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Papers

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Physics Condensed Matter

cond-mat Watch Condensed Matter

Wednesday 5 August

Materials Science

An Exchange-Correlation Functional for Fast and Accurate Modeling of Ferroelectric Perovskites

Owain T. Beynon and Chiara Gattinoni

We present a novel exchange correlation functional, C09x-PBEc, which combines C09 exchange with PBE correlation, to accurately model the ferroelectric properties of perovskites while retaining the computational efficiency of GGA functionals. With a growing interest in developing machine learning interatomic potentials (MLIPs) to model large-scale ferroelectric systems of technological relevance, it is important to scrutinise the density functional theory exchange-correlation functionals which are used to compute the forces, energies and stresses the MLIP is trained on. Using the example of the prototypical ferroelectrics lead titanate, PbTiO3, and barium titanate, BaTiO3 we show that many widely used functionals tend to overestimate their lattice constants and spontaneous polarization. Conversely, non-local van der Waals functionals with C09 exchange accurately capture these properties compared to experiment, but with a larger computational overhead than, for example, GGA. We show that C09x-PBEc combines the accuracy provided by the C09 exchange with the computational affordability of GGA, making it an excellent candidate to be used in the training of MLIPs for ferroelectric perovskites. We also demonstrate that an MLIP trained using C09x-PBEc accurately reproduces the ferroelectric-to-paraelectric phase transition temperature of PbTiO3 with respect to experiment, showing a marked improvement on MLIPs trained using GGA.
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Superconductivity

In search of novel ductile superconductors

Yiming Zhang and Samuel Poncé

We performed a first-principles high-throughput screening of the mechanical properties of phonon-mediated superconductors selected from the recent experimentally synthesized superconducting materials database PRX Energy 4, 033012 (2025). We developed the workflows that combine first-principles calculations of elastic constants and generalized stacking fault energies to assess the ductility of superconducting candidates. Starting from the 250 materials identified with promising superconducting critical temperatures, we computed their elastic tensors to evaluate bulk and shear moduli, Pugh's ratio, and Pettifor's ratio from first principles. To further characterize their plastic deformation behavior, we calculated the stacking fault energy and surface energy for selected materials and slip directions, allowing the estimation of Rice's ratio and ductility indicators. We found that several new materials simultaneously exhibit high-T$_c$ and ductility including HfPd$_2$Al, TiRuSb, and ZrNi$_2$Ga with predicted isotropic T$_c$= 6.80K, 12.88K, and 8.23K, respectively. This work offers a quantitative mapping of mechanical performance across a wide range of superconductors and provides a reference to identify new mechanically promising superconductors.
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Strongly Correlated Electrons

Lifting the degeneracy of quantum spin liquid phase by uniaxial pressure

Shams Sohel Islam, Zurab Guguchia, Orion Gerguri et al. (+13 more)

We report muon spin relaxation/rotation ($\mu$SR) measurements of the candidate three-dimensional (3D) quantum spin liquid (QSL) PbCuTe$_2$O$_6$, hosting $S=1/2$ moments, under controlled in situ [110] uniaxial compression up to $\sigma_{[110]}=37.7$~MPa. A small directional lattice perturbation significantly modifies the local magnetic response, while above $\sigma_{\rm cr}\sim10.8$\,MPa the relaxation rates are strongly enhanced and the internal-field distribution is substantially broadened. These changes occur along with the local crystalline symmetry breaking. While, no evidence for conventional static long-range magnetic order is observed, the compression drives the system towards a structurally modified and strongly correlated state in which enhanced quasi-static correlations coexist with persistent slow spin dynamics. This work demonstrates a clean and symmetry-selective route to control frustrated exchange landscape and access hidden magnetic instabilities in a 3D QSL candidate opening up the possibilities to tune other correlated systems where intrinsic coupling between magnetic and lattice degrees of freedom are relevant.
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All authors Shams Sohel Islam, Zurab Guguchia, Orion Gerguri, Petr Král, Maxime Lamotte, Toni Shiroka, Jonas A. Krieger, Thomas J. Hicken, Tina Arh, Gediminas Simutis, Abanoub Hanna, Nazmul Islam, Bella Lake, Hubertus Luetkens, Hans Henning Klauss, Rajib Sarkar

Materials Science

A phase field model of coupled crack and dislocations: emission, blunting, and the necessity of dissipative toughening

Khanh Chau Le and Thi My Kieu Tran

We propose a phase field model of a macrocracked single crystal in which the crack and the geometrically necessary dislocations descend from a single energy functional. Energy minimization alone then decides dislocation nucleation, through an integral criterion evaluated in closed form along slip chords. The criterion yields a size effect inaccessible to point-wise strength conditions: a grain-size-dependent yield stress. With slip suppressed the model reproduces Griffith fracture; with fracture suppressed, the nucleation load measured by the full non-smooth solver agrees with the closed-form nucleation criterion to four percent. The coupled computations produce a two-stage response: at loads an order of magnitude below cleavage, dislocation bands emitted from the notch tip blunt and shield it, raising the initiation load; once the crack grows, the bands heal; a compact cluster of like-signed dislocations travels with the tip, its canceling partner walls pinned at the grain boundary, and the dissipated fracture resistance equals the elastic one. In the purely energetic, dissipationless limit, emission shields the crack but does not toughen it; toughening requires dissipation, incorporated in the sequel through the threshold resistance to dislocation motion.
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Disordered Systems and Neural Networks

