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Physics Condensed Matter

cond-mat Watch Condensed Matter

Friday 14 November

Mesoscale and Nanoscale Physics

Nearly semi-elliptic relation between the minimal conductivity and Hall conductivity in unpaired Dirac fermions

Bo Fu, Kai-Zhi Bai, Shi-Hao Bi et al. (+1 more)

Electric conductivities may reveal the topological and magnetic properties of band structures in solids, especially for two-dimensional unpaired Dirac fermions. In this work, we evaluate the longitudinal and Hall conductivity for unpaired Dirac fermions in the framework of the self-consistent Born approximation and find a nearly semi-elliptic relation between the minimal conductivity and Hall conductivities in the Dirac fermions. Near the charge neutrality point, disorder may drive a metal-insulator transition, and enhance the longitudinal conductivity. For the massless case, the minimal conductivity $\sigma_{xx}^*$ coexists with the half-quantized Hall conductivity $e^2/2h$, forming an indicator for the parity anomalous semimetal. The relation signals a disorder-induced metallic phase that bridges two topologically distinct insulating phases, and agrees with the recent experimental observation in magnetic topological insulators.
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All authors Bo Fu, Kai-Zhi Bai, Shi-Hao Bi, Shun-Qing Shen

Soft Condensed Matter

Mastering Rheology: A Strategic and practical guide for empowering all users

Khushboo Suman

Rheology, the study of flow, plays a vital role in diverse industries such as pharmaceuticals, cosmetics and food. In this work, we provide a comprehensive introduction to fundamental rheological experiments and offer a strategic approach to understand the rheological behavior of any soft condensed material. We emphasize the importance of design of a good rheological experiment, input parameter and analysis of the obtained output parameter. Through the standard design of the experiment and systematic conduction of rheological experiments, we can estimate a broad range of rheological parameters such as viscosity, modulus, stability, yield stress, nonlinear behavior of the material. Furthermore, we also discuss methods for detecting and good practices for reducing some of the common experimental errors that may arise. We also present a comprehensive range of advanced rheological characterization techniques that can be pursued to gain deeper insights into the mechanical behavior and structural evolution of soft materials. Through this work, we attempt to make rheology more accessible to researchers from various domains to incorporate rheology into their characterization study and enable them to confidently navigate rheological studies and contribute to material development.
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Mesoscale and Nanoscale Physics

Effect of doping on hot-carrier thermal breakdown in perforated graphene metasurfaces

M. Ryzhii, V. Ryzhii, C. Tang et al. (+2 more)

We examine the robustness of the S-shaped current-voltage characteristics associated with hot-carrier-induced electrical breakdown in perforated graphene metasurfaces (PGMs) as a function of doping. The perforation of the graphene layer forms interdigital arrays of graphene microribbons (GMRs) interconnected by graphene nanoribbon (GNR) bridges. These GNR constrictions act as energy barriers for electrons and holes emitted from the GMRs and govern the inter-GMR thermionic current under an applied bias voltage. The doping and the voltage bias establish distinct electron and hole populations in adjacent GMRs. Peltier heating of these carriers within the GMRs increases their effective temperatures, thereby enhancing the inter-GMR current. The resulting positive feedback between carrier heating and current amplification can trigger an electrothermal breakdown, transforming a superlinear current-voltage dependence into an S-shaped characteristic exhibiting negative differential resistance. The degree of electron-hole asymmetry significantly influences this positive feedback and strongly modifies the overall current-voltage response. These results provide a framework for optimizing PGM-based devices employing GMR/GNR architectures, including voltage-controlled fast switches, incandescent emitters, and terahertz bolometric detectors.
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All authors M. Ryzhii, V. Ryzhii, C. Tang, T. Otsuji, M. S. Shur

Strongly Correlated Electrons

Kapitza-Dirac interference of Higgs waves in superconductors

Daemo Kang, Tien-Tien Yeh, Takahiro Morimoto et al. (+1 more)

We present a novel framework for controlling Higgs mode and vortex dynamics in superconductors using structured light. We propose a phenomenon analog of the Kapitza-Dirac effect in superconductors, where Higgs waves scatter off light-induced vortex lattices, generating interference patterns akin to matter wave diffraction. We also find that the vortices enable the linear coupling of Higgs mode to the electromagnetic field. This interplay between light-engineered Higgs excitations and emergent vortex textures opens a pathway to probe nonequilibrium superconductivity with unprecedented spatial and temporal resolution. Our results bridge quantum optics and condensed matter physics, offering new examples of quantum printing where one uses structured light to manipulate the collective modes in correlated quantum fluids.
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All authors Daemo Kang, Tien-Tien Yeh, Takahiro Morimoto, Alexander V. Balatsky

Materials Science

Ripple-assisted adsorption of noble gases on graphene at room temperature

Weilin Liu, Xianlei Huang, Li-Guo Dou et al. (+17 more)

