Showing 1–50 of 367 results for author: Sato, T

Room-temperature magnon-phonon transduction in high-damping Co/Pt structures

Authors: Gauravkumar Patel, Takuma Sato, Maximilian Frenzel, Prakriti P. Joshi, Ruslan Salikhov, Ievgeniia Korniienko, Dominik Legut, Olav Hellwig, Sebastian F. Maehrlein, Kilian Lenz, Jürgen Lindner

Abstract: Quantum communication and information processing strongly benefit from the coupling between different quasi-particles, offering complementary advantages. Magnetoelastic materials inherently allow for direct coupling between magnetization dynamics and quantized lattice vibrations, called phonons. Near the ferromagnetic resonances, phonons may thus trade energy and angular momentum with uniformly pr… ▽ More Quantum communication and information processing strongly benefit from the coupling between different quasi-particles, offering complementary advantages. Magnetoelastic materials inherently allow for direct coupling between magnetization dynamics and quantized lattice vibrations, called phonons. Near the ferromagnetic resonances, phonons may thus trade energy and angular momentum with uniformly precessing magnetization, called magnons, and enable transduction of information from magnetic to phononic modes, thereby paving the way for long-range transport of magnetic information without the need of magnetic material. Here, we employ tailored magnetic-nonmagnetic heterostructures, which simultaneously act as cavities for standing shear waves, to bring selective phonons and magnons into resonance. These Co films with Pt seed layers show extended linewidth and reduced amplitude of the phonon-resonant FMR lines, providing a hallmark of energy and angular momentum exchange. Complementarily, by theoretical modeling and ultra-fast coherent phonon spectroscopy, we identify the responsible transverse acoustic phonons as standing shear waves in the combined Co and Pt structure. We find a high crystal quality in conjunction with a large magnetoelastic coupling constant as a prerequisite for efficient magnon-phonon coupling of this type. Such resonant enhancement of magnon-phonon coupling in CMOS-compatible material provides an ideal material platform for future quantum transducers. △ Less

Submitted 26 June, 2026; originally announced June 2026.

Dynamically suppressed lattice rotations in SrTiO$_3$ as a basis for photo-induced ferroelectricity

Authors: Huaiyu Hugo Wang, Michael Fechner, Giovanni De Vecchi, Sylvia L. Griffitt, Gal Orenstein, Jade Stanton, Viktor Krapivin, Man T. Wong, Zhuquan Zhang, Mina Bionta, Vincent Esposito, Meredith Henstridge, Matthias C. Hoffmann, Patrick L. Kramer, Zach Porter, Ryan A. Duncan, Takahiro Sato, Soyeun K. Kim, Hasan Yavas, Samuel Teitelbaum, Keith Nelson, Ankit S. Disa, Michael F"orst, Mariano Trigo, Andrea Cavalleri

Abstract: Photo-induced ferroelectricity in the quantum paraelectric SrTiO$_3$ involves the dynamical interplay between a coherently driven Ti-O stretching vibration and multiple structural degrees of freedom, including antiferrodistortive rotations, strain, and the polar mode instability. In the high-temperature cubic phase, in the absence of average antiferrodistortion, time-resolved X-ray diffuse scatter… ▽ More Photo-induced ferroelectricity in the quantum paraelectric SrTiO$_3$ involves the dynamical interplay between a coherently driven Ti-O stretching vibration and multiple structural degrees of freedom, including antiferrodistortive rotations, strain, and the polar mode instability. In the high-temperature cubic phase, in the absence of average antiferrodistortion, time-resolved X-ray diffuse scattering has evidenced a correlation between a photo-induced reduction in antiferrodistortive fluctuations and the emergence of ferroelectric order. Here, we complement these measurements with time-resolved elastic X-ray diffraction in the low-temperature tetragonal phase, in which antiferrodistortive fluctuations are small but a finite average rotation has set in. In this phase, we observe a long-lived reduction of the equilibrium antiferrodistortive rotation angle. A unified theory of the nonlinear lattice dynamics based on first-principles calculations describes the dynamics in both high-temperature cubic and low-temperature tetragonal phases, providing a basis for light-induced ferroelectricity in SrTiO$_3$. △ Less

Submitted 15 June, 2026; originally announced June 2026.

Comments: Huaiyu Hugo Wang and Michael Fechner contributed equally to this work

Field-induced asymmetric band flattening and ideal quantum geometry in rhombohedral graphene

Authors: Hongyun Zhang, Jinxi Lu, Size Wu, Yijie Wang, Kai Liu, Fei Wang, Wanying Chen, Lingzhi Wen, Jinling Zhou, Kenji Watanabe, Takashi Taniguchi, Jose Avila, Pavel Dudin, Matthew D. Watson, Takafumi Sato, Pu Yu, Wenhui Duan, Zhida Song, Guorui Chen, Shuyun Zhou

Abstract: Rhombohedral graphene exhibits an exceptionally diverse array of correlated phases that depend sensitively on the displacement field. Compiling reported phases into a unified phase diagram reveals a pronounced field-dependent electron-hole asymmetry: correlated states on the hole-doped side emerge at small displacement fields, whereas the fractional quantum anomalous Hall effect (FQAHE) is observe… ▽ More Rhombohedral graphene exhibits an exceptionally diverse array of correlated phases that depend sensitively on the displacement field. Compiling reported phases into a unified phase diagram reveals a pronounced field-dependent electron-hole asymmetry: correlated states on the hole-doped side emerge at small displacement fields, whereas the fractional quantum anomalous Hall effect (FQAHE) is observed exclusively on the electron-doped side under large displacement fields. This stark asymmetry highlights the need to understand how flat bands evolve with displacement fields. Here, we directly visualize the field-induced electron-hole asymmetric band flattening in rhombohedral pentalayer graphene (R5G) using nanospot angle-resolved photoemission spectroscopy with electrostatic gating. Beyond gap opening and spectral weight redistribution indicative of layer polarization, the gating field drives a strongly asymmetric modification of the flat bands: the flat valence band (FVB) evolves into an M-shaped dispersion at high field, whereas the flat conduction band (FCB) progressively flattens with increasing field. Comparison with calculations identifies critical parameters governing the band curvature of R5G, from which the resulting finite Berry curvature and near-ideal quantum geometry support the emergence of topological phases under electron doping at large fields. These results establish a direct link between the asymmetric phase diagram, band structure evolution, and quantum geometry, providing a microscopic framework for understanding correlated and topological phases in rhombohedral graphene. △ Less

Submitted 6 May, 2026; originally announced May 2026.

Comments: 17 pages, 5 figures

Building a physics-aware AI ecosystem for solid-state hydrogen storage materials

Authors: Seong-Hoon Jang, Yiwen Yao, Chuanyu Liu, Linda Zhang, Di Zhang, Xue Jia, Hung Ba Tran, Eric Jianfeng Cheng, Ryuhei Sato, Yusuke Ohashi, Toyoto Sato, Yusuke Hashimoto, Mark Allendorf, Nongnuch Artrith, Marcello Baricco, Andreas Borgschulte, Darren P. Broom, Ang Cao, Benjamin W. J. Chen, Lixin Chen, Ping Chen, Eun Seon Cho, Stefano Deledda, Zhao Ding, Martin Dornheim , et al. (44 additional authors not shown)

Abstract: Hydrogen storage remains a central bottleneck for scalable hydrogen energy systems due to the multiscale and coupled nature of the thermodynamics, kinetics, and microstructural evolution of hydrogen storage materials (HSMs). Although artificial intelligence (AI) has accelerated materials discovery, current approaches remain constrained by fragmented data, limited physical consistency, and weak int… ▽ More Hydrogen storage remains a central bottleneck for scalable hydrogen energy systems due to the multiscale and coupled nature of the thermodynamics, kinetics, and microstructural evolution of hydrogen storage materials (HSMs). Although artificial intelligence (AI) has accelerated materials discovery, current approaches remain constrained by fragmented data, limited physical consistency, and weak integration with experimental validation. Here, we propose a unified framework that integrates coherent data infrastructure, physics-grounded modeling, and AI-driven inverse design within a closed-loop discovery paradigm. By embedding physical constraints and experimental feedback, this approach enables adaptive, physically consistent optimization, thereby establishing a pathway toward autonomous, digital-twin-enabled discovery of HSMs. △ Less

Submitted 19 May, 2026; v1 submitted 4 May, 2026; originally announced May 2026.

