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Physics Condensed Matter

cond-mat Watch Condensed Matter

Thursday 17 September

Mesoscale and Nanoscale Physics

Transport Relaxation Mechanisms in Bilayer Graphene: Effects of Pauli Blocking

Amit Varshney and SSZ Ashraf

We present a unified analytical treatment of carrier transport relaxation in bilayer graphene (BLG), including scattering by surface roughness, charged and neutral impurities, acoustic and optical phonons, and substrate polar phonons. The calculation is formulated within the low-energy two-band approximation using Fermi's golden rule and the semiclassical Boltzmann transport equation. Closed-form expressions for the transport relaxation rates are derived and verified by numerical evaluation of the corresponding scattering integrals. Particular attention is given to Pauli blocking, whose influence becomes important under degenerate carrier conditions owing to the approximately parabolic low-energy dispersion and nearly constant density of states of BLG. We find that Pauli blocking modifies the relaxation rates in a mechanism- and energy-dependent manner for both suspended and substrate-supported BLG. For the parameters considered, neutral-impurity scattering provides the dominant contribution over a substantial energy range, while acoustic-phonon and charged-impurity scattering remain important competing mechanisms. Optical-phonon scattering is suppressed below its emission threshold, whereas substrate polar phonons provide an additional relaxation channel in supported BLG. Comparison with monolayer graphene and a conventional two-dimensional electron gas highlights the role of band dispersion, density of states, screening, and chirality in determining the distinct scattering behavior of BLG. These results provide analytical insight into the relative importance of the principal momentum-relaxation mechanisms and the role of Pauli blocking in BLG transport.
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Strongly Correlated Electrons

Fractional quantum spin Hall crystals from hidden density-wave order

Matthew Shammami and Sudip Chakravarty

Broken symmetry and topology are distinct descriptions for how matter organizes itself. Here, we propose one gives rise to the other. A mixed singlet-triplet $d$-density wave, which is a particle-hole condensate, has a hidden bond order that breaks translation symmetry and generates a time-reversed pair of Chern bands, without producing conventional charge or spin density order. At fractional fillings with odd denominators, finite size exact diagonalization studies show how repulsive interactions can transform this symmetry-broken topological state into a fractional quantum spin Hall liquid. Its opposite chiralities produce a crossed response in which charge flux transports spin, and spin flux transports charge. The same setting also reveals what borders such a phase. Strong interspin coupling and increasingly dilute fillings favor charge order and make residual band dispersion decisive, while at half filling, the spin-decoupled limit gives a homogeneous correlated liquid with signatures consistent with the physics of a composite Fermi liquid.
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Statistical Mechanics

Collective Symmetry and Criticality from Single-Component Fluctuations

Chunhao Guo, Zhe Wang and Zheng Yan

Symmetry plays a fundamental role in modern physics by guiding the classification of phases and the construction of effective theories of critical systems. Identifying collective symmetry remains challenging because tests based on preselected order parameters can overlook hidden components. A central goal is therefore to extract symmetry and critical information from a single accessible component. We pursue this goal using Lee–Yang zero ratios generated by a single ordering field. Unmeasured components shape the projected fluctuation distribution and hence the relative zero positions. Deep in a phase with spontaneous symmetry breaking, we show analytically that the zero ratios for an isotropic order parameter approach a parameter-free Bessel sequence fixed by the number of collective components. Near quantum criticality, fluctuations deform this distribution. The resulting ratio deviations and their finite-size coupling dependence carry additional critical information. We demonstrate these results with large-scale quantum Monte Carlo simulations of many-body systems with conventional and enlarged continuous symmetries. This approach provides quantitative access to collective symmetry and criticality without measuring every competing order.
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Mesoscale and Nanoscale Physics

Quantum transport across normal-superlattice-normal graphene junctions: Fabry-Pérot interference, Hofstadter butterfly, and supersnake states

Che-Pin Hsu, Alina Mreńca-Kolasińska, Aitor Garcia-Ruiz et al. (+4 more)

Electrostatic modulation of graphene provides a tunable route to engineering miniband structures. We perform quantum transport simulations on a gate-defined graphene superlattice junction, formed by confining a two-dimensional superlattice graphene (SGr) region between two normal graphene (NGr) regions. In the low-field regime at low carrier densities, robust Fabry-Pérot interference fringes emerge even in the unipolar regime due to Fermi-velocity renormalization in the SGr region. At stronger magnetic fields but only up to 3 T, the conductance map clearly reveals the Hofstadter butterfly spectrum. At intermediate fields, our finite-width transport simulations reveal a new type of snake state, the supersnake state, composed of alternating anomalous cyclotron arcs on the SGr side and conventional semicircular arcs on the NGr side, forming a weaving trajectory along the junction. The resulting conductance oscillations agree well with geometrical conditions derived from semiclassical cyclotron orbits. Our results demonstrate that gate-defined NGr-SGr-NGr junctions provide a versatile platform hosting multiple transport regimes within a single device architecture and can be generalized to other types of superlattices not restricted to graphene.
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All authors Che-Pin Hsu, Alina Mreńca-Kolasińska, Aitor Garcia-Ruiz, Szu-Chao Chen, Denis Kochan, Klaus Richter, Ming-Hao Liu

