Mesoscale and Nanoscale Physics
Topological Band Theory for High-Dimensional Parameter Spaces
Vivek Chakrabhavi, Walt Chavarria, Jonathan J. Heckman et al. (+1 more)
We study the topological band theory of
quantum Hall systems in which the adiabatic flux parameters of an external gauge potential form a high-dimensional
manifold. This situation arises both for strongly correlated matter on spatial Riemann surfaces of genus $g > 1$, as well as in the formal study of matter on compact oriented
manifolds of dimension $4\ell+2 = 2p$ coupled to $p$-form gauge potentials. In these cases, the underlying topological invariants defined over the parameter space are significantly richer than the low-dimensional (i.e., two-dimensional) case and involve both the
curvature (i.e., first Chern class) as well as higher order
curvature invariants. These higher
curvature invariants correspond to topologically protected contributions to Kubo-like formulae, constructed from correlation functions of the physical current operators. We illustrate these general considerations with an explicit example from
hyperbolic band theory based on the genus-two Bolza Riemann surface, a case which has recently been simulated on a synthetic-dimension platform.
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All authors Vivek Chakrabhavi, Walt Chavarria, Jonathan J. Heckman, Steven Rayan
Statistical Mechanics · Cross-listed from quant-ph
One-dimensional quantum Gibbs states in constant circuit depth
Saúl Pilatowsky-Cameo, Georgios Styliaris, Ainesh Bakshi et al. (+1 more)
We prove that the Gibbs state of any
spin chain can be exactly decomposed into a mixture of product states over contiguous blocks of
qubits with bounded length, at any constant temperature. As a consequence, any such Gibbs state can be prepared by local
unitary circuits of constant depth, with efficient classical preprocessing. We place strong limits on the spatial structure of thermal
entanglement: both the
entanglement depth and
entanglement width (measures of multiparticle
entanglement) are bounded by constants, while the localizable
entanglement (the bipartite
entanglement that can be generated with the aid of local measurements and classical communication) vanishes exactly beyond a constant threshold distance. Our result also has implications for
quantum thermalization: for any translation-invariant
spin-chain
Hamiltonian with nondegenerate spectral gaps, typical low-complexity pure states drawn from a maximally entropic ensemble at any constant effective temperature become locally indistinguishable from the Gibbs state upon
unitary evolution. Additionally, we obtain an analogous decomposition for fermionic systems: the Gibbs state of any 1D local fermionic
Hamiltonian is exactly a mixture of constant-depth local fermionic
circuits acting on Gaussian states.
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All authors Saúl Pilatowsky-Cameo, Georgios Styliaris, Ainesh Bakshi, Daniel Malz
Strongly Correlated Electrons
Advancing Fuzzy-Sphere CFTs: $\mathrm{SO}(3)$-Rotation-Resolving Exact Diagonalization, 3D Ising Heavy Primaries, and Quasi-Hole-Space Projection
Zheng Zhou and Yin-Chen He
Fuzzy-sphere regularization has recently emerged as a powerful framework for studying three-dimensional conformal field theories (CFTs). In this work, we exploit
symmetry and
Hilbert-space structures to substantially advance this framework. We develop an exact-diagonalization
algorithm that resolves the full $\mathrm{SO}(3)$ rotational
symmetry, reducing computational costs by factors of several hundred. For the three-dimensional Ising CFT, we identify around 100 primary operators, including a large number of previously unknown heavy scalar primaries up to scaling dimension $\Delta\approx16.5$. We also resolve the tension between previous fuzzy-sphere and bootstrap results regarding the parity-odd scalar primary. Furthermore, we identify a branch of scalar primaries associated with $\phi^n$ operators in the $\phi^4$ Lagrangian description, beginning with the identity, $\sigma$, $\epsilon$, $\epsilon'$, and $\sigma'$. Their wave functions exhibit a semi-classical structure closely related to states invariant under area-preserving diffeomorphisms of the sphere. Finally, we show that critical points involving fractional
quantum Hall states can be studied within a substantially reduced
Hilbert space spanned by quasi-hole states of the corresponding model
quantum Hall wave functions like the Laughlin state. This suggests a general strategy for exploring CFTs intertwined with topologically ordered phases.
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Disordered Systems and Neural Networks
Discrete Scale Invariance of Ising Mesons
Denis V. Vasilyev and Abbas Ali Saberi
Discrete scale invariance often arises from resonant few-body or singular scale-invariant dynamics. We show that it can instead emerge in the internal motion of a composite excitation in an ordered
quantum magnet. In the marginal $1/r^4$ transverse-field Ising chain, long-range bonds generate an asymptotic inverse-square attraction between dressed domain walls, with strength fixed by the spontaneous magnetization. The dressed two-kink threshold
curvature sets the relative kinetic scale. Together, these independently accessible quantities determine the scale-anomaly exponent and hence the
meson hierarchy. In the supercritical regime, the same exponent governs
geometric binding-energy and size ratios, logarithmic level accumulation, and log-periodic threshold scattering. Full-
spin exact diagonalization and a fourth-order weak-field
Hamiltonian support the coupled energy–size scaling in the finite window between core and ring effects. This provides a microscopic many-body realization of
quantum limit-cycle physics, with its universal scaling fixed by
magnetic order and kink mobility.
