Disordered Systems and Neural Networks · Cross-listed from physics.comp-ph
Optimized Machine Learning Methods for Studying the Thermodynamic Behavior of Complex Spin Systems
Dmitrii Kapitan, Pavel Ovchinnikov, Konstantin Soldatov et al. (+2 more)
This paper presents a systematic study of the application of convolutional neural networks (CNNs) as an efficient and versatile tool for the analysis of critical and low-temperature phase states in
spin system models. The problem of calculating the dependence of the average energy on the spatial distribution of exchange integrals for the Edwards-Anderson model on a square
lattice with frustrated interactions is considered. We further construct a single convolutional classifier of phase states of the
ferromagnetic Ising model on square, triangular, honeycomb, and kagome
lattices, trained on configurations generated by the Swendsen-Wang cluster
algorithm. Computed temperature profiles of the averaged posterior probability of the high-temperature phase form clear S-shaped curves that intersect in the vicinity of the theoretical critical temperatures and allow one to determine the critical temperature for the kagome
lattice without additional retraining. It is shown that convolutional models substantially reduce the root-mean-square error (RMSE) compared with fully connected architectures and efficiently capture complex correlations between thermodynamic characteristics and the structure of
magnetic correlated systems.
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All authors Dmitrii Kapitan, Pavel Ovchinnikov, Konstantin Soldatov, Petr Andriushchenko, Vitalii Kapitan
Statistical Mechanics
Generalized density functional theory framework for the non-linear density response of quantum many-body systems
Zhandos A. Moldabekov, Cheng Ma, Xuecheng Shao et al. (+5 more)
A density functional theory (DFT) framework is presented that links functional derivatives of free-energy functionals to non-linear static density response functions in
quantum many-body systems. Within this framework, explicit expressions are derived for various higher-order response functions of systems that are homogeneous on average, including the first theoretical result for the cubic response at the first harmonic $\chi_0^{(1,3)}(\vec{q})$. Specifically, our framework includes hitherto neglected mode-coupling effects that are important for the non-linear density response even in the presence of a single harmonic perturbation. We compare these predictions for $\chi_0^{(1,3)}(\vec{q})$ to new Kohn-Sham DFT simulations, leading to excellent agreement between theory and numerical results. Exact analytical expressions are also obtained for the long-wavelength limits of the ideal quadratic and cubic response functions. Particular emphasis is placed on the connections between the third- and fourth-order functional derivatives of the non-interacting free-energy functional $F_s[n]$ and the ideal quadratic and cubic response functions of the uniform electron gas, respectively. These relations provide exact constraints that may prove useful for the future construction of improved approximations to $F_s[n]$, in particular for warm dense matter applications at finite temperatures. Here, we use this framework to assess several commonly employed approximations to $F_s[n]$ through orbital-free DFT simulations of the harmonically perturbed ideal electron gas. The results are compared with Kohn-Sham DFT calculations across temperatures ranging from the ground state to the warm dense regime. Additionally, we analyze in detail the temperature- and wavenumber-dependent non-monotonic behavior of the ideal quadratic and cubic response functions.
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All authors Zhandos A. Moldabekov, Cheng Ma, Xuecheng Shao, Sebastian Schwalbe, Pontus Svensson, Panagiotis Tolias, Jan Vorberger, Tobias Dornheim
Materials Science
Improving the Stability of Colloidal CsPbBr3 Nanocrystals with an Alkylphosphonium Bromide as Surface Ligand Pair
Meenakshi Pegu, Hossein Roshan, Clara Otero-Martinez et al. (+9 more)
In this study, we synthesised a phosphonium-based ligand, trimethyl(tetradecyl)phosphonium bromide (TTP-Br), and employed it in the post-synthesis surface treatment of Cs-oleate-capped CsPbBr3 NCs. The photoluminescence
quantum yield (PLQY) of the NCs increased from 60% to more than 90%, as a consequence of replacing Cs-oleate with TTP-Br ligand pairs. Density functional theory calculations revealed that TTP+ ions bind to the NC surface by occupying Cs+ surface sites and orienting one of their P-CH3 bonds perpendicular to the surface, akin to quaternary ammonium passivation. Importantly, TTP-Br-capped NCs exhibited higher stability in air compared to didodecyldimethylammonium bromide-capped CsPbBr3 NCs (which is considered a
benchmark system), retaining 90% of their PLQY after six weeks of air exposure. Light-emitting diodes fabricated with TTP-Br-capped NCs achieved a maximum external
quantum efficiency of 17.2 %, demonstrating the potential of phosphonium-based molecules as surface ligands for CsPbBr3 NCs in optoelectronic applications.