Soft Condensed Matter

Viscoelastocapillary extensional De-Oh or VE-DeOh stringiness of unentangled polymer solutions

Louis B. Edano and Vivek Sharma

Characterization, control, and calibration of stringiness, the propensity to form long, thin, persistent threads, are key to the design and application of polymer solutions and formulations transferred to substrates by dropwise dispensing, jetting, spraying, or coating, and are used in manufacturing fibers, membranes, or spray-dried products. For Newtonian fluids, enhancing shear viscosity translates into increasing stringiness as can be easily perceived during dripping of water vs. sugar syrups or glycerol-water mixtures. In contrast, even polymer solutions with comparable shear viscosity can display a significant contrast in extensional rheology and apparent stringiness. However, there are no quantitative maps, formulas, or scales for stringiness, which motivates this study. In this contribution, we contrast the stringiness for a series of aqueous polymer solutions by determining the filament lifespan using dripping-onto-substrate (DoS) rheometry and filament length and lifespan in using dripping-into-air (DiA). We investigate the influence of polymer chemistry and molecular weight on stringiness and rely on DoS rheometry to characterize pinching dynamics and extensional rheology response. We show that an increase in stringiness correlates with steady, terminal extensional viscosity, and extensional relaxation time, respectively. Lastly, we provide a framework, which compares the stringiness of different polymer solutions through an extensional Ohnesorge-Deborah ($Oh-De$) plot or $Oh_E-De_E$ map, by computing the two dimensionless groups $Oh_E$ using steady, terminal extensional viscosity and $De_E$ using extensional relaxation time, respectively
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Strongly Correlated Electrons

Vestigial chirality from fluctuating loop currents on the kagome lattice

Yin Shi, Xuande Bu and Sheng Meng

Multicomponent order can melt in stages, leaving a composite order after its primary constituents become short ranged. We study this possibility for commensurate three-$Q$ loop-current order on the kagome lattice. Large-scale cluster and parallel-tempering Monte Carlo simulations reveal direct and two-stage melting regimes in an effective fixed-amplitude sign model. In the latter regime, translation-sector domain walls proliferate before chirality-changing walls, restoring lattice translational symmetry while preserving long-range time-reversal-odd order. The primary $M$-point correlations are short ranged in this intermediate phase. The two-stage regime begins when the lowest-energy chirality-changing wall is only about $12$–$13\%$ more costly than a same-chirality translation wall. Finite-size scaling of a stable amplitude-resolved Ginzburg–Landau theory shows that the phase survives amplitude relaxation. Our results establish a quantitative domain-wall criterion for vestigial loop-current order and a fluctuation route to time-reversal symmetry breaking without long-range loop-current Bragg order. This separation provides a possible thermodynamic framework for time-reversal-odd responses recently reported above the critical temperature for conventional charge-density-wave ordering in kagome metals.
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Materials Science

Grain Boundary Distortion Reorients the Local 4f Easy Axis and Splits Light from Heavy Lanthanides in Nd2Fe14B

Avik Mahataa and M. Zakotnik

The coercivity of rare-earth permanent magnets originates from the crystal field acting on localized 4f electrons, yet how grain boundaries modify the underlying single-ion Hamiltonian remains largely unknown. Here we determine the 4f crystal field in bulk and grain-boundary environments of Nd2Fe14B using embedded multireference electronic-structure calculations for the substitutional series Ce, Pr, Nd, Gd, Tb, and Dy. Grain-boundary distortion produces a crossover across the lanthanide series: the crystal field weakens by up to 36% for light lanthanides but strengthens by about 25% for heavy lanthanides, with Nd marking the crossover. The ground-state doublet of Dy becomes more isolated and axial, whereas Ce exhibits a collapsed low-lying excitation that suppresses anisotropy. The local easy axis rotates from the bulk c axis toward the basal plane, identifying grain-boundary sites as favorable nucleation centers for magnetization reversal. These results provide an atomistic basis for heavy rare-earth grain-boundary diffusion and furnish transferable single-ion parameters for spin-lattice simulations.
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Materials Science

Hydrogen Reorganization in Hot Dense Ammonia

Yu Tao, Jingyi Liu and Li Lei

Hydrogen is known to form stable compounds with a variety of simple molecular solids under high pressure. However, ammonia hydride has not been experimentally observed. Here, through a series of laser-heating diamond anvil cell (LHDAC) experiments, we observed the transparent ionic ammonia transformed into an opaque phase above 150 GPa and 1000 K. Raman spectroscopy reveals a pronounced structural change, accompanied by the emergence of two distinct asymmetric H$_2$ vibrons ($\nu_1$ and $\nu_2$). A new ionic phase produced following thermally induced dissociation and hydrogen reorganization in hot dense ammonia. The combination of experimental results and first-principles calculations identify this phase as the theoretically predicted ionic $P4_12_12$ phase of NH$_7$ , which consists of NH$_4^+$, H$^-$and H$_2$ units. Our results suggest that the formation of ammonia hydrides follows a thermodynamic pathway distinct from those of other H$_2$-containing mixtures, revealing a new ionic transformation pathway driven by hydrogen reorganization in hot dense ammonia.
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Strongly Correlated Electrons

