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Physics Condensed Matter

cond-mat Watch Condensed Matter

Saturday 3 January

Materials Science

Efficient magnetization switching driven by orbital torque originating from light 3d-transition-metal nitrides

Gaurav K. Shukla, Yoshio Miura, Mayank K. Singh et al. (+1 more)

The orbital Hall effect (OHE) in light transition metals offers a promising route to generate orbital torques for efficient magnetization control, providing an alternative to conventional spin Hall effect approaches that rely on heavy metals. We demonstrate perpendicular magnetization switching in [Co/Pt]3 multilayers driven by the OHE in a light 3d transition metal nitride, VN, with 111-texture of face-center cubic structure. Second harmonic Hall measurement reveals a large torque efficiency of -0.41 in the VN(7.5 nm)/[Co(0.35nm)/Pt(0.3 nm)]3, which significantly surpasses that in the control samples with Co, Py, and CoFeB ferromagnets, suggesting strong conversion of orbital current originating from VN to spin current by [Co/Pt]3 ferromagnet. Full switching by in-plane current is achieved with an in-plane magnetic field, while partial field-free switching occurs without it. The critical current density for the switching is found to be comparable to that of the W-based spin-orbit torque device. First-principles calculations confirm a large orbital Hall conductivity in VN, with a small spin Hall conductivity around the Fermi energy. Our results highlight the potential in the combination of light 3d transition metal nitrides and Co/Pt ferromagnetic multilayer with 111-texture to maximize the magnetization switching efficiency of orbitronic devices.
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All authors Gaurav K. Shukla, Yoshio Miura, Mayank K. Singh, Shinji Isogami

Materials Science

Landscape of grain boundary migration in polycrystals

Jianfeng Hu

Grain boundary (GB) migration is a pivotal process that significantly impacts the development of microstructures, thereby influencing the practical performance of polycrystalline materials. Recent advances in 3D experimental techniques have revealed conflicts between observed GB migration behaviors and classical theoretical models. These contradictions raise two fundamental questions, namely, whether GB migration is linearly related to curvature, and how GB energy affect GB migration? Here, we provide a comprehensive analysis of GB migration dynamics in polycrystals and resolve these conflicts within a theoretical framework. Unexpectedly, in a polycrystalline system, the range of GB migration velocities shows little correlation with the magnitude of its curvature. The extent of the influence of GB energy on GB migration is revealed to mostly depend on GB step energy. Finally, a more general GB migration formula is derived to incorporate various driving forces beyond curvature.
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Superconductivity

Strain-triggered high-temperature superconducting transition in two-dimensional carbon allotrope

Tian Yan, Ru Zheng, Jin-Hua Sun et al. (+5 more)

Driving non-superconducting materials into a superconducting state through specific modulation is a key focus in the field of superconductivity. Pressure is a powerful method that can switch a three-dimensional (3D) material between non-superconducting and superconducting states. In the two-dimensional (2D) case, strain engineering plays a similar role to pressure. However, purely strain-induced superconductivity in 2D systems remains exceedingly scarce. Using first-principles calculations, we demonstrate that a superconducting transition can be induced solely by applying biaxial tensile strain in a 2D carbon allotrope, THO-graphene, which is composed of triangles, hexagons, and octagons. Free-standing THO-graphene is non-superconducting. Surprisingly, the electron-phonon coupling in strained THO-graphene is enhanced strong enough to pair electrons and realize superconductivity, with the highest superconducting transition temperature reaching 45 K. This work not only provides a notable example of controlling metal-superconductor transition in 2D system just via strain, but also sets a new record of superconducting transition temperature for 2D elemental superconductors.
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All authors Tian Yan, Ru Zheng, Jin-Hua Sun, Fengjie Ma, Xun-Wang Yan, Miao Gao, Tian Cui, Zhong-Yi Lu

Strongly Correlated Electrons

Spectral Visualization of Excitonic Pair Breaking at Individual Impurities in Ta2Pd3Te5

Lianzhi Yang, Deguang Wu, Hanbo Zhang et al. (+9 more)