Dynamical magnetotropic susceptibility as a new probe of Kitaev materials and beyond

Authors: João C. Inácio, J. Schwab, G. Rakhmanova, S. Safari, V. Zambra, H. Nasir, S. Paschen, K. A. Modic, Fakher F. Assaad, Toshihiro Sato

Abstract: The magnetotropic susceptibility $k(ω)$ probes ultra-low-frequency uniform fluctuations. For a crystal mounted on an oscillating cantilever in a magnetic field, it is defined as the ratio of torque to angular-displacement amplitude. Its real and imaginary parts determine the oscillation-frequency shift and crystal-induced damping. It is a low-energy probe of uniform $q=0$ spin and charge degrees o… ▽ More The magnetotropic susceptibility $k(ω)$ probes ultra-low-frequency uniform fluctuations. For a crystal mounted on an oscillating cantilever in a magnetic field, it is defined as the ratio of torque to angular-displacement amplitude. Its real and imaginary parts determine the oscillation-frequency shift and crystal-induced damping. It is a low-energy probe of uniform $q=0$ spin and charge degrees of freedom. We demonstrate this by deriving $k(ω)$ within linear response theory for a generic correlated-electron Hamiltonian with charge and spin degrees of freedom. Although it covers metallic and insulating magnets, correlated paramagnets, and exotic quantum critical points, we focus on limiting cases. For insulating spin systems $k(0)$ is sensitive to magnetic anisotropy whereas its finite-frequency imaginary part probes uniform dynamical spin susceptibility even in spin-symmetric models. For metallic systems we identify when eddy currents cause low-frequency damping. Our numerical results focus on Kitaev-material magnetotropic response. Using auxiliary-field quantum Monte Carlo with machine-learning-based sign-problem optimization we compute $k(ω)$ for several models proposed for $α$-RuCl$_3$. The observed low-temperature scaling of $k(0)/T$ with $B/T$ results from dominant Kitaev couplings: parameter sets without dominant Kitaev coupling do not exhibit this scaling. It remains robust upon inclusion of optical phonons. Beyond the static response, $k''(ω)$ for the $α$-RuCl$_3$ parameter set reproducing the experimental $k(0)$ data shows local-moment features at high and low $T$, with a single peak at the Larmor frequency. Beyond Kitaev systems we highlight broader applications. Probing ultra-low-energy uniform charge fluctuations is pertinent to Kondo destruction quantum criticality, of broad interest in strange metallicity and unconventional superconductivity. △ Less

Submitted 1 May, 2026; originally announced May 2026.

Comments: 35 pages, 8 figures

Robust topological surface states in skyrmion-host magnets Eu(Ga,Al)4: evidence for dual topology

Authors: Yuki Arai, Kosuke Nakayama, Takemi Kato, Tomonori Nakamura, Asuka Honma, Seigo Souma, Kenichi Ozawa, Kiyohisa Tanaka, Daisuke Shiga, Hiroshi Kumigashira, Yoshinori Okada, Kouji Segawa, Takafumi Sato

Abstract: The interplay between real-space topology such as magnetic skyrmions and momentum-space topology characterized by topological surface states (TSSs) is predicted to realize novel phenomena and functionalities, yet materials hosting both topologies are scarce. Skyrmion-hosting helimagnet family EuGa$_2$Al$_2$ and EuAl$_4$ has been a prime candidate for such a dual-topology system, but conclusive evi… ▽ More The interplay between real-space topology such as magnetic skyrmions and momentum-space topology characterized by topological surface states (TSSs) is predicted to realize novel phenomena and functionalities, yet materials hosting both topologies are scarce. Skyrmion-hosting helimagnet family EuGa$_2$Al$_2$ and EuAl$_4$ has been a prime candidate for such a dual-topology system, but conclusive evidence for its momentum-space topology has remained elusive. We provide this evidence by directly observing TSSs that stem from bulk Dirac nodal lines using high-resolution angle-resolved photoemission spectroscopy. These TSSs are exceptionally robust against various perturbations such as a 2$\times$1 surface reconstruction, a chemical change in the termination of the crystal surface, and the onset of helical antiferromagnetic order. Crucially, below the Neel temperature, we observe replica bands driven by the magnetic ordering. Moreover, we demonstrate clear surface-termination dependence of this magneto-topological coupling. Our findings establish Eu(Ga$_{1-x}$Al$_x$)$_4$ as a dual-topology material and offer a rare platform to explore and control the interaction between the two fundamental topological realms. △ Less

Submitted 14 April, 2026; originally announced April 2026.

Comments: 17 pages, 5 figures

Origin of multiple skyrmion phases in EuAl4

Authors: Y. Arai, K. Nakayama, A. Honma, S. Souma, D. Shiga, H. Kumigashira, T. Takahashi, K. Segawa, T. Sato

Abstract: The Dzyaloshinskii-Moriya (DM) interaction has been considered essential for skyrmion formation, however, the discovery of skyrmion lattices (SkLs) in nominally centrosymmetric materials where the DM interaction is forbidden, such as Eu(Ga$_{1-x}$Al$_x$)$_4$, has challenged this established view. Recent structural investigations of Eu(Ga$_{1-x}$Al$_x$)$_4$ have further complicated this issue by re… ▽ More The Dzyaloshinskii-Moriya (DM) interaction has been considered essential for skyrmion formation, however, the discovery of skyrmion lattices (SkLs) in nominally centrosymmetric materials where the DM interaction is forbidden, such as Eu(Ga$_{1-x}$Al$_x$)$_4$, has challenged this established view. Recent structural investigations of Eu(Ga$_{1-x}$Al$_x$)$_4$ have further complicated this issue by revealing that the charge-density wave breaks local symmetry, theoretically allowing DM interaction. This raises a fundamental question: are the complex magnetic phases driven by the DM interaction or by alternative mechanisms? Here, using soft-x-ray angle-resolved photoemission spectroscopy, we determine the three-dimensional bulk electronic structure of Eu(Ga$_{1-x}$Al$_x$)$_4$, and elucidate the electronic origins of its rich magnetic orders. We directly observe an x-dependent Lifshitz transition leading to the emergence of a Fermi-surface pocket. Importantly, multiple nesting vectors derived from this pocket match the symmetries and periodicities of the multiple SkLs. Moreover, these nesting vectors can also account for other magnetic orders, such as the zero-field helical magnetism, suggesting a common electronic origin of the complex magnetic phases. These findings suggest that competing nesting-induced Ruderman-Kittel-Kasuya-Yosida interactions and their engineering can generate and control various SkLs and related topological spin textures. △ Less

Submitted 14 April, 2026; originally announced April 2026.

Comments: 27 pages, 4 figures, author's version

Journal ref: Nature Communications 17, 3162 (2026)

A unified descriptor framework for hydrogen storage capacity and equilibrium pressure in interstitial hydrides

Authors: Seong-Hoon Jang, Di Zhang, Xue Jia, Hung Ba Tran, Linda Zhang, Ryuhei Sato, Yusuke Hashimoto, Yusuke Ohashi, Toyoto Sato, Kiyoe Konno, Shin-ichi Orimo, Hao Li

Abstract: Hydrogen is a promising energy carrier, yet its practical deployment is limited by the lack of storage materials that simultaneously achieve high storage capacity ($w$) and practical equilibrium pressure at room temperature ($P_{\rm eq,RT}$). Interstitial metal hydrides offer fast kinetics and favorable thermodynamics (high $P_{\rm eq,RT}$) but suffer from intrinsically low w. Here, we establish a… ▽ More Hydrogen is a promising energy carrier, yet its practical deployment is limited by the lack of storage materials that simultaneously achieve high storage capacity ($w$) and practical equilibrium pressure at room temperature ($P_{\rm eq,RT}$). Interstitial metal hydrides offer fast kinetics and favorable thermodynamics (high $P_{\rm eq,RT}$) but suffer from intrinsically low w. Here, we establish a physically interpretable, data-driven framework to uncover descriptor-property relationships in interstitial hydrides using a curated database of pressure-composition-temperature measurements (Digital Hydrogen Platform, DigHyd) and white-box symbolic regression. Strikingly, the analysis reveals a clear separation of governing mechanisms, in which $w$ is governed by geometric and lattice conditions, captured by the average atomic radius ($\left\langle r_M \right\rangle$) and average thermal conductivity ($\left\langleκ\right\rangle$), with an optimal regime of $r_M \sim 1.47 Å$ and relatively low $\left\langleκ\right\rangle$. In contrast, $P_{\rm eq,RT}$ is governed by elastic properties, captured by the average shear modulus ($\left\langle G \right\rangle$) and average Poisson's ratio ($\left\langle ν\right\rangle$), reflecting the role of lattice rigidity and mechanical compliance. These relationships are translated into compositional optimization pathways that follow the descriptor trends above, enabling the design of candidate materials with enhanced w under practical equilibrium conditions ($P_{\rm eq,RT} \sim 0.1$ MPa). This work establishes a general, interpretable strategy for physics-informed design of energy materials systems. △ Less

Submitted 13 April, 2026; originally announced April 2026.