Emergent Replica Clock Unifies Many-Body Localization and Thermalization

Tong Liu

Many-body localization (MBL) and eigenstate thermalization (ETH) are traditionally distinguished by a collection of separate diagnostics, not by a single order parameter. We show that replication and folding of isolated unitary dynamics generate a complex statistical mechanics of forward–backward history pairings and, in a closed cyclic infrared sector, an emergent clock $r_x\in\mathbb Z_t$. Its correlator has three distinct asymptotics: exponential decay in MBL, scale-free decay at criticality, and long-range locking in the thermal phase. A controlled $l$-bit reduction makes the clock action quasi-local, while its inverse correlation length obeys $\xi_{\rm pair}^{-1}=\kappa_{\rm el}=\ln|\rho_0/\rho_q|$, unifying history coherence, the defect line tension, and the transfer spectrum. Replica order supplies a second, discrete coordinate—the first order at which a hidden dynamical invariant becomes visible—and can distinguish localized dynamics that share the same spatial correlation length.
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Materials Science

Unifying microscopic theories for the phono-magnetic effect

Natalia Shabala, Finja Tietjen, Ylva Liljegren et al. (+1 more)

Phonon angular momentum induces an effective magnetic field, a phenomenon called phono-magnetic effect, which has been measured to vary significantly in size. Here, we compare three approaches for deriving the effective magnetic field of a phonon, using electron-phonon coupling and orbital magnetism. The adiabatic approach assumes a slow ionic motion, keeping electrons in the ground state. The perturbative approach treats the electron-phonon interaction as a perturbation to the electronic ground state, and the Floquet approach is based on the time-periodicity of the circular ionic motion. We show that all three approaches lead to the same effective Hamiltonian in the low-frequency limit, which moves us closer towards a unified theory of the phono-magnetic effect. Furthermore, we identify two phononic contributions to the sample magnetization, spontaneous and induced. Thus, we clarify the role of the effective magnetic field in the phonon-induced magnetization. We conclude by providing a numerical estimate for the effective magnetic field in SrTiO$_3$.
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All authors Natalia Shabala, Finja Tietjen, Ylva Liljegren, R. Matthias Geilhufe

Statistical Mechanics

Residual Saturation under Pressure-Controlled Drainage

Fernando Alonso-Marroquin and Hans J. Herrmann

Here, pressure-controlled drainage is formulated as bond percolation with trapping on the pore-network graph, establishing a direct connection between percolation theory and pressure–saturation relations. In two dimensions, the deviation of the residual saturation from its non-vanishing thermodynamic limit obeys a finite-size scaling law with exponent $\delta \approx 0.25$, independent of microscopic details of the lattice. In three dimensions, finite-size corrections decay more rapidly ($\delta \approx 0.75$), while the asymptotic residual saturation remains finite and depends on coordination number. This extends the standard invasion-percolation picture beyond the breakthrough state, where the invading cluster is fractal and the invaded-phase saturation vanishes in the infinite-size limit.
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Materials Science

Atomic Scale Ordering of Sulfur Vacancies Enhances Charge Transport in Monolayer MoS$_2$

Alessandro Pecchia, Andrea Lorenzoni, Alexander Croy et al. (+2 more)

Defect engineering in two-dimensional semiconductors has primarily focused on controlling the nature and concentration of atomic defects. Here, we show that the spatial arrangement of defects can be equally decisive in determining electronic transport. Using sulfur vacancies in monolayer MoS$_2$ as a model system, we investigate the impact of vacancy ordering through density functional theory, density functional tight-binding calculations, and quantum transport simulations. We demonstrate that a periodic vacancy arrangement at a concentration of 11.1% transforms isolated defect states into a narrow dispersive in-gap miniband, whereas randomly distributed vacancies generate only localized electronic states. This electronic transition fundamentally alters charge transport, enabling band-like propagation through the defect network rather than transport limited by disconnected localized states. A systematic analysis of the complete symmetry-reduced ensemble of 94 non-adjacent four-vacancy configurations shows that the ordered pattern lies within a broad low-energy manifold and is not energetically anomalous, although it is not the thermodynamic ground state. Device-level simulations of Au/MoS$_2$/Au junctions reveal efficient alignment of the metal Fermi level with vacancy-derived states, promoting charge injection into the defect miniband. As a result, ordered vacancy arrays exhibit electrical currents up to five orders of magnitude higher than statistically equivalent random distributions and can approach, or locally exceed, the transport performance of pristine MoS$_2$. These findings establish atomic-scale defect ordering as a powerful design principle for two-dimensional materials, demonstrating that the organization of defects, beyond their concentration alone, provides a route to simultaneously preserve functionality and high electrical conductivity in highly defective semiconductors.
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All authors Alessandro Pecchia, Andrea Lorenzoni, Alexander Croy, Francesco Mercuri, Massimiliano Cavallini