Excitonic insulators host the condensates of bound electron-hole pairs, offering a platform for studying correlated bosonic quantum states. Yet, how macroscopic coherence emerges from locally collapsed pairing remains elusive. Here, using scanning tunnelling spectroscopy, we report the impurity-induced pair breaking in an excitonic insulator Ta2Pd3Te5. Individual Te vacancies are found to generate a pair of spectral peaks within the excitonic gap. Their energies depend sensitively on the defect configurations and are continuously tunable by tip electric field, indicating controllable impurity scatterings. Spectral mapping shows spatially anisotropic and electronically coupled electron-hole components of the subgap states. These observations, together with mean-field modelling, suggest an excitonic pair-breaking origin. In the strongly electron-hole imbalanced region, a secondary pair-breaking effect, manifesting as an additional pair of subgap states with distinctly lower energies, can emerge, presenting the interplay of pairing breakings with different excitonic order parameters. Our findings demonstrate the spectroscopic 'fingerprint' of local excitonic depairing at the atomic level, offering a crucial clue to the critical behavior across excitonic condensation.
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All authors Lianzhi Yang, Deguang Wu, Hanbo Zhang, Yao Zhang, Xiutong Deng, Chao Zhang, Tianyou Zhai, Wenhao Zhang, Youguo Shi, Rui Wang, Chaofei Liu, Ying-Shuang Fu

Mesoscale and Nanoscale Physics

Family of High-Chern-Number Orbital Magnets in Twisted Rhombohedral Graphene

Xirui Wang, L. Antonio Benítez, Vo Tien Phong et al. (+5 more)

Realizing Chern insulators with Chern numbers greater than one remains a major goal in quantum materials research. Such platforms promise multichannel dissipationless chiral transport and access to correlated phases beyond the conventional C = 1 paradigm. Here, we discover a family of high-Chern-number orbital magnets in twisted monolayer-multilayer rhombohedral graphene, denoted (1+n) with n = 3, 4, and 5. Magnetotransport measurements show pronounced anomalous Hall effects at one and three electrons per moiré unit cell when they are polarized away from the moiré interface. Across the (1+n) systems, we observe a clear topological hierarchy C = n, revealed by the Středa trajectories and the quantized Hall resistance. Our experimental observations are supported by self-consistent mean-field calculations. Moreover, we realize both electrical and magnetic switching of the high-Chern-number states by flipping the valley polarization. Together, these results establish a tunable hierarchy of orbital Chern magnets in twisted rhombohedral graphene, offering systematic control of Chern number and topology through layer engineering in pristine graphene moiré systems.
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All authors Xirui Wang, L. Antonio Benítez, Vo Tien Phong, Wai In Chu, Kenji Watanabe, Takashi Taniguchi, Cyprian Lewandowski, Pablo Jarillo-Herrero

Mesoscale and Nanoscale Physics

Detection of MEMS Acoustics via Scanning Tunneling Microscopy

R. J. G. Elbertse, M. Xu, A. Keşkekler et al. (+2 more)

Scanning tunneling microscopy (STM) and micro-electromechanical systems (MEMS) have traditionally addressed vastly different length scales - one resolving atoms, the other engineering macroscopic motion. Here we unite these two fields to perform minimally invasive-measurements of high aspect-ratio MEMS resonators using the STM tip as both actuator and detector. Operating at cryogenic temperatures, we resolve acoustic modes of millimeter-scale, high-Q membranes with picometer spatial precision, without making use of lasers or capacitive coupling. The tunneling junction introduces negligible back-action or heating, enabling direct access to the intrinsic dynamics of microgram-mass oscillators. In this work we explore three different measurement modalities, each offering unique advantages. Combined, they provide a pathway to quantum-level readout and exquisite high-precision measurements of forces, displacements, and pressures at cryogenic conditions. This technique provides a general platform for minimally-perturbative detection across a wide range of nanomechanical and quantum devices.
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All authors R. J. G. Elbertse, M. Xu, A. Keşkekler, S. Otte, R. A. Norte

Materials Science

Photogalvanic currents from first-principles real-time density-matrix dynamics

Junting Yu, Andrew Grieder, Jacopo Simoni et al. (+3 more)

The photogalvanic effect is the generation of a second-order direct current by illumination of a non-centrosymmetric material. In this work, we develop a first-principles real-time density matrix (FPDMD) formalism enabling the calculations of the photogalvanic current in all time regimes: transient and steady. Unlike past ab-initio studies which focused only on the photo-excitation process, our first-principles theory framework encodes all quantum scatterings (intra/interband relaxation and electron-hole recombination) mediated by bosons (photons and phonons), and is thus predictive of photogalvanic currents in realistic materials. In particular, for the linear photogalvanic effect, we find electron scatterings mediated by phonons contribute significantly to the shift current for prototypical piezoelectrics like BaTiO$_3$. For the circular photogalvanic effect, we develop a self-consistent theory of a steady injection current that incorporates realistic scattering mediated by phonons. Our formulation developed for photogalvanic current elucidates its connection with fundamental quantum-geometric quantities such as the Berry curvature and the quantum metric. A phonon-based explanation is proposed for the bipolar transient photogalvanic current observed by the THz emission spectroscopy.
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All authors Junting Yu, Andrew Grieder, Jacopo Simoni, Ravishankar Sundararaman, Aris Alexandradinata, Yuan Ping

