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Physics Condensed Matter

cond-mat Watch Condensed Matter

Saturday 29 November

Materials Science

Emergent Anomalous and Topological Hall Responses in an Epitaxial Ferromagnetic Weyl Nodal-Line metal Fe5Si3

Shubhashish Pati, Sonali Srotaswini Pradhan, Abhay Pandey et al. (+9 more)

The interplay between real and reciprocal space topology yields intrinsically linked transport phenomena in magnetic Weyl systems, wherein the broken time-reversal symmetry, strong Dzyaloshinskii-Moriya interaction, and pronounced uniaxial anisotropy stabilize the momentum-space Berry-curvature monopoles (Weyl nodes) and real-space chiral spin textures. We present a combined first-principles and experimental study of epitaxial Fe5Si3 thin films, establishing them as a magnetic Weyl nodal-line material. First-principles Density Functional Theory (DFT) calculations unambiguously reveal that Fe5Si3 hosts a topologically nontrivial electronic structure containing six pairs of Weyl nodes at or near the Fermi level, accompanied by pronounced Berry curvature at high-symmetry points of the Brillouin Zone. High-quality epitaxial films exhibit robust ferromagnetism with a Curie temperature of ~370 K and strong magneto crystalline anisotropy. The magneto transport measurements on epitaxial films reveal the corresponding Berry curvature-driven responses, including a significantly large intrinsic anomalous Hall conductivity of 504 S/cm and a high anomalous Hall angle of 5.5%, which is in good agreement with DFT calculations. A negative and non-saturating longitudinal magnetoresistance is observed, consistent with a chiral-anomaly contribution from Weyl fermions near the Fermi level (EF). Furthermore, a substantial topological Hall resistivity of 1.6 {\mu}{\Omega} cm robust across a wide temperature range, indicating the possibility of robust chiral spin textures in the thin-film geometry. These combined theoretical and experimental results establish Fe5Si3 as a unique, low-cost, centrosymmetric magnetic Weyl nodal-line material, providing a versatile platform for exploring coupled real and reciprocal space topologies in topological spintronic applications.
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All authors Shubhashish Pati, Sonali Srotaswini Pradhan, Abhay Pandey, Nikita Sharma, Nanhe Kumar Gupta, Nakul Kumar, Nidhi Shukla, Saurav Singh, Vidhi Jain, Mitali, V. Kanchana, Sujeet Chaudhary

Mesoscale and Nanoscale Physics

Band inversion transition in HgTe nanowire grown along the [001] direction

Rui Li

The low-energy effective Hamiltonian of a cylindrical HgTe nanowire grown along the [001] crystallographic direction is constructed by using the perturbation theory. Both the anisotropic term and the bulk inversion asymmetry term of the Kane model are taken into account. Although the anisotropic term has converted the crossing between the $E_{1}$ and $H_{1}$ subbands into an anticrossing at $k_{z}R\!=\!0$, the gap-closing-and-reopening transition in the subband structure can still occur at finite wave vectors $k_{z}R\!\approx\!\pm0.24$ for critical nanowire radius $R\!\approx\!3.45$ nm. The bulk inversion asymmetry does not contribute to the low-energy effective Hamiltonian, i.e., there is no spin splitting in the $E_{1}$, $H_{1}$, and $H_{2}$ subbands for a [001] oriented cylindrical nanowire.
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Mesoscale and Nanoscale Physics

Classification of coherent peaks in two-terminal quantum devices into normal and anomalous Kondo peaks

Jongbae Hong

Coherent peaks arising in the differential conductance of quantum dot (QD) and quantum point contact (QPC) devices are classified into two categories, normal and anomalous Kondo peaks, according to the underlying spin dynamics and the form of the scaling function to which the scaled temperature-dependent linear conductance collapses. The zero-bias peaks (ZBPs) observed in QPCs and in the triplet state of the even sector of quantum dot single-electron transistors (QDSETs) are identified as normal Kondo peaks, formed by spin dynamics involving spin exchange, a symbolic characteristic of the Kondo effect. For these ZBPs, the scaling temperature coincides with half the full width at half maximum (FWHM). In contrast, the ZBP observed in the odd sector of QDSETs and all finite-bias coherent peaks, including the coherent side peaks of QPCs and the split ZBP in the singlet state of the QDSET even sector, are identified as anomalous Kondo peaks, because they arise from spin dynamics without spin exchange, and their scaling temperature does not coincide with half the FWHM. To support these findings, we reproduce gate-voltage-dependent differential conductance line shapes measured in the odd sector of a QDSET, demonstrating that its ZBP originates from a combination of two coherent side peaks explicitly observed in QPCs.
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Superconductivity

Dual instability of superconductivity from oxygen defects in La$_3$Ni$_2$O$_{7+\delta}$

Peiheng Jiang, Jie Li, Yu-Han Cao et al. (+4 more)