Type-I and Type-II Saddle Points and a Topological Flat Band in a Bi-Pyrochlore Superconductor CsBi2

Authors: Yusei Morita, Yongkai Li, Yu-Hao Wei, Kosuke Nakayama, Zhiwei Wang, Hua-Yu Li, Takemi Kato, Seigo Souma, Kiyohisa Tanaka, Kenichi Ozawa, Jia-Xin Yin, Takashi Takahashi, Min-Quan Kuang, Yugui Yao, Takafumi Sato

Abstract: The divergence of the electron density of states (DOS) plays an important role in enhancing many-body interactions and inducing various quantum phases in low-dimensional systems. However, such unique electronic structures remain experimentally elusive in three-dimensional (3D) systems, particularly those with strong spin-orbit coupling (SOC). Using angle-resolved photoemission spectroscopy and fir… ▽ More The divergence of the electron density of states (DOS) plays an important role in enhancing many-body interactions and inducing various quantum phases in low-dimensional systems. However, such unique electronic structures remain experimentally elusive in three-dimensional (3D) systems, particularly those with strong spin-orbit coupling (SOC). Using angle-resolved photoemission spectroscopy and first-principles calculations for a Laves-phase superconductor CsBi$_2$, which features a Bi-pyrochlore 3D network with strong SOC, we identify two characteristic electronic structures with a large DOS. One is a dispersionless topological flat band with p-orbital character, locally formed around the U-K line, which enhances DOS near the Fermi level. The other involves type-I and type-II saddle points connected by a flat band, which cooperatively produce an enhancement in the DOS. Our findings suggest a novel mechanism for achieving a DOS enhancement and lay a foundation for exploring exotic phenomena driven by the interplay of multiple singularities with a large DOS, nontrivial topology, and strong SOC in 3D pyrochlores. △ Less

Submitted 14 April, 2026; v1 submitted 9 April, 2026; originally announced April 2026.

Comments: 18 pages, 3 figures

Magnetic toroidal monopoles from relativistic polarization responses to magnetic field gradients

Authors: Taisei Yamanaka, Takumi Sato, Satoru Hayami

Abstract: The magnetic toroidal monopole, a time-reversal-odd scalar, has attracted attention through its characteristic responses, such as electric-field-induced nonreciprocal directional dichroism observed in Co$_2$SiO$_4$. However, its evaluation in crystalline solids remains unresolved, as it cannot be defined within conventional multipole expansions or thermodynamic formulations. In this paper, we prop… ▽ More The magnetic toroidal monopole, a time-reversal-odd scalar, has attracted attention through its characteristic responses, such as electric-field-induced nonreciprocal directional dichroism observed in Co$_2$SiO$_4$. However, its evaluation in crystalline solids remains unresolved, as it cannot be defined within conventional multipole expansions or thermodynamic formulations. In this paper, we propose a theoretical framework to evaluate the magnetic toroidal monopole in periodic crystals based on the response of relativistic electric polarization to a magnetic field gradient. By incorporating the magnetic-field-gradient correction to the relativistic polarization, we derive an explicit expression for the magnetic toroidal monopole beyond symmetry arguments. The resulting expression is formulated in terms of geometric quantity such as Berry curvatures and orbital magnetic moment defined in an extended parameter space spanning momentum, magnetic field, and electric field. We further perform model calculations for an antiferromagnetic system hosting a magnetic toroidal monopole and confirm that the proposed quantity is finite. These results provide a practical route to characterize magnetic toroidal monopoles in crystalline solids and clarify their quantum geometric nature. △ Less

Submitted 7 April, 2026; originally announced April 2026.

Comments: 9 pages, 4 figures

Thermodynamic Multipoles and Dissipative Conductivities in Metallic Systems

Authors: Takumi Sato, Satoru Hayami

Abstract: Multipoles provide a systematic framework for describing the electronic structures of quantum materials from a symmetry perspective. Thermodynamic multipole moments in crystalline solids exhibit direct microscopic connections to certain allowed physical responses beyond symmetry; however, such relations have thus far been limited to dissipationless responses in equilibrium insulating systems. Here… ▽ More Multipoles provide a systematic framework for describing the electronic structures of quantum materials from a symmetry perspective. Thermodynamic multipole moments in crystalline solids exhibit direct microscopic connections to certain allowed physical responses beyond symmetry; however, such relations have thus far been limited to dissipationless responses in equilibrium insulating systems. Here, this framework is extended at a heuristic level by focusing on the Fermi-surface contributions to thermodynamic multipole moments. These contributions establish direct relations to dissipative transport responses characteristic of metals, including charge and spin conductivities. A key consequence is that the conductivities exhibit extrema, typically maxima, at chemical potentials where the corresponding Fermi-surface contributions to the multipoles vanish, specifically, the electric quadrupole for charge conductivity and the magnetic octupole for spin conductivity. These findings uncover a previously overlooked aspect of thermodynamic multipole moments, opening a new perspective on dissipative transport in metallic systems. △ Less

Submitted 26 June, 2026; v1 submitted 31 March, 2026; originally announced March 2026.

Digital Hydrogen Platform (DigHyd): A Rigorously Curated Database for Hydrogen Storage Materials Empowered by AI-Assisted Literature Mining

Authors: Seong-Hoon Jang, Di Zhang, Xue Jia, Hung Ba Tran, Linda Zhang, Ryuhei Sato, Yusuke Hashimoto, Toyoto Sato, Kiyoe Konno, Shin-ichi Orimo, Hao Li

Abstract: Solid-state hydrogen storage materials are promising candidates for safe and compact hydrogen storage; however, data-driven discovery in this field remains limited by the availability of large-scale, well-curated datasets. Here, we present the Digital Hydrogen Platform (DigHyd: www.dighyd.org), a rigorously curated database comprising $>4,000$ experimental literature sources and $>30,000$ data ent… ▽ More Solid-state hydrogen storage materials are promising candidates for safe and compact hydrogen storage; however, data-driven discovery in this field remains limited by the availability of large-scale, well-curated datasets. Here, we present the Digital Hydrogen Platform (DigHyd: www.dighyd.org), a rigorously curated database comprising $>4,000$ experimental literature sources and $>30,000$ data entries on hydrogen storage materials, constructed through AI-assisted literature mining combined with human-in-the-loop validation. In addition to gravimetric hydrogen storage density ($w$), DigHyd also covers thermodynamic parameters, specifically the enthalpy ($ΔH$) and entropy ($ΔS$) changes associated with hydrogenation reactions, primarily defined as $M + \frac{1}{2} {\rm H}_2 \rightleftarrows M{\rm H}$. These parameters were obtained by manually analyzing multi-temperature pressure-composition-temperature (PCT) data using van't Hoff analysis. By focusing on $ΔH$ and $ΔS$ rather than fixing equilibrium pressure at a single temperature, DigHyd enables flexible evaluation of equilibrium behavior under application-specific operating conditions. Statistical analyses reveal distinct distributions of thermodynamic parameters across material classes, together with broad compositional variability within representative hydride systems. Furthermore, both physically interpretable symbolic regression and black-box XGBoost models achieve comparable predictive performance for $w$ and equilibrium pressure at room temperature ($P_{\rm eq,RT}$), demonstrating internal consistency and learnable composition-property relationships within the curated dataset. Overall, DigHyd provides a rigorously curated thermodynamic dataset that serves as a reliable basis for data-driven analyses of hydrogen storage materials and supports systematic exploration of structure-property relationships. △ Less

Submitted 14 March, 2026; originally announced March 2026.

Time Resolved Study of Laser Induced Ultrafast Alloying Processes in Au/Pd Core Shell Nanorods

Authors: Abhisakh Sarma, Jayanath C. P. Koliyadu, Romain Letrun, Egor Sobolev, Trupthi Devaiah C, Agnieszka Wrona, Katerina Doerner, Diogo V. M. Melo, Marco Kloos, Huijong Han, Marcin Sikorski, Konstantin Kharitonov, Juncheng E, Joana Valerio, Pralay K. Santra, Erik M. J. Johansson, Richard Bean, Chan Kim, Tokushi Sato

Abstract: Femtosecond laser-induced alloying presents a novel approach to modifying bimetallic systems. Visualizing ultrafast processes during laser-induced alloying is essential to uncover fundamental mechanisms associated with phase transformations, which enables precise control over material composition and structure at the atomic level. In this study, we investigated the ultrafast dynamics of laser-indu… ▽ More Femtosecond laser-induced alloying presents a novel approach to modifying bimetallic systems. Visualizing ultrafast processes during laser-induced alloying is essential to uncover fundamental mechanisms associated with phase transformations, which enables precise control over material composition and structure at the atomic level. In this study, we investigated the ultrafast dynamics of laser-induced alloying of Au/Pd core-shell nanorods using a time-resolved X-ray diffraction technique at an X-ray free-electron laser facility, capturing the structural evolution from picoseconds to microsecond timescales. We found that a laser fluence threshold of ~ 48 mJ/cm2 with 800 nm excitation is sufficient for melting and subsequent alloy formation. Above this threshold, the formation of Au1.51Pd0.49 was observed, and we found that alloying is not a single-step phenomenon; instead, it is a dynamic process involving interdiffusion. △ Less

Submitted 2 March, 2026; originally announced March 2026.