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Materials Science · Cross-listed from astro-ph.GA
Photoelectron and electron microscopy investigation of laboratory grown Mg-silicate space dust analogues
Steffen Friis Holleufer, Mie Andersen, Frederik Granzow Doktor et al. (+9 more)
The atomic structure at the surface of interstellar silicate dust particles likely plays a key role in a variety of chemical processes occurring in
star- and
planet-forming regions of the interstellar medium (ISM). Here, we use the photoelectric effect to characterize, in situ, the local chemical structure of Mg-silicate nanoparticulate films, and ex situ electron microscopy to trace changes to morphology resulting from different starting compositions. Nanoparticulate films are prepared via co-deposition of Si, Mg and O atoms on a graphitic substrate under ultra-high
vacuum (UHV) and are characterised with X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure measurements. Analysis, supported by density functional theory calculations, shows that adjusting the Mg-to-Si ratio from 3.5 to 1.8 changes particle composition from a mixture of MgO and Mg-silicate towards predominantly Mg-silicate. Annealing in UHV also pushes the composition towards Mg-silicate, however, non-stoichiometric chemical motifs are always observed, presenting a distribution of potentially catalytically active sites. Electron microscopy images show composition-dependent film morphologies that restructure differently upon thermal annealing. The Mg-silicate nanoparticulate films presented here are characterised as a 2D version of ISM analogue dust particles prepared via
laser ablation and are therefore a more readily suitable model for surface science investigations into the catalytic properties of interstellar Mg-silicate dust.
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All authors Steffen Friis Holleufer, Mie Andersen, Frederik Granzow Doktor, Lars Eric Borchert, Martin Wibrand Larsen, Duncan S. Sutherland, Zheshen Li, Alexei Preobrajenski, Jeppe Vang Lauritsen, Cornelia Jäger, Liv Hornekær, Andrew Cassidy
Strongly Correlated Electrons
Perfect Born Sampling of Symmetric Thermal Tensor Network for Quantum Lattice Models
Jianxin Gao, Qiaoyi Li, Yuan Gao et al. (+5 more)
Accurate calculations of
quantum lattice models at low temperatures constitute a major challenge in many-body physics.
Stochastic sampling of tensor-network states offers a promising route to tackle this problem; however, existing schemes have long faced a fundamental dilemma—sampling efficiency and
symmetry acceleration cannot be achieved simultaneously. Here we propose a perfect Born sampling approach for thermal tensor networks, which performs importance sampling directly from the purified density
matrix via the Born rule and incorporates Abelian and non-Abelian
symmetries by sampling
symmetry quantum numbers. We
benchmark the method, realized as both
stochastic matrix product states (stoMPS) and
stochastic projected entangled pair states (stoPEPS), on large-scale
quantum lattice models. Using stoMPS, we accurately simulate the square-
lattice Hubbard model on cylinders up to width $W=10$, and study the triangular-
lattice Hubbard model down to $T/t = 1/64$, revealing scalar chiral order at half filling and kinetic ferromagnetism upon electron doping. We further extend the stoMPS method to compute finite-temperature
quantum dynamics, as demonstrated by the optical conductivity of the Hubbard model, and generalize it to stoPEPS, as showcased on the $20\times20$ square-
lattice quantum Ising model at its
quantum critical point. Our method combines high sampling efficiency with full
symmetry acceleration, and can be used as a state-of-the-art framework for studying both
equilibrium and dynamical properties down to ultralow temperatures.
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All authors Jianxin Gao, Qiaoyi Li, Yuan Gao, Chuanshu Xu, Guoliang Wu, Su Yi, Bin-Bin Chen, Wei Li
Materials Science
Catalogue of Collinear Magnetic Structures in Stoichiometric Materials
Liangliang Huang, Houhao Wang, Yuanze Song et al. (+3 more)
Magnetic materials persistently serve as a fertile platform for exploring novel
quantum emergent phenom ena and advancing information technologies. Reliably determining their
magnetic structures (MSs), which is essential for understanding their emergent
quantum properties, usually depends on experiments such as
neutron scattering. Here, noting that around half of the MSs in MAGNDATA (a database of experimentally determined MSs) are collinear, we adopt the
spin space groups (SSGs) to preset the candidate MSs for a given nonmagnetic
crystal structure to be collinear from the outset. On the other hand, more than 80% of the collinear magnets in MAGNDATA are subject to maximal subgroups of nonmagnetic groups which are collinear SSGs, and we then enumerate all collinear MSs formed at Wyckoff positions by the maximal collinear SSG subgroups of the 230 nonmagnetic space groups. With such exhaustive enumeration, we categorize all
symmetry-allowed collinear MSs for the 72,075 nonmagnetic stoichiometric structures in Inorganic
Crystal Structure Database, obtaining 365,639 collinear MSs. We then determine the ground-state SSG
symmetries of 7,824 collinear magnets among the 21,315 selected materials using first-principles calculations. These magnets then undergo a comprehensive topological classification combining SSGs and
magnetic space groups, outputting thousands of new
magnetic topological materials. The experimentally determined MSs in MAGNDATA represent the tip of the iceberg in our computational database of collinear magnets, expected to guide MS characterization by experimentalists and further investigation towards spintronics applications.
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All authors Liangliang Huang, Houhao Wang, Yuanze Song, Ruixi Pu, Xiangang Wan, Feng Tang
Materials Science
A Si-on-SiC Platform for Interfacing with Vanadium Dopants in the Telecom O-Band
Timothy Draher, Nolan Bitner, Vasileios Niaouris et al. (+6 more)
Vanadium color centers in silicon carbide (SiC) offer telecom O-band emission, sub-microsecond optical lifetimes, and second-scale
spin relaxation times, rendering them promising candidates for
quantum network nodes. However, efficient
photon extraction from the high-refractive-index SiC host remains challenging. Here, we introduce a silicon-on-SiC heterogeneous photonic platform in which silicon nanocavities evanescently couple to shallow vanadium dopants in commercial 4H-SiC, enabling efficient zero-
phonon line (ZPL) collection under resonant excitation. In as-grown ensembles, we observe Purcell-enhanced photoluminescence with transient spectral hole burning linewidths of 50 MHz on microsecond timescales. In dilute implanted samples, we isolate individual vanadium centers with high-purity single
photon emission and a Purcell-enhanced lifetime of 109 ns. The entire optical interface operates through a single lensed fiber, including a 905 nm repump
laser that recovers the vanadium charge state with 95% efficiency. These results establish Si-on-SiC as a scalable, foundry-compatible platform for telecom-wavelength
spin-
photon interfaces.