Materials Science · Cross-listed from hep-ph

Search for Axions and Dark Photons Using Single Molecule Magnets

Jose R. Alves, Manfred Lindner, Farinaldo S. Queiroz et al. (+1 more)

Molecular magnets, although analogous to familiar macroscopic magnets, offer a platform for next generation magnetic storage technologies with far higher data densities and prospective applications in quantum information science. When exposed to an external magnetic field, single molecule magnets enter a frustrated magnetic configuration that is exceptionally sensitive to low energy excitations. Energy deposited by a dark matter particle can trigger the relaxation of a metastable molecule, releasing Zeeman energy that subsequently propagates through neighboring molecules. This magnetic avalanche encodes the energy deposited in the initial excitation. By combining concepts from chemistry, condensed matter physics, and particle physics, we show that dysprosium and manganese molecules can achieve more than an order of magnitude improvement in sensitivity to dark photon and QCD axion models, respectively, compared with existing detection methods.
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All authors Jose R. Alves, Manfred Lindner, Farinaldo S. Queiroz, Manoel S. Vasconcelos

Quantum Gases

Ground State and Collective Modes of Bose-Einstein Condensates in Newtonian and MOND-inspired gravitational potentials

Ning Liu

We analytically and numerically study the ground state and collective dynamics of Bose-Einstein condensates in two traps: a Newtonian potential and a logarithmic potential inspired by Modified Newtonian Dynamics (MOND). In the ground state, the MOND potential supports bound states only in the deep-MOND regime, where the condensate becomes significantly larger than its Newtonian counterpart. The size increases with repulsive coupling parameter $\beta$ in both potentials. A clear scaling law of the size with $\beta^{1/3}$ emerges in the MOND case and is confirmed numerically over a wide parameter range, while for the Newtonian potential no simple scaling law exists as the Thomas-Fermi approximation ceases to be valid. For the dynamics, we derive and solve equations for the monopole collective mode. The larger MOND-bound condensate oscillates at a lower frequency, which scales as $\beta^{-1/3}$ in the strong-interaction limit. These scaling laws provide insights for quantum-simulation experiments aiming to probe modified-gravity scenarios with cold atoms.
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Mesoscale and Nanoscale Physics

Terahertz magneto-photocurrents in the topological insulator Bi$_2$Se$_3$ probe its topological surface states

Chihun In, Genaro Bierhance, Deepti Jain et al. (+5 more)

We study ultrafast magneto-photocurrents in a three-dimensional topological insulator. For this purpose, we excite (In$_r$Bi$_{1-r}$)$_2$Se$_3$ thin films with a femtosecond laser pulse in the presence of an external magnetic field $B_{\text{ext}}$ up to 0.3 T parallel to the film plane. The resulting in-plane photocurrent is measured by detecting the emitted terahertz (THz) electromagnetic pulse. It scales linearly with $B_{\text{ext}}$ and is perpendicular to $B_{\text{ext}}$. Strikingly, for $r\ge$4%, we observe an abrupt photocurrent reduction, which is strongly correlated with the Indium-induced quenching of the topological surface states. The rise time, decay time and amplitude of the THz magneto-photocurrent can consistently be explained by a scenario in which optically excited spin-polarized electrons propagate toward the film surface where the accumulated spin is converted into an in-plane charge current due to spin-velocity locking. Our results are highly relevant for contact-free probing of spin-charge conversion in systems with paramagnetic rather than spontaneous magnetic order.
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All authors Chihun In, Genaro Bierhance, Deepti Jain, Tom S. Seifert, Oliver Gueckstock, Roberto Mantovan, Seongshik Oh, Tobias Kampfrath

Quantum Gases

Tunable chiral spiral phases in a non-Hermitian Ising-Gamma spin chain

Run-Dong Huang, Wei-Lin Li and Zhi Li

We study the influence of dissipation on the Ising-Gamma model. Through observables such as ground-state energy, order parameters, entanglement entropy, etc., we identify each phase region and provide the global phase diagram of the system. The results show that the region of the spiral phase will continuously expand with the increase of dissipation, gradually squeezing the original paramagnetic and antiferromagnetic phase regions. Remarkably, unlike the conservative system, the introduction of dissipation will cause two spiral phases with opposite chiralities to emerge simultaneously in the system, which provides a possibility for the manipulation of spiral chirality in cold atomic experiments. Moreover, we reveal the mechanism of the dependence of the transformation between these two spiral phases with distinct chirality on the strength of the relative coefficient of off-diagonal Gamma interactions in the Ising-Gamma model. Since both the relevant order parameters and dissipation can be well controlled within a detectable range, these phenomena can be observed in ultracold atomic experiments.
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Disordered Systems and Neural Networks