Controllable gas adsorption is critical for both scientific and industrial fields, and high-capacity adsorption of gases on solid surfaces provides a significant promise due to its high-safety and low-energy consumption. However, the adsorption of nonpolar gases, particularly noble gases, poses a considerable challenge under atmospheric pressure and room temperature (RT). Here, we theoretically simulate and experimentally realize the stable adsorption of noble gases like xenon (Xe), krypton (Kr), argon (Ar), and helium (He) on highly rippled graphene at RT. The elemental characteristics of adsorbed Xe are confirmed by electron energy loss spectroscopy and X-ray photoelectron spectroscopy. The adsorbed gas atoms are crystalized with periodic arrangements. These adsorbed noble gases on graphene exhibit high stability at RT and can be completely desorbed at approximately 350 {\deg}C without damaging the intrinsic lattice of graphene. The structural and physical properties of graphene are significantly influenced by the adsorbed gas, and they fully recover after desorption. Additionally, this controllable adsorption could be generalized to other layered adsorbents such as NbSe2, MoS2 and carbon nanotubes. We anticipate that this ripple-assisted adsorption will not only re-define the theoretical framework of gas adsorption, but also accelerate advancements in gas storage and separation technologies, as well as enhance the applications in catalysis, surface modification, and other related fields.
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All authors Weilin Liu, Xianlei Huang, Li-Guo Dou, Qianglong Fang, Ang Li, Guowen Yuan, Yongjie Xu, Zhenjia Zhou, Jun Li, Yu Jiang, Zichong Huang, Zihao Fu, Peng-Xiang Hou, Chang Liu, Jinlan Wang, Wu Zhou, Ming-Gang Ju, Shao-Chun Li, Hui-Ming Cheng, Libo Gao

Strongly Correlated Electrons

Probing universal imaginary-time relaxation critical dynamics with infinite projected entangled pair states

He-Yu Lin, Shuai Yin, Z. Y. Xie et al. (+1 more)

We investigate the imaginary-time relaxation critical dynamics of the two-dimensional transverse-field Ising model using infinite projected entangled pair states (iPEPS) with the full-update strategy. Simulating directly in the thermodynamic limit, we explore the relaxation process near the critical point with two types of initial states: a fully polarized state and a product state with a small magnetization. For the fully polarized state, the magnetization shows a power law scaling $M\propto \tau^{-\beta/(\nu z)}$ in the imaginary-time evolution, from which both the critical point and critical exponent can be determined with high accuracy. For the nearly paramagnetic state, the relaxation process exhibits a behavior of $M\propto \tau^\theta$ with $\theta=0.1958$ being the critical initial-slip exponent, which is in good agreement with that obtained from the dynamic scaling of the self-correlation in quantum Monte Carlo method. These universal features emerge well before the system converges to the ground state, demonstrating the efficiency of imaginary-time evolution for probing quantum criticality. Our results demonstrate that iPEPS can serve as a robust and scalable method for studying dynamical critical phenomena in two-dimensional quantum many-body systems.
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All authors He-Yu Lin, Shuai Yin, Z. Y. Xie, Zhong-Yi Lu

Materials Science

Impact of Nitrogen Atom Clusters and Vacancy Defects on Graphene: A Molecular Dynamics Investigation

Indranil Rudra, Md. Moktadir Billah Tahmid, Jahid Emon et al. (+1 more)

Graphene's exceptional mechanical properties are crucial for its integration into advanced technological applications. However, real-world synthesis and functionalization processes introduce structural modifications that can compromise its mechanical integrity. Nitrogen doping, while beneficial for electronic property tuning, often results in atomic clustering rather than uniform distribution, while concurrent vacancy defect formation represents another common structural alteration during processing. This study systematically investigates the comparative effects of nitrogen atom clusters and equivalent sized vacancy defects on the mechanical behavior of graphene sheets through molecular dynamics simulations. The Nitrogen clustering significantly degraded mechanical performance almost similarly to random doping. In comparison, systems with equivalent-sized vacancy defects showed higher stiffness and lower ductility than those with clusters. The study revealed distinct failure mechanisms between doped and defective configurations, with nitrogen clusters showing modified crack propagation patterns while vacancies acted as pronounced stress concentrators, leading to premature failure. However, this study also showed that defect morphology critically influences mechanical properties. These findings provide important insights for optimizing graphene synthesis and processing protocols, highlighting the differential mechanical risks associated with dopant clustering versus vacancy formation. The results inform defect-tolerant design strategies for graphene-based nanoelectronics, composites, and sensors, where mechanical reliability is paramount for device performance and longevity.
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All authors Indranil Rudra, Md. Moktadir Billah Tahmid, Jahid Emon, Mohammad Jane Alam Khan

Soft Condensed Matter · Cross-listed from cs.RO

Terradynamics and design of tip-extending robotic anchors

Deniz Kerimoglu, Nicholas D. Naclerio, Sean Chu et al. (+5 more)