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All authors Timothy Draher, Nolan Bitner, Vasileios Niaouris, Claire E. McDermott, David Czaplewski, Supratik Guha, David D. Awschalom, F. Joseph Heremans, Alan M. Dibos
Quantum Gases
Structure and Dynamics of Bose Polarons across the Mott-Insulator to Superfluid Transition
Ragheed Alhyder, Alessio Recati and Georg M. Bruun
A mobile impurity particle immersed in a
quantum degenerate gas leads to the formation of a quasiparticle, which can serve as a sensor for its environment. Here, we develop a unified wave function description of an impurity in a two-dimensional Bose-Hubbard model across the Mott-
insulator to superfluid transition. Using a multi-mode variational ansatz based on the
quantum Gutzwiller approach, we show that the impurity can form several kinds of polaronic and molecular states. In addition to their spectral properties, the wave function provides direct access to the microscopic structure of these states, and we demonstrate that strong correlations in the critical regime of the
quantum phase transition give rise to several non-analytic features in the polaron properties. Increasing the impurity-
boson interaction leads to transitions between polaron and molecular ground states, and the number of
bosons in the impurity dressing cloud grows rapidly at the
quantum critical point as the bath modes soften. We furthermore reveal qualitatively distinct non-
equilibrium many-body dynamics after the injection of the impurity in the different phases of the Bose-Hubbard model. Our spectral, real space, and dynamical predictions describe complementary properties of polarons in a strongly correlated bosonic bath, which can be observed using current techniques with atoms in optical
lattices.
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Statistical Mechanics
Speed-Fisher Information: Chaos and Irreversibility in Classical and Quantum Dynamics
Nachiket Karve, Nathan Rose, David Campbell et al. (+1 more)
We propose a unified perspective on classical and
quantum chaos based on the sensitivity of a system's stationary states to slow driving. We probe this sensitivity via the system's susceptibility to the average protocol speed, which we call the “speed-Fisher information," and relate it to irreversible
entropy production in the system. We show that chaotic dynamics manifests as a divergence of the speed-Fisher information with the protocol time, and that this response is controlled by the perturbation's low-frequency spectral weight. This approach to chaos applies to both classical and
quantum Hamiltonian systems, and naturally extends to non-
Hamiltonian classical flows. Additionally, we identify a
quantum regime at times shorter than the Planckian timescale, in which the speed-Fisher information exhibits a uniquely
quantum scaling. We illustrate this framework with simple classical and
quantum examples
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All authors Nachiket Karve, Nathan Rose, David Campbell, Anatoli Polkovnikov
Materials Science
Pyroelectric thermo-wave probing of BaZr0.2Ti0.8O3 ceramics
Nicholas V. Morozovsky, Yuriy O. Zagorodniy, Mykola Yelisieiev et al. (+9 more)
BaxZr1-xTiO3 ceramics with 0.15 < x < 0.25, which have high dielectric permittivity, small leakage currents and low dissipation factor, are promising materials for tunable capacitor devices, multilayered ceramic capacitors and piezoelectric actuators. These materials reveal relaxor properties due to the local chemical strains caused by the isovalent substitution of Ti4+ ions by Zr4+ ions with a larger ionic size. However, to the best of our knowledge, the pyroelectric properties of BaxZr1-xTiO3 ceramics with x~0.2 have not been studied. To fill the gap in knowledge, we perform the pyroelectric thermo-wave probing of BaZr0.2Ti0.8O3 ceramics prepared by the solid-state synthesis. Results of the thermo-wave probing of the most interesting polar states, namely polarized, depolarized, relaxed and restored polarized states, reveal the pronounced pyroelectric response that can be strongly asymmetric with respect to the opposite surfaces of the ceramics. The asymmetry and profiles of pyroelectric response depend significantly on the pre-history of the electric field cycling indicating possible non-ergodic relaxor-type polar states in the ceramic sample. X-ray diffraction spectrum, recorded at room temperature, reveals the virtual absence of the macroscopic tetragonality and the weak asymmetry of the (200) peak, which indicate the small tetragonality inside the Ti-enriched polar nanoregions. Decrease of the relative intensity of the Raman band at 714 cm-1 occurring upon heating evidences the diffuse ferroelectric-paraelectric phase transition between 30 C and 40 C. The temperature dependence of the dielectric permittivity obeys modified Curie-Weiss law with power 1.4, indicating that both ordered ferroelectric and relaxor states coexist in the BaTi0.8Zr0.2O3 ceramics.Results can be useful for elaboration of lead-free relaxor ferroelectric ceramics for advanced pyroelectric applications.
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All authors Nicholas V. Morozovsky, Yuriy O. Zagorodniy, Mykola Yelisieiev, Oleksandr S. Pylypchuk, Mykola O. Semenenko, Iryna V. Kondakova, Lesya P. Yurchenko, Viktor Bovtun, Valentin V. Laguta, Eugene A. Eliseev, Volodymyr M. Dzhagan, Anna N. Morozovska
Statistical Mechanics · Cross-listed from quant-ph
Optimal performance in a chaotic Floquet quantum battery
Lakshmija N K and Uma Divakaran
We study a
quantum battery consisting of $N$
qubits interacting uniformly with each other. Starting from its ground state, the battery is charged by introducing periodic kicks in the transverse direction along with adding disorder in the interaction direction. Although the battery
Hamiltonian along with the initial state preserves the permutation
symmetry, the dynamics in presence of disorder during the charging phase takes the system out of the permutation symmetric subspace, resulting to a larger amount of stored energy as compared to that without disorder. In the limit of large disorder, this interplay brings in a possibility of a chaotic charging phase when stored energy can be exactly calculated using random
matrix theory. More importantly, we also find that the energy fluctuations decrease and become negligible in the limit of large disorder, which is also when the energy stored is maximum, a condition highly desirable for the optimum functioning of the battery. We complete the battery cycle by considering the storage phase in the presence of dephasing noise. We find that for larger disorder strengths, the ergotropy remains close to the stored energy, indicating nearly complete energy extractability with an efficiency approaching unity.