Materials Science

Performance Analysis of Double Perovskite-Based Solar Cells Using SCAPS-1D Simulation: A brief review

H. Laltlanmawii, Lalrem Kima, Mahabur Rahman et al. (+7 more)

Lead-free double perovskites are among the rapidly developing next-generation solar cell technologies, providing the required low toxicity, stability, as well as high optoelectronic potential. So far, experimentally prepared lead-free perovskite solar cell devices are reported to have low power conversion efficiency (PCE) for practical application as compared to the lead-based perovskites. In recent years, numerical simulations have emerged as a cost-effective approach that plays a crucial role in expediting scientific research, can bridge the gap between experiment and theory, and provide predictive information regarding the preparation of solar cells and their PCEs without undergoing real-time experiments. The tools, such as 1D numerical simulation software SCAPS-1D, are now needed to test newer architectures and determine what exactly is holding them back. So far in the field of solar cell research, SCAPS-1D has been extensively used and looks like a powerful software due to its user-friendliness and simulation of results in a few seconds. The speed and ease of simulation make SCAPS-1D a very popular tool; as a result, it enables rapid optimization of a large number of photovoltaic devices and their performances without undergoing any experimental work, which can save time and money. However, one serious drawback is that the SCAPS-1D simulator works only for 1D configurations. It is ineffective in incorporating atomistic interactions and 3D effects. Hence, the efficacy of the SCAPS-1D simulator solely relies on the accuracy of the input parameters that the user provides, failing which may give wrong results and large deviations from accuracy.
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All authors H. Laltlanmawii, Lalrem Kima, Mahabur Rahman, Md. Ferdous Rahman, Dilshod Nematov, S. Bhattarai, C. V. M. Chaturvedi, Yazen M. Alawaideh, A. Laref, D. P. Rai

Statistical Mechanics

An improved car-oriented mean-field theory for stochastic traffic flow models

Yasar Efe Dai, Andreas Schadschneider and Michael Schreckenberg

We propose an improved mean-field analysis of cellular automata models of single-lane vehicular traffic. By combining aspects of the Car-Oriented-Mean-Field (COMF) theory and the 2-site cluster method, which have been previously successfully applied to similar models, we aim to capture both short- and long-range correlations more accurately. In contrast to classical mean-field theories, the improved method is well suited for models with inhomogeneous stationary states and able to capture the essential properties of phase separation, e.g. in models with slow-to-start rules. The improved accuracy and new physical insights are illustrated through an application to the VDR model with $v_{\text{max}}=1$.
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Materials Science

Nanomechanics of MXene flakes at gold interfaces

Pierluigi Bilotto, Sabine Schwarz, Marko Piljevic et al. (+3 more)

The demand for novel self-powering and sustainable electronics requires major efforts in identifying new advanced materials for nano-applications. MXene have gathered attention due to their electronic and mechanical properties, however, their nanomechanical compliance against a metal-like interface is still not clearly identified. In this work, we employ atomic force microscopy to characterize the nanomechanical properties of a self-assembled thin flake of titanium carbide MXene (Ti3C2Tx) against a gold probe. The investigation returns an interfacial shear stress of 399 MPa, and the observation of nano-wear localized in the center of the MXene flake. Nevertheless, MXene flakes retained their crystallinity in the tribofilms as confirmed by transmission electron microscopy and electron energy loss spectroscopy. The outcomes of this work set the basis for the use of Ti3C2Tx in novel nano harvesting systems involving metal interfaces (e.g., tribovoltaic nanogenerators) with large scope in nanoelectronics, wearable sensing, electric vehicles, and robotics.
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All authors Pierluigi Bilotto, Sabine Schwarz, Marko Piljevic, Iago Peters, Michael Stöger-Pollach, Carsten Gachot

Statistical Mechanics · Cross-listed from quant-ph

Single-eigenstate test of eigenstate thermalization hypothesis via perturbed eigenstate quench

Zhouhao Guo and Jiaju Zhang

We propose and numerically validate an efficient single-eigenstate diagnostic for the eigenstate thermalization hypothesis (ETH) based on a perturbed eigenstate quench protocol. By introducing a weak random perturbation to an energy eigenstate to break its stationarity, we characterize the time-averaged subsystem evolution speed as a function of the subsystem-to-total system size ratio. The diagnostic relies on a robust qualitative distinction: eigenstates satisfying ETH exhibit an S-shaped curve with a clear inflection point near half the system size, while ETH-violating eigenstates display a convex J-shaped profile. We benchmark the criterion across paradigmatic one-dimensional spin chains covering chaotic, integrable, many-body localized, and quantum many-body scar regimes, obtaining full agreement with established thermalization phenomenology. Our method circumvents the need for explicit thermal ensemble construction, providing a robust, experimentally feasible probe of eigenstate thermalization at the single-eigenstate level.
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Materials Science