Comments: The submission consists of a single PDF file containing both the main text and the Supporting Information. The main text includes 20 pages and 5 figures, while the Supporting Information comprises 21 pages and 10 figures

Revealing altermagnetic Fermi surfaces with two Kondo impurities

Authors: Qiong Qin, Toshihiro Sato, Marcin Raczkowski, Jeroen van den Brink, Congjun Wu, Fakher F. Assaad

Abstract: Motivated by recent advances in the study of altermagnetism, or unconventional magnetism, and in the realization and manipulation of two-impurity Kondo physics in real materials, we propose a phase-sensitive method to explore unconventional magnetic symmetries. Our method can be implemented with spin-resolved scanning tunneling microscopy to study two-impurity Kondo phenomena on altermagnetic meta… ▽ More Motivated by recent advances in the study of altermagnetism, or unconventional magnetism, and in the realization and manipulation of two-impurity Kondo physics in real materials, we propose a phase-sensitive method to explore unconventional magnetic symmetries. Our method can be implemented with spin-resolved scanning tunneling microscopy to study two-impurity Kondo phenomena on altermagnetic metals by varying the distance and orientation between magnetic impurities. Using quantum Monte Carlo simulations, we analyze the spin splitting of the Kondo resonance, whose spatial distribution sensitively captures the symmetry of the underlying altermagnetic order. Furthermore, the impurity spin correlations reflects the anisotropy of the RKKY interaction due to the altermagnetic Fermi surface splitting. This work provides a framework for studying the competition between the Kondo effect, the RKKY interaction and altermagnetism, in the simplest possible system. △ Less

Submitted 11 January, 2026; originally announced January 2026.

Orbital magnetic octupole in crystalline solids and characterization of orbital altermagnetism

Authors: Takumi Sato, Satoru Hayami

Abstract: Magnetic multipole moments beyond dipoles have emerged as key descriptors of unconventional electromagnetic responses in crystalline solids. However, a gauge-invariant bulk expression for orbital magnetic multipole moments has remained elusive, hindering a unified understanding of their physical consequences. Here we formulate a gauge-invariant expression for the orbital magnetic octupole moment i… ▽ More Magnetic multipole moments beyond dipoles have emerged as key descriptors of unconventional electromagnetic responses in crystalline solids. However, a gauge-invariant bulk expression for orbital magnetic multipole moments has remained elusive, hindering a unified understanding of their physical consequences. Here we formulate a gauge-invariant expression for the orbital magnetic octupole moment in periodic crystals and investigate its behavior in two models with distinct origins of magnetism: a spinless two-orbital model with orbital magnetic order arising from complex hopping and a two-sublattice $d$-wave altermagnetic model based on antiferromagnetic spin order. We also show that the orbital magnetic octupole is naturally linked to a higher-rank Hall response induced by spatially nonuniform electric fields, leading to a generalized Středa-type relation. Our results further demonstrate that the orbital magnetic octupole provides a quantitative characterization of \textsl{orbital altermagnetism}. △ Less

Submitted 7 June, 2026; v1 submitted 30 December, 2025; originally announced December 2025.

Multipolar orbital relaxation of the $t_{2g}$ states

Authors: Aurélien Manchon, Xiaobai Ning, Chi Sun, Tetsuya Sato, Takeo Kato, Tatiana Rappoport

Abstract: Using a nonperturbative approach, the relaxation rate of orbital dipolar and quadrupolar moments is computed analytically for the t2g states. In the presence of short-range impurities and in the absence of spin-orbit coupling, the orbital relaxation emerges from the competition between momentum scattering and the effect of the crystal field. In the case of weak disorder, the orbital relaxation tim… ▽ More Using a nonperturbative approach, the relaxation rate of orbital dipolar and quadrupolar moments is computed analytically for the t2g states. In the presence of short-range impurities and in the absence of spin-orbit coupling, the orbital relaxation emerges from the competition between momentum scattering and the effect of the crystal field. In the case of weak disorder, the orbital relaxation time is proportional to the momentum scattering time: each scattering event contributes to destroying the orbital moment. In the case of strong disorder, the effect of the crystal field is averaged out, and the orbital relaxation time is inversely proportional to the momentum scattering. We finally find that the dipolar and quadrupolar orbital moments are coupled by the crystal field, resulting in a complex dynamical behavior upon orbital injection. △ Less

Submitted 6 December, 2025; originally announced December 2025.

Comments: 5 pages, 4 figures

A Combined Theoretical and Experimental Study of Oxygen Vacancies in Co$_3$O$_4$ for Liquid-Phase Oxidation Catalysis

Authors: Amir Omranpour, Lea Kämmerer, Catalina Leiva-Leroy, Anna Rabe, Takuma Sato, Soma Salamon, Joachim Landers, Benedikt Eggert, Eugen Weschke, Jean Pascal Fandré, Ashwani Kumar, Harun Tüysüz, Martin Muhler, Heiko Wende, Jörg Behler

Abstract: In the present work, we investigate oxygen vacancies (V$_\mathrm{O}$) in Co$_3$O$_4$, both in the bulk phase and under liquid-phase ethylene glycol oxidation, by combining theoretical and experimental techniques. Density functional theory calculations for bulk Co$_3$O$_4$ show that introducing an oxygen vacancy reduces two adjacent Co$^{3+}$ ions to Co$^{2+}$ and narrows the band gap. The newly fo… ▽ More In the present work, we investigate oxygen vacancies (V$_\mathrm{O}$) in Co$_3$O$_4$, both in the bulk phase and under liquid-phase ethylene glycol oxidation, by combining theoretical and experimental techniques. Density functional theory calculations for bulk Co$_3$O$_4$ show that introducing an oxygen vacancy reduces two adjacent Co$^{3+}$ ions to Co$^{2+}$ and narrows the band gap. The newly formed Co$^{2+}$ ions adopt high-spin configurations in distorted octahedral sites and remain stable in this state in ab initio molecular dynamics simulations at $300$ K. Computed O and Co K-edge X-ray absorption spectra for ideal and vacancy-containing Co$_3$O$_4$ show excellent agreement with the experimental data and serve as references to analyze the liquid-phase ethylene glycol oxidation. The comparison with experimental O K-edge spectra of fresh and post-reaction catalysts shows that fresh samples resemble the vacancy-containing reference, whereas post-reaction spectra shift toward the ideal reference. These results suggest that under liquid-phase ethylene glycol oxidation conditions, Co$_3$O$_4$ becomes more oxidized rather than reduced, by refilling preexisting oxygen vacancies. This is further supported by the observation that higher O$_2$ pressures increase the conversion and that the catalyst remains stable and active over several cycles. △ Less

Submitted 24 November, 2025; originally announced November 2025.

Many-body electronic structure in pyrochlore superconductor CsBi2 and spin liquid Pr2Ir2O7

Authors: Wei Song, Guowei Liu, Hanbin Deng, Tianyu Yang, Yongkai Li, Xiao-Yu Yan, Ruoxing Liao, Qianming Wang, Jiayu Xu, Chao Yan, Yuanyuan Zhao, Hailang Qin, Da Wang, Wenchuan Jing, Dawei Shen, Kosuke Nakayama, Takafumi Sato, Chandan Setty, Desheng Wu, Boqing Song, Tianping Ying, Zhaoming Tian, Akito Sakai, Satoru Nakatsuji, Harish Kumar , et al. (4 additional authors not shown)

Abstract: The pyrochlore lattice materials can exhibit geometrical frustration, while the related many-body electronic states remain elusive. In this work, we performed scanning tunneling microscopy measurements on the pyrochlore superconductor CsBi2 and spin liquid Pr2Ir2O7 at 0.3 K. For the first time, we obtained atomically resolved images of their (111) surfaces, revealing a hexagonal lattice or a kagom… ▽ More The pyrochlore lattice materials can exhibit geometrical frustration, while the related many-body electronic states remain elusive. In this work, we performed scanning tunneling microscopy measurements on the pyrochlore superconductor CsBi2 and spin liquid Pr2Ir2O7 at 0.3 K. For the first time, we obtained atomically resolved images of their (111) surfaces, revealing a hexagonal lattice or a kagome lattice. Tunneling spectroscopy in CsBi2 reveals a nearly fully opened superconductivity gap. The ratio of 2Δ/kBTC = 4.7 suggests relatively strong coupling superconductivity, as compared with that in kagome superconductors AV3Sb5 (A = K, Rb, Cs). In contrast to the previous study categorizing CsBi2 as a type-I superconductor, the applied magnetic field induces a hexagonal vortex lattice in which each vortex core exhibits an intriguing three-fold symmetry state. In Pr2Ir2O7, we observed a spatially homogeneous Kondo-lattice resonance, which is compared with that in the kagome Kondo-lattice material CsCr6Sb6. We further discover that the Kondo resonance exhibits a spatial modulation with three-fold symmetry, and the applied magnetic field induces a Zeeman splitting of the Kondo resonance with intriguing atomic site dependence. We discuss the relations of these many-body electronic phenomena with the pyrochlore lattice geometry and its charge or spin frustration. Our systematic observations offer atomic-scale insights into the many-body electronic structures of the geometrically frustrated pyrochlore superconductors and spin liquids. △ Less

Submitted 22 November, 2025; originally announced November 2025.

