymmetric in both
magnetic field and Néel order, but symmetric in its two indices. It can be understood in terms of the contribution induced by both
magnetic field and
geometric quantities, as confirmed by our theoretical calculations. Our discovery of a new Néel-order-dependent transverse transport effect provides opportunities to the advancing
antiferromagnetic spintronics.
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All authors Qi Feng, Yilin Han, Yongkai Li, Yuqing Hu, Mo Tian, Qiuli Li, Huimin Peng, Jinrui Zhong, Zhiwei Wang, Zhi-Ming Yu, Junxi Duan, Yugui Yao
Superconductivity
Magnetism and Electrical Conduction in Lightly-Doped Single-Layer High-$T_c$ Cuprate $\mathrm{La}_2\mathrm{CuO}_{4+\delta}$
Kota Miyakoshi, Yuya Kinugawa, Shusei Mizuta et al. (+6 more)
The temperature dependences of magnetization and electrical resistivity as well as their
magnetic field dependences have been examined in lightly-doped single-layer cuprate $\mathrm{La}_2\mathrm{CuO}_{4+\delta}$ (LCO, hole-doping level $p \, (2\delta) \cong 0.03$) single
crystals, in comparison with those in the extremely low doping region of $p \lesssim 0.015$ to uncover the intrinsic magnetism and electrical conduction of the $\mathrm{Cu\text{-}O}$ plane that exhibits both
antiferromagnetic (AF) and superconducting (SC) orders simultaneously. In $p \cong 0.03$ SC LCO, the sub-
lattice moments on $\mathrm{Cu}$ sites and their AF couplings are only $\sim 15\,\%$ smaller than those of the Mott-
insulator parent material, suggesting that the localization of $\mathrm{Cu}$ $3d$ electrons remains very strong. Furthermore, we report that in the SC LCO, two-dimensional AF
spin correlations develop rapidly from $T^* \cong 280\text{ K}$ towards Néel temperature $T_{\mathrm{N}} = 266\text{ K}$, where the out-of-plane resistivity starts to decrease largely. This might be responsible for the AF ordering at such a high temperature in the SC single-layer cuprate with $p \cong 0.03$.
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All authors Kota Miyakoshi, Yuya Kinugawa, Shusei Mizuta, Yoshihiko Ihara, Hiroyuki K. Yoshida, Tohru Kurosawa, Yasunori Toda, Naoki Momono, Migaku Oda
Mesoscale and Nanoscale Physics
Floquet engineering of topological bands in semiconductor van der Waals heterobilayers
Eréndira Santana-Suárez, Brayan E. Walteros-Mendivelso, A. Jazmín Tapia-de-la-Rosa et al. (+2 more)
We show that periodic driving with near-infrared to visible light drives topological phase transitions in the
photon-dressed band structure of transition-metal dichalcogenide heterobilayers. We apply the Floquet formalism to a light-coupled lowest-order $\mathbf{k}\cdot\mathbf{p}$
Hamiltonian, and obtain an effective four-band model that correctly captures the essential
photon-dressed bands of type-II TMD heterobilayers in the vicinity of the first
photon resonance. Crossings between the zero- and one-
photon sectors effectively invert the bands, yielding topological phases with Chern numbers up to $\pm 2$ and gaps of order 10 meV. Our results establish TMDs as
prime candidates for engineering bands with higher Chern numbers, and exploring driven topological states in solid state media.
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All authors Eréndira Santana-Suárez, Brayan E. Walteros-Mendivelso, A. Jazmín Tapia-de-la-Rosa, Mahmoud M. Asmar, David A. Ruiz-Tijerina
Materials Science
Three-temperature atomistic spin-lattice dynamics in LAMMPS: a moment-consistent, fluctuation-dissipation-correct extension and its validation on ultrafast demagnetization and all-optical switching of GdFeCo
Chun-Yeol You, Jiwan Kim and Dong-Hyun Kim
Atomistic
spin dynamics (ASD) codes like VAMPIRE simulate femtosecond-
laser-induced ultrafast demagnetization and all-optical switching (AOS) on a rigid
lattice; the
SPIN package of the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) instead propagates
spins and
lattice together, but until now could couple its
spin thermostat only to a single global temperature, precluding three-temperature (3TM) simulations. We present a validated 3TM extension with two new fixes: langevin/
spin/ttm, coupling the
stochastic spin bath to the local electron-temperature field of fix ttm; and moment/scale/
spin, supplying the per-atom moment-dependent ($1/\mu_i$) prefactor required for heterogeneous-moment systems, with deterministic terms scaling as $1/\mu_i$ and, as required by the fluctuation-dissipation
theorem,
stochastic noise scaling as $1/\sqrt{\mu_i}$. We validate the framework on single-species
benchmarks-precession, the bcc-Fe Curie curve, a continuous demagnetization-remagnetization-precession trajectory, and a
lattice-strain acoustic-
phonon pulse unavailable to
spin-only ASD codes-then on heat-induced AOS of GdFeCo with literature parameters (Radu/Ostler exchange constants, moments, and damping). The corrected integrator reproduces the experimentally reported transient
ferromagnetic-like sublattice
alignment, field-insensitive thermal switching, a switching probability approaching unity across a genuine 8x8 literature damping phase diagram, a non-monotonic critical-cooling-duration boundary that disappears above $T_e^0 \approx 2000 \text{ K}$, a composition-dependent switching window centered on angular-momentum compensation, and convergence in system size with monotonic, physical damping dependence. These results establish LAMMPS as a quantitatively validated platform for three-temperature
spin-
lattice simulations of ultrafast magnetism.