Disordered Dynamics in High Dimensions: Connections to Random Matrices and Machine Learning

Blake Bordelon and Cengiz Pehlevan

We provide an overview of high dimensional dynamical systems driven by random matrices, focusing on applications to simple models of learning and generalization in machine learning theory. Using both cavity method arguments and path integrals, we review how the behavior of a coupled infinite dimensional system can be characterized as a stochastic process for each single site of the system. We provide a pedagogical treatment of dynamical mean field theory (DMFT), a framework that can be flexibly applied to these settings. The DMFT single site stochastic process is fully characterized by a set of (two-time) correlation and response functions. For linear time-invariant systems, we illustrate connections between random matrix resolvents and the DMFT response. We demonstrate applications of these ideas to machine learning models such as gradient flow, stochastic gradient descent on random feature models and deep linear networks in the feature learning regime trained on random data. We demonstrate how bias and variance decompositions (analysis of ensembling/bagging etc) can be computed by averaging over subsets of the DMFT noise variables. From our formalism we also investigate how linear systems driven with random non-Hermitian matrices (such as random feature models) can exhibit non-monotonic loss curves with training time, while Hermitian matrices with the matching spectra do not, highlighting a different mechanism for non-monotonicity than small eigenvalues causing instability to label noise. Lastly, we provide asymptotic descriptions of the training and test loss dynamics for randomly initialized deep linear neural networks trained in the feature learning regime with high-dimensional random data. In this case, the time translation invariance structure is lost and the hidden layer weights are characterized as spiked random matrices.
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Friday 2 January

Materials Science · Cross-listed from physics.chem-ph

Co-doping of silicate bioceramics as a potential strategy to further enhance mono-doping consequences

Sohrab Mofakhami and Erfan Salahinejad

Silicate bioceramics have attracted significant attention in medical applications, particularly in hard tissue regeneration, because of their controllable chemical, physical, and biological functionalities, while ensuring biocompatibility. The coordination of silicate bioceramics with mono-dopants has been extensively studied to enhance their osteogenesis, angiogenesis, and antibacterial activity. However, the concept of employing dual or multiple co-doping to further enhance these biomaterials still demands more attention. This review paper originally focuses on the effect of chemical interactions among co-dopants and the principal constituents of the silicates, on the bio-behavior of these bioceramics. Additionally, future prospects of co-doped silicate bioceramics are outlined, including in vivo studies, clinical trials, and potential commercialization.
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Materials Science · Cross-listed from physics.chem-ph

AI-Guided Discovery of Novel Ionic Liquid Solvents for Industrial CO2 Capture

Davide Garbelotto, Alexander Lobo, Urvi Awasthi et al. (+8 more)

We present an AI-driven approach to discover compounds with optimal properties for CO2 capture from flue gas-refinery emissions' primary source. Focusing on ionic liquids (ILs) as alternatives to traditional amine-based solvents, we successfully identify new IL candidates with high working capacity, manageable viscosity, favorable regeneration energy, and viable synthetic routes. Our approach follows a five-stage pipeline. First, we generate IL candidates by pairing available cation and anion molecules, then predict temperature- and pressure-dependent CO2 solubility and viscosity using a GNN-based molecular property prediction model. Next, we convert solubility to working capacity and regeneration energy via Van't Hoff modeling, and then find the best set of candidates using Pareto optimization, before finally filtering those based on feasible synthesis routes. We identify 36 feasible candidates that could enable 5-10% OPEX savings and up to 10% CAPEX reductions through lower regeneration energy requirements and reduced corrosivity-offering a novel carbon-capture strategy for refineries moving forward.
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All authors Davide Garbelotto, Alexander Lobo, Urvi Awasthi, Oleg Medvedev, Srayanta Mukherjee, Anton Aristov, Konstantin Polunin, Alex De Mur, Leonid Zhukov, Azad Huseynov, Murad Abdullayev

Strongly Correlated Electrons

Tuneable skyrmion and anti-skyrmion fluids via mechanical strain in chiral kagome lattice

Gonzalo dos Santos, Flavia A. Gómez Albarracín, Ludovic D. C. Jaubert et al. (+3 more)