Comments: To appear in PRB

Journal ref: Phys. Rev. B 112, 245131 2025

Impact of Carrier Injector Design on the Threshold of Interband Cascade Lasers

Authors: T. Sato, B. Petrović, R. Weih, F. Hartmann, S. Höfling, S. Birner, C. Jirauschek, T. Grange

Abstract: We theoretically investigate how the injector region design of interband cascade lasers (ICLs) impacts the threshold carrier and current densities. The model combines a polarization-sensitive 8-band $\mathbf{k}\cdot\mathbf{p}$ calculation, electrostatics, and a microscopic calculation of Auger recombination rates. The inelastic carrier-carrier scattering is included to lowest order using quasi-equ… ▽ More We theoretically investigate how the injector region design of interband cascade lasers (ICLs) impacts the threshold carrier and current densities. The model combines a polarization-sensitive 8-band $\mathbf{k}\cdot\mathbf{p}$ calculation, electrostatics, and a microscopic calculation of Auger recombination rates. The inelastic carrier-carrier scattering is included to lowest order using quasi-equilibrium Green's functions. It captures the combined effects of charge-carrier redistribution, parasitic absorption, and bias voltage on the Auger recombination rate. We show that heavily doping the electron injector suppresses the dominant multi-hole Auger recombination by reducing the hole population of the recombination quantum wells. This agrees with the experimental observation that the heavy doping reduces threshold currents. Unlike the measurements, however, they do not increase at high doping concentrations in our model, which does not include scattering-mediated carrier escape and/or light absorption. Furthermore, by introducing indium to the conventional $\mathrm{Ga}\mathrm{Sb}$ hole injector wells, we explain the rule of thumb from experiments that raising the hole injector levels does not outperform the doping strategy. Our model provides physical insights for optimizing ICL carrier injectors. △ Less

Submitted 19 February, 2026; v1 submitted 19 November, 2025; originally announced November 2025.

Comments: Revised argument in section III, results unchanged. Higher image resolution for Fig. 5

Orbital Accumulation Induced by Chiral Phonons

Authors: Tetsuya Sato, Takeo Kato, Aurelien Manchon

Abstract: We theoretically investigate orbital accumulation driven by chiral phonons via orbital-dependent electron-lattice coupling. We derive a formula for the orbital accumulation induced by classical lattice dynamics or nonequilibrium phonons, emphasizing the rectified second-order response of the orbital moment to lattice displacement. We show that chiral phonons primarily couple to orbital quadrupole… ▽ More We theoretically investigate orbital accumulation driven by chiral phonons via orbital-dependent electron-lattice coupling. We derive a formula for the orbital accumulation induced by classical lattice dynamics or nonequilibrium phonons, emphasizing the rectified second-order response of the orbital moment to lattice displacement. We show that chiral phonons primarily couple to orbital quadrupole moments and that static orbital dipole accumulation can be generated at second order in the lattice displacement. Our study provides a useful method for generating orbital accumulation without using spin-orbit interactions and suggests a strategy to boost its magnitude by harnessing band structure hot spots associated with orbital degeneracy. △ Less

Submitted 25 July, 2026; v1 submitted 14 November, 2025; originally announced November 2025.

Comments: 25 pages, 6 figures

Tuning Stability of AB3-Type Alloys by Suppressing Magnetism

Authors: Hung Ba Tran, Toyoto Sato, Ryuhei Sato, Hiroyuki Saitoh, Shin-ichi Orimo, Hao Li

Abstract: Hydrogen is a promising clean energy carrier, yet effective and reversible storage remains challenging. AB3-type intermetallic alloys are promising for solid-state hydrogen storage due to intermediate thermodynamic stability and rapid hydrogen uptake. Optimizing stability and gravimetric density is hindered by competing thermodynamic and magnetic effects. Here, we analyze AB3 compounds (A = Ca, Y,… ▽ More Hydrogen is a promising clean energy carrier, yet effective and reversible storage remains challenging. AB3-type intermetallic alloys are promising for solid-state hydrogen storage due to intermediate thermodynamic stability and rapid hydrogen uptake. Optimizing stability and gravimetric density is hindered by competing thermodynamic and magnetic effects. Here, we analyze AB3 compounds (A = Ca, Y, Mg; B = Co, Ni) and ternary alloys CaxYyMg1-x-yB3 using first-principles calculations and Monte Carlo simulations. We find a direct correlation between formation energy and total magnetic moment that dictates alloy stability, explaining the trade-off in hydrogen storage. In Co-rich systems with large lattice volumes, formation energy rises with magnetization, showing magnetism as the dominant factor. Mg-rich compositions achieve high gravimetric densities, but strong magnetism destabilizes the system, requiring Y substitution to suppress magnetic moments. Replacing Co with Ni weakens magnetism: YNi3 is nonmagnetic, while CaNi3 and MgNi3 are weakly polarized, allowing thermodynamic stability across compositions. Notably, CaMg2Ni9 combines high theoretical capacity (3.32 wt%) with good reversibility. Mg-rich Ni-based alloys are predicted to offer negative formation energies with the highest gravimetric densities (up to 3.40 wt%). These results show that controlling magnetism via transition-metal substitution is key to overcoming the stability-capacity trade-off in AB3 hydrogen storage materials. △ Less

Submitted 24 December, 2025; v1 submitted 11 November, 2025; originally announced November 2025.

Coupled dimerized alternating-bond quantum spin chains in the distorted honeycomb-lattice magnet Cu$_5$SbO$_6$

Authors: C. Piyakulworawat, K. Morita, Y. Fukumoto, W. -Y. Hsieh, W. -T. Chen, K. Nakajima, S. Ohira-Kawamura, Y. Zhao, S. Wannapaiboon, P. Piyawongwatthana, T. J. Sato, K. Matan

Abstract: We analyze powder-averaged inelastic neutron scattering and magnetization data for the distorted honeycomb compound Cu$_5$SbO$_6$ using a first-order dimer expansion calculation and quantum Monte Carlo simulations. We show that, in contrast to the previously proposed honeycomb lattice model, Cu$_5$SbO$_6$ accommodates interacting dimerized spin chains with alternating ferromagnetic-antiferromagnet… ▽ More We analyze powder-averaged inelastic neutron scattering and magnetization data for the distorted honeycomb compound Cu$_5$SbO$_6$ using a first-order dimer expansion calculation and quantum Monte Carlo simulations. We show that, in contrast to the previously proposed honeycomb lattice model, Cu$_5$SbO$_6$ accommodates interacting dimerized spin chains with alternating ferromagnetic-antiferromagnetic couplings along the chain. Moreover, unlike the typical couplings observed in other Cu$^{2+}$-based distorted honeycomb magnets, the spin chains in Cu$_5$SbO$_6$ primarily couple through an antiferromagnetic coupling that arises between the honeycomb layers, rather than the expected interchain coupling in the layers. This finding reveals a different magnetic coupling scheme for Cu$_5$SbO$_6$. In addition, utilizing x-ray spectroscopy and transmission electron microscopy, we also refine the crystal structure and stacking-fault model of the compound. △ Less

Submitted 4 March, 2026; v1 submitted 7 November, 2025; originally announced November 2025.

Comments: 17 pages, 8 figures

Journal ref: Phys. Rev. Research 8, 013247 (2026)

Generating ferro-spinetic polarizations in altermagnetic insulators

Authors: Toshihiro Sato, Mengli Hu, Ion Cosma Fulga, Oleg Janson, Jorge I. Facio, Alessandro Stroppa, Fakher F. Assaad, Jeroen van den Brink

Abstract: Altermagnets are a novel class of fully spin-compensated magnetic materials that nevertheless have spin-split electronic bands, offering novel perspectives for spintronics applications. Based on a rigorous analysis of altermagnetic many-body models and their symmetry we establish the important role of two fundamental types of polarizations in altermagnetic insulators: the charge and the spinetic o… ▽ More Altermagnets are a novel class of fully spin-compensated magnetic materials that nevertheless have spin-split electronic bands, offering novel perspectives for spintronics applications. Based on a rigorous analysis of altermagnetic many-body models and their symmetry we establish the important role of two fundamental types of polarizations in altermagnetic insulators: the charge and the spinetic one, where the latter corresponds to a macroscopic spin-displacement field. First principles calculations confirm and quantify their presence in real materials. The two polarizations are directly coupled and emerge in orthogonal directions when inversion symmetry is broken, either by the system developing a spontaneously ferroelectric polarization (in ferroelectric altermagnets), or by a charge displacement induced by an external electric field (for inversion invariant altermagnetic insulators). This presence of large and switchable spin accumulations constitute an attractive fundamental feature of altermagnetic insulators. △ Less

Submitted 26 February, 2026; v1 submitted 21 October, 2025; originally announced October 2025.