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All authors Meenakshi Pegu, Hossein Roshan, Clara Otero-Martinez, Luca Goldoni, Juliette Zito, Nikolaos Livakas, Pascal Rusch, Francesco De Boni, Francesco Di Stasio, Ivan Infante, Luca De Trizio, Liberato Manna
Soft Condensed Matter
Simple models for the trapping of charged particles and macromolecules by diffusiophoresis in salt gradients
Richard P. Sear and Patrick B. Warren
We study the trapping of charged particles and macromolecules (such as DNA) in salt gradients in aqueous solutions. The source for the salt gradient can be as simple as a dissolving ionic
crystal, as shown by McDermott et al. [Langmuir 28, 15491 (2012)]. Trapping is due to a competition between localisation due to diffusiophoresis in the salt gradient, and spreading out by
diffusion. The size of the trap is typically 1 to 100 micrometres. We further predict that at steady state, the particle (macromolecule) number density is a power law of the salt concentration, with an exponent that is the ratio of the diffusiophoretic mobility to the
diffusion coefficient of the trapped species. This ratio increases with size and typically becomes much greater than 1 for particles or macromolecules with hydrodynamic radii of hundreds of nanometres and above. Thus large particles or macromolecules are easily caught and trapped at steady state by salt gradients.
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Statistical Mechanics
Prediction and inference in complex networks: a brief review and perspectives
Francisco A. Rodrigues
Inference and prediction are fundamental to the study of complex systems, where network data are often incomplete, inaccurate or obtained indirectly. In this paper, we review recent advances in network sampling and comparison, as well as in link prediction and network reconstruction from time series. We summarise key methodological developments and emerging approaches that integrate statistical and machine learning perspectives. We also outline promising research directions for enhancing the inference and prediction of complex networked systems.
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Disordered Systems and Neural Networks
Non-Hermitian off-diagonal disordered optical lattices
E. T. Kokkinakis, I. Komis, K. G. Makris et al. (+1 more)
Within the framework of non-Hermitian photonics, we investigate the spectral and dynamical properties of one- and two-dimensional non-Hermitian off-diagonal disordered optical
lattices, where randomness is applied to the couplings rather than to the on-site potential terms. We analyze
eigenvalue distributions and the localization properties of the eigenmodes, comparing them with those of the corresponding Hermitian
lattices. Furthermore, we study their transport behavior under single-channel excitation and identify unconventional phenomena such as jumps between distant
lattice regions in systems with a purely real spectrum, as well as complex spectrum-induced Anderson jumps, reported here for the first time in two dimensions. Our results establish a reference framework for non-Hermitian off-diagonal disorder and open new directions for future studies of localization phenomena.
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All authors E. T. Kokkinakis, I. Komis, K. G. Makris, E. N. Economou
Strongly Correlated Electrons
Single-Q and Double-Q magnetic orders: A Theoretical Analysis of Inelastic Neutron Scattering in a Centrosymmetric Structure
Artem O. Nosenko and Dmitri V. Efremov
Recent discoveries of multi-Q
magnetic structures in centrosymmetric compounds have stimulated growing interest in their microscopic origin and observable properties. Here, we calculate the dynamical
magnetic structure factor for a double-Q
magnetic structure and compare it with that of a single-Q configuration. Our results demonstrate how inelastic
neutron scattering can distinguish the double-Q state from competing single-Q states in this square-
lattice setting.
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Materials Science
Multiplet structure of chromium(III) dopants in wide band gap materials
Ilya Popov, Petros-Panagis Filippatos, Shayantan Chaudhuri et al. (+3 more)
Transition metal doping is commonly used for altering the properties of solid-state materials to suit applications in science and technology. Partially filled $d$-shells of transition metal atoms lead to electronic states with diverse spatial and
spin symmetries. Chromium(III) cations have shown great potential for designing
laser materials and, more recently, for developing
spin qubits in
quantum applications. They also represent an intriguing class of chemical systems with strongly correlated multi-reference excited states, due to the $d^3$ electron configuration. These states are difficult to describe accurately using single-reference
quantum chemical methods such as density functional theory (DFT), the most commonly used method to study the electronic structures of solid-state systems. Recently, the periodic effective
Hamiltonian of
crystal field (pEHCF) method has been shown to overcome some limitations arising in the calculations of excited $d$-states. In this work, we assess the suitability of DFT and pEHCF to calculate the electronic structure and $d$-$d$ excitations of chromium(III) dopants in wide band gap host materials. The results will aid computational development of novel transition metal-doped materials and provide a deeper understanding of the complex nature of transition metal dopants in solids.