Most engineered pilings require substantially more force to be driven into the ground than they can resist during extraction. This requires relatively heavy equipment for insertion, which is problematic for anchoring in hard-to-access sites, including in extraterrestrial locations. In contrast, for tree roots, the external reaction force required to extract is much greater than required to insert–little more than the weight of the seed initiates insertion. This is partly due to the mechanism by which roots insert into the ground: tip extension. Proof-of-concept robotic prototypes have shown the benefits of using this mechanism, but a rigorous understanding of the underlying granular mechanics and how they inform the design of a robotic anchor is lacking. Here, we study the terradynamics of tip-extending anchors compared to traditional piling-like intruders, develop a set of design insights, and apply these to create a deployable robotic anchor. Specifically, we identify that to increase an anchor's ratio of extraction force to insertion force, it should: (i) extend beyond a critical depth; (ii) include hair-like protrusions; (iii) extend near-vertically, and (iv) incorporate multiple smaller anchors rather than a single large anchor. Synthesizing these insights, we developed a lightweight, soft robotic, root-inspired anchoring device that inserts into the ground with a reaction force less than its weight. We demonstrate that the 300 g device can deploy a series of temperature sensors 45 cm deep into loose Martian regolith simulant while anchoring with an average of 120 N, resulting in an anchoring-to-weight ratio of 40:1.
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All authors Deniz Kerimoglu, Nicholas D. Naclerio, Sean Chu, Andrew Krohn, Vineet Kupunaram, Alexander Schepelmann, Daniel I. Goldman, Elliot W. Hawkes

Mesoscale and Nanoscale Physics

Anomalous parametric resonance in a spin-1/2 chain: dynamical effects of nontrivial topology

Mahmoud T. Elewa and M. I. Dykman

Resonant parametric modulation is a major tool of studying magnetic systems. For a spin-1/2 chain in a strong magnetic field, the resulting excitations can be mapped on fermionic excitations in the Kitaev chain. We show that the response to turning on the modulation reveals dynamical bulk aspects of the nontrivial topology of the closed chain. In the topological regime, depending on the turn-on rate, the system displays an absence of frequency dispersion of the time-averaged magnetization and an absence or a suppression of its spatial correlations near resonance. The transition between the topological and trivial regimes is controlled by the modulation frequency.
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Materials Science

Lead-Free Europium Halide Perovskite Nanoplatelets

Sebastian Fernández, Divine Mbachu, Manchen Hu et al. (+9 more)

Metal halide perovskites possess desirable optical, material, and electrical properties which have had substantial impact on next-generation optoelectronics. However, given the toxicity of lead, alternative lead-free perovskite semiconductors are needed. By fully replacing lead with rare-earth elements, one can simultaneously address toxicity concerns and achieve comparable optoelectronic performance. Here, we demonstrate the synthesis of two-dimensional europium halide perovskite nanoplatelets governed by the formula $\mathrm{L_{2}EuX_{4}}$ where L is an organic ligand and X is a halide anion. The structure, morphology, and composition of the nanoplatelets are confirmed by XRD, AFM, and XPS. Deep blue-emitting $\mathrm{PEA_{2}EuBr_{4}}$ perovskite nanoplatelets are synthesized in both the solution- and solid-states with photoluminescence emission centered at 446 nm and CIE color coordinates of (0.1515, 0.0327) and (0.1515, 0.0342), respectively. Additionally, near-ultraviolet $\mathrm{PEA_{2}EuCl_{4}}$ perovskite nanoplatelets are synthesized in both the solution- and solid-states with photoluminescence emission centered at 400 nm and 401 nm, respectively. Overall, europium halide perovskite nanoplatelets offer a lead-free alternative for deep blue, violet, and near-ultraviolet light emission $\unicode{x2013}$ charting new pathways for optoelectronics in this energy regime.
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All authors Sebastian Fernández, Divine Mbachu, Manchen Hu, Han Cui, William Michaels, Pournima Narayanan, Tyler K. Colenbrander, Qi Zhou, Da Lin, Ona Segura Lecina, Guosong Hong, Daniel N. Congreve

Materials Science

Mechanistic Transition from Screw to Edge Dislocation Glide Enhances High-Temperature Strength in Refractory Complex Concentrated Alloys

Tamanna Zakia, Ayeman Nahin, Dunji Yu et al. (+8 more)

The strength of body-centered cubic materials is traditionally known to be governed by screw dislocations. However, recent findings reveal that in certain refractory complex concentrated alloys, edge dislocations can instead control strength. This work integrates high-temperature mechanical testing, in-situ neutron scattering during heating and tension, scanning transmission electron microscopy, and molecular dynamics simulations to uncover the mechanism behind this behavior. In the Nb-Ta-Ti-V system, increasing the V content, due to its smaller atomic size, induces substantial atomic misfit that raises the glide barrier for edge dislocations relative to screw dislocations. This effect drives a gradual transition from screw to edge dislocation-controlled deformation, leading to markedly enhanced strength at elevated temperatures.
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All authors Tamanna Zakia, Ayeman Nahin, Dunji Yu, Jacob Pustelnik, Juntan Li, Mason Kincheloe, Lia Amalia, Yan Chen, Peter K. Liaw, Haixuan Xu, Mingwei Zhang

Thursday 13 November

Materials Science · Cross-listed from cs.DC

Noise-Aware Optimization in Nominally Identical Manufacturing and Measuring Systems for High-Throughput Parallel Workflows