We uncover a dual mechanism by which oxygen defects suppress superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_{7+\delta}$ using density functional theory, dynamical mean-field theory, and functional renormalization group analysis. Apical vacancies and interbilayer interstitials emerge as the dominant low-energy defect species and are further stabilized by orthorhombic domain walls. These two defect classes drive the electronic structure in opposing directions. Vacancy-induced disorder generates local magnetic moments and promotes Anderson localization at moderate concentrations, whereas periodic interstitial ordering yields a coherent but weakly correlated metallic background that fails to support superconductivity. These findings highlight the decisive role of oxygen defects in shaping the superconducting and provide microscopic guidance for improving superconductivity through controlled defect engineering.
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All authors Peiheng Jiang, Jie Li, Yu-Han Cao, Xiaodong Cao, Zhicheng Zhong, Yi Lu, Qiang-Hua Wang

Materials Science

Interpretable Graph Neural Networks for Classifying Structure and Magnetism in Delafossite Compounds

Jovin Ryan Joseph, Do Hoon Kiem, Sinchul Yeom et al. (+1 more)

Delafossites (ABC2, where A and B are metals and C is a chalcogen) are a versatile family of quantum materials and layered oxides/chalcogenides whose properties are highly sensitive to atomic composition and stacking geometry. Their broad chemical tunability makes them an ideal platform for large-scale combinatorial exploration and high-throughput computational screening with desirable quantum properties. In this work, we employ a Concept Whitening Graph Neural Network, a gray-box AI model, to classify delafossite structures by stacking sequence and magnetic states. By aligning learned representations with human-interpretable physical concepts, this gray-box approach enables both accurate prediction and insight into the structural and chemical features driving magnetic behavior. The magnetic-ordering models achieved validation accuracies exceeding 80 percent, with a further slight uptick observed in the model incorporating the largest number of concepts. Concept alignment analysis revealed measurable learning across nine physically meaningful descriptors, with coefficients of determination ranging from approximately 0.6 for the d-shell valence-electron concepts to 0.4-0.5 for the magnetic coupling parameters. Furthermore, we mapped the concept importances onto the material graph representation, elucidating interpretable physical trends and the progression of stable concept-aligned regions across training. These results demonstrate the potential of interpretable graph-based learning to capture the underlying physics of complex materials systems and provide an interpretable framework for accelerating the discovery and understanding of delafossites and related crystalline materials.
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All authors Jovin Ryan Joseph, Do Hoon Kiem, Sinchul Yeom, Mina Yoon

Quantum Gases

Roles of Polarization and Detuning in the Noise-induced Relaxation Dynamics of Atomic-Molecular Bose Condensates

Avinaba Mukherjee and Raka Dasgupta

We study the relaxation process of a resonant Bose gas under the influence of Gaussian white noise. We characterize the system dynamics in terms of the polarization or imbalance between the atoms and molecules, and the system coherence. The relaxation times corresponding to these two quantities are studied both using a mean-field model, and a Born Green Kirkwood Yvon hierarchy that takes into account the higher-order correlations. The role of the initial polarization and the Feshbach detuning are investigated. It is found with an increasing initial population imbalance, the longituidinal relaxation time (that governs the dyanamics of the polarization) grows, while the transverse relaxation time (that governs the dynamics of the coherence) decays. As for the varying Feshbach detuning, it is observed that the longituidinal relaxation time reaches its minima and its transverse counterpart reaches its maxima near the resonance. We also study how the initial polarization and the detuning affect physical quantities like drift speed, condensate fraction, fidelity and entanglement entropy etc, and find the results to be fully consistent with the behavior of the relaxation dynamics of the system.
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Soft Condensed Matter

Bend Instabilities and Topological Turbulence in Shear-Aligned Living Liquid Crystal

Hend Baza, Fei Chen, Taras Turiv et al. (+2 more)

Flagellated microswimmers B. Subtilis dispersed in a nematic phase of a lyotropic chromonic liquid crystal form a living liquid crystal (LLC). The combination of the passive and active components allows us to analyze how the active component transitions from the shear-imposed alignment into topological turbulence. The lateral extension of the experimental cell is 1000 times larger than the 10-micrometer thickness, to avoid the effect of lateral confinement on the dynamic patterns. The surface anchoring is azimuthally degenerate to avoid permanent anisotropy. After the shear cessation, active forces produced by the microswimmers trigger self-amplifying bend undulations. The amplitude of undulations grows with time and then saturates, while the wavelength increases only slightly. Strong bend stresses at the extrema of undulations are released by nucleating disclination pairs, which multiply to produce topological turbulence. The nucleating pairs and the symmetry axes of the +1/2 disclinations are orientationally ordered. In highly active LLCs, this alignment causes a nonmonotonous time dependence of the disclinations' number, as a fast-moving +1/2 disclination of one pair annihilates a -1/2 disclination of the neighboring pair. The spectra of elastic and kinetic energies exhibit distinct wavevector dependencies caused by the energy cost of the deformed passive nematic background. The study demonstrates how applied shears and a passive viscoelastic background affect the dynamics of active matter. Combined with the previously determined viscoelastic properties of the lyotropic chromonic liquid crystal, the results complete a comprehensive description of the experimentally assessable type of active matter.
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All authors Hend Baza, Fei Chen, Taras Turiv, Sergij V. Shiyanovskii, Oleg D. Lavrentovich