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All authors Ilya Popov, Petros-Panagis Filippatos, Shayantan Chaudhuri, Andrei L. Tchougréeff, Katherine Inzani, Elena Besley
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Intrinsic non-Markovian magnetisation dynamics
Felix Hartmann, Vivek Unikandanunni, Matias Bargheer et al. (+3 more)
Memory effects are widespread in complex systems while evidence of non-Markovian dynamics in fundamental physical systems has been limited to specifically engineered cases. Here we report on the experimental observation of non-Markovian magnetisation dynamics in crystalline cobalt. Intense terahertz electromagnetic driving brings the system into a non-
equilibrium state, and allows us to follow its magnetisation dynamics in real time at low temperatures. The response reveals a striking multi-peaked frequency spectrum that defies conventional descriptions. Using open
quantum system theory we evidence that a non-Markovian
memory kernel, arising from
spin-
phonon bath coupling, naturally explains both the spectral structure and its temperature dependence. Theoretical prediction of the spectrum is accomplished by simulating the non-Markovian Landau-Lifshitz-Gilbert equation, while analytics quantitatively determines the frequencies of three spectral peaks. The results demonstrate that intrinsic
memory effects can emerge in a simple ferromagnet, opening new opportunities to explore and control non-Markovian dynamics in condensed-matter systems.
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All authors Felix Hartmann, Vivek Unikandanunni, Matias Bargheer, Eric E. Fullerton, Stefano Bonetti, Janet Anders
Disordered Systems and Neural Networks
Edge-Aware Graph Attention Model for Structural Optimization of High Entropy Carbides
Neethu Mohan Mangalassery and Abhishek Kumar Singh
Predicting relaxed atomic structures of chemically complex materials remains a major computational challenge, particularly for high-
entropy systems where traditional first-principles methods become prohibitively expensive. We introduce the edge-aware graph
attention model, a physics-informed graph neural network tailored for predicting relaxed atomic structures of high-
entropy systems. the edge-aware graph
attention model employs chemically and geometrically informed descriptors that capture both atomic properties and local structural environments. To effectively capture atomic interactions, our model integrates a multi-head self-
attention mechanism that adaptively weighs neighbouring atoms using both node and edge features. This edge-aware
attention framework learn complex chemical and structural relationships independent of global orientation or position. We trained and evaluated the edge-aware GAT model on a
dataset of carbide systems, spanning binary to high-
entropy carbide compositions, and demonstrated its accuracy, convergence efficiency, and transferability. The architecture is lightweight, with a very low computational footprint, making it highly suitable for large-scale materials screening. By providing invariance to rigid-body transformations and leveraging domain-informed
attention mechanisms, our model delivers a fast, scalable, and cost-effective alternative to DFT, enabling accelerated discovery and screening of
entropy-stabilised materials.
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Materials Science · Cross-listed from physics.chem-ph
Two-dimensional RMSD projections for reaction path visualization and validation
Rohit Goswami
Transition state or minimum energy path finding methods constitute a routine component of the computational chemistry toolkit. Standard analysis involves trajectories conventionally plotted in terms of the relative energy to the initial state against a cumulative displacement variable, or the image number. These dimensional reductions obscure structural rearrangements in high dimensions and are often history dependent. This precludes the ability to compare optimization histories of different methods beyond the number of calculations, time taken, and final saddle geometry. We present a method mapping trajectories onto a two-dimensional projection defined by a permutation corrected root mean square deviation from the reactant and product configurations. Energy is represented as an interpolated color-mapped surface constructed from all optimization steps using a gradient-enhanced Gaussian Process with the inverse multiquadric kernel, whose posterior variance contours delineate data-supported regions from extrapolated ones. A rotated coordinate frame decomposes the RMSD plane into reaction progress and orthogonal distance. We show the utility of the framework on a cycloaddition reaction, where a machine-learned potential saddle and density functional theory reference lie on comparable energy contours despite
geometric displacements, along with the ratification of the visualization for more complex reactions, a Grignard rearrangement, and a conrotatory bicyclobutane ring opening.
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Disordered Systems and Neural Networks
Towards probing velocity distributions in dense granular matter: Utilizing Fiber Bragg Gratings
Marlo Kunzner, Luis Henriques, Fahad Puthalath et al. (+8 more)
Granular gases are commonly characterized through their velocity distribution, which provides access to the granular temperature. In experiments, velocity distributions are typically obtained by particle tracking, which however becomes limited at moderate and high particle densities. As a way forward, we propose a new technique for measuring particle velocities in situ by using a Fiber Bragg Grating (FBG) sensor, which remains applicable at significantly higher particle densities.The FBG sensor detects strain pulses induced by particle-fiber collisions, from which the velocity of the impacting particle can be derived. Applying this method to an ensemble of granular particles allows to extract its velocity distributions as we present for a granular system excited by a vibrational shaker. We validate the extracted velocity distribution against conventional particle-tracking measurements, confirming the reliability of the FBG-based technique.