Universality of the $1/9$ Magnetization Plateau and Quantum-Disordered States in the Kagome Family $\mathrm{Cs_8AB_3Ti_{12}F_{48}}$ ($A=\mathrm{Rb},\mathrm{Li}$; $B=\mathrm{K},\mathrm{Na}$)

Prena Chaudhary, Asiri Thennakoon, Tommy Park et al. (+17 more)

The microscopic origin of the low-field $1/9$ magnetization plateau in spin-$1/2$ kagome antiferromagnets remains unresolved. Here, we show that chemical pressure reshapes the hierarchy of fractional magnetization plateaus in the titanium-based kagome family $\mathrm{Cs_8AB_3Ti_{12}F_{48}}$ ($A=\mathrm{Rb},\mathrm{Li}$; $B=\mathrm{K},\mathrm{Na}$). High-field magnetization measurements up to 60 T reveal a robust $1/9$ plateau-like phase in the expanded $\mathrm{Cs_8RbK_3Ti_{12}F_{48}}$ and $\mathrm{Cs_8LiK_3Ti_{12}F_{48}}$ compounds, despite the absence of the conventionally more robust $1/3$ plateau. In contrast, compressed $\mathrm{Cs_8LiNa_3Ti_{12}F_{48}}$ exhibits neither the $1/9$ plateau-like phase nor a quantum-disordered ground state. Specific-heat measurements and first-principles calculations show that lattice expansion preserves a frustrated, fully connected kagome exchange network and gapless quantum-disordered ground states, whereas compression reorganizes the exchange network into weakly coupled quasi-one-dimensional subsystems and induces successive magnetic transitions. These results demonstrate that the $1/9$ and $1/3$ plateaus need not share a common microscopic origin and suggest that the $1/9$ plateau may represent a more universal feature of frustrated spin-$1/2$ kagome magnetism.
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All authors Prena Chaudhary, Asiri Thennakoon, Tommy Park, Hanru Wang, Leshan Zhao, Laurel Winter, Neil Herrison, Christina Hoffmann, Junghong H. He, Harald O. Jeschke, Hiroyuki Nojiri, Akira Matsuo, Koichi Kindo, Miwako Takahashi, Yukio Noda, Taku J. Sato, Shiyan Li, Hiroaki Ueda, Gia-Wei Chern, Seung-Hun Lee

Materials Science

Portfolio-Based Constrained Multi-Objective Bayesian Optimization for Materials Design

Sushant Sinha, Christofer Hardcastle, Robert Robinson et al. (+4 more)

Materials discovery and design campaigns can be formulated as constrained multi-objective Bayesian optimization (CMOBO) problems, within which each experimental decision negotiates between two coupled but competing goals: discovering feasible candidates and refining the underlying Pareto front. Here we recast acquisition-function choice as an adaptive policy-selection problem over a portfolio of conventional and feasibility-focused acquisition functions. This was done using two controllers: UCB-Bandit, a modified UCB multi-armed bandit, and Agentic-Switch, a multi-agent decision system driven by a large language model (LLM). Both were evaluated against fixed-policy baselines in silico across five synthetic benchmark functions and two materials design case studies. The adaptive policies performed competitively in terms of both cumulative feasibility count and feasible hypervolume improvement, while each individual acquisition function performed well for only one metric, suggesting that adaptive policies are better suited for constrained materials science problems.
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All authors Sushant Sinha, Christofer Hardcastle, Robert Robinson, Shakti Prasad Padhy, Brent Vela, Douglas Allaire, Raymundo Arroyave

Wednesday 16 September

Disordered Systems and Neural Networks

Gradient-estimator design overcomes trainability barriers in neural-network-based variational optimization

Yi-Ran Xue, Rui Wang, Baigeng Wang et al. (+1 more)

Neural networks provide expressive representations for scientific computing. However, even sufficiently expressive networks can suffer training failure in weak-gradient regimes, limiting their practical use in quantum many-body physics and ab initio quantum chemistry. Here we derive an unbiased direct gradient estimator and introduce the adaptive minimum-variance phase (AMVP) estimator for neural-network variational optimization. By improving the signal-to-noise ratio of weak gradients, these methods enable reliable scientific calculations where training previously failed, while substantially reducing computational cost. The framework enables compact networks to outperform larger and fine-tuned default standard-estimator models with over an order of magnitude less GPU time on correlated flux models, and ultimately exceed the density matrix renormalization group (DMRG) accuracy. It further achieves chemical accuracy in N$_2$ bond breaking and, for the first time, in heavy-element I$_2$ with explicit spin-orbit coupling. These results demonstrate that gradient-estimator design expands the capabilities of neural-network variational methods for accurate scientific computing.
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All authors Yi-Ran Xue, Rui Wang, Baigeng Wang, Chenan Wei

Materials Science

NTBuilder: Commensurate Construction of Nanotubes from Arbitrary Two-Dimensional Crystals and a Catalog of 20 Million Structures