Strongly Correlated Electrons

Magnetosynthesis effect on the structure and ground state of Cu$^{2+}$-based antiferromagnets

Micaela E. Primer, Anna A. Berseneva, Ayesha Ulde et al. (+2 more)

Subtle synthetic variables can have an outsizes influence on the crystal structure and magnetic properties of a material, particularly those of quantum materials. In this work, we investigate the impact of synthesis under a magnetic field (magnetosynthesis) on the crystal structure and magnetic properties of several Cu$^{2+}$ ($S=1/2$) based materials with antiferromagnetic interactions and varying levels of magnetic frustration, from simple antiferromagnets to a quantum spin liquid. We employ small (0.09 - 0.37 T) magnetic fields applied during low-temperature hydrothermal or evaporative synthesis of the simple antiferromagnet CuCl$_2\cdot$2H$_2$O, the canted antiferromagnet (Cu,Zn)$_3$Cl$_4$(OH)$_2\cdot$2H$_2$O, the frustrated and canted antiferromagnet atacamite Cu$_2$(OH)$_3$Cl, and the highly frustrated quantum spin liquid herbertsmithite Cu$_3$Zn(OH)$_6$Cl$_2$. We found that (Cu,Zn)$_3$Cl$_4$(OH)$_2\cdot$2H$_2$O experiences structural changes well above its magnetic transition. Atacamite Cu$_2$(OH)$_3$Cl synthesized under a 0.19 T field experiences a 0.15 K (~3%) decrease in its Néel transition temperature and a significant strengthening of its antiferromagnetic interactions, suggesting that magnetosynthesis can influence the ground state of moderately frustrated materials.
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All authors Micaela E. Primer, Anna A. Berseneva, Ayesha Ulde, Wenhao Sun, Rebecca W. Smaha

Friday 28 November

Materials Science · Cross-listed from cs.LG

A self-driving lab for solution-processed electrochromic thin films

Selma Dahms, Luca Torresi, Shahbaz Tareq Bandesha et al. (+5 more)

Solution-processed electrochromic materials offer high potential for energy-efficient smart windows and displays. Their performance varies with material choice and processing conditions. Electrochromic thin film electrodes require a smooth, defect-free coating for optimal contrast between bleached and colored states. The complexity of optimizing the spin-coated electrochromic thin layer poses challenges for rapid development. This study demonstrates the use of self-driving laboratories to accelerate the development of electrochromic coatings by coupling automation with machine learning. Our system combines automated data acquisition, image processing, spectral analysis, and Bayesian optimization to explore processing parameters efficiently. This approach not only increases throughput but also enables a pointed search for optimal processing parameters. The approach can be applied to various solution-processed materials, highlighting the potential of self-driving labs in enhancing materials discovery and process optimization.
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All authors Selma Dahms, Luca Torresi, Shahbaz Tareq Bandesha, Jan Hansmann, Holger Röhm, Alexander Colsmann, Marco Schott, Pascal Friederich

Mesoscale and Nanoscale Physics · Cross-listed from physics.ins-det

Detuning-insensitive wide-field imaging of vector microwave fields with diamond sensors

Xiu-Qi Chen, Rui-Zhi Zhang, Gang-Qin Liu et al. (+1 more)

Nitrogen vacancy (NV) centers in diamond have precipitated profound advances in microwave detection, manifesting themselves both in spatial resolution and sensitivity. However, typical methods based on Rabi oscillations are subject to detunings due to thermal and magnetic fluctuations and/or gradients, which introduce systematic errors and render the measurements susceptible to environmental perturbations. Here, we propose and demonstrate a novel approach for determining both the magnitude and direction of microwaves, by exploiting the spectral line broadening effect in the optically detected magnetic resonance of NV centers. This method eliminates the requirement of aligning the MW frequency to the spin transitions and is therefore immune to variations and inhomogeneities of the magnetic field and temperature, providing an optimal tool for fast imaging applications. With this method, we achieved wide-field imaging of near field microwaves generated with a microscale $\rm{\Omega}$-pattern antenna with a resolution of 800\,nm. Combining with the vector detection using multi-axis NVs, a full reconstruction of the vector microwave fields is obtained. Besides, our scheme also exhibits excellent linearity over a broad range of MW amplitudes, and the scale is theoretically calculated to be more than four orders. Our results augment the applicability of diamond-based microwave devices in applications under complex scenarios, especially where large dynamic range, fast test speed, and high spatial resolution are demanded.
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All authors Xiu-Qi Chen, Rui-Zhi Zhang, Gang-Qin Liu, Huijie Zheng