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All authors Marlo Kunzner, Luis Henriques, Fahad Puthalath, Leonardo Facchini, Mohammadhossein Shahsavari, Léa Gommeringer, Martin Angelmahr, Peidong Yu, Matthias Sperl, Till Böhmer, Jan Philipp Gabriel
Strongly Correlated Electrons
Quantum coherent states of mass-imbalanced electron-hole system within optical microcavities
Thi-Hau Nguyen, Thi-Hong-Hai Do and Van-Nham Phan
The interplay of the excitoniclike polariton, polariton, and photoniclike polariton coherent states in mass-imbalanced electron-hole systems within optical microcavities is theoretically examined. Utilizing the unrestricted Hartree-Fock approximation, we derive a set of self-consistent equations that evaluate the excitonic and photonic order parameters in a two-band electronic model, accounting equally for both electron-hole Coulomb attraction and light-matter coupling. Analyzing the competition among these condensate order parameters reveals a complex phase structure of coherent states in the ground state. As the mass imbalance is reduced, we observe a transition from a normal disordered electron-hole-
photon system to excitoniclike, polariton, and ultimately photoniclike polariton condensation states. The distinct features of these robust condensates can be identified in the momentum distribution of the electron-hole pair amplitude and the photonic density, as well as in the wave-number-resolved photoemission spectra of electrons, holes, and
photons. Increasing the excitation density further expands the range of condensation states. Additionally, lowering the mass imbalance leads to the emergence of
quantum coherent bound states prior to the formation of robust condensates, which are evidenced by the static and dynamical excitonic and photonic susceptibility functions.
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Materials Science
Equivariant Diffusion for Crystal Structure Prediction
Peijia Lin, Pin Chen, Rui Jiao et al. (+6 more)
In addressing the challenge of
Crystal Structure Prediction (CSP),
symmetry-aware deep learning models, particularly
diffusion models, have been extensively studied, which treat CSP as a conditional generation task. However, ensuring permutation, rotation, and periodic translation equivariance during
diffusion process remains incompletely addressed. In this work, we propose EquiCSP, a novel equivariant
diffusion-based generative model. We not only address the overlooked issue of
lattice permutation equivariance in existing models, but also develop a unique noising
algorithm that rigorously maintains periodic translation equivariance throughout both training and inference processes. Our experiments indicate that EquiCSP significantly surpasses existing models in terms of generating accurate structures and demonstrates faster convergence during the training process.
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All authors Peijia Lin, Pin Chen, Rui Jiao, Qing Mo, Jianhuan Cen, Wenbing Huang, Yang Liu, Dan Huang, Yutong Lu
Materials Science
Impact of Lattice Distortions on Magnetocrystalline Anisotropy and Magnetization in (Nd$_{1-x}$Pr$_x$)$_2$Fe$_{14}$B Alloys
Haruki Okumura, Takashi Miyake, Taro Fukazawa et al. (+6 more)
Nd$_{2}$Fe$_{14}$B – a widely used permanent magnet – has magnetocrystalline anisotropy constants that differ between the bulk and interface regions. This study explores the effects of
lattice distortion on the magnetocrystalline anisotropy ($K_{\rm u}$) and magnetization of (Nd$_{1-x}$Pr$_x$)$_2$Fe$_{14}$B. Nd$_2$Fe$_{14}$B
alloys were fabricated; scanning transmission electron microscopy revealed a compressive strain of up to 25% near grain boundaries. Using the full-potential Korringa–Kohn–Rostoker method, we calculated the strain dependence of $K_{\rm u}$, showing that although $K_{\rm u}$ is 4.2 MJ/m$^3$ under strain-free conditions at 0 K, it becomes negative in regions with 25% compressive strain. Additionally, Pr$_{2}$Fe$_{14}$B exhibits a larger $K_{\rm u}$ than Pr$_{2}$Fe$_{14}$B under undistorted conditions, whereas Pr-rich
alloys exhibit a more pronounced reduction in $K_{\rm u}$ under strain. These findings highlight the critical influence of
lattice distortions on
magnetic properties. The calculated strain-dependent
magnetic anisotropy parameters provide valuable inputs for future micromagnetic simulations, aiding the design of advanced
magnetic materials.