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Materials Science · Cross-listed from cs.LG
Bounding Retraining Equivalence and the Deletion Floor in Materials Machine Unlearning
Can Polat, Mustafa Kurban, Erchin Serpedin et al. (+1 more)
In materials machine learning, closely related retained structures can sustain accurate property predictions even after removing a specific record, rendering post-deletion prediction error an ambiguous metric for machine unlearning. To resolve this ambiguity, we define the deletion floor as the expected target loss under a specified retraining procedure at the deleted request. Standard indistinguishability constraints yield a sharp interval bounding an update's target loss around this baseline reference. Theoretically, a conditional neighbor bound links a low deletion floor directly to retained fit, prediction regularity, and local label agreement, while an exact ridge identity isolates residual fit from the prediction change induced by record deletion. Empirically, controlled redundancy sweeps show an $\approx 8\times$ drop in median normalized retraining loss when one retained relative remains after deletion. Across two distinct fitting regimes in a paired Materials Project study, the lower-floor regime also exhibits a larger prediction change on more than 50% of the shared requests. Systematic comparisons against approximate updates and the original model decouple deliberate target suppression from preserved overall model utility. Consequently, request-level unlearning evaluations should report reference loss, prediction change, and retained utility together, interpreting post-deletion accuracy against what retraining itself leaves behind.
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All authors Can Polat, Mustafa Kurban, Erchin Serpedin, Hasan Kurban
Strongly Correlated Electrons
Chiral Weyl–Kondo semimetal and circular photogalvanic effect in a prototype Kondo lattice system
Yuan Fang, Kuan-Sen Lin, Mounica Mahankali et al. (+5 more)
Chiral Weyl–Kondo semimetals (cWKSM) provide a setting in which chiral Weyl quasiparticles emerge in the immediate vicinity of the Fermi energy from a Kondo-driven reconstruction of the strongly correlated electronic states in chiral heavy
fermion systems (K.-S. Lin et al., arXiv:2602.22185). A defining characteristic of this strongly correlated topological state is the Kramers chiral Weyl
fermions in the low-energy quasiparticle states. Recently, experiments in CeGaGe have emerged as a concrete realization of the proposed effect (Arushi et al., preprint). Motivated by these findings, here we go beyond the materials-specific effects by constructing a prototype Kondo
lattice model; it incorporates only the essential couplings that respect the associated tetragonal crystalline
symmetries. This simplification allows us to robustly demonstrate the
symmetry-enforced Kramers Weyl
fermions and related topological nodal states in the spectrum of heavy quasiparticles. Furthermore, the simplification provides a tractable setting to determine the salient features in the system's nonlinear optical response, the circular photogalvanic effect, in chiral Weyl–Kondo semimetals. Both analytical and numerical calculations identify sharp peaks in the frequency domain as signatures of the Kondo-driven chiral Weyl nodes; the sharpness of the spectrum reflects the resonant nature of the underlying strongly correlated electronic excitations. Thus, cWKSM provides a unique setting to spectroscopically identify topological
fermions that are induced by strong electron correlations. As such, our results are expected to bring about much needed new insights into the understanding of strongly correlated gapless topological matter.
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All authors Yuan Fang, Kuan-Sen Lin, Mounica Mahankali, Arushi, Kevin Allen, Sanu Mishra, Emilia Morosan, Qimiao Si
Superconductivity
Periodic Trends and a Physics-Based Multistage Search for High-$T_c$ Hydride Superconductors
Tianran Chen and Taner Yildirim
Hydrogen-rich materials under pressure are candidates for conventional
phonon-mediated superconductivity, but first-principles electron–
phonon-coupling (EPC) calculations are computationally demanding. We develop an interpretable multistage workflow for prioritizing binary hydride
superconductors before full EPC calculations. Using controlled cubic $X_m$H$_n$ prototypes at 200~GPa, we introduce the spectral moment $A_1$ as an intermediate measure of electron–
phonon perturbation strength. In its linewidth-based construction, $A_1$ remains finite as harmonic
phonons soften, allowing related structures with imaginary harmonic modes to be screened; it does not establish superconductivity or physical realizability for unstable structures. For dynamically stable cubic systems, $A_1$ correlates strongly with the H-$1s$ density-of-states fraction at the Fermi level, $\alpha$, a projected Fermi-surface descriptor, $\beta$, and atomic number density, $\rho$. These trends are summarized heuristically by $A_1\approx h\alpha\beta\rho$, where $h$ is framework- and pressure-dependent; the descriptor is used for ranking, not as a universal $T_c$ predictor. We incorporate $\alpha\beta\rho$ and enthalpic competitiveness into a three-stage evolutionary search comprising multiobjective structural screening, harmonic-
phonon stability filtering, and coarse density-functional perturbation-theory (DFPT) EPC calculations. Application to more than 100,000 binary hydride structures at 50 and 200~GPa identifies candidates and prioritizes NaH$_6$ structural families. Higher-accuracy calculations show that $Pm\bar{3}m$ NaH$_6$ is dynamically stable down to 50~GPa and has a harmonic Allen–Dynes estimate of $T_c\approx230$~K. The framework combines electronic descriptors with progressively more expensive stability and EPC calculations for scalable hydride exploration.