Christina Schenk, Miguel Hernández-del-Valle, Luis Calero-Lumbreras et al. (+2 more)

Device-to-device variability in experimental noise critically impacts reproducibility, especially in automated, high-throughput systems like additive manufacturing farms. While manageable in small labs, such variability can escalate into serious risks at larger scales, such as architectural 3D printing, where noise may cause structural or economic failures. This contribution presents a noise-aware decision-making algorithm that quantifies and models device-specific noise profiles to manage variability adaptively. It uses distributional analysis and pairwise divergence metrics with clustering to choose between single-device and robust multi-device Bayesian optimization strategies. Unlike conventional methods that assume homogeneous devices or generic robustness, this framework explicitly leverages inter-device differences to enhance performance, reproducibility, and efficiency. An experimental case study involving three nominally identical 3D printers (same brand, model, and close serial numbers) demonstrates reduced redundancy, lower resource usage, and improved reliability. Overall, this framework establishes a paradigm for precision- and resource-aware optimization in scalable, automated experimental platforms.
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All authors Christina Schenk, Miguel Hernández-del-Valle, Luis Calero-Lumbreras, Marcus Noack, Maciej Haranczyk

Materials Science

High Mobility Multiple-Channel AlScN/GaN Heterostructures

Aias Asteris, Thai-Son Nguyen, Chuan F.C. Chang et al. (+4 more)

Aluminum scandium nitride (AlScN) is a promising barrier material for gallium nitride (GaN)-based transistors for the next generation of radio-frequency electronic devices. In this work, we examine the transport properties of two dimensional electron gases (2DEGs) in single- and multi-channel AlScN/GaN heterostructures grown by molecular beam epitaxy, and demonstrate the lowest sheet resistance among AlScN-based systems reported to date. Assorted schemes of GaN/AlN interlayers are first introduced in single-channel structures between AlScN and GaN to improve conductivity, increasing electron mobility up to $1370$ cm$^{2}$/V$\cdot$s at 300 K and $4160$ cm$^{2}$/V$\cdot$s at 77 K, reducing the sheet resistance down to 170 $\Omega/\square$ and 70 $\Omega/\square$ respectively. These improvements are then leveraged in multi-channel heterostructures, reaching sheet resistances of 65 $\Omega/\square$ for three channels and 45 $\Omega/\square$ for five channels at 300 K, further reduced to 21 $\Omega/\square$ and 13 $\Omega/\square$ at 2 K, respectively, confirming the presence of multiple 2DEGs. Structural characterization indicates pseudomorphic growth with smooth surfaces, while partial barrier relaxation and surface roughening are observed at high scandium content, with no impact on mobility. This first demonstration of ultra-low sheet resistance multi-channel AlScN/GaN heterostructures places AlScN on par with state-of-the-art multi-channel Al(In)N/GaN systems, showcasing its capacity to advance existing and enable new high-speed, high-power electronic devices.
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All authors Aias Asteris, Thai-Son Nguyen, Chuan F.C. Chang, Chandrashekhar Savant, Pierce Lonergan, Huili Grace Xing, Debdeep Jena

Statistical Mechanics · Cross-listed from math-ph

Phase transitions in the Ising model on random graphs

Artem Alexandrov and Georgi S. Medvedev

We study phase transitions in the Ising model on random graphs using graph limits. We show that the critical temperatures are determined by the eigenvalues of the kernel operator associated with the graph limit. Bifurcation diagrams for Erdos-Renyi, small-world, and power-law graphs illustrate the theory. In the small-world case, we identify metastable behavior in both ferromagnetic and antiferromagnetic regimes.
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Mesoscale and Nanoscale Physics

Anyonic Chern insulator in graphene induced by surface electromagnon vacuum fluctuations

Xinle Cheng, Emil Viñas Boström, Frank Y. Gao et al. (+3 more)

Sub-wavelength cavities have emerged as a promising platform to realize strong light-matter coupling in condensed matter systems. Previous studies are limited to dielectric sub-wavelength cavities, which preserve time-reversal symmetry. Here, we lift this constraint by proposing a cavity system based on magneto-electric materials, which host surface electromagnons with non-orthogonal electric field and magnetic field components. The quantum fluctuations of the surface electromagnons drive a nearby graphene monolayer into an anyonic Chern insulator, characterized by anyonic quasi-particles and a topological gap that decays polynomially with the graphene-substrate distance. Our work opens a path to controllably break time-reversal symmetry and induce exotic quantum states through cavity vacuum fluctuations.
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All authors Xinle Cheng, Emil Viñas Boström, Frank Y. Gao, Edoardo Baldini, Dante M. Kennes, Angel Rubio