Comments: 26 pages, 7 figures

Direct observation of band structure modifications from monolayer WSe2 to Janus WSSe

Authors: Masato Sakano, Shunsuke Akatsuka, Takato Yamamoto, Tianyishan Sun, Dingkun Bi, Hiroto Ogura, Naoya Yamaguchi, Fumiyuki Ishii, Natsuki Mitsuishi, Kenji Watanabe, Takashi Taniguchi, Miho Kitamura, Koji Horiba, Kenichi Ozawa, Katsuaki Sugawara, Seigo Souma, Takafumi Sato, Yuta Seo, Satoru Masubuchi, Tomoki Machida, Toshiaki Kato, Kyoko Ishizaka

Abstract: Janus monolayer transition metal dichalcogenides (TMDs), created by post-growth substitution of the top chalcogen layer, represent a new direction for engineering 2D crystal properties. However, their rapid ambient degradation and the difficulty of obtaining large-area monolayer samples have limited the available experimental probes, leaving their detailed electronic structure near the Fermi level… ▽ More Janus monolayer transition metal dichalcogenides (TMDs), created by post-growth substitution of the top chalcogen layer, represent a new direction for engineering 2D crystal properties. However, their rapid ambient degradation and the difficulty of obtaining large-area monolayer samples have limited the available experimental probes, leaving their detailed electronic structure near the Fermi level largely unexplored. In this work, by performing micro-focused angle-resolved photoemission spectroscopy (μ-ARPES) on an identical sample transformed from monolayer WSe2 to Janus WSSe via a H2 plasma-assisted chalcogen-exchange method, we reveal the evolution of its electronic band structure. We observe ARPES signature consistent with the Rashba-type spin splitting due to broken horizontal mirror symmetry, and a significant upward shift of the highest valence band at the Γ-point by approximately 160 meV. These direct observations clarify the key electronic modifications that govern the material's properties and provide a pathway for band engineering in Janus TMDs. △ Less

Submitted 5 October, 2025; originally announced October 2025.

Comments: 16 pages, 4 figures

Enhanced premelting at the ice-rubber interface using all-atom molecular dynamics simulation

Authors: Takumi Kojima, Ikki Yasuda, Takumi Sato, Noriyoshi Arai, Kenji Yasuoka

Abstract: The ice-rubber interface is critical in applications such as tires and shoe outsoles, yet its molecular tribology remains unclear. Using all-atom molecular dynamics simulations, we studied premelting layers at the basal face of ice in contact with styrene-butadiene rubber from 254 to 269 K. Despite its hydrophobicity, rubber enhances structural disorder of interfacial water, promoting premelting.… ▽ More The ice-rubber interface is critical in applications such as tires and shoe outsoles, yet its molecular tribology remains unclear. Using all-atom molecular dynamics simulations, we studied premelting layers at the basal face of ice in contact with styrene-butadiene rubber from 254 to 269 K. Despite its hydrophobicity, rubber enhances structural disorder of interfacial water, promoting premelting. In contrast, water mobility is suppressed by confinement from polymer chains, leading to glassy dynamics distinct from the ice-vapor interface. Near the melting point, rubber chains become more flexible and penetrate the premelting layer, forming a mixed rubber-water region that couples the dynamics of both components. These results suggest that nanoscale roughness and morphology of hydrophobic polymers disrupt ice hydrogen-bond networks, thereby enhancing premelting. Our findings provide molecular-level insight into ice slipperiness and inform the design of polymer materials with controlled ice adhesion and friction. △ Less

Submitted 30 December, 2025; v1 submitted 28 August, 2025; originally announced August 2025.

Journal ref: Langumir (2026)

"DIVE" into Hydrogen Storage Materials Discovery with AI Agents

Authors: Di Zhang, Xue Jia, Tran Ba Hung, Seong Hoon Jang, Linda Zhang, Ryuhei Sato, Yusuke Hashimoto, Toyoto Sato, Kiyoe Konno, Shin-ichi Orimo, Hao Li

Abstract: Data-driven artificial intelligence (AI) approaches are fundamentally transforming the discovery of new materials. Despite the unprecedented availability of materials data in the scientific literature, much of this information remains trapped in unstructured figures and tables, hindering the construction of large language model (LLM)-based AI agent for automated materials design. Here, we present… ▽ More Data-driven artificial intelligence (AI) approaches are fundamentally transforming the discovery of new materials. Despite the unprecedented availability of materials data in the scientific literature, much of this information remains trapped in unstructured figures and tables, hindering the construction of large language model (LLM)-based AI agent for automated materials design. Here, we present the Descriptive Interpretation of Visual Expression (DIVE) multi-agent workflow, which systematically reads and organizes experimental data from graphical elements in scientific literatures. We focus on solid-state hydrogen storage materials-a class of materials central to future clean-energy technologies and demonstrate that DIVE markedly improves the accuracy and coverage of data extraction compared to the direct extraction by multimodal models, with gains of 10-15% over commercial models and over 30% relative to open-source models. Building on a curated database of over 30,000 entries from 4,000 publications, we establish a rapid inverse design workflow capable of identifying previously unreported hydrogen storage compositions in two minutes. The proposed AI workflow and agent design are broadly transferable across diverse materials, providing a paradigm for AI-driven materials discovery. △ Less

Submitted 24 September, 2025; v1 submitted 18 August, 2025; originally announced August 2025.

Comments: 23 pages, 5 figures. The supplementary video is available at the GitHub link provided in the manuscript

Journal ref: Chemical Science 2026

Edge modes of topological Mott insulators and deconfined quantum critical points

Authors: Yuhai Liu, Toshihiro Sato, Disha Hou, Zhenjiu Wang, Wenan Guo, Fakher F. Assaad

Abstract: Topology and anomalies lead to edge modes that can interact with critical bulk fluctuations. To study this setup, pertaining to boundary criticality, we consider a model exhibiting a deconfined quantum critical point (DQCP) between a dynamically generated quantum spin Hall state (i.e.a topological Mott insulator) and an s-wave superconductor. For the topological Mott insulator, the bulk Goldstone… ▽ More Topology and anomalies lead to edge modes that can interact with critical bulk fluctuations. To study this setup, pertaining to boundary criticality, we consider a model exhibiting a deconfined quantum critical point (DQCP) between a dynamically generated quantum spin Hall state (i.e.a topological Mott insulator) and an s-wave superconductor. For the topological Mott insulator, the bulk Goldstone modes are shown to be irrelevant at the helical Luttinger liquid fixed points. The deconfined quantum critical point is an instance of an emergent anomaly, and we observe a sharp localized edge state at this point. The sharpness of the edge mode is consistent with an ordinary phase in which electronic edge modes decouple from critical edge bosonic fluctuations. At the DQCP, the scaling dimension of the edge electron shows a jump, a feature argued to be a signature of the emergent anomaly. Our results are based on large-scale auxiliary-field quantum Monte Carlo simulations.We also carry out calculations for the Kane-Mele-Hubbard model to confirm spectral features of the ordinary and extraordinary-log phases in the vicinity of the bulk critical point. △ Less

Submitted 8 September, 2025; v1 submitted 6 August, 2025; originally announced August 2025.

Comments: 8 pages, 8 figures

Second-order microscopic nonlinear susceptibility in a centrosymmetric material: application to imaging valence electron motion

Authors: Chance Ornelas-Skarin, Tatiana Bezriadina, Matthias Fuchs, Shambhu Ghimire, J. B. Hastings, Quynh L Nguyen, Gilberto de la Peña, Takahiro Sato, Sharon Shwartz, Mariano Trigo, Diling Zhu, Daria Popova-Gorelova, David A. Reis

Abstract: We report measurements of phase-matched nonlinear x-ray and optical sum-frequency generation from single-crystal silicon using sub-resonant 0.95 eV laser pulses and 9.5 keV hard x-ray pulses from the LCLS free-electron laser. The sum-frequency signal appears as energy and momentum sidebands to the elastic Bragg peak. It is proportional to the magnitude squared of the relevant temporal and spatial… ▽ More We report measurements of phase-matched nonlinear x-ray and optical sum-frequency generation from single-crystal silicon using sub-resonant 0.95 eV laser pulses and 9.5 keV hard x-ray pulses from the LCLS free-electron laser. The sum-frequency signal appears as energy and momentum sidebands to the elastic Bragg peak. It is proportional to the magnitude squared of the relevant temporal and spatial Fourier components of the optically induced microscopic charges/currents. We measure the first- and second-order sideband to the 220 Bragg peak and find that the efficiency is maximized when the applied field is along the reciprocal lattice vector. For an optical intensity of $\sim10^{12} \text{W}/\text{cm}^2$, we measure peak efficiencies of $3\times 10^{-7}$ and $3\times 10^{-10}$ for the first and second-order sideband respectively (relative to the elastic Bragg peak). The first-order sideband is consistent with induced microscopic currents along the applied electric field (consistent with an isotropic response). The second-order sideband depends nontrivially on the optical field orientation and is consistent with an anisotropic response originating from induced charges along the bonds with C$_{3v}$ site symmetry. The results agree well with first-principles Bloch-Floquet calculations. △ Less

Submitted 1 July, 2025; originally announced July 2025.