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Strongly Correlated Electrons
Hubb_DMFT and Wan2mb_DMFT: Continuous-Time Quantum Monte Carlo Solvers for Single- and Multi-Orbital Hubbard Model
A. A. Katanin
We present two related customized software packages, Hubb_DMFT and Wan2mb_DMFT, designed to solve the Dynamical Mean-Field Theory (DMFT) equations for strongly correlated electron systems. Hubb_DMFT is adjusted for the single-band Hubbard model, providing a fast way to calculate the local self-energy, as well as the two-particle
fermion and triangular
fermion-
boson charge and
spin vertices, while Wan2mb_DMFT extends this capability to realistic multi-orbital systems, directly interfacing Wannier tight-binding
Hamiltonians with many-body solvers. Both codes are based on iQIST v.0.7
impurity solver and utilize a modified and internally integrated Continuous-Time
Quantum Monte Carlo (CT-QMC) core with the hybridization expansion (CT-HYB) framework and improved self-energy and vertex
estimators for density-density interaction, ensuring numerically exact solutions for
quantum impurity problems.
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Strongly Correlated Electrons · Cross-listed from quant-ph
Good Stabilizer Codes from Shallow Clifford Circuits with Random Matchings
Emile Anand, Elia Gorokhovsky, Jennifer Hritz et al. (+1 more)
Encoding
quantum information with low
circuit overhead is a fundamental challenge in fault-tolerant
quantum computation. Random
circuits provide a natural mechanism for rapidly spreading logical information through simple gates applied in parallel. Brown and Fawzi showed that random Clifford
circuits on two-
qubit Clifford gates provide such encoders that achieve the
quantum Gilbert-Varshamov rate-distance tradeoff with depth $O(\log^3 n)$. We show that the same asymptotic tradeoff is attained in optimal $O(\log n)$ depth under a gate distribution with a more restricted support. For every fixed $\delta>0$ and sufficiently large $n$, if $\frac kn < 1 - H(\frac{d}{n}) - \frac{d}{n}\log_2 3 - \delta$, we can construct random
circuits of depth $O(\log n)$ which define, with high probability, an $[n,k]$ stabilizer code of distance at least $d+1$, which matches the $\Omega(\log n)$ light-cone lower bound for linear distance encoders. Our ensemble employs a random matching
circuit architecture consisting of $T$ independent permutation-invariant layers. In each layer, the
qubits are paired up by a uniformly random perfect matching, and a random independent two-
qubit Clifford gate is applied to each pair. The gate distribution need not be uniform over, or even have full support on, the two-
qubit Clifford group; rather, we allow for very general distributions on Clifford gates satisfying three regularity conditions. In particular, the construction can be implemented using $n/2$ CNOT gates on randomly matched pairs in each layer, with parallel one-
qubit Clifford twirls. These regularity conditions allow us to reduce the second-moment dynamics of our random
circuits to a reversible Markov chain on binary support strings. We establish logarithmic hitting-time bounds for this Markov chain and comparisons of its stationary distribution to prove the coding properties of the
circuits.