Statistical Mechanics · Cross-listed from quant-ph

Universal Thermodynamic Uncertainty Relation for Quantum $f-$Divergences

Domingos S. P. Salazar

We show that any Petz $f$-divergence (where $f$ is operator convex) between quantum states admits a universal $\chi^2$-mixture representation: the distinguishability of $\rho$ from $\sigma$ is obtained as a positive superposition of quadratic contrasts $\chi^2_\lambda$, with nonnegative weights $w_f(\lambda)$ determined explicitly from the Stieltjes representation of the generator $f$. This identifies $\chi^2_\lambda$ as atomic building blocks for quantum $f$-divergences and yields closed-form $w_f$ for canonical choices (relative entropy/KL, Hellinger/Bures, R'{e}nyi). By mapping $\chi^2_\lambda$ into a classical Pearson $\chi^2$, we leverage the Chapman-Robbins variational representation and obtain a tight and universal quantum thermodynamic uncertainty relation: any $f$-divergence is lower bounded by a function of the statistics of quantum observables (mean and variance), reproducing previous and novel results in quantum thermodynamics as applications.
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Statistical Mechanics · Cross-listed from q-bio.PE

Invading activity fronts or near-critical clusters may stabilize vulnerable excitable systems against stochastic trajectories terminating in an absorbing state

Kenneth A. V. Distefano, Sara Shabani and Uwe C. Täuber

Stochastic chemical reaction or population dynamics in finite systems often terminates in an absorbing state. Yet in large spatially extended systems, the time to reach the absorbing configuration (e.g., species extinction or fixation) becomes exceedingly long. Tuning control parameters may effectively diminish the survival probability, rendering active states (species coexistence) susceptible to stochastic extinction events. In inhomogeneous settings, where a vulnerable subsystem is diffusively coupled to an adjacent stable patch, the former is re-activated through influx from the interfaces, provided the vulnerable region either sustains spreading activity fronts or near-critical large active clusters. Either dynamical structure resets the system to correlated initial conditions favorable for survival. We demonstrate this generic elimination of finite-size extinction instabilities via immigration flux in stochastic predator-prey, cyclic competition, and epidemic spreading models.
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Quantum Gases

Paramagnetic phases of strongly correlated ultracold fermions coupled to an optical cavity

Renan da Silva Souza, Youjiang Xu and Walter Hofstetter

We numerically study a gas of two-component fermions coupled to a transversely pumped optical cavity and confined to a two-dimensional static square optical lattice. In the dispersive regime, the steady state of the system is described by an extended Hubbard Hamiltonian with cavity-mediated long-range interactions. Using real-space dynamical mean-field theory (RDMFT), we investigate the formation of the (superradiant) checkerboard density-wave phase. Our analysis focuses on paramagnetic phases both at quarter and half filling. At quarter filling, we find a reentrant homogeneous Fermi liquid to density wave phase transition with increasing temperature, which is due to the higher entropy of the ordered phase. At half filling, in addition to the Fermi liquid to Mott insulator phase transition, marked by a vanishing quasiparticle residue at the Fermi level, we identify the transition into a density-wave phase. Due to perfect Fermi surface nesting at half filling, we find that arbitrarily small long-range interactions destabilize the system towards the density-wave phase in the absence of short-range interactions. By varying short- and long-range interactions at a fixed low temperature, we obtain the full phase diagram and identify a region of coexistence between the homogeneous Fermi liquid and Mott insulating phase with the density-wave phase. In this region, we determine the thermodynamic phase transition by comparing the energies of the different RDMFT solutions.
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Superconductivity

Raman fingerprint of high-temperature superconductivity in compressed hydrides

Philip Dalladay-Simpson, Guglielmo Marchese, Zi-Yu Cao et al. (+5 more)

The discovery of high-temperature superconductivity in hydrogen-rich compounds under extreme pressures has prompted great excitement, intense research, but also debate over the past decade. Electrical transport has been the primary diagnostic tool for identifying superconductivity in these systems, whereas complementary probes, including magnetic, spectroscopic, tunnelling and ultrafast methods, remain mostly qualitative due to experimental constraints and sample heterogeneity. Recent concerns over their reliability have fuelled controversy, leading to scepticism and pointing out the need for alternative, quantitative approaches. In this study, we acquired unprecedented high-quality Raman spectra of hexagonal LaH10 at approximately 145 GPa and low temperatures, in conjunction with electrical transport measurements. Upon cooling, we observe a drop of resistivity and simultaneous remarkable variations of phonon frequencies and linewidths. These effects are interpreted and perfectly reproduced by the Migdal-Eliashberg theory, providing a definitive proof of phonon-mediated superconductivity and enabling a quantitative determination of the superconducting energy gap. Our results establish Raman spectroscopy as a robust, contact-free probe with micrometric resolution for studying high temperature superconductivity, opening a powerful route to its discovery and characterization.
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All authors Philip Dalladay-Simpson, Guglielmo Marchese, Zi-Yu Cao, Paolo Barone, Lara Benfatto, Gaston Garbarino, Francesco Mauri, Federico Aiace Gorelli

Materials Science · Cross-listed from hep-th

Anomalous resonance in Weyl semimetals: A holographic study of non-linear effects