Marcelo Lopes Pereira Junior

Although tens of thousands of two-dimensional (2D) crystals are now known, the nanotubes they can form are usually constructed for individual materials, using procedures derived for graphene. Here, we develop a general theory of the rolling construction for arbitrary 2D crystals and implement it in \ntbuilder{} (NanoTube Builder), an open-source library with desktop and web interfaces. We express the size of the exact unit cell of a tube through the reduced form of a single rational number built from the lattice metric, which explains why triangular and square lattices always close with small cells, whereas rectangular, centered rectangular, and oblique lattices can require cells larger by many orders of magnitude. The optimal approximate cells are the best rational approximations of a second number, so that a cell small enough for first-principles calculations can be chosen with a controlled departure from periodicity, as in the biphenylene network, where such a cell is eight orders of magnitude smaller than the exact one. The 17 plane groups reduce to seven distinct chirality maps once rolling is taken into account, the rolling sense of layers without a horizontal mirror defines distinct tubes, and bonds formed or broken by curvature are detected pair by pair against the flat layer. The same framework assembles multiwalled tubes whose walls share one axial period, periodic bundles, and strained or twisted structures that remain periodic. Applied to 46,403 distinct 2D systems, it produced a public catalog of 20,334,372 nanotubes, of which 57.1% show no bond change upon rolling. All structures can be exported directly as input for first-principles and molecular dynamics codes.
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Disordered Systems and Neural Networks

A rank-two spherical transform for eigenvector and singular-vector overlaps

Pierre Bousseyroux

We define a rank-two extension $\mathcal{H}^{(2)}$ of the spherical $\mathcal{H}$-transform, together with its associated transform $\mathcal{R}^{(2)}$. For sums, both transforms are additive. The transform $\mathcal{R}^{(2)}$ yields a $4\times4$ formula for the average product of two hermitized resolvents, and hence for off-diagonal eigenvector overlaps, recovering the Ginibre, elliptic, and bi-invariant formulas. We also treat correlated pairs of matrices, and obtain overlaps between their eigenvectors or singular vectors.
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Statistical Mechanics

Universal Response Functions in Driven Dissipative Tunneling Dynamics

Krishna Kingkar Pathak

Universality in nonlinear nonequilibrium systems is typically expressed through scaling laws that render macroscopic behavior insensitive to microscopic details. Whether this universality survives when periodic forcing and nonlocal dissipative memory act simultaneously remains an open question in driven open dynamics. Here, we demonstrate that barrier-crossing processes in periodically driven dissipative systems are governed not merely by modified exponential scaling, but by an explicit two-parameter universal response function. Within a semiclassical instanton framework incorporating Floquet modulation and Ohmic environmental coupling, the tunneling exponent factorizes into a system-dependent static contribution and a universal function of two dimensionless control parameters: normalized driving frequency and dissipation strength. This factorization arises from the combined modification of a single saddle-point trajectory and introduces no additional independent scaling variables. Weak-to-moderate dissipation acts as a smooth dynamical renormalization of the effective action, preserving the saddle-point structure and enabling controlled analytical expansion. In the high-frequency regime, the response exhibits universal dynamical averaging, while an explicit integral representation establishes a continuous adiabatic to Floquet crossover. Direct numerical evaluation of the nonlocal instanton action confirms that the normalized tunneling exponent exhibits a universal dependence on the driving parameters across different model systems. These results identify driven dissipative barrier crossing as a distinct two-parameter universality class within nonlinear nonequilibrium dynamics and provide a predictive functional framework for response phenomena in driven systems with memory.
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Quantum Gases

Radial dam breaks in a two-dimensional droplet bearing environment

F. Bristy, S. Chandramouli, G. A. Bougas et al. (+3 more)

The controlled dynamical generation of radial dam break flows (DBFs) is demonstrated in two-dimensional droplet environments. These ultracold mixtures feature competing mean-field and quantum fluctuation effects and are initialized on a disk-shaped uniform density. Depending on the value of the latter, reflecting also the dominance of attractive or repulsive interactions, three dynamical response regimes are identified. At small densities a moving wavetrain of bright ring solitons is observed (dubbed ring DBF), at intermediate densities a ring kink-dispersive shock wave, while at large densities the flows encompass composite ring kink-rarefaction waves moving towards the center. The emergence of these patterns, whose formation would be prohibited in two-dimensional Kerr media due to wave-collapse, is rooted in the competition between attraction and repulsion inherent to droplet environments, modeled by the extended Gross-Pitaevskii equation. The characterization of the ensuing nonlinear waveforms is further corroborated by reduced models based on the cubic-quintic nonlinear Schrödinger equation. The different wave patterns emanating from dam break flows can be experimentally realized using current state-of-the-art ultracold atom experiments.
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All authors F. Bristy, S. Chandramouli, G. A. Bougas, G. C. Katsimiga, P. G. Kevrekidis, S. I. Mistakidis

Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Engineering and Probing a One-Dimensional Dipolar Spin Ensemble in Diamond

Lingjie Chen, Shreyas Parthasarathy, Simon A. Meynell et al. (+11 more)