Raman Signatures of Lithium Ion Dynamics in LLZO Garnet Electrolytes: Atomistic Insights from MD-Raman Calculations

Takeru Miyagawa, Willis O'Leary, Manuel Grumet et al. (+4 more)

Lithium lanthanum zirconate (LLZO) garnets are among the most promising solid electrolytes for next-generation batteries owing to their high ionic conductivity, chemical stability, and compatibility with lithium metal. Raman spectroscopy is commonly employed to distinguish the highly conductive cubic phase from the poorly conductive tetragonal phase of LLZO, yet the atomistic origin of these spectral differences and their direct connection to Li-ion transport remain unresolved. Here, we close this gap by comparing computed and experimental Raman spectra for the tetragonal, cubic, and Ta-doped variants of LLZO, with the computed spectra obtained from the MD-Raman approach that combines machine-learning molecular dynamics with first-principles polarizability calculations. We show that the contrasting ionic transport behavior across these LLZO variants is encoded in the vibrational dynamics of the lithium sublattice and gives rise to distinct features in their Raman spectra. A symmetry-resolved analysis further reveals that experimentally observed Raman peaks do not correspond to individual normal modes, but instead arise from overlapping contributions of multiple symmetry-allowed vibrations, challenging conventional peak-assignment approaches. By explicitly connecting experimentally accessible Raman signatures to the underlying atomic-scale dynamics, our results show how Raman spectroscopy can move beyond empirical phase identification toward a microscopic probe of Li-ion dynamics in lithium garnet electrolytes.
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All authors Takeru Miyagawa, Willis O'Leary, Manuel Grumet, Hyunwon Chu, Jennifer L.M. Rupp, Waldemar Kaiser, David A. Egger

Materials Science · Cross-listed from cs.LG

Active Learning Guided Design Space Refinement for Scalable Multi-Objective Bayesian Optimization in Materials Discovery

Alexandros Ntagiantas, Panagiotis Tsilimidos, George Giannakopoulos et al. (+2 more)

Advanced materials discovery increasingly relies on machine learning and Bayesian optimization to explore large discrete design spaces under limited evaluation budgets. However, conventional Bayesian optimization (BO) can become inefficient as candidate spaces grow, often evaluating low-value regions before reaching informative areas. We propose an active-learning (AL)-guided adaptive search-space refinement framework combined with multi-objective BO to accelerate materials optimization while preserving Pareto-relevant regions. We evaluate the approach on CH4/N2 separation in covalent-organic frameworks and pressure-vessel design with material-direction stress components and thickness objectives. Results show that the AL-guided refinement reduces the candidate space by approximately half while preserving more than 99 percent of the original hypervolume. The reduced-space strategy improves early convergence and cumulative Pareto-front discovery from the BO, demonstrating efficient large-scale materials optimization across constrained autonomous materials discovery settings.
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All authors Alexandros Ntagiantas, Panagiotis Tsilimidos, George Giannakopoulos, Christoforos Rekatsinas, Panagiotis Krokidas

Materials Science

Nanosecond timescale plasticity in shock-compressed polycrystalline MgO: evidence for transition in mechanism above 100 GPa

A. Chakraborti, H. Ginestet, F. Bertrand et al. (+80 more)

The mechanical properties of ceramics under extreme conditions directly impact applications ranging from shielding spacecrafts, designing plasma facing materials in nuclear fusion to understanding the rheology of deep planetary interiors. Here, we use polycrystalline MgO as a model ceramic to understand the high-pressure-temperature mechanical behaviour of such materials under extreme strain rates. We use laser-driven shock compression up to 175(15) GPa on the principal Hugoniot along with ultrafast diagnostics at the European X-ray Free Electron Laser to probe the dominant deformation mechanisms with changing P -T conditions. These near-instantaneous time-resolved snapshots, coupled with elasto-viscoplastic self-consistent (EVPSC) simulations, strongly suggest that MgO attains plastic regime in the nanoseconds scale accompanied by a pressure-mediated change in dominant slip system between 95 and 175 GPa. This work provides a new direct window into the deformation dynamics of polycrystalline ceramics under high-velocity impacts.
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All authors A. Chakraborti, H. Ginestet, F. Bertrand, J. Chantel, S. Merkel, M. Harmand, S. Pandolfi, A. Amouretti, M. Andrzejewski, K. Appel, E. Barraud, A.B. Belonoshko, E. Brambrink, K. Buakor, C. Camarda, O. Castelnau, D.M. Cheshire, G. Collins, T.E. Cowan, C. Crépisson, J. Deng, X. Fang, M. Fitzgerald, A. Gleason, F. Hanby, N.J. Hartley, P.G. Heighway, H. Höppner, N. Jaisle, J. Kim, Z. Konopkova, D. Kraus, A. Krygier, L. Libon, C.M. Lonsdale, S-N. Luo, W. Lynn, M. Masruri, E.E. McBride, D. McGonegle, J.D. McHardy, M.I. McMahon, R.S. McWilliams, T. Michelat, B. Nagler, M. Nakatsutsumi, A-M. Norton, I. Ocampo, I.I. Oleynik, C. Otzen, S.E. Parsons, D.J. Peake, A. Pelka, A. Phelipeau, C. Prescher, T.R. Preston, N. Pulver, L. Rogal, J-P. Schwinkendorf, G. Shoulga, R.F. Smith, S. Singh, C.N. Somarathna, T. Stevens, C.V. Storm, C. Strohm, T-A. Suer, M. Tang, A. Tipeev, M. Toncian, T. Toncian, U. Trdan, T. Tschentscher, J.D. Umpleby-Thorp, L. Wang, J.S. Wark, A. Descamps, A. Higginbotham, T.M. Hutchinson, C. McGuire, A. Sollier, G. Morard, J.H. Eggert