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Materials Science
Density functional perturbation theory of meta-generalized gradient approximations using algorithmic differentiation
Bruno Ploumhans, Niklas Frederik Schmitz and Michael F. Herbst
Density functional perturbation theory (DFPT) is an established framework for the computation of derivatives in plane-wave density functional theory (DFT). We present an implementation of DFPT for exchange-correlation (XC) functionals $E_\mathrm{xc}(\rho,\tau)$ that incorporate an explicit dependence on both the density $\rho$ and the kinetic energy density $\tau$. This covers the popular class of semilocal meta-generalized gradient approximations (meta-GGAs) as well as broader nonlocal parametrizations. We sidestep the derivation of cumbersome XC second energy derivative expressions by recasting these derivatives as a Jacobian-vector product of the XC potentials, which we evaluate with algorithmic differentiation (AD) techniques. Integration with our previously developed AD-DFPT framework [N. F. Schmitz et al., npj Comput. Mater. 12, 6 (2026)] provides access to derivatives of arbitrary ground state quantities with respect to arbitrary perturbations. We employ AD-DFPT to compute a range of response properties for ZnO and BaTiO3, and find that the recent r2SCAN01 meta-GGA functional generally outperforms LDA and PBE. Finally, we showcase the optimization of a neural-network meta-GGA to self-consistently reproduce hybrid-DFT reference densities of bulk silicon, using AD-DFPT gradients. Overall, these results establish AD-DFPT as a versatile route for computing DFT derivatives at the meta-GGA level, be they common response properties or the unusual derivatives required for the gradient-based training of novel XC functionals.
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Materials Science
AI-Assisted Identification of Magnetic Orders and Skyrmions
Haowen Yang, Sophia Huerta and Yingying Wu
Exotic
magnetic orders in two-dimensional (2D) materials are attracting huge interest for energy-efficient spintronic applications, yet realizing robust high-temperature van der Waals antiferromagnets and topological
magnetic states remains challenging. In this work, we develop a machine-learning framework for identifying
magnetic orders and predicting magnetization using structural, compositional, and electronic information derived from the Materials Project. Fixed-length descriptors are constructed for two complementary tasks: ferromagnetic (FM) versus antiferromagnetic (AFM) classification and quantitative magnetization prediction.
Magnetic order is classified using a LightGBM model trained on structure-derived descriptors without explicitly including
magnetic descriptors. Five-fold cross-validation grouped by chemical system is used to reduce chemical leakage, together with hyperparameter and classification-threshold optimization. On an isolated test set, the classifier achieves a balanced accuracy of 93.6% in the testing set. Magnetization is predicted using a residual multilayer perceptron with compositional, structural, electronic, and task-specific initial-state descriptors. A logarithmic target transformation and robust weighted loss are used to account for the broad magnetization distribution, while three independently trained models are combined into a final ensemble. The model achieves a mean absolute error of 0.686 {\mu}B/formula unit. For selected
magnetic candidates, simulated
magnetic imaging and phase-reconstruction analysis are further used to investigate magnetization textures and skyrmion-like features through normalized magnetization profiles and topological charge. This framework provides an efficient approach for screening 2D
magnetic materials and prioritizing candidates for antiferromagnetic and topological spintronic applications.
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Materials Science
Ultra-high vacuum Raman platform for in situ characterization of graphene nanoribbons
Jeong Ha Hwang, Amogh Kinikar, Lukas Rotach et al. (+9 more)
Atomically precise
graphene nanoribbons (GNRs) exhibit tunable electronic and
magnetic properties governed by edge
topology and finite-size effects, which make them versatile platforms for next-generation electronic and spintronic applications. However, the unpaired pi-electrons responsible for their
magnetic properties simultaneously make them highly susceptible to chemical degradation under ambient conditions. This intrinsic reactivity poses a central experimental challenge: accessing vibrational and electronic signatures of air-sensitive GNRs during synthesis and under controlled environments without breaking
vacuum. Once the material has been exposed to air, standard characterization techniques would probe oxidized or chemically modified species rather than the pristine form. Here, we overcome this limitation by developing a home-built ultra-high
vacuum (UHV) Raman platform designed to preserve sample integrity by preventing air exposure and to enable in situ investigation of material properties. The portable Raman
vacuum suitcase (RVS) integrates temperature control and precise gas dosing, allowing direct monitoring of growth kinetics,
lattice dynamics, and reactive-edge responses under well-defined thermal and chemical environments. Using this platform, we monitor the on-surface synthesis of 7- and 9-atom-wide armchair GNRs (7- and 9-AGNRs), quantify the
evolution of 7-AGNR Raman modes over a wide temperature range (162-748 K), and resolve chemical changes upon controlled O2 exposure that are consistent with oxidation at the reactive zigzag sites. These results establish UHV Raman spectroscopy with the RVS as a route to accessing the intrinsic vibrational signatures of low-dimensional
quantum materials under controlled environments.
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All authors Jeong Ha Hwang, Amogh Kinikar, Lukas Rotach, Andres Ortega-Guerrero, Carlo A. Pignedoli, Klaus Muellen, Thorsten G. Englmann, Xinliang Feng, Pascal Ruffieux, Roman Fasel, Mickael L. Perrin, Gabriela Borin Barin
Mesoscale and Nanoscale Physics
Superfluid Spin Transport in the Van der Waals Antiferromagnet CrCl$_3$
Peisen Yuan, Xiaomin Guo, Vincent Flynn et al. (+3 more)
Over the past decade, the quest for
spin superfluidity has moved to the forefront of spintronics, driven by the promise of phase-gradient-driven, ultra-low-loss
spin transport. However, experimental investigations remain limited, primarily due to the lack of suitable material systems. Here, we report on the discovery and control of a superfluid
spin transport in the easy-plane van der Waals antiferromagnetic (AFM)
insulator CrCl$_3$ by a nonlocal device structure. Combining nonlocal
magnon transport measurements with theoretical modelling, we demonstrate that
spin superfluidity emerges in CrCl$_3$ under canted AFM
spin configurations, where it gives rise to ultra-long range (around 90 $\mu$m), weakly decaying
spin transport. We also provide direct evidence that strong
magnetic fields and elevated temperatures suppress the superfluid state, restoring the rapid exponential decay with distance of incoherent
magnons. These findings underscore the potential of
spin superfluidity in two-dimensional
magnetic insulators and establish CrCl$_3$ as a promising platform for energy-efficient, long-distance
spin transport in next-generation spintronic applications.