Maximilian Gaschler, Andreas Schäfer and Sebastian Waeber

We consider hairy, magnetic black brane solutions to Einstein-Maxwell-Chern-Simons-dilaton theory with full back-reaction of the scalar and gauge fields and compute the time evolution with time dependent sources. The dual field theory's 't Hooft anomaly leads to long-lived current oscillations in the low temperature limit long after the electric pulse which created these modes has been switched off. We find that, generally, non-linear effects increase the decay rates of long-lived modes by a temperature dependent amount. Nonetheless, our results suggest that the life-time of time-translation symmetry breaking states can be made arbitrarily large if the temperature after the quench is sufficiently small. Experimentally this might be realizable in Weyl semimetals.
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Soft Condensed Matter

Solid-angle based nearest-neighbor algorithm adapted for systems with low coordination number

Alptuğ Ulugöl, Frank Smallenburg and Laura Filion

Nearest-neighbor identification is central to the analysis of local structure in condensed matter systems. The solid-angle-based nearest-neighbor (SANN) algorithm is widely used offering a parameter-free and computationally efficient alternative to cutoff- or Voronoi-based methods. Unfortunately, however, in systems with low coordination numbers, SANN tends to identify many particles as neighbors that are outside the nearest neighbor shell. Here, we propose a solution to this problem. Specifically, we propose a geometric modification, the “inscribed circle modification”, that resolves systematic overcounting in low-coordination lattices without introducing free parameters. We benchmark the modified algorithm (mSANN) against Voronoi and the original SANN algorithm in crystalline, quasicrystalline, and heterogeneous systems, and demonstrate that it provides robust and low-cost neighbor identification across both two and three dimensions.
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Mesoscale and Nanoscale Physics

Ordinary lattice defects as probes of topology

Aiden J. Mains, Jia-Xin Zhong, Yun Jing et al. (+1 more)

In addition to topological lattice defects such as dislocations and disclinations, crystals are also accompanied by unavoidable ordinary defects, devoid of any non-trivial geometry or topology, among which vacancies, Schottky defects, substitutions, interstitials, and Frenkel pairs are the most common. In this work, we demonstrate that these ubiquitous ordinary lattice defects, although geometrically trivial, can nonetheless serve as universal probes of the non-trivial topology of electronic Bloch bands, and any change in the local topological environment in an otherwise normal insulator in terms of mid-gap bound states in their vicinity. We theoretically establish these generic findings by implementing a minimal model Hamiltonian describing time-reversal symmetry breaking topological and normal insulators on a square lattice, fostering such point defects. The defect-bound mid-gap modes are also shown to be robust against sufficiently weak random point-like charge impurities. Furthermore, we showcase experimental observation of such bound states by embedding ordinary crystal defects in two-dimensional acoustic Chern lattices, where precision-controlled hopping amplitudes are implemented via active meta-atoms and Green's-function-based spectroscopy is used to reconstruct spectra and eigenstates. Our combined theory-experiment study establishes ordinary lattice defects as probes of topology that should be germane in crystals of any symmetry and dimension, raising the possibility of arresting localized Majorana modes near such defects in the bulk of topological superconductors and to emulate ordinary-defect-engineered topological devices.
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All authors Aiden J. Mains, Jia-Xin Zhong, Yun Jing, Bitan Roy

Strongly Correlated Electrons

Supernematic

Dan Mao and Eun-Ah Kim

Quantum theory of geometrically frustrated systems is usually approached as a gauge theory where the local conservation law becomes the Gauss law. Here we show that it can do something fundamentally different: enforce a global conserved quantity via a non-perturbative tiling invariant, rigorously linking microscopic geometry to a new macroscopically phase-coherent state. In a frustrated bosonic model on the honeycomb lattice in the cluster-charging regime at fractional filling, this mechanism protects a conserved global quantum number, the sublattice polarization $\tilde{N} = N_A - N_B$. Quantum fluctuation drives the spontaneous symmetry breaking of this global U(1) symmetry to result in a supernematic (SN) phase – an incompressible yet phase-coherent quantum state that breaks rotational symmetry without forming a superfluid or realizing topological order. This establishes a route to a novel quantum many-body state driven by combinatorial constraints.
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Materials Science

Uniaxial strain tuning of polar lattice vibrations in KTaO$_3$ and SrTiO$_3$

I. Khayr, N. Somun, S. Hameed et al. (+11 more)

The interplay of electronic and structural degrees of freedom is a prominent feature of many quantum materials and of particular interest in systems with strong ferroelectric fluctuations, such as SrTiO$_3$ (STO) and KTaO$_3$ (KTO). Both materials are close to a ferroelectric transition, but despite six decades of extensive research, pivotal questions regarding the nature of this transition and of the associated fluctuations remain debated. Here we combine inelastic neutron scattering, Raman spectroscopy, and ab initio calculations to study the evolution of soft polar phonons across the strain-induced ferroelectric transition in STO and KTO. We find that the modes remain underdamped and at nonzero energy, consistent with a first-order quantum phase transition. We also reveal a strong violation of the well-known Lyddane-Sachs-Teller relation between the phonon energies and static dielectric permittivities in insulating KTO and STO, which is not captured by ab initio calculations and points to the presence of slow mesoscale fluctuations induced by long-range interactions. In metallic STO, we uncover a first-order transition at a remarkably low critical stress, in qualitative agreement with recent theoretical predictions. The present work resolves several long-standing questions pertaining to the model systems STO and KTO and is relevant to numerous other materials with soft polar phonons.
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All authors I. Khayr, N. Somun, S. Hameed, Z. Van Fossan, X. He, R. Spieker, S. Chi, E. Clements, D. M. Pajerowski, M. Minola, B. Keimer, T. Birol, D. Pelc, M. Greven