Materials Science

The Frequency-Dependent Spin Contribution to the Magnetoelectric Tensor of Cr$_2$O$_3$: A First-Principles Study

Torsten Geirsson, Davide Sangalli, Alberto García-Cristóbal et al. (+1 more)

The magnetoelectric (ME) effect provides a promising pathway for controlling magnetic functionalities using electric fields. While first-principles methods for the static linear ME response are well established, comparable approaches for the frequency-dependent response remain less developed, despite experiments showing pronounced finite-frequency resonances. Here, we investigate the dynamical spin-induced linear ME response from first principles and systematically compare the independent-particle approximation (IPA), random-phase approximation (RPA), time-dependent density functional theory (TDDFT), and the Bethe-Salpeter equation (BSE). We apply these methods to the prototypical ME material Cr$_2$O$_3$ and compare the results with available experimental and theoretical studies. We find that the IPA and RPA fail to reproduce the previously reported finite static limit of the spin-induced response. Within the BSE framework, pronounced excitonic resonances emerge in the ME spectrum, in qualitative agreement with experiment. We also identify a magnon-like peak that coincides with a pole of the transverse spin susceptibility while remaining essentially dark in optical absorption, highlighting the sensitivity of the ME response to spin excitations. TDDFT places this mode closer to the expected low-energy magnonic regime and yields a sizable static spin-induced response. Our results show that these frameworks capture complementary aspects of the dynamical ME response. Low-energy collective spin excitations are required to recover the static limit, whereas electron-hole interactions are essential for reproducing the excitonic resonances.
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All authors Torsten Geirsson, Davide Sangalli, Alberto García-Cristóbal, Alejandro Molina-Sánchez

Soft Condensed Matter

Structural and dynamical behavior of methane-water systems under nanoconfinement

José Torres-Arenas, Ángel M. Fernández-Fernández, Martín Pérez-Rodríguez et al. (+1 more)

We investigate the structural and dynamical behavior of methane water systems under nanoconfinement using molecular dynamics simulations across pore widths from 1 to 5 nm. Structural analysis reveals a strong and nonmonotonic dependence on confinement: while tetrahedral ordering partially recovers as confinement is reduced, cubic like order associated with clathrate precursors is maximized at intermediate pore sizes. Radial distribution functions show that three dimensional correlations are suppressed under strong confinement, whereas lateral ordering persists, indicating a reduction in the effective dimensionality of structural organization. Transport properties reflect the same structural competition. Parallel diffusion is nonmonotonic with pore size, while perpendicular motion is subdiffusive due to confinement induced trapping and heterogeneity. Methane exhibits stronger subdiffusion and remains dynamically coupled to the water matrix. A characteristic confinement length scale emerges at which structural ordering, dynamical heterogeneity, and solvent solute decoupling are simultaneously maximized. At strong confinement, three dimensional correlations are suppressed, leading to dimensional reduction, frustrated ordering, and inhibited nucleation.
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All authors José Torres-Arenas, Ángel M. Fernández-Fernández, Martín Pérez-Rodríguez, Manuel M. Piñeiro

Mesoscale and Nanoscale Physics

Methods for traceable scanning magnetometry using single nitrogen vacancy centers in diamond: determining orientation, distance and localization

Nikhita Khera, Ephraim Spindler, Yanis Abdedou et al. (+7 more)