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All authors Peisen Yuan, Xiaomin Guo, Vincent Flynn, Benedetta Flebus, Fèlix Casanova, Luis E. Hueso
Materials Science
High-Throughput Imaging of Degradation-Inducing Microscopic Impurities in Perovskite Solar Cells
Sofiia Kosar, Anil R. Pininti, Vladyslav Hnapovskyi et al. (+6 more)
The scalable fabrication of high-quality, large-area perovskite thin films is hindered by microscopic inhomogeneities, particularly residual compositional impurities formed during processing. Identifying these impurities, understanding their impact on device operation, and enabling their rapid detection are essential for upscaling perovskite solar
cells (PSCs). Here, nano-
Fourier transform infrared spectroscopy combined with high-resolution optical and scanning probe techniques was used to identify detrimental impurities in wide-bandgap perovskite films relevant to tandem solar
cells. Photo-stress experiments revealed that degradation of the perovskite layer initiates at impurity-perovskite interfaces, demonstrating that impurities act as failure nucleation sites. Leveraging these insights, a rapid, non-invasive, and high-throughput framework based on high-resolution reflected light microscopy and machine learning-supported image analysis was developed, enabling detection and quantification of harmful impurities in as-prepared films within seconds. Films with higher impurity density show accelerated early degradation, establishing this parameter as an early-warning metric for stability screening. Extending this framework to degradation tracking further reveals coupled photo- and thermo-chemical contributions to impurity-mediated instability. Overall, this work establishes a practical chemically-validated diagnostic imaging framework for rapid pre-screening of perovskite films to enable stable and scalable PSCs.
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All authors Sofiia Kosar, Anil R. Pininti, Vladyslav Hnapovskyi, José P. Jurado, Subhashri Mannar, Lorenzo Mardegan, Anand S. Subbiah, Frédéric Laquai, Stefaan De Wolf
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Electron decoherence in cylindrical holes and circular apertures
Cruz I. Velasco and F. Javier García de Abajo
The coherence of free electrons sets fundamental limits on the resolution and contrast of phase-sensitive electron microscopy because coherence is lost whenever the electron leaves a distinguishing excitation in its environment. We develop a quantitative theory of fast-electron decoherence in two canonical geometries: a cylindrical hole drilled through a realistic metal and a circular aperture in a thin perfectly conducting film. By combining an electromagnetic Green-tensor formulation with the fluctuation–dissipation
theorem while fully retaining retardation, we obtain the decoherence probability and elastic phase as functions of the electron trajectory, temperature, and material response. For the cylindrical hole, we obtain a closed-form, azimuthally resolved expression that separates the dependences on path position, temperature, and conductivity. In the high-conductivity and high-temperature limits, this result reduces to a universal expression that is linear in temperature and independent of both conductivity and electron velocity. For the circular aperture, which we solve using a cylindrical-wave modal expansion, the energy-loss probability diverges as $1/\omega$ at low frequency, whereas the decoherence probability remains finite and reaches a maximum when the aperture radius is comparable to the relevant electromagnetic wavelength. Because the interaction is spatially localized, aperture-induced decoherence is generally weaker than that produced by a cylindrical hole, except for sufficiently large apertures at high temperature. Finally, we show that, for sufficiently large holes, decoherence can dominate the spatial broadening of a focused electron probe and must therefore be incorporated into the design of coherent electron-beam instruments.
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Mesoscale and Nanoscale Physics
Dynamical transition in non-Hermitian Chern insulator
Zhi-Qiang Zhang, Yuan-Hang Ren, Ming Lu et al. (+1 more)
We unveil a peculiar dynamical transition for the propagation of wave packets in non-Hermitian Chern
insulators, where the
evolution of the topological wave packets at the edge is not solely determined by the topological Chern number. Unlike the Hermitian Chern
insulator, where a wave packet initiated at the edge propagates along the system boundary, here it may instead penetrate into the bulk. This behavior is attributed to the competition between the localization induced by conventional
topology and the non-Hermitian skin effect. Specifically, when the former dominant, the wave packet will evolve along the boundary; otherwise, it will spread into the bulk. These features demonstrate that while the generalized-Brillouin zone framework reliably predicts topological phase transitions, it does not on its own specify the dynamical transitions.