Magnetic skyrmions are nanometric swirling spin textures that exhibit remarkable stability at finite temperatures, making them promising candidates for spintronic applications. Achieving controllable stability and transitions between distinct topological structures is crucial for practical implementations. In this work, we investigate the effect of uniaxial mechanical strain on a magnetic model on the kagome lattice, focusing on skyrmion stability and emergent topological phases. To this end, we consider a Heisenberg model that includes exchange interactions and both in-plane and out-of-plane Dzyaloshinskii-Moriya interactions. Using a combination of Spin-Lattice Dynamics and Monte Carlo simulations, we explore uniaxial strain variations in the range of $-10\%$ to $10\%$, showing important effects on the phase diagram. For compressive strain, we find that the density of skyrmions in the skyrmion gas (SkG) phase can be tuned and that the stability of this phase extends to higher temperatures. Tensile strain, in contrast, reduces the number of skyrmions and promotes transitions to other magnetic states. Within this regime, strain levels of about ($\sim4-6\%$) lead to a change in topological charge, turning skyrmions ($Q=-1$) into antiskyrmions ($Q=+1$). We also examine how strain affects other phases commonly appearing in skyrmion-hosting systems, such as the helical and fully polarized states, showing that mechanical deformation alters their stability and characteristic properties. Finally, we compare these results with the strain response of a more conventional skyrmion model, in order to clarify the role of the different interactions involved. Our results identify strain as an experimentally accessible route for engineering topological spin textures.
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All authors Gonzalo dos Santos, Flavia A. Gómez Albarracín, Ludovic D. C. Jaubert, Pierre Pujol, Eduardo M. Bringa, H. Diego Rosales

Mesoscale and Nanoscale Physics

Symmetry and Topology in a Non-Hermitian Kitaev chain

Ayush Raj, Soham Ray and Sai Satyam Samal

We investigate the non-Hermitian Kitaev chain with non-reciprocal hopping amplitudes and asymmetric superconducting pairing. We work out the symmetry structure of the model and show that particle-hole symmetry (PHS) is preserved throughout the entire parameter regime. As a consequence of PHS, the topological phase transition point of a finite open chain coincides with that of the periodic (infinite) system. By explicitly constructing the zero-energy wave functions (Majorana modes), we show that Majorana modes necessarily occur as reciprocal localization pairs accumulating on opposite boundaries, whose combined probability density exhibits an exact cancellation of the non-Hermitian skin effect for the zero energy modes. Excited states, by contrast, generically display skin-effect localization, with particle and hole components accumulating at opposite ends of the system. At the level of bulk topology, we further construct a $\mathbb{Z}_2$ topological invariant in restricted parameter regimes that correctly distinguishes the topological and trivial phases. Finally, we present the topological phase diagram of the non-Hermitian Kitaev chain across a broad range of complex parameters and delineate the associated phase boundaries.
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Mesoscale and Nanoscale Physics

Moiré-driven equilibrium of perturbations in moiré systems

Federico Escudero, Zhen Zhan, Pierre A. Pantaleón et al. (+1 more)

Perturbations in moiré materials, such as due to substrates or strain, are common in many experiments and can significantly modify the electronic properties of the system. Here, we show that perturbations in twisted bilayer graphene tend to be transferred between the coupled Dirac cones, eventually reaching an equilibrium near the magic angle. We connect our results to experiments and show that this equilibrium behavior remains robust even when the moiré potential itself is perturbed. Our findings extend the notion of the magic angle to a more general regime governed by moiré-driven equilibrium.
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All authors Federico Escudero, Zhen Zhan, Pierre A. Pantaleón, Francisco Guinea

Superconductivity

AI-Guided Computational Design of a Room-Temperature, Ambient- Pressure Superconductor Candidate: Grokene

DEARDAO DeSci Collaborative Team and Yanhuai Ding

We introduce Grokene, a novel two-dimensional superlattice derived from graphene, which was identified through an AI-guided materials discovery workflow utilizing a large language model. Grokene is predicted to exhibit ambient-pressure, room-temperature superconductivity, with computational simulations revealing a high electron-phonon coupling constant and a substantial logarithmic-averaged phonon frequency (~1650 K), leading to a mean-field critical temperature of approximately 325 K. Full isotropic Eliashberg solutions further support a critical temperature around 310 K, underscoring its strong potential for room-temperature superconductivity. However, the strict two-dimensional nature of Grokene introduces phase fluctuations, limiting the observable superconducting transition to a Berezinskii-Kosterlitz-Thouless (BKT) temperature of about 120 K in monolayers. To elevate TBKT toward room temperature, strategies such as few-layer stacking, substrate or gate engineering, and optimization of superlattice structure and doping levels are proposed. Our integrated workflow, combining AI-driven materials discovery with advanced many-body theories (DFPT/EPW, Eliashberg, and RPA), provides a systematic and reproducible framework for exploring novel superconductors. We suggest that experimental synthesis and comprehensive characterization of Grokene will be essential to assess these computational predictions and to explore routes toward practical superconductivity under ambient pressure.
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Mesoscale and Nanoscale Physics