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All authors Haruki Okumura, Takashi Miyake, Taro Fukazawa, Noritsugu Sakuma, Yuta Suzuki, Tetsuya Shoji, Hisazumi Akai, Masako Ogura, Tetsuya Fukushima
Superconductivity
Pauli Master Equation numerical analysis of coherent and incoherent dressed fermions in triplet unconventional superconductors
Pedro L. Contreras E
We report two types of dressed
fermions in a triplet
superconductor with an in-situ disorder effective field. They are obtained numerically by analyzing with the Pauli Master Equation, the self-consistent imaginary part data of the elastic-scattering cross-section. We use a two-component irreducible representation of the order parameter with quasi-point nodes and study the quasi-classical effective probabilistic density distribution as function of disorder. We find a stable coherent
quantum state of dressed
fermions, and slow characteristic decay time for dilute disorder. Also, we find an incoherent
quantum state with diffused dressed
fermions and enriched disorder with self-consistency increasing decoherence, a fast characteristic decay time, and a diffuse
unitary resonance. We conclude that the most stable dressed
fermion states are those in which the field has dilute disorder, the threshold zero gap, and some dressed
fermions behave like an s-wave
superconductor, showing a tiny gap.
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Superconductivity
Novel coupling between charge order and time-reversal-symmetry-breaking superconductivity
Quanxin Hu, Lingfeng Zhang, Yu Zheng et al. (+9 more)
The interplay between charge-density waves (CDWs), which break translational
symmetry, and spatially homogeneous superconductivity, which breaks global U(1) gauge
symmetry, can give rise to an intriguing phenomenon: the pair-density wave, characterized by a spatial modulation of the superconducting order parameter. Yet how CDWs couple to unconventional superconducting states-particularly those with time-reversal
symmetry breaking (TRSB)-remains largely unexplored. Here, using scanning tunneling microscopy on heavily hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$, which hosts an s $\pm$ is superconducting state, we reveal a previously unobserved coupling between a surface CDW and TRSB superconductivity. Experimentally, the TRSB superconductivity imparts "chirality" to the CDW, which manifests as commensurate domains separated by domain walls with $\pi$-phase slips-forming what we term a bipolar CDW. The domain walls delineate TRSB domains of opposite chirality, consistent with spontaneous breaking of U(1) $\times$ Z2. Supported by theoretical modelling, we construct a framework in which a hidden interfacial pair-density modulation (PDM) mediates a linear coupling between the surface CDW and interband Josephson currents of TRSB superconductivity. Crucially, the theory shows that realizing this linear coupling requires a controlled global phase difference $\delta$ $\phi$ = $\pi$/2 between the PDM and CDW states. Our results uncover a previously overlooked connection between charge ordering and TRSB superconductivity, opening a pathway to explore intertwined
quantum orders in iron-based
superconductors and other strongly correlated systems.
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All authors Quanxin Hu, Lingfeng Zhang, Yu Zheng, Yongwei Li, Qiheng Wang, Xinyu Liang, Baiqing Lv, Chi-Ming Yim, Takuto Kawakami, Vadim Grinenko, Xiao Hu, Hong Ding
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Quantum geometrical effects in non-Hermitian systems
Anton Montag and Tomoki Ozawa
We explore the relation between
quantum geometry in non-Hermitian systems and physically measurable phenomena. We highlight various situations in which the behavior of a non-Hermitian system is best understood in terms of
quantum geometry, namely the notion of adiabatic potentials in non-Hermitian systems and the localization of Wannier states in periodic non-Hermitian systems. Further, we show that the non-Hermitian
quantum metric appears in the response of the system upon time-periodic modulation, which one can use to experimentally measure the non-Hermitian
quantum metric. We validate our results by providing numerical simulations of concrete exemplary systems.
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Strongly Correlated Electrons · Cross-listed from hep-th
The late-time attractor structure of dynamical black branes
Yan Liu and Hao-Tian Sun
The ringdown phase of a perturbed black hole is conventionally described by a linear superposition of quasinormal modes. However, as the AdS black brane approaches its final global
equilibrium, this linear quasinormal mode description becomes inadequate, and nonlinear dynamics play a significant role in the late-stage
evolution. We show that the interplay between nonlinear
evolution and horizon dissipation in general relativity drives dynamical AdS black branes towards the final state along a unique path, independent of their initial perturbations. Through numerical simulations, we identify this late-time attractor and uncover the associated universal nonlinear behavior, characterized by a simple dimensionless relative amplitude in the dual hydrodynamic variables.