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All authors Zhi-Qiang Zhang, Yuan-Hang Ren, Ming Lu, Hua Jiang
Statistical Mechanics
Out-of-equilibrium finite-time percolation transitions and spinodal-like behaviors after quenches across magnetic first-order transitions of Ising systems
Andrea Pelissetto, Davide Rossini and Ettore Vicari
We analyze the out-of-
equilibrium relaxational dynamics of ferromagnetic Ising-like systems driven across their low-temperature
magnetic first-order transition (FOT) line, by sudden and slow variations of an external homogenous
magnetic field $h$. As a paradigmatic example, we consider a two-dimensional Ising system driven across its low-temperature FOT line by suddently quenching $h$ from $h_i<0$ to $h>0$, inducing a transition from the negatively to the positively magnetized phase. We show the emergence of a dynamic percolation transition at a finite critical time $t_c(h)$ along the post-quench
evolution for finite (sufficiently small) values of $h$, which is marked by the percolation of the largest positive-
spin cluster and the antipercolation of the largest negative-
spin cluster. This out-of-
equilibrium percolation transition displays a finite-size scaling behavior as in the standard random-percolation case. However, while the fractal dimension of the percolating clusters is consistent with the random-percolation value, the exponent controlling the approach to criticality differs and depends on $h$. We also show that the percolation transition marks the passage from the metastable negatively-magnetized phase to the stable positively-magnetized one. Therefore, in the small-$h$ limit, the percolation critical behavior is related to the spinodal-like behavior of the magnetization, implying that the percolation time $t_c(h)$ exhibits a spinodal-like exponential dependence on $h$. The existence of percolation transitions is likely a generic phenomenon at
magnetic Ising-like FOTs. For example, we observe an analogous behavior in dynamic (Kibble-Zurek-like) protocols entailing slow variations of the
magnetic field driving the crossing of the FOT line.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Improving the Stability of the Hierarchical Equations of Motion for Open Quantum Systems with Strong Coupling to Structured Bosonic Baths
Salvatore Gatto, Samuel L. Rudge, Bokang Hou et al. (+2 more)
The hierarchical equations of motion (HEOM) approach is one of the most powerful methods avail- able to simulate the dynamics of open
quantum systems. Although the HEOM approach has been shown to be stable and efficient in many parameter regimes and physical scenarios, it is also well known that the unavoidable finite truncation of the hierarchy can lead to fundamental numeri- cal instabilities for bosonic environments. In this work, we apply recently developed techniques to analyze these issues. We use an auxiliary Fock-space picture of the hierarchy to first identify the unbalanced hierarchy-raising terms responsible for the strong transient amplification associated with the non-normal algebraic structure of the HEOM generator. We then apply a non-
unitary similarity transformation that converts these terms into balanced combinations of hierarchy-raising and hierarchy-lowering terms, thereby suppressing their numerical impact. We first demonstrate the efficacy of this approach via a
spin-
boson model with a simple Brownian oscillator spectral den- sity and subsequently extend the analysis to a structured spectral density, showing for both cases that the transformed HEOM approach allows simulations of much larger system-bath coupling than would otherwise be possible.
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All authors Salvatore Gatto, Samuel L. Rudge, Bokang Hou, Eran Rabani, Michael Thoss
Mesoscale and Nanoscale Physics
Extracting the anyonic exchange phase from co-tunneling through an open quantum dot
Felix Puster, Matthias Thamm and Bernd Rosenow
The elementary excitations of fractional
quantum Hall states obey anyonic statistics, described by an exchange phase $\theta$ intermediate between the bosonic and fermionic values. We study an interferometer in which three chiral edge segments are pairwise connected by three
quantum point contacts, so that, depending on the applied voltages, one of the segments acts as an open
quantum dot: its spectrum is continuous, and tunneling through it is resonant without any gate tuning. This setup realizes the co-tunneling mechanism of a recently proposed antidot interferometer in a particularly simple setting. Using a non-
equilibrium Keldysh calculation to third order in the tunneling amplitudes, we show that the interference current contains a co-tunneling contribution in which two quasiparticles are exchanged, so that for Laughlin states the exchange phase $\theta=\pi\nu$ appears as the phase shift between the Aharonov-Bohm oscillations measured in two voltage configurations. We obtain a closed analytic expression for the interference current at finite temperature and finite separation between the
quantum point contacts and show that, provided voltage-induced changes of the enclosed area are sufficiently screened, the exchange phase can be extracted from a robust phase-plateau difference. Alternatively, simultaneous measurements of the interference currents at two drains avoid this screening assumption and time-of-flight phases can be quantified analytically, allowing the exchange phase to be extracted by a controlled extrapolation with a few-percent systematic error.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.optics
Plasmon-Enhanced Second-Harmonic Generation in Atomically Thin Crystalline Silver Nanostructures
Saad Abdullah, Philipp K. Jenke, Andrew P. Weber et al. (+8 more)
The intrinsically weak nonlinear optical response of existing materials, further constrained by
symmetry-forbidden second-order processes in centrosymmetric media, severely limits efficient frequency conversion in deeply subwavelength, ultrathin volumes. Addressing this challenge is crucial for the development of nonlinear nanophotonics. Here, we show that atomically thin, epitaxially grown crystalline silver films circumvent these restrictions through the interplay of vertical electronic
quantum confinement and lateral plasmonic enhancement. We fabricate atomically thin films that exhibit an enhanced nonlinear response associated with electronic
quantum wells, and subsequently pattern them into periodic nanoribbon and nanotriangle arrays sustaining infrared localized surface plasmon resonances. Strong near-field confinement in these structures further boosts second-harmonic generation compared to unpatterned films. Precise control over nanostructure geometry enables spectral tuning of the plasmonic resonance, and consequently, the enhanced harmonic frequency. Our findings establish an approach for activating robust second-order nonlinearities in
quantum-confined metals, where intrinsic size effects and plasmonic resonances act synergistically. The compatibility of high-quality epitaxial growth with microchip fabrication technology offers a scalable route toward ultracompact nonlinear optical components for on-chip frequency conversion, sensing, and
quantum photonic applications.