Quantum Gases

Competition of fermion pairing, magnetism, and charge order in the spin-doped attractive Hubbard gas

Thomas Hartke, Botond Oreg, Chunhan Feng et al. (+7 more)

The tension between fermion pairing and magnetism affects numerous strongly correlated electron systems, from high-temperature cuprates to twisted bilayer graphene. Exotic forms of fermion pairing and superfluidity are predicted when attraction between fermions competes with spin doping. Here, we follow the evolution of fermion pairing and charge and spin order in a spin-imbalanced attractive Hubbard gas of fermionic $^{40}$K atoms, covering a wide range of densities, magnetizations, and interactions with single-atom resolution. At low spin imbalance and weak interactions, we find a mixture of nonlocal fermion pairs coexisting with itinerant excess fermions. For stronger interactions an effective hard-core Bose-Fermi mixture emerges. Spin doping drives a crossover from charge-density wave correlations to a Fermi liquid of polarons. Beyond a certain spin imbalance and interaction strength, we find evidence for the onset of combined spin- and pair-density wave order, a possible precursor for the existence of magnetized superfluidity in the attractive Hubbard system.
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All authors Thomas Hartke, Botond Oreg, Chunhan Feng, Carter Turnbaugh, Jens Hertkorn, Yuan-Yao He, Ningyuan Jia, Ehsan Khatami, Shiwei Zhang, Martin Zwierlein

Materials Science

Excitonic Landscapes in Monolayer Lateral Heterostructures Revealed by Unsupervised Machine Learning

Maninder Kaur, Nicolas T. Sandino, Jason P. Terry et al. (+2 more)

Two-dimensional (2D) in-plane heterostructures including compositionally graded alloys and lateral heterostructures with defined interfaces display rich optoelectronic properties and offer versatile platforms to explore one-dimensional interface physics and many-body interaction effects. Graded \(\mathrm{Mo}_x\mathrm{W}_{1-x}\mathrm{S}_2\) alloys show smooth spatial variations in composition and strain that continuously tune excitonic emission, while \(\mathrm{MoS}_2\)–\(\mathrm{WS}_2\) lateral heterostructures contain atomically sharp interfaces supporting one-dimensional excitonic phenomena. These single-layer systems combine tunable optical and electronic properties with potential for stable, high-performance optoelectronic devices. Hyperspectral and nano-resolved photoluminescence (PL) imaging enable spatial mapping of optical features along with local variations in composition, strain, and defects, but manual interpretation of such large datasets is slow and subjective. Here, we introduce a fast and scalable unsupervised machine-learning (ML) framework to extract quantitative and interpretable information from hyperspectral PL datasets of graded \(\mathrm{Mo}_x\mathrm{W}_{1-x}\mathrm{S}_2\) alloys and \(\mathrm{MoS}_2\)–\(\mathrm{WS}_2\) heterostructures. Combining principal-component analysis (PCA), t-distributed stochastic neighbor embedding (t-SNE), and density-based spatial clustering (DBSCAN), we uncover spectrally distinct domains associated with composition, strain, and defect variations. Decomposition of representative spectra reveals multiple emission species, including band-edge excitons and defect-related transitions, demonstrating that ML-driven analysis provides a robust and automated route to interpret rich optical properties of 2D materials.
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All authors Maninder Kaur, Nicolas T. Sandino, Jason P. Terry, Mahdi Ghafariasl, Yohannes Abate

Disordered Systems and Neural Networks · Cross-listed from quant-ph

Information phases of partial projected ensembles generated from random quantum states and scrambling dynamics

Alan Sherry, Saptarshi Mandal and Sthitadhi Roy

The projected ensemble – an ensemble of pure states on a subsystem conditioned on projective measurement outcomes on its complement – provides a finer probe of ergodicity and information structure than the reduced density matrix of the subsystem in bipartite quantum states. This framework can be generalised to partial projected ensembles in tripartite settings, where outcomes from part of the measured subsystem are discarded, leading to ensembles of mixed states. We show that information measures defined for such ensembles, in particular the Holevo information, yield a more detailed characterisation of how quantum information is distributed between subsystems compared to conventional entanglement measures. Using exact analytical results supported by numerical results, we uncover a qualitative change in the scaling of the Holevo information with system size in partial projected ensembles generated by Haar-random states, as the relative sizes of the subsystem are varied. In one phase, the Holevo information decays exponentially with system size, while in the other it grows linearly, thereby defining distinct information phases separated by sharp transitions signalled by non-analyticities in the Holevo information. The exponentially decaying phase rigorously establishes the existence of a measurement-invisible quantum-correlated phase – a manifestation of many-body information scrambling with no bipartite analogue. We contrast this information-phase diagram with the entanglement-phase structure of tripartite Haar-random states obtained from logarithmic negativity, and show that the Holevo information reveals additional fine structure beyond conventional entanglement measures. Finally, we show that these information phases, as characterised by the Holevo information, emerge in the dynamics of chaotic quantum circuits and discuss the associated timescales.
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Other Condensed Matter · Cross-listed from physics.optics