Individual, scannable nitrogen vacancy (NV) centers in single crystal diamond nanostructures enable nanoscale, quantitative imaging of magnetic stray fields. Nevertheless, important parameters like distance between the NV center and the sample and the orientation of the NV high symmetry axis are often not known precisely and enter data evaluation as free fitting parameters. We here use scanning NV imaging on micro-patterned, perpendicularly magnetized stripes and discs. From these measurements, we directly infer NV - sample distance d_NV and the NV's azimuthal orientation without the need for an external vector magnet control. We determine d_NV = 31.5 nm, while we infer the azimuthal orientation with a precision of 3{\deg}. We additionally employ commercially available silicon needles to image the apex topography of our diamond nanostructures to detect surface contamination. Simultaneously, monitoring NV fluorescence as a function of the needle's position allows us to estimate the lateral placement of the NV inside the diamond nanostructure.
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All authors Nikhita Khera, Ephraim Spindler, Yanis Abdedou, Marcel Gasser, Sandra Wolff, Bert Lägel, Robert Frömter, Mathias Weiler, Mathias Kläui, Elke Neu

Statistical Mechanics

Analytical Floquet Quantum Statistics from Nonequilibrium Green's Functions

Yuhua Ren, Gaomin Tang, Hui Pan et al. (+1 more)

We derive an analytical expression for the steady-state quantum statistics of periodically driven quantum systems coupled to a bath using the nonequilibrium Green's function (NEGF) formalism. By embedding Floquet theory into NEGF, we obtain closed expressions for the retarded, advanced, and lesser Green's functions in the Floquet representation, yielding the Floquet Fermi distribution in which the steady-state occupation is expressed as a weighted sum of Fermi functions shifted by integer multiples of the driving frequency. The weights are determined solely by the Fourier components of the micromotion operator, providing a transparent interpretation of Floquet sideband occupations. Our analysis extends beyond the diagonal commuting Hamiltonians treated in earlier work, and further shows that the robust Floquet distribution remains valid for a broad class of weakly coupled bath spectral functions beyond the ideal featureless-bath approximation. Finally, we establish a Floquet version of the Landauer formula for the DC part of the current, in which the equilibrium Fermi functions are replaced by their Floquet-modified counterparts. Together, these results provide a coherent description of Floquet quantum statistics and transport in periodically driven open quantum systems.
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All authors Yuhua Ren, Gaomin Tang, Hui Pan, Jian-Sheng Wang

Strongly Correlated Electrons

Wavepacket Approach for Spin Transport in Zigzag Spin Chain

Hiroaki Onishi

We study the spin transport property of a spin nematic liquid in a frustrated zigzag spin chain in a magnetic field from the perspective of the time evolution of wavepackets by a time-dependent density-matrix renormalization group method. We use the periodic boundary condition to avoid an edge-induced magnetization structure in the open boundary condition. We find that at the saturation, a magnon-pair wavepacket of momentum k=pi stays localized, since the gapless dispersion due to the antiferro-quadrupole quasi-long-range order has a rather flat structure, indicating zero propagation velocity.
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Strongly Correlated Electrons · Cross-listed from hep-th

Critical and multicritical Kasner scaling in holographic phase transitions

Ling-Long Gao, Yan Liu and Hong-Da Lyu

We study how holographic phase transitions imprint their scaling laws on the local Kasner geometry inside Einstein-scalar black holes. At fixed double-trace coupling, we derive the near-critical scaling of the deviation of the first-epoch Kasner exponent from the Schwarzschild value: $p_t^{(1)}+\frac{1}{3} \propto O^2 \propto (T_c-T)^{1/r}$, where $O$ denotes the boundary condensate and $r$ labels ordinary criticality ($r=1$), tricriticality ($r=2$), and higher multicriticality ($r\geq3$). A super-exponential scalar potential generates a sequence of Kasner epochs separated by scalar-field bounces. We further find that any later epoch that can be tracked continuously across the transition inherits the same temperature exponent, while its coefficient depends on the epoch. Numerical solutions confirm these predictions for the ordinary and tricritical cases. Finally, we show analytically that the leading irrelevant exponent of the infrared fixed point governs the low-temperature Kasner scaling, and verify the resulting scaling numerically for the first Kasner exponent.
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Superconductivity

Microscopic theory of the field-induced instability of the vortex-free state in superconducting thin-film strips

Takayuki Kubo

In the Pearl–London theory, the edge-barrier-disappearance field of a superconducting thin-film strip depends on an arbitrary short-distance core cutoff because the vortex is treated as a point object. The theory does not determine the cutoff or how it depends on temperature $T$, and therefore cannot determine the $T$ dependence of the instability field. Here we formulate the microscopic stability problem directly for the vortex-free superconducting state. This removes the core-cutoff ambiguity and determines the instability field $B_s$ over the full temperature range and across all width regimes considered here. For a homogeneous dirty strip with negligible self-field, three width regimes occur. For $W<W_1(T)$, superconductivity disappears continuously into the normal state through a one-dimensional (1D) instability. For $W_1(T)<W<W_2(T)$, an edge-selective two-dimensional (2D) long-wavelength mode becomes unstable. For $W>W_2(T)$, the critical wave number is finite and the unstable mode is localized near an edge. In the wide-strip limit, $B_s\propto1/W$, recovering the Pearl–London scaling. In sufficiently narrow strips, however, the Pearl–London edge-barrier picture fails qualitatively.
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Statistical Mechanics