Dimensionality plays a central role in determining the collective behavior of interacting quantum systems. Engineering strongly interacting ensembles of solid-state spin defects in reduced dimensions remains a significant challenge at the interface between the applied and fundamental sciences. Here, we create and characterize a positionally disordered, quasi-one-dimensional spin chain in diamond, consisting of optically dark substitutional nitrogen defects (P1 centers) and optically addressable probe nitrogen-vacancy (NV) centers. Our approach exploits the preferential incorporation of nitrogen along step bunches formed during chemical vapor deposition to achieve both lateral and vertical confinement. Combining spatially resolved materials characterization with nanoscale quantum sensing, we establish the one-dimensional character of the optically dark, unpolarized P1 spin ensemble. We then use correlation spectroscopy to probe local spin autocorrelations and investigate infinite-temperature dipolar spin transport. Our results establish a materials-based route for engineering low-dimensional quantum spin systems.
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All authors Lingjie Chen, Shreyas Parthasarathy, Simon A. Meynell, Lillian B. Hughes Wyatt, Eveline Postelnicu, Haopu Yang, Zilin Wang, Weijie Wu, Winston V. Peloso, Casey K. Kim, Chris R. Laumann, Kunal Mukherjee, Norman Y. Yao, Ania C. Bleszynski Jayich

Superconductivity

Pairing symmetry and superconductivity from long-range Coulomb interactions in the extended t-t'-t”-Jz model for cuprates

U.A. Diaz-Reynoso and F. Mireles

We investigate the pairing symmetry of the ground-state phase diagram of an extended two-dimensional $t$-$t'$-$t''$-$J_z$ model, in which the first-, second-, and third-nearest neighbor electron hopping terms ($t,t'$ and $t''$) and an anisotropic Ising-like antiferromagnetic interaction $J_z$ are treated on the same framework. We find that the dominant pairing symmetry depends sensitively on the sign and magnitude of the hopping parameters $t'$ and $t''$, showing pure $p$-wave, pure $d$-wave, or coexisting pairing channels, highlighting the decisive role of these terms. We further explore the role of the long-range and short-range repulsive interactions in the formation of pairs and analyze the specific case of hole-doped cuprates. Our results indicate that repulsive interactions favors hole pair escaping of the stripe domains. Furthermore, pairing correlation calculations strongly suggest that repulsive Coulomb interactions drive reentrant superconducting behavior at experimentally observed underdoped regime ($\delta\approx0.07$ to $0.15$), as in Neodymium-based cuprates.
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Materials Science

ALIGNN 2.0: A Unified Line-Graph Neural Network Framework for Materials Screening, Force Fields, Inverse Design, Spectroscopy, and Microscopy

Jaehyung Lee, Charles Rhys Campbell, Akshaya Ajith et al. (+3 more)

Graph neural networks are central to materials property prediction and machine-learning interatomic potentials, yet their reliance on specialized graph libraries hampers portability and reproducibility, and property and force-field models have historically required separate graph pipelines. We present ALIGNN 2.0, a dependency-free, pure-PyTorch reimplementation of the Atomistic Line Graph Neural Network, with the line graph and its batching built from scratch, running on current-generation accelerators and unifying scalar, spectral, tensorial, per-atom, and force-field prediction behind a single graph, a combination that to our knowledge no existing framework provides. Comparing radius and k-nearest-neighbor (kNN) graphs, the wider kNN graph is more accurate for properties while the smoothly varying radius graph is required for energy-conserving molecular dynamics. On the JARVIS-Leaderboard, ALIGNN 2.0 leads on 26 of 30 single-property benchmarks against the original ALIGNN, with large gains for piezoelectric and dielectric maxima, exfoliation energy, moduli, and superconducting Tc. The LAMMPS- and OpenMM-compatible ALIGNN-FF matches leading universal potentials on the Matbench-Discovery and CHIPS-FF benchmarks at a small fraction of their parameters while scaling to hundred-thousand-atom cells. We further use ALIGNN 2.0 as the denoiser in a conditional crystal-diffusion model, where explicit line-graph message passing consistently lowers structural denoising loss. We also show, as work in progress, that an independently diffused, redundant bond-angle state is learnable but does not uniformly improve reconstruction or combine additively with the line graph. Finally, from a single relaxed structure the same framework reconstructs infrared, Raman, optical-dielectric, and neutron spectra in agreement with experiment and DFT, and drives frozen-phonon electron-microscopy image simulation.
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All authors Jaehyung Lee, Charles Rhys Campbell, Akshaya Ajith, Sergei V. Kalinin, Christopher Wolverton, Kamal Choudhary

Superconductivity

Surface-condition-mediated supercooling of the A-B transition in confined superfluid helium-3

Daksh Malhotra, Aymar Muhikira, Alexander J. Shook et al. (+1 more)

Chiral $p$-wave superconductors and superfluids are central model systems in the search for topological quantum matter, but in reduced dimensions their properties are inseparable from the boundary conditions imposed by surrounding surfaces. Superfluid $^3$He provides a uniquely clean, well-established spin-triplet $p$-wave condensate in which these boundary conditions can be engineered directly. Here, we use a fourth-sound Helmholtz resonator to study the A–B phase transition of $^3$He confined to a $1.8\,\mu$m cavity after preplating the internal surfaces with $\sim4$ atomic layers of $^4$He. In contrast to our previous fourth-sound measurements, which showed no resolvable hysteresis in the transition temperature, the preplated device exhibits a clear separation between cooling and warming transition temperatures together with stochastic run-to-run variations. The hysteresis decreases toward the highest pressures studied, consistent with previous observations of the pressure dependence of $^4$He-mediated quasiparticle boundary scattering. These results demonstrate that surface preparation can qualitatively modify first-order transition kinetics in confined $^3$He, making boundary conditions an experimentally accessible control parameter for metastability and for future studies of reduced-dimension topological superfluid states.
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All authors Daksh Malhotra, Aymar Muhikira, Alexander J. Shook, John P. Davis