Symmetric approximant formalism for statistical topological matter

R. Johanna Zijderveld, Adam Yanis Chaou, Isidora Araya Day et al. (+1 more)

The standard approach to characterizing topological matter, computing topological invariants, fails when the symmetry protecting the topological phase is preserved only on average in a disordered system. Because topological invariants rely on enforcing the symmetry exactly, they can overcount phases by incorrectly identifying certain non-robust features as robust. Moreover, in intrinsic statistical topological insulators, enforcing the symmetry exactly is guaranteed to destroy the topological phase. We define a mapping that addresses both issues and provides a unified framework for describing disordered topological matter.
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All authors R. Johanna Zijderveld, Adam Yanis Chaou, Isidora Araya Day, Anton R. Akhmerov

Other Condensed Matter

The Thomas-Reiche-Kuhn sum rule as a consequence of a non-singular optical susceptibility in semiconductors

Angiolo Huamán

The Thomas-Reiche-Kuhn optical (TRK) sum rules for bulk materials have customarily been obtained by combining the Kramers-Kronig relations with the high frequency limit of the optical susceptibility tensor $\chi_{ij}$. Also, a non-singular expression for $\chi_{ij}$ involve the reduction of some its parts to an effective mass tensor. In this paper we show that the latter procedure is intimately connected to the TRK sum rules, and in fact these sum rules can be obtained from it. In reaching this result, we present before a thorough description of the momentum matrix elements of Bloch eigenfunctions bypassing the so-called $\bf{k}-$representation.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Exponentially Accelerated Sampling of Pauli Strings for Nonstabilizerness

Zhenyu Xiao and Shinsei Ryu

Quantum magic, quantified by nonstabilizerness, measures departures from stabilizer structure and underlies potential quantum speedups. We introduce an efficient classical framework for computing stabilizer Rényi entropies and stabilizer nullity of generic $N$-qubit wavefunctions. The method combines the fast Walsh-Hadamard transform with an exact partition of Pauli operators, reducing the average cost per sampled Pauli string from $\mathcal{O}(2^N)$ to $\mathcal{O}(N)$. We further develop a Monte Carlo estimator with Clifford preconditioning and find that the required number of samples shows no visible growth with $N$ in our benchmarks. Applying the method to $T$-doped random Clifford circuits, we identify the scrambling ratio $\eta$ (Clifford gates per $T$ gate) as the key parameter governing magic growth. Each $T$ gate approaches its dilute-limit nonstabilizerness power with only modest Clifford scrambling. Our approach enables quantitative studies of magic in highly entangled states and long-time nonequilibrium dynamics.
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Statistical Mechanics · Cross-listed from hep-th

Spin-operator form factors of the critical Ising chain and their finite volume scaling limits

Yizhuang Liu

In this work, we provide a self-contained derivation of the spin-operator matrix elements in the fermionic basis, for the critical periodic Ising chain at a generic system length $N\in 2Z_{\ge 2}$. The approach relies on the near-Cauchy property of certain matrices formed by the Toeplitz symbols in the critical model, and leads to a few square-root products for the leg functions. The square root products allow simple integral representations, that further reduce to the Binet's second integral and its generalization by Hermite, in the finite volume scaling limit. This leads to product formulas for the spin operator matrix elements in the scaling limit, providing explicit expressions for the spin-operator form factors of the Ising CFT in the fermionic basis, that were computed iteratively in Yurov:1991my. They are all rational numbers up to $\sqrt{2}$. We also determine the normalization factor of the spin-operator and show explicitly how the coefficient $G(\frac{1}{2})G(\frac{3}{2})$ appear through a ground state overlap. Moreover, by expanding the spin-spin two point correlator in the fermionic basis, we observed a Fredholm determinant identity that allows to show the convergence of the rescaled two-point correlator to the CFT version on a cylinder.
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Quantum Gases · Cross-listed from math-ph

The Ground State Energy of a Mean-Field Fermi Gas in Two Dimensions

Gregorio Casadei and Sascha Lill

We rigorously establish a formula for the correlation energy of a two-dimensional Fermi gas in the mean-field regime for potentials whose Fourier transform $\hat{V}$ satisfies $\hat{V}(\cdot) | \cdot | \in \ell^1$. Further, we establish the analogous upper bound for $\hat{V}(\cdot)^2 | \cdot |^{1 + \varepsilon} \in \ell^1$, which includes the Coulomb potential $\hat{V}(k) \sim |k|^{-2}$. The proof is based on an approximate bosonization using slowly growing patches around the Fermi surface. In contrast to recent proofs in the three-dimensional case, we need a refined analysis of low-energy excitations, as they are less numerous, but carry larger contributions.
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Strongly Correlated Electrons