The two-particle-irreducible vertex of the two-dimensional lattice $\phi^4$ model across the Ising transition

Lode Pollet

We reconstruct the 2PI vertex $\Gamma(k,p;q)$ from Monte Carlo measurements of the connected two-particle correlator for the two-dimensional single-component $\phi^4$ lattice field theory and follow it across the Ising transition. Resolving the vertex in the irreducible representations of the point group $C_{4v}$, we find that the instability is driven by the $A_1$ (ferromagnetic) channel at zero transfer, whose leading eigenvalue of the symmetrized Bethe–Salpeter kernel approaches unity. Substantial $B_1$ (nematic) and $B_2$ (diagonal nematic) contributions cooperate with $A_1$ across all system sizes, highlighting that the soft sector is multidimensional. In real space, the vertex is short-ranged away from criticality while it develops a power-law tail at the critical point. In the ordered phase, the $q=0$ eigenvalue collapses because the ferromagnetic weight has condensed into the (one-particle-reducible) order parameter (or collective coordinate for a finite system), although finite-momentum fluctuations persist. By stripping the crossed-channel ladders, we obtain the fully irreducible vertex, which is a local contact – to a very good approximation. Inserted into the parquet and Schwinger–Dyson equations, this contact reproduces the Monte Carlo self-energy with an accuracy better than one-tenth of a percent. This provides a first-principles benchmark of the dynamical local-vertex approximation (D$\Gamma$A). Additionally, we demonstrate that in the critical region, the physical solution of the parquet equations behaves as a repulsive fixed point, driven initially by a single order-parameter mode.
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Materials Science

Nano-scale visualization of magnetic vortices in metal nanoparticles

Satoko Toyama, Yoshiki O. Murakami, Ayako Nishikawa et al. (+4 more)

Magnetic vortices in individual nanoparticles are fundamental spin structures that govern the properties of next-generation magnetic devices and biomedical applications. However, directly imaging their complete structure, from the circulating in-plane magnetization to the nanometer scale out-of-plane core, remains challenging. Here, direct and quantitative visualization of magnetic vortex structures in individual cobalt nanoparticles is achieved by integrating a time-reversal methodology with tilt-scan-averaged differential phase contrast scanning transmission electron microscopy in a magnetic-field-free environment. This approach enables magnetic imaging in conjunction with atomic-scale analysis and reveals a correlation between particle geometry and internal demagnetizing fields. In addition, dynamic evolution of the vortex under in situ magnetic-field application is tracked, enabling unambiguous determination of the out-of-plane core polarity. This approach provides a powerful platform for correlating atomic-scale structure and magnetism within individual nanoparticles and for understanding the origins of nanoscale magnetic properties, thereby supporting the rational design of advanced nanomagnetic materials and devices.
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All authors Satoko Toyama, Yoshiki O. Murakami, Ayako Nishikawa, Takehito Seki, Akihito Kumamoto, Yuichi Ikuhara, Naoya Shibata

Materials Science

Monolithic integration of optically anisotropic GeSe-based films on GaAs by templated solid-phase epitaxy

Kira J. Martin, Autumn Y. Lee, Pranav Mahaadev et al. (+6 more)

Layered IV-VI semiconductors such as GeSe exhibit strong in-plane optical anisotropy, making them promising candidates for polarization-sensitive photonic devices. However, realizing these properties in scalable platforms requires heteroepitaxial integration on technologically relevant substrates like GaAs. Direct growth of GeSe is complicated by its glass formation at low temperatures and high vapor pressure at elevated temperatures. To overcome this, we develop a method for ex-situ solid-phase epitaxy utilizing a SnSe buffer and offcut GaAs substrate to enable single-orientation crystalline GeSe films. Using polarized reflection measurements, we find that stabilizing a single-in-plane-orientation results in a 2x increase in anisotropic response between the armchair and zigzag directions. This work provides a new integration route to harness the anisotropic optical properties of GeSe and its alloys for polarization-sensitive technologies.
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All authors Kira J. Martin, Autumn Y. Lee, Pranav Mahaadev, Pooja D. Reddy, Kelly Xiao, Tri Nguyen, Ashlee M. García, Aaron M. Lindenberg, Kunal Mukherjee

Strongly Correlated Electrons

Zero-Clustering Geometry in Realistic Fractional Quantum Hall Wave Functions

Xin Wan, Ziang Wang, Zi-Xiang Hu et al. (+1 more)

The clustering pattern of zeros in the ground state of a fractional quantum Hall system is a defining feature of its topological properties. We analyze the geometrical fluctuations of the zeros around individual electrons and propose to use the displacement ratio of the zeros to visualize and measure the distance of a realistic state to a model wave function. The distribution of the zero displacement ratio behaves like an order parameter in the transition from a Laughlin phase to a topologically trivial one. The statistical comparison between quantum Hall states belonging to different Jain sequences leads to a composite fermion fluid description of the $\nu = 1/5$ ground state with long-range Coulomb interaction that agrees almost perfectly for as few as $3$-$5$ electrons, overcoming the long-standing difficulties of accommodating the competing liquid and crystal orders at short distances.
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All authors Xin Wan, Ziang Wang, Zi-Xiang Hu, Zhao Liu