Terahertz-field activation of polar skyrons

Authors: Huaiyu Wang, Vladimir Stoica, Cheng Dai, Marek Paściak, Sujit Das, Tiannan Yang, Mauro A. P. Gonçalves, Jiri Kulda, Margaret R. McCarter, Anudeep Mangu, Yue Cao, Hari Padma, Utkarsh Saha, Diling Zhu, Takahiro Sato, Sanghoon Song, Mathias Hoffmann, Patrick Kramer, Silke Nelson, Yanwen Sun, Quynh Nguyen, Zhan Zhang, Ramamoorthy Ramesh, Lane Martin, Aaron M. Lindenberg , et al. (5 additional authors not shown)

Abstract: Unraveling collective modes arising from coupled degrees of freedom is crucial for understanding complex interactions in solids and developing new functionalities. Unique collective behaviors emerge when two degrees of freedom, ordered on distinct length scales, interact. Polar skyrmions, three-dimensional electric polarization textures in ferroelectric superlattices, disrupt the lattice continuit… ▽ More Unraveling collective modes arising from coupled degrees of freedom is crucial for understanding complex interactions in solids and developing new functionalities. Unique collective behaviors emerge when two degrees of freedom, ordered on distinct length scales, interact. Polar skyrmions, three-dimensional electric polarization textures in ferroelectric superlattices, disrupt the lattice continuity at the nanometer scale with nontrivial topology, leading to previously unexplored collective modes. Here, using terahertz-field excitation and femtosecond x-ray diffraction, we discovered subterahertz collective modes, dubbed 'skyrons', which appear as swirling patterns of atomic displacements functioning as atomic-scale gearsets. Momentum-resolved time-domain measurements of diffuse scattering revealed an avoided crossing in the dispersion relation of skyrons. We further demonstrated that the amplitude and dispersion of skyrons can be controlled by sample temperature and electric-field bias. Atomistic simulations and dynamical phase-field modeling provided microscopic insights into the three-dimensional crystallographic and polarization dynamics. The discovery of skyrons and their coupling with terahertz fields opens avenues for ultrafast control of topological polar structures. △ Less

Submitted 1 September, 2025; v1 submitted 15 May, 2025; originally announced May 2025.

Report number: ISSN 2041-1723

Journal ref: Nat Commun 16, 8994 (2025)

Quantum theory of magnetic octupole in periodic crystals and application to $d$-wave altermagnets

Authors: Takumi Sato, Satoru Hayami

Abstract: Magnetic multipoles have been recognized as order parameters characterizing magnetic structure in solids. Recently, magnetic octupoles have been proposed as the order parameters of time-reversal-symmetry breaking centrosymmetric antiferromagnets exhibiting nonrelativistic spin splitting, which is referred to as ``altermagnet''. However, a gauge-invariant formulation of magnetic octupoles in crysta… ▽ More Magnetic multipoles have been recognized as order parameters characterizing magnetic structure in solids. Recently, magnetic octupoles have been proposed as the order parameters of time-reversal-symmetry breaking centrosymmetric antiferromagnets exhibiting nonrelativistic spin splitting, which is referred to as ``altermagnet''. However, a gauge-invariant formulation of magnetic octupoles in crystalline solids remains elusive. Here, we present a gauge-invariant expression of spin magnetic octupoles in periodic crystals based on quantum mechanics and thermodynamics, which can be used to quantitatively characterize time-reversal-symmetry breaking antiferromagnets including $d$-wave altermagnets. The allowed physical response tensors are classified beyond symmetry considerations, and direct relationships are established for some of them in insulators at zero temperature. Furthermore, our expression reveals a contribution from an anisotropic magnetic dipole, which has the same symmetry as conventional spin and orbital magnetic dipoles but carries no net magnetization. We discuss the relation between the anisotropic magnetic dipole and the anomalous Hall effect. △ Less

Submitted 4 February, 2026; v1 submitted 30 April, 2025; originally announced April 2025.

Molecular Dynamics Investigation of Static and Dynamic Interfacial Properties in Ice-Polymer Premelting Layers

Authors: Takumi Sato, Ikki Yasuda, Noriyoshi Arai, Kenji Yasuoka

Abstract: Premelting at the ice-polymer interfaces, in which a quasi-liquid layer (QLL) forms below the melting point, is strongly influenced by polymer surface chemistry; however, the molecular-scale mechanisms underlying these effects remain poorly understood. This study employs large-scale molecular dynamics simulations combined with machine learning-assisted analysis to elucidate how polymer type (hydro… ▽ More Premelting at the ice-polymer interfaces, in which a quasi-liquid layer (QLL) forms below the melting point, is strongly influenced by polymer surface chemistry; however, the molecular-scale mechanisms underlying these effects remain poorly understood. This study employs large-scale molecular dynamics simulations combined with machine learning-assisted analysis to elucidate how polymer type (hydrophilic vs hydrophobic) modulates interfacial premelting. Our simulations reveal that hydrophilic and hydrophobic polymer surfaces have distinct effects on the QLL thickness, interfacial water structure, and diffusivity. Specifically, a hydrophilic polymer interface promotes a thicker QLL with more ordered interfacial water and lower diffusivity, whereas a hydrophobic interface induces a thinner QLL with a less ordered interfacial water structure and higher diffusivity. These results advance the understanding of polymer-mediated interfacial melting phenomena and offer guidance for designing anti-icing and low-friction materials. △ Less

Submitted 28 April, 2025; originally announced April 2025.

Comments: 5 pages, 7 figures

Tuning chirality amplitude at ultrafast timescales

Authors: Hiroki Ueda, Takahiro Sato, Quynh L. Nguyen, Elizabeth Skoropata, Ludmila Leroy, Tim Suter, Elsa Abreu, Matteo Savoini, Vincent Esposito, Matthias Hoffmann, Carl P. Romao, Julien Zaccaro, Diling Zhu, Steven Lee Johnson, Urs Staub

Abstract: Chirality is a fundamental symmetry concept describing discrete states, i.e., left-handed, right-handed, or achiral, and existing at disparate scales and in many categories of scientific fields. Even though symmetry breaking is indispensable for describing qualitatively distinct phenomena, symmetry cannot quantitatively predict measurable quantities. One can continuously distort an object, introdu… ▽ More Chirality is a fundamental symmetry concept describing discrete states, i.e., left-handed, right-handed, or achiral, and existing at disparate scales and in many categories of scientific fields. Even though symmetry breaking is indispensable for describing qualitatively distinct phenomena, symmetry cannot quantitatively predict measurable quantities. One can continuously distort an object, introducing the concept of chirality amplitude, similar to representing magnetization as the amplitude of time-reversal symmetry breaking. Considering the role of magnetization in emergent phenomena with time-reversal symmetry breaking, chirality amplitude is intuitively a key quantity for controlling chirality-related emergent phenomena. Here, we propose two types of chiral lattice distortions and demonstrate the tunability of their amplitude in ultrafast timescales. Resonant X-ray diffraction with circular polarization is an established technique to measure crystal chirality directly. We quantify the ultrafast change in chirality amplitude in real time after an optical excitation. Using instead a THz excitation, we observe oscillations in the resonant diffraction intensities corresponding to specific phonon frequencies. This indicates the creation of additional asymmetry, which could also be described as an enhancement in chirality amplitude. Our proposed concept of chirality amplitude and its ultrafast control may lead to a unique approach to control chirality-induced emergent phenomena in ultrafast timescales. △ Less

Submitted 10 April, 2025; originally announced April 2025.

Comments: 14 pages, 5 figures

Giant Rashba splitting in PtTe/PtTe$_2$ heterostructure

Authors: Runfa Feng, Yang Zhang, Jiaheng Li, Qian Li, Changhua Bao, Hongyun Zhang, Wanying Chen, Xiao Tang, Ken Yaegashi, Katsuaki Sugawara, Takafumi Sato, Wenhui Duan, Pu Yu, Shuyun Zhou

Abstract: Achieving a large spin splitting is highly desirable for spintronic devices, which often requires breaking of the inversion symmetry. However, many atomically thin films are centrosymmetric, making them unsuitable for spintronic applications. Here, we report a strategy to achieve inversion symmetry breaking from a centrosymmetric transition metal dichalcogenide (TMDC) bilayer PtTe$_2$, leading to… ▽ More Achieving a large spin splitting is highly desirable for spintronic devices, which often requires breaking of the inversion symmetry. However, many atomically thin films are centrosymmetric, making them unsuitable for spintronic applications. Here, we report a strategy to achieve inversion symmetry breaking from a centrosymmetric transition metal dichalcogenide (TMDC) bilayer PtTe$_2$, leading to a giant Rashba spin splitting. Specifically, the thermal annealing turns one layer of PtTe$_2$ sample into a transition metal monochalcogenide (TMMC) PtTe through Te extraction, thus forming PtTe/PtTe$_2$ heterostructure with inversion symmetry breaking. In the naturally-formed PtTe/PtTe$_2$ heterostructure, we observe a giant Rashba spin splitting with Rashba coefficient of $α_{R}$ = 1.8 eV$\cdot$$Å$, as revealed by spin- and angle-resolved photoemission spectroscopy measurements. Our work demonstrates a convenient and effective pathway for achieving pronounced Rashba splitting in centrosymmetric TMDC thin films by creating TMMC/TMDC heterostructure, thereby extending their potential applications to spintronics. △ Less

Submitted 8 April, 2025; originally announced April 2025.