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All authors Emile Anand, Elia Gorokhovsky, Jennifer Hritz, Jingtong Sun
Strongly Correlated Electrons
High-field fate of the Kitaev quantum spin liquid in $\alpha$-RuCl$_3$
K. Imamura, R. Ohno, Y. C. Tsuzuki et al. (+15 more)
Kitaev
quantum spin liquids (KQSLs) host fractionalized excitations described by itinerant Majorana quasiparticles and gapped $Z_2$ fluxes (visons), providing a platform for emergent topological matter. Whether such a state survives under strong
magnetic fields, however, remains an open question. The layered honeycomb magnet $\alpha$-RuCl$_3$ is a leading candidate material: an in-plane field of $\sim$ 7 T suppresses
antiferromagnetic order and induces a
quantum-disordered phase exhibiting signatures consistent with Majorana excitations, including an anomalous thermal
Hall effect and field-angle-dependent specific heat. At higher fields, the magnetization approaches saturation, suggesting a transition to a
spin-polarized state, yet the microscopic
evolution between these limits remains unresolved. Here we report high-field specific heat measurements up to 24 T that reveal a distinct crossover at $\mu_0H^*\approx$15 T, beyond which the perturbative Kitaev description breaks down. Above $H^*$, the characteristic six-fold angular modulation of the specific heat collapses and the excitation gap deviates from the predicted $H^3$ scaling. Meanwhile, the gap decreases with increasing field and the in-plane magnetization anisotropy persists up to $\sim$ 24 T, both in sharp contrast to a trivial
spin-polarized state, indicating that KQSL signatures are preserved even at $\sim$ 90 % of magnetization saturation. These results reveal that the KQSL in $\alpha$-RuCl$_3$ extends well beyond the perturbative window, persisting as a nonperturbative regime in which the Majorana and vison energy scales merge, before eventually giving way to
spin polarization. This thermodynamic roadmap provides a basis for understanding how fractionalized phases evolve under strong
magnetic fields.
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All authors K. Imamura, R. Ohno, Y. C. Tsuzuki, Y. Akui, R. Namba, K. Ishihara, M. Akaki, M. Kimata, N. Kurita, H. Tanaka, N. Kimura, S. Imajo, A. Matsuo, K. Kindo, Y. Matsuda, K. Hashimoto, Y. Mizukami, T. Shibauchi
Strongly Correlated Electrons
Universal thermal breakdown of polaron coherence in one, two, and three dimensions
Jeet Shannigrahi, Janez Bonča and Mona Berciu
How a polaron loses its quasiparticle coherence with increasing temperature is a long-standing open problem. Holstein addressed it in 1959 only in the extreme antiadiabatic, strong-coupling limit, while more recent numerically exact approaches are largely restricted to one dimension. Here we solve this problem on square and simple cubic
lattices across the weak-, intermediate-, and strong-coupling regimes. We show that the polaron effective mass $m^*$ and inverse lifetime $1/\tau$ increase monotonically with temperature until, at $T \sim 0.5\,\Omega$, the quasiparticle peak dissolves into a broad incoherent thermal continuum. The
phonon frequency $\Omega$ therefore defines a universal coherence scale, independent of dimensionality and coupling strength, validating Holstein's prediction far beyond the regime in which it was derived. These results follow from a finite-temperature generalization of the Momentum Average (MA) approximation, yielding a closed-form, diagrammatically derived self-energy that is asymptotically exact in the strong-coupling limit at all temperatures.
Benchmark comparisons demonstrate excellent quantitative agreement of the resulting 1D spectral functions with the numerically exact Variational Exact Diagonalization-Finite-Temperature Lanczos Method (VED-FTLM) and finite-$T$ Density
Matrix Renormalization Group (DMRG).
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Strongly Correlated Electrons
Extracting Chern numbers from the equilibrium transport noise of multiband bosons
Zhi-Wei Wang and Samuel L. Braunstein
We show that the
equilibrium transport noise of a multiband bosonic system carries a measurable imprint of its Chern numbers, and we give the protocol that extracts them. For electrons the topological part of the Berry
curvature is absent from the transport noise: over a filled band it yields the quantized anomalous
Hall conductivity, yet at a
Fermi surface the Chern number is invisible to the fluctuations of the
Hall current. That cancellation is an accident of the
Fermi surface:
magnons and
phonons populate every band thermally, each with its own Chern number, so the harmonic sector of the Hodge decomposition is not a single constant, and a variance does not annihilate it. What survives is the occupation-weighted dispersion of Chern numbers. That current noise sees band geometry is known, and at zero temperature the antisymmetric part of the noise sum rule already returns a Chern number; the new element is an object with no zero-temperature analogue, existing only when several bands are populated and weighted differently. We derive it from a fluctuating Boltzmann equation and resolve the energy-magnetization subtraction at the level of fluctuations: the subtracted term is a curl, contributing exactly zero to the current a transport measurement records, and no magnetization auto-correlation enters the antisymmetric cross-spectrum at any wavevector. Measurement uses an
equilibrium sum rule on that cross-spectrum, with no drive and hence no suppression: its frequency-integrated form cannot in practice separate topological from
geometric content, for a rank reason, whereas the frequency-resolved form does, at accessible resolution. The protocol is specified for Cu(1,3-bdc), with calibration and robustness characterized. A driven alternative lies twelve orders below the
equilibrium floor. Two independent implementations agree on every convention-free quantity.
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