Origin of the magnetic ground state of Tb$_{2}$Ir$_{2}$O$_{7}$

Y. Alexanian, E. Lhotel, J. Robert et al. (+10 more)

Magnetic-rare-earth pyrochlore iridates exhibit a rich variety of unconventional phases, driven by the complex interactions within and between the rare-earth and the iridium sublattices. In this study, we investigate the peculiar magnetic state of Tb$_{2}$Ir$_{2}$O$_{7}$, where a component of the Tb$^{3+}$ moment orders perpendicular to its local Ising anisotropy axis. By means of neutron diffraction and inelastic neutron scattering down to dilution temperatures, complemented by specific heat measurements, we show that this intriguing magnetic state is fully established at 1.5 K and we characterize its excitation spectrum across a broad range of energies. Our calculations reveal that bilinear interactions between Tb$^{3+}$ ions subjected to the Ir molecular field capture several key features of the experiments, but need to be supplemented to fully reproduce the observed behavior.
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All authors Y. Alexanian, E. Lhotel, J. Robert, S. Petit, E. Lefrançois, P. Lejay, A. Hadj-Azzem, F. Damay, J. Ollivier, B. Fåk, S. De Brion, R. Ballou, V. Simonet

Soft Condensed Matter

Strong anchoring boundary conditions in nematic liquid crystals: Higher-order corrections to the Oseen-Frank limit and a revised small-domain theory

Prabakaran Rajamanickam

Strong anchoring boundary conditions are conventionally modelled by imposing Dirichlet conditions on the order parameter in Landau-de Gennes theory, neglecting the finite surface energy of realistic anchoring. This work revisits the strong anchoring limit for nematic liquid crystals in confined two-dimensional domains. By explicitly retaining a Rapini-Papoular surface energy and adopting a scaling where the extrapolation length $l_{ex}$ is comparable to the coherence length $\xi$, we analyse both the small-domain ($\ep = h/\xi\to 0$; $h$ is the domain size) and Oseen-Frank $(\ep \to \infty$) asymptotic regimes. In the small-domain limit, the leading-order equilibrium solution is given by the average of the boundary data, which can vanish in symmetrically frustrated geometries, leading to isotropic melting. In the large-domain limit, matched asymptotic expansions reveal that surface anchoring introduces an $O(1/\ep)$ correction to the director field, in contrast to the $O(1/\ep^2)$ correction predicted by Dirichlet conditions. The analysis captures the detailed structure of interior and boundary defects, showing that mixed (Robin-type) boundary conditions yield smoother defect cores and more physical predictions than rigid Dirichlet conditions. Numerical solutions for square and circular wells with tangential anchoring illustrate the differences between the two boundary condition treatments, particularly in defect morphology. These results demonstrate that a consistent treatment of anchoring energetics, together with stability considerations, is essential for accurate modelling of nematic equilibria in micro- and nano-scale confined geometries.
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Materials Science

Electronic-Entropy-Driven Solid-Solid Phase Transitions in Elemental Metals

S. Azadi, S.M. Vinko, A.Principi et al. (+2 more)

We compute the thermodynamic phase diagram of seventeen elemental metals with hexagonal close-packed (hcp), face-centered cubic (fcc), and body-centered cubic (bcc) crystal structures using finite-temperature density functional theory. Helmholtz free-energy differences between competing hcp, fcc, and bcc phases are evaluated as functions of electronic temperature up to 7 eV, allowing us to identify solid-solid phase transitions driven by electronic entropy. The systems studied include Zr, Ti, Cd, Zn, Co, and Mg (hcp), Ni, Cu, Ag, Al, Pt, and Pb (fcc), and Cr, W, V, Nb, and Mo (bcc) in their ground-state structures. From the free-energy crossings, we extract the transition electronic temperatures and analyze systematic trends across the metallic systems. We found that all the studied systems go through one or two solid-solid phase transition caused purely by electronic entropy except Mg and Pb. Our results establish electronic entropy as a key factor governing structural stability in metals under strong electronic excitation.
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All authors S. Azadi, S.M. Vinko, A.Principi, T.D. Kuehne, M.S. Bahramy

Superconductivity

Nematic-fluctuation-mediated superconductivity in CuxTiSe2

Xingyu Lv, Yang Fu, Shangjie Tian et al. (+5 more)