Soft Condensed Matter

Polar and apolar light-induced alignment of ferroelectric nematic at photosensitive polymer substrate

Ruslan Kravchuk, Oleksandr Kurochkin, Vassili G. Nazarenko et al. (+4 more)

Surface alignment of a recently discovered ferroelectric nematic liquid crystal (NF) is usually achieved by buffed polymer films, which produce a unidirectional polar alignment of the spontaneous electric polarization. We demonstrate that photosensitive polymer substrates could provide a wider variety of alignment modes. Namely, a polyvinyl cinnamate polymer film irradiated by linearly polarized ultraviolet (UV) light yields two modes of surface orientation of the NF polarization. (1) A planar apolar mode, in which the equilibrium NF polarization aligns perpendicularly to the polarization of normally impinging UV light; the NF polarization adopts either of the two antiparallel states. (2) A planar polar mode, produced by an additional irradiation with obliquely impinging UV light. In this mode, there is only one stable azimuthal direction of polarization in the plane of the substrate. The two modes differ in their response to an electric field. In the planar apolar mode, polarization can be switched between two states of equal surface energy. In the planar polar mode, the field-perturbed polarization relaxes back to the single photoinduced easy axis once the field is switched off. The versatility of modes and absence of mechanical contact make the photoalignment of the NF attractive for practical applications.
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All authors Ruslan Kravchuk, Oleksandr Kurochkin, Vassili G. Nazarenko, Volodymyr Sashuk, Mykola Kravets, Bijaya Basnet, Oleg D. Lavrentovich

Strongly Correlated Electrons

Robust semiclassical magnetization plateau in the kagome lattice

Gabriel Capelo and Eric C. Andrade

Inspired by recent observations of the $1/3$ magnetization plateau in kagome-based magnets, we investigate the $J_1-J_2$ Heisenberg model on the kagome lattice under the influence of an external magnetic field. Although the classical ground state at zero field depends on the sign of $J_2$, we find a robust $1/3$ semiclassical magnetization plateau in both cases. The mechanism that stabilizes this plateau is analogous to that observed in the triangular lattice, where quantum fluctuations select a collinear state from the degenerate classical manifold. We calculate the plateau width, which shows a weak dependence on $J_2$, using nonlinear spin-wave theory. Additionally, we find that a straightforward approach based on linear spin-wave yields quantitatively accurate results even for $S=1/2$. Furthermore, we identify a magnetization jump at the saturation field when $J_2=0$; this jump is related to the presence of a flat band and disappears for $J_2 \neq 0$. Our study demonstrates that a semiclassical approach effectively captures the $1/3$ plateau in the kagome lattice and serves as a valuable tool for interpreting experimental and numerical results alike.
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Soft Condensed Matter

Understanding the Role of Particle Deformability on the Crystal and Glass formation using Two-dimensional Ring Polymer Model

Padmanabha Bose and Smarajit Karmakar

Soft matter systems are common in nature and make up nearly all the essential components necessary for life, from cells to the organelles within those cells. The ability of these soft materials to deform is crucial for the proper functioning of various biological processes, such as blood flow in our veins and arteries. It is vital to understand how deformability influences the normal functioning of these processes. We have investigated an assembly of two-dimensional (2D) polymeric non-overlapping rings via extensive molecular dynamics simulations. The main idea is to study an assembly of model particles with anisotropic deformability using polymer rings. By tuning the degree of deformability of these model deformable particles, we study the dynamic and static properties of the assembly at different densities and temperatures. This deformable particle model might correspond to an assembly of epithelial cells or similar biologically soft bodies. In the limit at which the rings are very rigid with very little deformability, one expects to see the formation of a triangular lattice by the centres of these polymer rings. On the other hand, if one increases the deformability of these polymer rings, due to increased disorder, one observes glass-like dynamical behaviour even for identically sized polymer rings. We also show a transition from a crystalline state to a disordered glassy state driven solely by particle deformability. We observe non-trivial finite-size effects in the dynamics of these glass-forming ring polymers, not seen in usual molecular glass-formers.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.app-ph

Nanoscale Magnonic Neurons

K. G. Fripp, A. V. Shytov and V. V. Kruglyak

We use micromagnetic simulations to demonstrate neuron functionality of two-dimensional (2D) chiral magnonic resonators. Our design exploits nonlinear resonant scattering of spin waves propagating in a YIG medium from an edge mode of a permalloy nano-element. The reduced frequency and volume of the edge mode facilitate matching it to the YIG modes and give rise to their wide-angle scattering. As the amplitudes of the incident spin waves increase, the edge mode exhibits a positive nonlinear frequency shift. This shift leads to a complex frequency-dependent nonlinear variation of the amplitude and phase of spin waves scattered in different directions. We show that the scattered waves are strong enough to activate secondary neurons. This provides the connectivity required for combining our proposed neurons into 2D magnonic neural networks.
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Statistical Mechanics