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Statistical Mechanics
Local Reversibility and Divergent Markov Length in 1+1-D Directed Percolation
Yu-Hsueh Chen and Tarun Grover
Recent progress in open many-body
quantum systems has highlighted the importance of the Markov length, the characteristic scale over which conditional correlations decay. It has been proposed that non-
equilibrium phases of matter can be defined as equivalence classes of states connected by short-time
evolution while maintaining a finite Markov length, a notion called local reversibility. A natural question is whether well-known classical models of non-
equilibrium criticality fit within this framework. Here we investigate the Domany-Kinzel model – which exhibits an active phase and an absorbing phase separated by a 1+1-D directed-percolation transition – from this information-theoretic perspective. Using tensor network simulations, we provide evidence for local reversibility within the active phase. Notably, the Markov length diverges upon approaching the critical point, unlike classical
equilibrium transitions where Markov length is zero due to their Gibbs character. Correspondingly, the conditional mutual information exhibits scaling consistent with directed percolation universality. Further, we analytically study the case of 1+1-D compact directed percolation, where the Markov length diverges throughout the phase diagram due to spontaneous breaking of domain-wall parity
symmetry from strong to weak. Nevertheless, the conditional mutual information continues to faithfully detect the corresponding phase transition.
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Mesoscale and Nanoscale Physics
Microscopic signatures of Chern number sign reversal in twisted bilayer WSe2
Ke Lv, Ya-Ning Ren and Lin He
The discovery of quantized Chern numbers in twisted transition metal dichalcogenide (TMD) homobilayers,including 3.7{deg} twisted MoTe2 and 1.23{deg} twisted WSe2,has emerged as a defining breakthrough in physics. A striking and unresolved puzzle from these studies is the unexpected opposite sign of the observed Chern numbers between the two systems. Recent theory has proposed a twist-angle-dependent Chern number sign reversal in both twisted MoTe2 and WSe2, offering a potential explanation for the disparate experimental observations6. However, a direct experimental verification of the twist-angle-dependent Chern number sign reversal in a specific twisted TMD homobilayer is still elusive. Here, we report the first experimental demonstration that the Chern numbers of the moire frontier bands undergo sign reversal at a critical twist angle 1.42{deg} in twisted WSe2
bilayers (tWSe2). Using scanning tunnelling microscopy and spectroscopy, we direct measure layer-pseudospin skyrmion textures of tWSe2 and our results reveal that tWSe2 in the vicinity of the 1.42{deg} exhibits a twist-angle-dependent layer-pseudospin polarization, an effect that serves as the fundamental origin of the observed Chern number sigh reversal6-12.
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Materials Science
Spin and Orbital Magnetism in UH2 Thin Films Studied by X-ray Magnetic Circular Dichroism
Evgenia A. Tereshina-Chitrova, Mykhaylo Paukov, Oleksandra Koloskova et al. (+8 more)
Uranium dihydride UH2 is a metastable phase unknown in bulk form but accessible through thin-film synthesis. We prepared UH2 films by reactive dc sputtering on CaF2(001) or Si(001) substrates, the latter equipped with a Mo buffer layer to suppress a U-Si interdiffusion. On CaF2, UH2 adopts the fluorite-type structure with a near-[1 1 1] out-of-plane texture, four rotational domains, and a
lattice parameter a = 539 +- 3 pm without measurable strain, whereas the Mo-buffered film is polycrystalline. X-ray photoelectron spectroscopy confirmed complete hydrogenation and minimal oxidation. Magnetization and XMCD measurements show
ferromagnetic ordering with Curie temperatures of 120-130 K and a uranium 5f moment of 0.9 {\mu}B/U, dominated by the orbital contribution ({\mu}L ~ 1.4 {\mu}B, {\mu}S ~ -0.5 {\mu}B), in a good agreement with GGA+U computations, which otherwise overestimate absolute values of the
spin and orbital components. The slightly reduced moment in thinner CaF2-supported films is attributed to surface U(IV) species. These results demonstrate that thin-film synthesis enables stabilization of UH2 and direct probing of 5f magnetism, opening pathways toward higher uranium hydrides and interface-engineered actinide systems.