Statistical Mechanics · Cross-listed from quant-ph

Equilibrium Thermodynamics of Non-Hermitian Dirac Fermions: Caloric and Magnetic Responses

Francisco J. Peña, Bastian Castorene, Juan Pablo Esparza et al. (+2 more)

We establish scaling relations governing the equilibrium thermodynamics of real-spectrum non-Hermitian Dirac fermions in a magnetic field. Assuming thermalization with respect to the quasi-Hermitian Hamiltonian, a similarity transformation maps the system at the same applied field onto a Hermitian Dirac model with reduced velocity, while the Landau-level spectrum also admits a representation in terms of a reduced effective magnetic field. This structure yields scaling relations for the chemical potential, entropy, heat capacities, and orbital magnetic response in different thermodynamic ensembles. At fixed projected filling factor (PFF), the self-consistent chemical potential follows the compressed ladder of Landau levels, and the canonical thermodynamic functions are rescaled Hermitian responses. At fixed chemical potential, Landau-level crossings generate oscillatory caloric and magnetic responses governed by the same spectral compression. Quasistatic non-Hermitian deformation at fixed PFF further yields adiabatic temperature scaling. More broadly, these results establish a thermodynamic framework for real-spectrum non-Hermitian quantum matter and provide a starting point for incorporating the effects of interactions and disorder within the same formalism.
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All authors Francisco J. Peña, Bastian Castorene, Juan Pablo Esparza, Vladimir Juričić, Patricio Vargas

Materials Science

Unveiling the Role of Friction in Coarse-Grained Clay: A Hybrid Framework Integrating Long-Range Interactions and Granular Contact Mechanics

Wang-Qi Xu, Yijie Wang and Zhen-Yu Yin

Given the predominant role of inter-particle physicochemical forces in governing clay behavior, researchers have increasingly utilized coarse-grained molecular dynamics (CGMD) simulations. However, inter-particle friction has been historically overlooked due to methodological limitations, and the extent to which this omission influences simulation accuracy remains an unresolved question. This study proposes a novel hybrid CGMD framework explicitly coupling long-range Buckingham potential with Hertzian granular contact mechanics. A baseline model was validated via isotropic compression, where the resulting compressibility and derived compression index (Cc) aligned with macroscopic geotechnical observations. Parametric analyses revealed that viscoelastic damping of particle contacts governs structural evolution. Elevated damping suppresses densification, trapping platelets in disorganized, high-void-ratio configurations. Furthermore, evaluating the interplay with thermal fluctuations underscores the necessity of precise temperature control to prevent such unphysical kinetic trapping. Finally, uniaxial compression tests demonstrate the critical importance of inter-particle friction. Explicit friction locks sliding interfaces and sustains significantly higher loads compared to frictionless models; the latter rely solely on geometric interlocking and ultimately exhibit unphysical fluid-like yielding. By bridging atomistic potentials with contact mechanics, this framework highlights the fundamental role of the inter-particle friction and offers essential guidelines for future multi-scale simulations of clay assemblies.
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Strongly Correlated Electrons

Quantized topological invariant of symmetry-projected Gibbs states

Weiguang Cao and Haruki Watanabe

We study Gibbs states projected onto the symmetric sector of contractible one-form symmetries. In a three-dimensional cluster-model interpolation, this projection stabilizes symmetry-protected topological and projected-paramagnetic phases, both sharply distinct from thermal disorder. A flux-twisted membrane invariant distinguishes these three phases by the values $-1,+1,0$, respectively. These values are exact on the endpoint, self-dual, zero-temperature, and infinite-temperature lines; elsewhere their quantization requires positive spatial-sheet, winding-line, and interface tensions. Quantum Monte Carlo supports the quantization through tension diagnostics and direct finite-size estimates.
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Statistical Mechanics · Cross-listed from math.PR

The rank and layer distributions in random recursive trees

Huck Stepanyants, P. L. Krapivsky, Harrison Hartle et al. (+1 more)

The distribution of node depths in a network is crucial for analyzing network structure. Two measures, rank and layer, quantify how deep inside a network a node is. The rank is the node's distance to the closest leaf, a node of degree 1. The layer is the number of times all leaves must be stripped off for the node to become a leaf. We derive exact recursive expressions for the rank and layer distributions in random recursive trees. We show that the rank and layer distributions decay factorially and geometrically, respectively, and prove the self-averaging of the numbers of nodes with fixed rank or layer. Unlike previous studies, our approach does not depend on labels or a root node, providing a more versatile framework for analyzing rank and layer distributions in complex networks.
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All authors Huck Stepanyants, P. L. Krapivsky, Harrison Hartle, Dmitri Krioukov