Dissipation and fluctuations of CMOS ring oscillators close to criticality

Ashwin Gopal, Massimiliano Esposito and Jan Meibohm

We analyze a thermodynamically consistent model of CMOS-based ring oscillators near the onset of coherent voltage oscillations. For driving voltages close to the critical value, we derive the normal form of the Hopf bifurcation that underlies the oscillation transition in the thermodynamic limit. Using this normal form, we determine the phase and amplitude dynamics, and demonstrate that entropy dissipation decreases in the oscillating state for ring oscillators with more than three inverters. These findings culminate in a stability-dissipation relation, which links the observed decrease in dissipation to an increase in the local stability of the oscillating state. Next, we characterize finite-size fluctuations of the amplitude and phase near the critical voltage, using a stochastic version of the normal form. We demonstrate that close to the transition, finite-size fluctuations remain important at arbitrary system size, introducing oscillations even below the critical voltage. We further show that these noise-induced oscillations have an anomalously short decoherence time that scales sub-linearly with the system-size, in contrast to the behavior far from criticality.
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Mesoscale and Nanoscale Physics

Resonant Dyakonov-Shur Magnetoplasmons in Graphene Terahertz Photodetectors

Juan A. Delgado-Notario, Cedric Bray, Elsa Perez-Martin et al. (+16 more)

Graphene plasmons confine incident terahertz fields far below the diffraction limit and, when hosted by a gate-defined Fabry-Perot cavity, enable electrically tunable, frequency-selective photodetectors. In a magnetic field, these plasmons hybridize with the cyclotron motion to form magnetoplasmons, offering a platform for fundamental studies and for nonreciprocal, spectrally selective, ultrasensitive terahertz photonics. However, implementing magnetoplasmon-assisted resonant transistors at terahertz frequencies has remained challenging so far. Here we use gate-dependent, on-chip terahertz photocurrent spectroscopy combined with a perpendicular magnetic field to resolve and probe the evolution of resonant magnetoplasmons in antenna-coupled monolayer and bilayer graphene TeraFETs. In monolayer graphene the dispersion reflects the Dirac nature of the carriers, exhibiting a non-monotonic density dependence due to the interplay of plasma resonance and cyclotron motion, with an inflection point at maximal plasmon-cyclotron coupling. In contrast, in bilayer graphene we recover and map a magnetoplasmon dispersion consistent with the conventional Schrödinger-type picture. These results establish graphene TeraFET devices as a robust on-chip platform for resonant magnetoplasmonics at terahertz frequencies, enabling magnetically programmable, frequency-selective photonics and opening avenues toward photodetectors with enhanced sensitivity.
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All authors Juan A. Delgado-Notario, Cedric Bray, Elsa Perez-Martin, Ben Benhamou-Bui, Namrata Saha, Sahil Parvez, Christophe.Consejo, Guillaume Sigu, Salah Benlemqwanssa, Laurent Bonnet, Takashi Taniguchi, Kenji Watanabe, José M. Caridad, Sergey S. Krishtopenko, Yahya M. Meziani, Benoit Jouault, Jérémie Torres, Sandra Ruffenach, Frédéric Teppe

Superconductivity

Purely even harmonic Josephson current due to crossed pair transmission across strongly spin-polarized materials

Niklas L. Schulz, Danilo Nikolić and Matthias Eschrig

We revisit the problem of the second harmonic generation in the current-phase relation across ferromagnetic bilayers placed between BCS superconductors. In particular, we consider a strongly spin-polarized metallic ferromagnet coupled to two superconducting leads via thin spin-active (left) and non-spin-active (right) insulating layers. The system is examined in the framework of the quasiclassical Green$^\prime$s function formalism, both in the ballistic (Eilenberger) and the diffusive (Usadel) limit. Strong spin polarization allows for neglecting short-range mixed-spin correlations, and the Josephson supercurrent across the ferromagnet is fully mediated by long-range equal-spin triplet correlations. Using a diagrammatic technique for ballistic propagators introduced in Refs. [1-3], we describe the relevant Andreev processes responsible for the effective conversion of two spin-singlet Cooper pairs in the superconductor into two $\uparrow\uparrow$ and $\downarrow\downarrow$ pairs in the ferromagnet. Contrary to the naive picture of direct conversion, we show that the lowest order process involves four Cooper pairs in the superconductor, among which three are incoming, and one is outgoing, giving rise to net charge transport of 4e across the non-spin-active interface. The self-consistent numerical treatment of the diffusive junction, typically more relevant in experiments, confirms this picture quantitatively.
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Strongly Correlated Electrons

Exploring the apparent violation of the Mott relation in a noncentrosymmetric kagome ferromagnet

Benjamin Kostroun, Tomoya Asaba, Sean M. Thomas et al. (+4 more)