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All authors Evgenia A. Tereshina-Chitrova, Mykhaylo Paukov, Oleksandra Koloskova, Amir Hen, Fabrice Wilhelm, Lukas Horak, Mayerling Martinez Celis, Miroslav Cieslar, Ladislav Havela, Andrei Rogalev, Thomas Gouder
Materials Science
Nonreciprocal charge transport in an iron-based superconductor with broken inversion symmetry engineered by a hydrogen-concentration gradient
Takayuki Nagai, Yukito Nishio, Jumpei Matsumoto et al. (+4 more)
The breaking of spatial inversion
symmetry in condensed matter gives rise to intriguing physical properties, such as ferroelectricity, piezoelectricity,
spin-momentum locking, and nonreciprocal responses. Here we propose that a concentration gradient, which often persists as a quasi-stable nonequilibrium state with long relaxation times in solids, can serve as a general platform for inversion
symmetry breaking. We demonstrate this concept in an epitaxial thin film of the hydrogen-doped SmFeAsO (Sm1111:H)
superconductor with a depthwise hydrogen-concentration gradient introduced via an optimized topotactic reaction. This film exhibits nonreciprocal charge transport, meaning that the electrical resistance depends on the direction of the applied current, which serves as a key signature of broken inversion
symmetry. A pronounced nonreciprocal signal emerges in the vicinity of the superconducting transition, which we attribute to
vortex-motion nonreciprocity arising from an asymmetric pinning landscape created by the hydrogen-concentration gradient. Owing to the high critical temperature of Sm1111:H,
vortex-origin nonreciprocity is observed above 40 K, representing the highest temperature reported to date among single bulk materials without an artificially hetero-layered structure. Our findings establish concentration-gradient engineering as a versatile and broadly applicable route for realizing inversion-broken states in otherwise centrosymmetric hosts, opening pathways toward a broader landscape of odd-parity-driven functionalities.
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All authors Takayuki Nagai, Yukito Nishio, Jumpei Matsumoto, Kota Hanzawa, Hidenori Hiramatsu, Hideo Hosono, Tsuyoshi Kimura
Other Condensed Matter
Experimental Evidence of Néel-order-driven Magneto-optical Kerr Effect in an Altermagnetic Insulator
Haolin Pan, Rui-Chun Xiao, Jiahao Han et al. (+5 more)
The magneto-optical Kerr effect (MOKE) is investigated in hematite, a collinear
antiferromagnetic insulator, across a broad wavelength spectrum. By combining the optical measurements with magnetometry results, we unambiguously demonstrate that the Néel-order contribution dominates the MOKE signal, while contributions from net magnetization and external
magnetic fields are negligible. This conclusion is quantitatively supported by first-principles calculations, and qualitatively by a
symmetry analysis that the Néel contribution appears at the first order in
spin-orbit coupling while the magnetization contribution starts only at the third order. This study clarifies the altermagnetic origin of the pronounced MOKE in hematite, underscoring the potential of altermagnets as a promising new class of magneto-optical materials.
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All authors Haolin Pan, Rui-Chun Xiao, Jiahao Han, Hongxing Zhu, Junxue Li, Qian Niu, Yang Gao, Dazhi Hou
Mesoscale and Nanoscale Physics
A fluorescent color center in meteoritic Lonsdaleite
Giannis Thalassinos, Alan G. Salek, Daniel Stavrevski et al. (+7 more)
Lonsdaleite – hexagonal diamond – has only recently been proposed as a wide-bandgap host capable of supporting optically active point defects, but no such centres have yet been observed. Here we provide the first experimental evidence that lonsdaleite does in fact host photoluminescent color centres. In meteoritic lonsdaleite grains from the ureilite NWA7983, we identify a new defect, RU1, which exhibits bright and stable emission across 550-800 nm, with optimal blue excitation (~455 nm) and a peak at ~700 nm. Time-resolved photoluminescence reveals an excited-state lifetime of 14 ns with no detectable blinking, bleaching, or charge conversion. From the excitation-emission energetics we infer an unresolved zero-
phonon line near 550 nm. Correlative electron microscopy confirms the lonsdaleite host
lattice, and compositional analysis suggests N, Si, or Ni as plausible defect constituents. These results suggest lonsdaleite could become a new
quantum-grade crystalline platform and indicate that hexagonal-diamond color centres may form a new and unexplored family of solid-state
quantum emitters.