Materials Science

The Origin of the Observed Raman Peaks in {\alpha}-MnTe

Nurul Azam, Syed Mohammad Shahed, Liam T. Schmidt et al. (+21 more)

The Raman spectrum of the room-temperature altermagnet $\alpha$-MnTe is reported to contain unassigned peaks at 120(3) cm$^{-1}$ and 140(3) cm$^{-1}$, absent from the predicted phonon spectrum of the material. This has generated considerable debate within the altermagnet community and necessitates urgent resolution. This work establishes that these peaks, together with the 90(5) cm$^{-1}$ peak that matches a theoretically predicted mode, are all extrinsic, originating from elemental tellurium formed during air exposure of the surface. Raman spectra of MBE-grown thin films show that these peaks closely match those of elemental tellurium, emerge soon after air exposure, and are absent in AlO$_x$-capped films. X-ray photoelectron spectroscopy shows that air exposure breaks Mn–Te bonds and oxidizes Mn within a minute. Cross-sectional scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy reveals that the oxidation leads to Mn out-diffusion, forming a few-nanometer-thick oxide layer above a buried, Te-enriched region, likely responsible for the anomalous Raman peaks. Additionally, no sample exhibited the 175 cm$^{-1}$ Raman peak which is commonly attributed to MnTe$_2$. The aggressive surface oxidation and associated elemental Te formation in MnTe have important implications for any surface-sensitive and optical characterization of MnTe (and other similar Te-containing materials) involving even the briefest air exposure.
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All authors Nurul Azam, Syed Mohammad Shahed, Liam T. Schmidt, Sara Bey, Oksana Yastrubchak, Maria F. Munoz, Riccardo Torsi, Thi Hai Yen Pham, Dushyanthini Balasundaram, Resham Babu Regami, Wentao Liang, Imrankhan Mulani, Matthew Matzelle, Vineet Kumar Sharma, Sougata Mardanya, Sugata Chowdhury, Nirmal Ghimire, Patrick M. Vora, Angela R. Hight Walker, Xinyu Liu, Badih A. Assaf, Arun Bansil, Alberto De la Torre, Swastik Kar

Strongly Correlated Electrons

Energy partitioning of wave packets in one-dimensional systems

Flavia B. Ramos, Imke Schneider, Sebastian Eggert et al. (+1 more)

We investigate the non-equilibrium dynamics of wave packets in a one-dimensional critical fermionic system and analyze the resulting partitioning of energy between emergent excitations. Starting from a Gaussian wave packet injected on top of the many-body ground state, we follow its real-time evolution using the time-dependent density-matrix renormalization group. We observe that interactions lead to fractionalization of the initial excitation into counter-propagating left- and right-moving modes, whose energies can be resolved in real space. To interpret these results, we employ Luttinger liquid theory, which allows us to derive analytical predictions for the energy carried by the emergent modes. We find good agreement between field-theoretical predictions and numerical simulations in the low-energy regime. In contrast to charge fractionalization, which is completely determined by the Luttinger liquid parameter, we show that energy partitioning is non-universal and depends on details of the injected wave packet, such as its width. Our results provide a real-space characterization of energy partitioning in one-dimensional systems and establish a quantitative comparison between non-equilibrium numerical simulations and the effective field-theory description.
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All authors Flavia B. Ramos, Imke Schneider, Sebastian Eggert, Rodrigo G. Pereira

Soft Condensed Matter · Cross-listed from nlin.PS

The Origin of Imperfection Sensitivity in the Buckling of Cylindrical Shells

Tian Yang and Tobias M. Schneider

Buckling of thin cylindrical shells under axial compression, a classical example of a subcritical instability, is highly sensitive to small imperfections, with minute geometric variations causing large changes in buckling threshold. To uncover the origin of this sensitivity, we use numerical continuation and bifurcation analysis while systematically varying the depth and size of a single localized Gaussian defect. We show that the instabilities of the imperfect shell originate from localized equilibria already present in the perfect shell. By breaking translation symmetry, the defect pins these equilibria and changes how they connect to the imperfect base state. Small changes in defect geometry can thereby switch the bifurcation that triggers buckling, producing non-monotonic and discontinuous changes in buckling threshold and abrupt changes in buckling mode. Imperfection sensitivity is therefore not simply sensitivity to imperfection magnitude, but sensitivity of the underlying bifurcation structure to imperfection geometry.
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Statistical Mechanics · Cross-listed from physics.flu-dyn

How universal are warm turbulent cascades?