In magnetic topological materials, time-reversal symmetry breaking gives rise to topological point and line nodes with distinctive signatures in the anomalous Hall and anomalous Nernst conductivity that satisfy the well-known Mott relation. However, this relationship can fail for doping-dependent transport measurements of materials with complex magnetism, topology, and electronic correlations. In this work, we present transport measurements of the correlated topological metal UCoAl doped with Ru, which appear to violate the Mott relation. We develop a model that captures the evolution of Stoner magnetism and topological Weyl points as a function of doping. Using this model, we show how the correlated flat band in this material pins the Weyl points to the Fermi energy, and demonstrate how this explains the unusual doping-dependent behavior of the anomalous Hall and anomalous Nernst conductivities in this material, while the Mott relation is in fact satisfied at each doping level.
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All authors Benjamin Kostroun, Tomoya Asaba, Sean M. Thomas, Eric D. Bauer, Sergey Y. Savrasov, Filip Ronning, Vsevolod Ivanov

Disordered Systems and Neural Networks

Tuning Universality in Deep Neural Networks

Arsham Ghavasieh

Deep neural networks (DNNs) exhibit crackling-like avalanches whose origin lacks a mechanistic explanation. Here, I derive a stochastic theory of deep information propagation (DIP) by incorporating Central Limit Theorem (CLT)-level fluctuations. Four effective couplings $(r, h, D_1, D_2)$ characterize the dynamics, yielding a Landau description of the static exponents and a Directed Percolation (DP) structure of activity cascades. Tuning the couplings selects between avalanche dynamics generated by a Brownian Motion (BM) in a logarithmic trap and an absorbed free BM, each corresponding to a distinct universality classes. Numerical simulations confirm the theory and demonstrate that activation function design controls the collective dynamics in random DNNs.
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Materials Science

Singularly isostatic and geometrically unstable rigidity of metal-organic frameworks

Christopher M. Owen and Michael J. Lawler

Metal-organic frameworks (MOFs) combine high porosity with structural fragility, raising important questions about their mechanical stability. We develop a rigidity-based framework in which spring networks parameterized by UFF4MOF are used to construct rigidity and dynamical matrices. Large-scale analysis of 5,682 MOFs from the CoRE 2019 database shows that most frameworks are formally over-constrained yet cluster sharply near the isostatic threshold, revealing accidental geometric modes and placing many MOFs near mechanical instability. In the representative case of UiO-66, we show that auxiliary long-range constraints introduced by tuning the neighbor cutoff lift these modes into soft, flat, finite-frequency bands. The results show that rigidity-matrix analysis can rapidly identify MOFs likely to remain mechanically stable. This near-criticality mirrors behavior known from topological mechanics and points to a deeper design principle in porous crystals.
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Materials Science

Topological passivation makes high strength alloys insensitive to hydrogen embrittlement

Huijie Cheng, Binhan Sun, Aochen Zhang et al. (+6 more)

Infrastructure parts for a hydrogen (H) economy need alloys that are mechanically strong and at the same time resistant to the most dangerous and abrupt type of failure mode, namely, H embrittlement. These two properties are in fundamental conflict, as increasing strength typically amplifies susceptibility to H-related failure. Here, we introduce a new approach to make alloys resistant to H embrittlement, by creating a topological passivation layer (up to a few hundred micrometers thick) near the material surface, the region that is most vulnerable to H ingress and attack. It features instead a layer of ultrafine laminated grains with tens of times higher dislocation density than conventional materials, altering H diffusion, trapping and crack evolution. We tested the concept on a face-centered cubic (FCC) CoCrNi medium entropy model alloy which undergoes severe H-induced intergranular cracking. Two key mechanisms create the topological passivation: First, the high density (up to ~1.3e15 m-2) of H-trapping dislocations within the passivating grain layer decelerates H migration by up to about an order of magnitude, delaying H-induced crack initiation at grain boundaries. More importantly, once unavoidable micro-sized H-induced intergranular cracks emerge in the topmost surface region, they become completely arrested by the laminated grains, due to a transition in the embrittlement mechanism from H-enhanced grain boundary decohesion to highly energy-dissipative dislocation-associated cracking. These effects almost completely eliminate H embrittlement, at even doubled yield strength, when exposing the so architected material to harsh H attack. Our approach leverages surface mechanical treatments to tailor metallic microstructures in surface regions most susceptible to H attack, providing a scalable solution to protect alloys from H-induced damage.
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All authors Huijie Cheng, Binhan Sun, Aochen Zhang, Dirk Ponge, Fengkai Yan, Tiwen Lu, Xian-Cheng Zhang, Dierk Raabe, Shan-Tung Tu

Mesoscale and Nanoscale Physics

Wilson loop invariants and bulk-boundary correspondence in higher-order topological insulators with two anticommuting mirror symmetries