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All authors Giannis Thalassinos, Alan G. Salek, Daniel Stavrevski, Qiang Sun, Mitchell O. de Vries, Colin M. MacRae, Nicholas C. Wilson, Andrew G. Tomkins, Dougal G. McCulloch, Andrew D. Greentree
Strongly Correlated Electrons
A Numerical Perspective on Moiré Superlattices: From Single-Particle Properties to Many-Body Physics
Xin Lu, Bo Xie and Jianpeng Liu
Moiré superlattices in two-dimensional materials provide a versatile platform to explore strongly correlated and topological phases. This work presents a practical theoretical workflow for studying the correlated and topological states in moiré systems, combining continuum modeling, Hartree-Fock mean-field approximations, many-body perturbation theory, and exact diagonalizations. We focus on the numerical implementation of these methods, emphasizing subtleties such as remote band effects, inhomogeneous and dynamical screening, double counting problem, etc., which are often swept under the rug in theoretical works. The workflow enables a systematic investigation of
symmetry-breaking ground state properties, quasiparticle excitation properties and fractional Chern
insulator phases emerging from moiré superlattices, providing insights that are directly relevant to experimental observations. By bridging technical details and physical interpretations, this work aims to guide both theorists and experimentalists in understanding and predicting correlated phenomena in moiré materials.
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Materials Science
Revisiting the theory of crystal polarization: The downside of employing the periodic boundary conditions
Qiu-Shi Huang and Su-Huai Wei
Periodic boundary condition (PBC) is a standard approximation for calculating crystalline materials properties. However, a PBC
crystal is not the same as the real macroscopic
crystal, therefore, if applied indiscriminately, it can lead to erroneous conclusions. For example, unlike other extensive observables such as total energy, the polarization of a macroscopic
crystal cannot always be described by a PBC model, because polarization is inherently nonlocal and strongly dependent on surface terminations, irrespective of
crystal size, and moreover, the
symmetry of the macroscopic
crystal can be altered when the PBC is applied to a macroscopic
crystal. We demonstrate in this paper that the polarization of a macroscopic
crystal receives contributions from both the repeating bulk units and the
crystal surfaces, which must be treated on an equal footing. When the combined system of the bulk and its surfaces are taken into account, materials traditionally classified as nonpolar can, in fact, admit polar
symmetry, thus explaining why experimentalists have observed polarization in some nominally “nonpolar” systems. Our study, thus, clarifies that polarization can only exist in polar group systems and that apparent violations of Neumann's principle reported in some recent works originate from misinterpreting bulk PBC
crystal as intrinsic macroscopic
crystal, ignoring the contribution from the surfaces. We demonstrate that when the full bulk-plus-surface system is considered, the
crystal polarization and
symmetry is fully consistent with Neumann's principle.
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Mesoscale and Nanoscale Physics
Evidence for strong localization of orbital polarization
Taiyang Zhang, Lujun Zhu, Zhihao Yan et al. (+1 more)
Whether orbital polarization propagates has become the most essential question of the blooming orbitronics that aims to generate non-local orbital torque and orbital pumping. Recent theories have suggested a strong orbital
Hall effect within the light metal Al and a strong orbital Rashba effect at Co/Al interfaces, providing ideal platforms for experimental verification of possible orbital transport effects. Here, we report robust experimental evidence for the strong localization of orbital polarization. We demonstrate that neither the bulk nor the interface of the Al contributes a detectable orbital torque on adjacent
magnetic layer with strong bulk and interfacial
spin-orbit coupling necessary for potential orbital-
spin conversion. These results have clarified that orbital polarization undergoes much faster relaxation than
spin polarization and hardly participates in non-local accumulation, transport, or torque generation. The experimental evidence for strong localization of orbital polarization represents a groundbreaking advance towards solving the essential orbital torque debate.
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All authors Taiyang Zhang, Lujun Zhu, Zhihao Yan, Lijun Zhu
Materials Science
Photodynamics and Temperature Dependence of Single Spin Defects in Hexagonal Boron Nitride
Benjamin Whitefield, Ivan Zhigulin, Nicholas P. Sloane et al. (+3 more)
Quantum emitters in hexagonal boron nitride (hBN) that exhibit optically detected
magnetic resonance (ODMR) signatures have recently garnered significant
attention as an emerging solid-state platform for
quantum technologies. However, the underlying
spin dynamics, and the mechanisms determining the
spin-dependent fluorescence in these defects are still poorly understood. In this work we perform detailed photodynamical studies of the
spin complexes in hBN. In particular, we show that
spin transitions are located within the metastable
manifold which can be explained by the rate model, populating in a cascading manner. In addition, we perform temperature dependent measurements on these defects and show that the
spin-
lattice relaxation and coherence times increase as the temperature reduces. Furthermore, we find that the ODMR frequencies of the S=1 transition show only a marginal frequency shift as a function of temperature, which makes them a robust sensor at cryogenic temperatures. These insights are crucial for further understanding of the
spin dynamics of
quantum emitters in hBN and their practical implementation in
quantum sensing.
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All authors Benjamin Whitefield, Ivan Zhigulin, Nicholas P. Sloane, Jean-Philippe Tetienne, Igor Aharonovich, Mehran Kianinia