Broadband ultrafast self-heterodyned chiro-optical spectroscopy

Francesco Gucci{\dag}, Andrea Iudica{\dag}, Andres Valladares Y Tacchi{\dag} et al. (+12 more)

Ultrafast chiro-optical spectroscopy provides unique access to the structural dynamics of molecules, spin-valley relaxation in semiconductors, and the non-equilibrium optical response of chiral nanophotonic systems. Yet, because chiral signals are intrinsically weak and time-resolved spectroscopy probes small photoinduced changes, transient chiro-optical responses are often difficult to isolate from parasitic achiral contributions. Here, we introduce a broadband ultrafast chiro-optical spectroscopy technique that integrates a birefringent common-path interferometer with an optical polarization bridge to sensitively detect photoinduced changes in the polarization state of light. Phase-sensitive self-heterodyned detection enables simultaneous measurement of transient circular dichroism and optical rotatory dispersion across a broad spectral range with ultrafast temporal resolution. Balanced detection suppresses excess laser noise, enabling exceptional sensitivity (<50 ${\mu}$deg) close to shot-noise limit. We demonstrate this approach on an array of gold nano-helicoids, supported by a full-wave time-resolved model of the spatiotemporal dynamics of plasmonic non-equilibrium carriers and their associated optical nonlinearities. The model traces the system's transient chiro-optical response back to photoinduced modulations of the electric-magnetic dipole interaction in the nano-helicoid, elucidating the connection of near- and far-field dynamics in the non-equilibrium regime. We further investigate spin excitation, thermalization, and relaxation in a lead halide perovskite, establishing a novel approach to broadband time-resolved Faraday rotation. The simplicity, sensitivity, and wide applicability of this detection scheme provide a powerful platform for broadband ultrafast chiro-optical spectroscopy, opening new opportunities in biochemistry, solid-state physics, and nanophotonics.
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All authors Francesco Gucci{\dag}, Andrea Iudica{\dag}, Andres Valladares Y Tacchi{\dag}, Andrea Schirato, Giulia Crotti, Ryeong Myeong Kim, Soo Min Lee, Jeong Hyun Han, Andrea Villa, Dawar Ali, Aurora Rizzo, Margherita Maiuri, Ki Tae Nam, Giuseppe Della Valle, Giulio Cerullo

Materials Science · Cross-listed from cs.LG

Maximizing Efficiency of Dataset Compression for Machine Learning Potentials With Information Theory

Benjamin Yu, Vincenzo Lordi and Daniel Schwalbe-Koda

Machine learning interatomic potentials (MLIPs) balance high accuracy and lower costs compared to density functional theory calculations, but their performance often depends on the size and diversity of training datasets. Large datasets improve model accuracy and generalization but are computationally expensive to produce and train on, while smaller datasets risk discarding rare but important atomic environments and compromising MLIP accuracy/reliability. Here, we develop an information-theoretical framework to quantify the efficiency of dataset compression methods and propose an algorithm that maximizes this efficiency. By framing atomistic dataset compression as an instance of the minimum set cover (MSC) problem over atom-centered environments, our method identifies the smallest subset of structures that contains as much information as possible from the original dataset while pruning redundant information. The approach is extensively demonstrated on the GAP-20 and TM23 datasets, and validated on 64 varied datasets from the ColabFit repository. Across all cases, MSC consistently retains outliers, preserves dataset diversity, and reproduces the long-tail distributions of forces even at high compression rates, outperforming other subsampling methods. Furthermore, MLIPs trained on MSC-compressed datasets exhibit reduced error for out-of-distribution data even in low-data regimes. We explain these results using an outlier analysis and show that such quantitative conclusions could not be achieved with conventional dimensionality reduction methods. The algorithm is implemented in the open-source QUESTS package and can be used for several tasks in atomistic modeling, from data subsampling, outlier detection, and training improved MLIPs at a lower cost.
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Materials Science

In-vacuum surface flashover of SiN, AlN, and etched SiO2 thin films at micrometre scales

Vijay Kumar, Martin Siegele-Brown, Matthew Aylett et al. (+2 more)

We investigate the surface flashover voltage threshold for SiO$_2$, SiN, and AlN thin films over micrometre scale lengths. Furthermore, we test the effects of different etching chemistries on SiO$_2$ layers. We find that there is little significant difference between untreated SiO$_2$ samples and those that have been etched with hydrogen fluoride or Transene AlPad Etch 639. SiN and AlN samples performed significantly better than all SiO$_2$ samples giving a 45% increase in surface flashover voltage at a distance of 5 $\mu$m with the difference increasing with electrode spacing.
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All authors Vijay Kumar, Martin Siegele-Brown, Matthew Aylett, Sebastian Weidt, Winfried Karl Hensinger