Suman Aich and Babak Seradjeh

We investigate the higher-order bulk-boundary correspondence in a family of chiral-symmetric Bloch Hamiltonians with anticommuting mirror symmetries. These models generalize the $\pi$-flux square lattice, the prototypical topological quadrupole insulator, and include both separable and nonseparable models with extended and diagonal hopping. For separable systems, the product of subsystem chiral winding numbers correctly predicts the number of zero-energy corner states. However, this invariant fails in nonseparable models, motivating the development of new momentum-space diagnostics. We introduce gauge-independent mirror-filtered winding numbers for Wannier Hamiltonians, constructed by projecting mirror eigenstates onto the occupied subspace. Furthermore, by adapting periodicized Wilson lines from chiral Floquet theory to the case with momentum-dependent chiral operator, we define new invariants associated directly with Wannier gaps. These invariants provide a detailed characterization of Wannier band topology. Our results clarify the interplay between chiral symmetry, mirror symmetries, and Wilson loops in higher-order topological phases and point to open challenges in formulating momentum-space invariants for general nonseparable models.
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Other Condensed Matter · Cross-listed from hep-th

Schwinger effect with backreaction in 1+1D massive QED with a strong external field

Samuel E. Gralla and Morifumi Mizuno

In the presence of a strong electric field, the vacuum is unstable to the production of pairs of charged particles – the Schwinger effect. The created pairs extract energy from the electric field, resulting in nontrivial backreaction. In this paper, we study 1+1D massive QED subject to strong external electric fields in a self-consistent and fully quantum manner. We use the bosonized version of the theory, which attains a cosine interaction term in the presence of nonzero fermion mass $m$. However, the assumption of strong electric field justifies a perturbative treatment of the cosine interaction, i.e., an expansion in $m$. We calculate the vacuum expectation value of the electric field to first order in $m$ and show that – surprisingly – it satisfies a classical nonlinear partial differential equation (related to the sine-Gordon equation). We show that the electric field exhibits dissipation-free oscillations (analogous to ordinary plasma oscillations) and calculate the plasma frequency analytically. We also compare to the semiclassical approximation commonly used to study backreaction, showing that it fails to capture the $O(m)$ shift in the plasma frequency.
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Other Condensed Matter · Cross-listed from quant-ph

Random purification channel made simple

Filippo Girardi, Francesco Anna Mele and Ludovico Lami

The recently introduced random purification channel, which converts $n$ i.i.d. copies of any mixed quantum state into a uniform convex combination of $n$ i.i.d. copies of its purifications, has proved to be an extremely useful tool in quantum learning theory. Here we give a remarkably simple construction of this channel, making its known properties – and several new ones – immediately transparent. In particular, we show that the channel also purifies non-i.i.d. states: it transforms any permutationally symmetric state into a uniform convex combination of permutationally symmetric purifications, each differing only by a tensor-product unitary acting on the purifying system. We then apply the channel to give a one-line proof of (a stronger version of) the recently established Uhlmann's theorem for quantum divergences.
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Statistical Mechanics · Cross-listed from math-ph

Extended regime of nematic order in an interacting monomer-dimer model of Heilmann and Lieb

Qidong He

We revisit a two-dimensional model of liquid crystals introduced by Heilmann–Lieb (1979), which consists of dimers on the square lattice at chemical potential $\lambda$, interacting via a hard-core repulsion and an attractive interaction of strength $a>0$ between adjacent, colinear dimers. The model is conjectured to exhibit nematic order at low temperatures, in the sense of orientational symmetry breaking coupled with the absence of translational order, provided that $\lambda+a>0$. We prove this conjecture under the additional condition $3a>\lambda$, substantially extending the parameter regime in which nematic order is known. The extra condition corresponds exactly to the regime in which misaligned dimers remain rare on scales comparable to the correlation length of the one-dimensional reference system. Our proof begins with the microscopic characterization of orientational order used by Heilmann–Lieb, strengthens it into a quantitative characterization of the rarity of misaligned dimers under the strong-percolation formalism of Hadas–Peled (2025), and uses the improved estimate to implement a disagreement-percolation argument in the sense of van den Berg (1993). As a consequence, we establish uniqueness of Gibbs measure and anisotropic exponential decay of correlations within each orientational phase.
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Superconductivity

Plasmon excitations and their attenuation in dirty superconductors

Daniil K. Karuzin and Mikhail A. Skvortsov

We develop a theory for the plasmon spectrum in dirty superconductors across the entire temperature range. Starting with the microscopic Keldysh sigma model description, we link the plasmon dispersion $\omega(q)$ to the optical conductivity $\sigma(\omega,T)$ of a superconductor, which requires analytical continuation to the lower half-plane of complex frequency. This approach reveals a discontinuity at the superconducting transition: a jump in both the real and imaginary parts of $\omega(q)$ at $T_c$. For any temperature below $T_c$, the plasmon dispersion terminates at a critical wave vector $q_c(T)$ where plasmons remains undamped, with weakly $T$-dependent $\omega[q_c(T)] \approx 2\Delta(0)$. Plasmons significantly attenuate only within a narrow 5% temperature window near $T_c$, with the propagating mode recovering at large $q$.
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Soft Condensed Matter