Ben Y. Israeli and Gregory Falkovich

It is often assumed that general stationary solutions to the wave kinetic equation are described solely by conserved fluxes and temperature. The families of solutions parameterized by these quantities are referred to as warm cascades, and are derivable from differential models of wave kinetics. These models rely on the assumption of local interactions, and we demonstrate by a time scale argument that the nonlocal effect of thermal damping on the cascade may result in dependence on an additional parameter, the UV cutoff, calling into question the validity of warm cascade solutions. This is shown for systems with both three-wave and four-wave interactions. We then consider the depletion of the cascade by this damping, and present a numerical demonstration of this nonlocal and nonuniversal behavior.
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Mesoscale and Nanoscale Physics

Multiferroic Quantum Dot in an Artificial van der Waals Heterostructure

Antti Karjasilta, Mohammad Amini, Liwei Jing et al. (+5 more)

Quantum dots (QDs) provide a versatile platform for engineering quantum-confined electronic states with functionalities relevant for optoelectronics, spintronics, and quantum technologies. While substantial progress has been achieved in coupling confined states to spin, valley, topological, or ferroelectric degrees of freedom, the realization of a multiferroic QD in which quantum confinement simultaneously intertwines with magnetism and ferroelectricity remains elusive. Here, we engineer a multiferroic QD in an artificial van der Waals heterostructure grown by molecular beam epitaxy under ultra-high-vacuum conditions. The heterostructure consists of ferroelectric SnTe nanoislands deposited on the layered magnet CrBr$_2$ supported on highly oriented pyrolytic graphite. Combining scanning tunneling microscopy and spectroscopy with ab initio calculations and low-energy tight-binding models, we demonstrate the emergence of spin-polarized discretized electronic states confined within the SnTe islands. Remarkably, the spectroscopic response of the QD strongly depends on the ferroelectric domain configuration of the SnTe nanoislands, demonstrating an interplay between quantum confinement, magnetic exchange, and ferroelectric order at the atomic scale. Our results establish engineered van der Waals heterostructures as a platform for multiferroic quantum confinement and open new routes toward electrically tunable quantum spintronic devices.
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All authors Antti Karjasilta, Mohammad Amini, Liwei Jing, Robert Drost, Jose Lado, Shawulienu Kezilebieke, Peter Liljeroth, Adolfo O. Fumega

Superconductivity

Multiple Andreev Reflection Spectroscopy of Spin-Resolved Proximity States

A.A. Golubov and M.Yu. Kupriyanov

We predict a coherent multiple Andreev reflection regime in superconducting nanoconstrictions with parallel proximity-induced exchange fields in the electrodes. The subharmonic gap structure is controlled by reconstruction of the spin-resolved electrode spectra, rather than by a rigid displacement of BCS gap edges. This produces split and redistributed anomalies governed by the interval between the outer gap edge and a shifted exchange induced density of states singularity. The effect survives channel transparency averaging and provides a direct transport probe of spin-resolved proximity states.
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Mesoscale and Nanoscale Physics

Displacement field stabilizes even-denominator and partonic fractional quantum Hall states in the $\mathcal{N}{=}2$ Landau levels of Bernal-stacked bilayer graphene

Rakesh K. Dora, Udit Khanna and Ajit C. Balram

Bernal-stacked bilayer graphene (BLG), in which a graphene layer is stacked atop another and laterally shifted by a lattice constant, offers remarkable tunability in its single-particle states under applied magnetic and displacement fields. Owing to this tunability, recent transport and scanning tunneling microscopy experiments in the presence of a perpendicular magnetic field and finite interlayer displacement fields have observed even-denominator fractional quantum Hall (FQH) states at half-filling in the first excited, namely, $\mathcal{N}{=}2$, Landau level (LL) of BLG. In contrast, at zero displacement field, a gapless composite fermion Fermi liquid (CFFL) is realized at half-filling of the $\mathcal{N}{=}2$ LL. Motivated by these experiments, we compute the phase diagram as a function of the displacement field in the half-filled $\mathcal{N}{=}2$ LL of BLG by studying the competition between the CFFL and the Moore-Read state—a candidate even-denominator FQH state—by calculating their thermodynamic energies in this setting. We find that the modified effective Coulomb interaction, induced by changes in the single-particle states with increasing displacement field, softens the inter-electronic repulsion at short distances, thereby stabilizing the Moore-Read state over the CFFL in the $\mathcal{N}{=}2$ LL of BLG. We also study the nature of FQH states at fillings $2/5$, $3/7$, $4/9$, and $6/13$ in the $\mathcal{N}{=}2$ LL of BLG. Our results suggest that, with increasing displacement field, the Jain composite-fermion states at $3/7$, $4/9$, and $6/13$ transition into states with distinct topological order that are well-captured by parton wave functions.
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Materials Science

Pressure-dependent melting and crystallization of B2-NiAl from neural-network molecular dynamics

A. S. Onegin, P. R. Levashov and N. M. Chtchelkatchev

We investigate pressure-dependent melting of ordered B2-NiAl using neural-network molecular dynamics with a Deep Potential interatomic model. Melting temperatures are determined from two-phase solid-liquid coexistence simulations over a broad pressure range, yielding the melting curve $T_m(P)$. Relative to available experimental and previous molecular-dynamics results, the present calculations predict a stronger increase of the melting temperature with pressure at elevated compression. To assess the thermodynamic consistency of the calculated melting line, we evaluate the enthalpy and volume changes upon melting and compare the Clapeyron slope with the derivative of the fitted $T_m(P)$ curve. The two estimates are in good agreement over most of the investigated pressure range, supporting the internal consistency of the coexistence results. To probe the character of melting, we perform a layer-resolved composition analysis of the coexistence configurations and find that the coexisting liquid remains essentially equiatomic at all studied pressures, with deviations of the aluminum fraction from the stoichiometric value not exceeding $5\times10^{-3}$. This provides direct atomistic evidence that melting of B2-NiAl remains congruent within the present model. Together, these results establish a thermodynamically consistent pressure-dependent melting description of B2-NiAl and clarify the character of its melting under compression.
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Materials Science