The interplay among electronic nematicity, charge density wave, and superconductivity in correlated electronic systems has induced extensive research interest. Here, we discover the existence of nematic fluctuations in TiSe2 single crystal and investigate its evolution with Cu intercalation. It is observed that the elastoresistivity coefficient mEg exhibits a divergent temperature dependence following a Curie-Weiss law at high temperature. Upon Cu intercalation, the characteristic temperature T* of nematic fluctuation is progressively suppressed and becomes near zero when the superconductivity is optimized. Further intercalation of Cu leads to the sign change of T* and the suppression of superconductivity. These results strongly indicate that nematic phase transition may play a vital role in enhancing superconductivity in CuxTiSe2. Therefore, CuxTiSe2 provides a unique material platform to explore the nematic-fluctuation-mediated superconductivity.
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All authors Xingyu Lv, Yang Fu, Shangjie Tian, Ying Ma, Shouguo Wang, Cedomir Petrovic, Xiao Zhang, Hechang Lei

Mesoscale and Nanoscale Physics

Pseudo-Hermitian Magnon Dynamics

Jamal Berakdar and Xi-guang Wang

A defining quantity of a physical system is its energy which is represented by the Hamiltonian. In closed quantum mechanical or/and coherent wave-based systems the Hamiltonian is introduced as a Hermitian operator which ensures real energy spectrum and secures the decomposition of any state over a complete basis set spanning the space where the states live. Pseudo-Hermitian, or PT symmetric, systems are a special class of non-Hermitian ones. They describe open systems but may still have real energy spectrum. The eigenmodes are however not orthogonal in general. This qualitative difference to Hermitian physics has a range of consequences for the physical behaviour of the system in the steady state or when it is subjected to external perturbations. This overview reviews the recent progress in the field of pseudo-Hermitian physics as it unfolds when applied to low-energy excitations of magnetically ordered materials. The focus is mainly on long wave length spin excitations (spin waves) with magnons being the energy quanta of these excitations. Various setups including ferromagnetic, antiferromagnetic, magnonic crystals, and hybride structures with different types of coupling to the environments as well as spatio-temporally engineered systems will be discussed with a focus on the particular aspects that are brought about by the pseudo-Hermiticity such as mode amplifications, non-reciprocal propagation, magnon cloaking, non-Hermitian skin effect, PT-symmetric assisted Floquet engineering, topological energy transfer, and field-induced enhanced sensitivity.
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Quantum Gases

Chiral Dynamics Near Intra- and Inter-Band Exceptional Points under Dissipative Spin-Orbital-Angular-Momentum Coupling

Bo-Wen Liu, Ke-Ji Chen, Fan Wu et al. (+1 more)

We study the parametric chiral dynamics of atoms under dissipative spin-orbital-angular-momentum coupling (SOAMC). With atoms confined in the ring-shaped potential of the Laguerre-Gaussian Raman beams, the SOAMC not only couples the atomic center-of-mass angular momentum to the hyperfine spins, but also mixes different bands in the radial direction. This gives rise to a series of exceptional points of two types, the intra-band and the inter-band. Leveraging the topology of the spectral Riemann surface close to these exceptional points, we demonstrate the path-dependent chiral transfer of atoms to the higher-lying bands, by evolving the system along closed loops in the parameter space. Specifically, we illustrate two distinct scenarios, characterized by different mechanisms, where the atoms can be transferred to designated SOAMC-dressed bands. Our work demonstrates the rich exceptional structure in atom gases under dissipative SOAMC, and offers a novel route toward populating higher bands.
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All authors Bo-Wen Liu, Ke-Ji Chen, Fan Wu, Wei Yi

Soft Condensed Matter

Reversibility, Chaos, and Attractors in Periodically Sheared Elastic Filaments

Francesco Bonacci, Brato Chakrabarti, Olivia du Roure et al. (+2 more)

The dynamics of filaments in flow are central to understanding a wide range of biological and soft-matter systems, yet their behavior under time-dependent forcing remains poorly understood. Here, we investigate the long-time dynamics of Brownian inextensible elastic filaments subjected to strong uniform oscillatory shear by combining microfluidic experiments on actin filaments with numerical simulations based on a fluctuating Euler-Bernoulli elastica model in a viscous fluid. As the oscillation period increases, irreversibility emerges from the interplay of flow-induced deformations and thermal noise. This leads to a departure from reversible, deterministic rigid-body dynamics: in this regime, the filaments cycle between nearly straight, flow-aligned conformations at full periods and buckled shapes at half periods. Owing to the time-glide symmetry of the system, two such attracting states in fact coexist with a phase shift of half a period. The system spontaneously selects one, but occasionally switches between them as a result of noise, producing intermittent transitions between apparent order and disorder. This system constitutes an experimentally accessible realization of stochastic symmetry breaking, attractor hopping, and intermittency in a minimal nonequilibrium soft-matter system, with novel implications for the design and control of soft matter systems under time-dependent flows.
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All authors Francesco Bonacci, Brato Chakrabarti, Olivia du Roure, Anke Lindner, David Saintillan