Non-reciprocal interactions between condensates in chemically active mixtures

Jacopo Romano, Martin Kjøllesdal Johnsrud, Benoît Mahault et al. (+1 more)

We study the behaviour of catalytically active droplets in multi-component conserved mixtures affected by noise. Working in the thin interface limit, we analytically determine the state diagram of the system, characterized by multiple dynamical regimes, and verify our findings using numerical simulations. In particular, we show the emergence of a non-reciprocal, chemically-mediated interaction between the droplets, which leads to the formation of (meta-)stable clusters of droplets of different species. We find that the clusters can display self-propulsion in a large part of the parameter space, including regions where the non-reciprocal interactions between the droplets are purely attractive. This surprising feature arises from the non-local nature of the chemical interactions, and points to locality violations as a general mechanism for energy dissipation and emergence of out-of-equilibrium steady states in active matter.
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All authors Jacopo Romano, Martin Kjøllesdal Johnsrud, Benoît Mahault, Ramin Golestanian

Soft Condensed Matter

Helix alignment, chevrons, and edge dislocations in twist-bend ferroelectric nematics

Bijaya Basnet, Priyanka Kumari, Sathyanarayana Paladugu et al. (+6 more)

We explore surface alignment and edge dislocations in the recently discovered twist-bend ferroelectric nematic, NTBF, in which the vector of spontaneous polarization follows an oblique helicoidal trajectory around a polar twist-bend axis. In a planar cell, the polar axis aligns at some angle to the rubbing direction to mitigate surface electric charge. We demonstrate that the pseudolayers in planar cells form chevron defects, a hallmark defect of one-dimensionally positionally ordered phases, such as smectic A and smectic C. The polar character of the twist-bend axis prevents the cores of NTBF edge dislocations from splitting into semi-integer disclinations, in stark contrast to dislocations in paraelectric and ferroelectric chiral nematics. The tilt of pseudolayers around the defect core allows us to estimate the elastic penetration length as being close to the pitch of NTBF. Compression/dilation stresses around the core modify the heliconical tilt angle of molecules as evidenced by a substantial variation in local birefringence. The climb of dislocations exhibits high mobility, allowing the system to equilibrate the temperature-dependent pitch. The uncovered properties facilitate the development of NTBF materials for electro-optical applications, such as electrically controlled diffraction lattices and structural colors.
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All authors Bijaya Basnet, Priyanka Kumari, Sathyanarayana Paladugu, Damian Pociecha, Jakub Karcz, Przemysław Kula, Nataša Vaupotič, Ewa Górecka, Oleg D. Lavrentovich

Materials Science

Non-local Chemistry Driven by Cation-Anion Size Disparity in Helium Inserted Compounds under High Pressure

Zhen Liu, Stefano Raciopp, Katerina P. Hilleke et al. (+6 more)

Opposing the theory that Helium (He) cannot be inserted into AB-type ionic compounds due to the Madelung energy increase, our crystal structure search and first-principles calculations found that He can form stable compounds with sodium halides (NaX, X=Cl, Br, I) under high-pressure. These reactions are driven by the non-local chemistry arising from the cation-anion size disparity, distinctly different from the He insertion reaction with A2B-type compounds. The large size differences between Na+ and X- enable structures that can effectively host He insertions through volume and inter-atomic distance disproportionation. Furthermore, the insertion of He atoms can significantly relieve the elevated Madelung energy that builds up in NaX under high pressure. This energy increase arises from structural transitions driven by cation-anion size disparity, which are necessary for reducing volume under pressure. The insertion of He allows the reduction of the total volume under high pressure without increasing the Madelung energy. Our predicted compounds and stability analysis reveal a new example of He reactivity governed not by local chemical bond formation, but by long-range electrostatic interactions.
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All authors Zhen Liu, Stefano Raciopp, Katerina P. Hilleke, Abhiyan Pandit, Shuran Ma, Andreas Hermann, Dadong Yan, Eva Zurek, Mao-sheng Miao

Statistical Mechanics · Cross-listed from quant-ph

Renormalisation of Fermionic Cellular Automata

Lorenzo Siro Trezzini, Andrea Pizzamiglio, Alessandro Bisio et al. (+1 more)

We present an exact renormalisation scheme for fermionic cellular automata on hypercubic lattices. By grouping neighbouring cells into tiles and selecting subspaces within them, multiple evolution steps on the original system correspond to a single step of an effective automaton acting on the subspaces. We derive a necessary and sufficient condition for renormalisability and fully characterise the renormalisation flow for two-cell tiles and two time steps of nearest-neighbour fermionic automata on a chain of spinless modes, identifying all fixed points.
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All authors Lorenzo Siro Trezzini, Andrea Pizzamiglio, Alessandro Bisio, Paolo Perinotti