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All authors Saad Abdullah, Philipp K. Jenke, Andrew P. Weber, Álvaro Rodríguez Echarri, Vahagn Mkhitaryan, Fadil Iyikanat, Frederik Schiller, Philip Walther, J. Enrique Ortega, Lee A. Rozema, F. Javier García de Abajo
Materials Science
Pressure-induced s,p-d electron redistribution accompanies structural transformation in amorphous Zr-Cu alloy under compression
P. Dziegielewski, J. Antonowicz, K. Georgarakis et al. (+6 more)
Extreme compression can reorganize atomic valence states, in some regimes driving charge from spatially extended orbitals into more compact ones. We investigate this effect in amorphous Zr67Cu33 metallic glass by combining high-pressure X-ray absorption fine structure measurements up to 69 GPa with molecular dynamics simulations and density functional theory calculations. The Zr K-edge XANES spectra show a pronounced increase in the pre-edge intensity and a change in its pressure dependence, whereas the Cu response is markedly weaker. Molecular-dynamics simulations reproduce the measured EXAFS spectra and provide reliable input for the electronic-structure calculations. The calculated Mulliken populations reveal pressure-induced depletion of Zr s and p-states, accompanied by an increase of the d-state population, while the density of states indicates enhanced p-d hybridization under compression. These results provide experimental and computational evidence for pressure-induced s,p-d electronic redistribution in zirconium and link this effect to the previously reported anomalous structural
evolution of the compressed amorphous
alloy.
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All authors P. Dziegielewski, J. Antonowicz, K. Georgarakis, O. Lord, M. Amboage, D. Daisenberger, L. Clough, T. Irifune, T. Shinmei
Disordered Systems and Neural Networks · Cross-listed from cs.LG
First Learn, Then Memorize: The Spectral Bias of Diffusion Models
Raphaël Urfin, Tony Bonnaire, Giulio Biroli et al. (+1 more)
Diffusion models trained on a finite
dataset first learn to generate novel, high-quality samples and only much later collapse onto their training set. We identify the mechanism behind this separation of timescales and the object that probes it. The training dynamics of the score function are governed—exactly, and at any width—by the Gram
matrix of the Neural Tangent Kernel (NTK) evaluated on the noisy training data, so the timescales of generalization and of memorization must be encoded in its spectrum. We show that they are, and that the structure responsible has no analogue in standard kernel settings. The use of multiple noise realizations per sample ($m$ noised copies at a fixed noise level) in the score-matching loss is what restructures the Gram
matrix spectrum into two distinct parts. The first, of large
eigenvalues, carries the global features of the target distribution and is present already for $m=1$. The second, which the repeated noising creates, consists of the smallest
eigenvalues and is supported on eigenvectors aligned with the sample-specific noise directions; it sets a memorization timescale parametrically larger in the training set size $n$. We establish this picture on two fronts. Analytically, we solve the spectrum in the lazy high-dimensional limit for both linear ($n \asymp d$) and
polynomial ($n \asymp d^k$) sample complexities, and prove through a bias–variance decomposition that the first bulk minimizes the approximation error while the second drives the error associated with memorization. Empirically, we show the same two-bulk structure in Convolutional NTKs on CelebA and in finite-width U-Nets trained well beyond the lazy regime, and we make the link causal: truncating the Gram
matrix at rank $r$ tunes the generalization–memorization transition, and an $L_2$ penalty targeting the second bulk suppresses memorization in feature-learning U-Nets.
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All authors Raphaël Urfin, Tony Bonnaire, Giulio Biroli, Marc Mézard
Mesoscale and Nanoscale Physics
Plasmon modes in tilted three-dimensional nodal-ring semimetals. I. $\mathcal{PT}$-symmetric nodal ring and its gapped cousin
Ipsita Mandal
We study collective charge excitations in three-dimensional $\mathcal{PT}$-symmetric nodal-ring semimetals (PTNRs) and their gapped counterparts (GNRs) in the presence of band tilt, together with Weyl semimetals (WSMs). Within a low-energy two-band description, we consider untilted systems as well as in-plane and axial tilts. Using the random-phase approximation, we obtain the density of states, Drude weights, dielectric functions, and plasmon dispersions at low doping and in the intrinsic limit. The mass gap and the two inequivalent tilt directions generate distinct Fermi-surface
topologies and thereby control the formation and damping of plasmons. By comparing the collective modes with the particle-hole continuum, we identify the regimes in which plasmons remain well defined. We also calculate the response of untilted and tilted WSMs and contrast it with that of PTNRs and GNRs. The two classes of semimetals differ in the carrier-density dependence and isotropy of the plasmon frequency as well as in interband screening and their response to tilt. These differences reflect their distinct Fermi-surface geometries and interband structures. Our results establish how mass and tilt govern collective charge dynamics through their interplay with intraband spectral weight and interband particle-hole excitations.
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Materials Science
Machine-learning-guided exploration of domain walls in the hybrid improper ferroelectric Ca3Ti2O7
Ida C. Skogvoll, Erik Fransson, Leo Ö. Westin et al. (+4 more)
Ruddlesden-Popper phases are highly tunable and naturally layered structures, in which polarization can arise via a hybrid improper ferroelectric mechanism. This enables a complex domain wall (DW) structure where multiple order parameters, like octahedral rotations, polar distortions and strain, interact. In this work, we explore the rich set of DW structures in prototypical Ca3Ti2O7, mapping out the DWs in the {100}, {110} and {001} pseudo-tetragonal planes using group theory and machine-learned interatomic potentials (MLIPs). The trained potential reproduces the density functional theory (DFT) order parameter and polarization profiles for all wall types and orientations considered. A charge-aware training framework combined with reference Born effective charges reduces the prediction errors for the DW formation energies by 70%, revealing the importance of including long-range electrostatics to model
symmetry-broken interfaces. Finally, the MLIP is used to identify minimum energy pathways at the atomic scale with nearly the precision of DFT calculations, revealing a low-energy antipolar configuration for polarization switching.
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All authors Ida C. Skogvoll, Erik Fransson, Leo Ö. Westin, Benjamin A. D. Williamson, Nicholas C. Bristowe, Sverre M. Selbach, Paul Erhart