Electron $g$-value comparison of paramagnetic muonium and hydrogen centers in rutile TiO$_2$

T. U. Ito, W. Higemoto and A. Koda

Muonium (Mu) is widely used as a model for isolated hydrogen in condensed matter, but whether its electronic state is truly equivalent to that of hydrogen remains to be tested with an orbital-sensitive probe. Here we use double electron-muon resonance to determine the electron $g$ values of the paramagnetic Mu center in rutile TiO$_2$ for magnetic fields along [100], [110], and [001] directions. The obtained $g$ values closely follow those of the H-related Ti$^{3+}$ paramagnetic center in both magnitude and anisotropy. This agreement tests the Mu–H correspondence at the level of the Ti $3d$ electronic state and supports a common localized Ti$^{3+}$ electronic core for the two centers.
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Materials Science · Cross-listed from physics.app-ph

Si- and C-Induced Modifications in Vapor-Solid-Grown InGaAs Nanowires: From Crystal Structure to Carrier Dynamics

Hamidreza Esmaielpour, Leopold Rothmayer, Thomas Trinkl et al. (+5 more)

Determining the impact of dopants in ternary III-V nanowires (NWs) is crucial for their applications in next-generation optoelectronic and photonic devices; however, the interplay between dopant species, crystal structure, and carrier dynamics in vapor-solid-grown InGaAs NWs remains underexplored. In this study, the influence of silicon (Si) and carbon (C) doping on the structural, morphological, and optical properties of catalyst-free, selective-area grown InGaAs NWs fabricated by molecular beam epitaxy is investigated. Scanning electron microscopy demonstrates that C-doping increases the NW aspect ratio by over 100% compared to undoped NWs, while a concomitant decrease in their planar defect density is observed, indicating enhanced crystal quality with C-doping. Despite their superior crystal quality, the C-doped NWs present a diminished photoluminescence intensity, due to point defects and increased surface recombination velocity. This is further supported by time-resolved photoluminescence spectroscopy, revealing accelerated non-radiative recombination in the C-doped NWs. Lastly, hot-carrier effects in these nanostructures are studied, and their signatures remain observable, albeit with reduced intensity in both Si- and C-doped NWs. This comprehensive study establishes clear correlations between doping parameters and the resulting material characteristics and carrier dynamics, offering valuable insights into the fundamental mechanisms governing the VS-grown InGaAs NWs and their implications in optoelectronic devices.
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All authors Hamidreza Esmaielpour, Leopold Rothmayer, Thomas Trinkl, Laura Niermann, Tore Niermann, Michael Lehmann, Jonathan J. Finley, Gregor Koblmüller

Superconductivity

Encapsulation epitaxy of air-stable monolayer superconducting films for quantum circuits and qubits

Xudong Zheng, Sameia Zaman, Kenan Zhang et al. (+22 more)

Two-dimensional (2D) superconductors are an emerging platform for strongly correlated physics and quantum information science. Their reduced dimensionality, atomically flat interfaces, and high crystallinity are attractive for realizing compact lumped-element devices in superconducting circuits. However, synthesizing large-area, monolayer 2D superconductors remains challenging because of their susceptibility to oxidation. Here, we report an "encapsulation epitaxy" mechanism that enables the growth of large-area, air-stable, monolayer superconducting NbSe2 films and explore their use in superconducting quantum circuits. A 2D encapsulation layer, such as graphene or hexagonal boron nitride (hBN), pre-deposited on a 3D substrate (e.g., SiO2 or Si3N4), serves both as a template for epitaxial growth of monolayer NbSe2 (1L-NbSe2) underneath it and as a protective cover. This approach produces uniform, large-area (>1-inch) 1L-NbSe2 with greatly enhanced ambient stability, enabling device fabrication in air. The resulting 1L-graphene/NbSe2 heterostructures exhibit robust superconductivity (Tc ~ 1 K) and enhanced charge density wave order (TCDW ~ 177 K), indicative of high material quality. We further integrate 1L-NbSe2 into superconducting circuits using oxidation-free transfer and superconducting edge-contact techniques. The 1L-NbSe2 exhibits a measured kinetic inductance LK ~ 0.7 nH/square, making it suitable for quantum circuits requiring high-kinetic-inductance elements. Encapsulation epitaxy thus provides a route to air-stable 2D superconductors and van der Waals heterostructures, with potential for wafer-scale, monolithic fabrication of superconducting quantum circuitry.
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All authors Xudong Zheng, Sameia Zaman, Kenan Zhang, Connor A Occhialini, Haowei Xu, Zhien Wang, Fangyuan Liu, Luiz Gustavo Pimenta Martins, Sejoon Lim, Tianyi Zhang, Tilo H. Yang, Jiangtao Wang, Yunyue Zhu, Zachariah Hennighausen, Sein Park, Steven Vitale, Kevin Tibbetts, Stephen Margiotta, Phillip Kim, Cong Su, Ju Li, Riccardo Comin, William D. Oliver, Joel Î-j. Wang, Jing Kong