Journal ref: Nat. Commun. 16, 2667 (2025)

Moiré enhanced flat band in rhombohedral graphene

Authors: Hongyun Zhang, Jinxi Lu, Kai Liu, Yijie Wang, Fei Wang, Size Wu, Wanying Chen, Xuanxi Cai, Kenji Watanabe, Takashi Taniguchi, Jose Avila, Pavel Dudin, Matthew D. Watson, Alex Louat, Takafumi Sato, Pu Yu, Wenhui Duan, Zhida Song, Guorui Chen, Shuyun Zhou

Abstract: The fractional quantum anomalous Hall effect (FQAHE) is a fascinating emergent quantum state characterized by fractionally charged excitations in the absence of magnetic field,which could arise from the intricate interplay between electron correlation, nontrivial topology and spontaneous time-reversal symmetry breaking. Recently, FQAHE has been realized in aligned rhombohedral pentalayer graphene… ▽ More The fractional quantum anomalous Hall effect (FQAHE) is a fascinating emergent quantum state characterized by fractionally charged excitations in the absence of magnetic field,which could arise from the intricate interplay between electron correlation, nontrivial topology and spontaneous time-reversal symmetry breaking. Recently, FQAHE has been realized in aligned rhombohedral pentalayer graphene on BN superlattice (aligned R5G/BN), where the topological flat band is modulated by the moiré potential. However, intriguingly, the FQAHE is observed only when electrons are pushed away from the moiré interface. The apparently opposite implications from these experimental observations, along with different theoretical models, have sparked intense debates regarding the role of the moiré potential. Unambiguous experimental observation of the topological flat band as well as moiré bands with energy and momentum resolved information is therefore critical to elucidate the underlying mechanism. Here by performing nanospot angle-resolved photoemission spectroscopy (NanoARPES) measurements, we directly reveal the topological flat band electronic structures of R5G, from which key hopping parameters essential for determining the fundamental electronic structure of rhombohedral graphene are extracted. Moreover, a comparison of electronic structures between aligned and non-aligned samples reveals that the moiré potential plays a pivotal role in enhancing the topological flat band in the aligned sample. Our study provides experimental guiding lines to narrow down the phase space of rhombohedral graphene, laying an important foundation for understanding exotic quantum phenomena in this emerging platform. △ Less

Submitted 8 April, 2025; originally announced April 2025.

Comments: 15 pages,4 figures

Multipolar Phase Transition in the 4$f^2$ fcc lattice compound PrCdNi$_{4}$

Authors: Yuka Kusanose, Yasuyuki Shimura, Kazunori Umeo, Naomi Kawata, Toshiro Takabatake, Taichi Terashima, Naoki Kikugawa, Takako Konoike, Yuya Hattori, Kazuhiro Nawa, Hung-Cheng Wu, Taku J Sato, Takahiro Onimaru

Abstract: Transport and magnetic properties of a 4$f^{2}$ fcc lattice compound, PrCdNi$_4$, were studied. The magnetic susceptibility, $χ(T)$, follows the Curie--Weiss law from 300 K to 20 K, as expected for a free Pr$^{3+}$ ion. As the temperature decreases below 5 K, $χ(T)$ approaches a constant, indicating van-Vleck paramagnetic behavior. The specific heat, $C(T)$, displays a broad shoulder at around 4 K… ▽ More Transport and magnetic properties of a 4$f^{2}$ fcc lattice compound, PrCdNi$_4$, were studied. The magnetic susceptibility, $χ(T)$, follows the Curie--Weiss law from 300 K to 20 K, as expected for a free Pr$^{3+}$ ion. As the temperature decreases below 5 K, $χ(T)$ approaches a constant, indicating van-Vleck paramagnetic behavior. The specific heat, $C(T)$, displays a broad shoulder at around 4 K, which can be reproduced by a doublet triplet two-level model with an energy gap of 12 K. These results suggest a non-magnetic $Γ_3$ doublet ground state of the Pr$^{3+}$ ion in the cubic crystalline electric field. $C(T)$ exhibits a peak at $T_{\rm O}$ = 1.0 K and this peak remains robust against magnetic fields up to 5 T. In powder neutron diffraction measurements, no magnetic reflection was observed at 0.32 K $<$ $T_{\rm O}$. Two anomalies at $B$ = 2.1 and 5.3 T in magnetoresistance $ρ(B)$ at 0.05 K likely originate from switching in the order parameter. These results suggest that the phase transition at $T_{\rm O}$ is ascribed to an antiferro-type order of the electric quadrupole or magnetic octupole of the $Γ_3$ doublet in the 4$f^2$ fcc lattice. △ Less

Submitted 8 April, 2025; originally announced April 2025.

Comments: 10 pages, 8 figures. Additional 5 pages, 4 figures 3 tables in supplemental material

A double-spiral spin ordering in the helimagnet YBaCuFeO$_{5}$

Authors: Yu-Hui Liang, Chun-Hao Lai, Chin-Wei Wang, Shinichiro Yano, Daisuke Okuyama, Taku J. Sato, Yusuke Nambu, Shih-Chang Weng, Yen-Chung Lai, Wei-Tin Chen, Kirrily C. Rule, Chao-Hung Du

Abstract: Materials with a spiral spin ordering always show a rich phase diagram and can be a playground for studying the exotic physical properties associated with spiral magnetism. Using neutron elastic and resonant x-ray scattering on a high-quality single crystal YBaCuFeO$_{5}$, we demonstrate YBaCuFeO$_{5}$ to be a helimagnet consisting of a double-spiral spin ordering. YBaCuFeO$_{5}$ undergoes a comme… ▽ More Materials with a spiral spin ordering always show a rich phase diagram and can be a playground for studying the exotic physical properties associated with spiral magnetism. Using neutron elastic and resonant x-ray scattering on a high-quality single crystal YBaCuFeO$_{5}$, we demonstrate YBaCuFeO$_{5}$ to be a helimagnet consisting of a double-spiral spin ordering. YBaCuFeO$_{5}$ undergoes a commensurate to incommensurate magnetic phase transition at $T_{N2}$$\sim$ 175 K, and the incommensurate phase consists of two spin-ordered components. Both components have different periodicities but with the same propagating direction along the {\it c}-axis below $T_{N2}$. Using resonant x-ray scattering at the Fe and Cu \textit{K}-edges, we further demonstrate that both spiral spin orderings result from the Fe$^{3+}$ and Cu$^{2+}$, respectively, forming a double-spiral spin ordering structure. This can be understood to be caused by the coupling between both sublattices of Fe$^{3+}$ and Cu$^{2+}$ with the atomic lattice. △ Less

Submitted 27 March, 2025; originally announced March 2025.

Comments: 29 pages, 6 figures

Discovery of a Highly Anisotropic Type-II Ferromagnetic Weyl State Exhibiting a 3D Quantum Hall Effect

Authors: Yingdong Guan, Abhinava Chatterjee, Trace Bivens, Seng Huat Lee, Asuka Honma, Hirofumi Oka, Jorge D Vega Bazantes, Ruiqi Zhang, David Graf, Jianwei Sun, Seigo Souma, Takafumi Sato, Yong P. Chen, Yuanxi Wang, Chaoxing Liu, Zhiqiang Mao

Abstract: Topological semimetals, particularly Weyl semimetals (WSMs), are crucial platforms for exploring emergent quantum phenomena due to their unique electronic structures and potential to transition into various topological phases. In this study, we report the discovery of a ferromagnetic (FM) type-II WSM in Mn(Bi1-xSbx)4Te7, which exhibits a remarkable three-dimensional (3D) quantum Hall effect (QHE).… ▽ More Topological semimetals, particularly Weyl semimetals (WSMs), are crucial platforms for exploring emergent quantum phenomena due to their unique electronic structures and potential to transition into various topological phases. In this study, we report the discovery of a ferromagnetic (FM) type-II WSM in Mn(Bi1-xSbx)4Te7, which exhibits a remarkable three-dimensional (3D) quantum Hall effect (QHE). By precisely tuning the chemical potential through Sb doping, we obtained samples with the Fermi level near the charge neutrality point for x = ~ 0.27. This was confirmed by spectroscopy measurements (ARPES and STS), and these samples showed strong quantum oscillations along with a key transport signature of a Weyl state - chiral anomaly, and Fermi surface reconstruction driven by FM ordering. Our theoretical analysis indicates that this Weyl state evolves from a parent nodal ring state, where higher-order k-terms split the nodal line into type-II Weyl nodes. The Weyl state exhibits significant anisotropy, characterized by a pronounced reduction in Fermi velocity along the kz-axis, likely accounting for the observed 3D QHE. These results not only highlight the exceptional tunability of the Mn(Bi1-xSbx)4Te7 system, where precise control of the chemical potential and magnetic properties opens access to novel quantum phases, but also advance the understanding of FM WSMs. △ Less

Submitted 10 March, 2025; originally announced March 2025.

Comments: 28 pages, 5 figures

Finite-momentum superconducting states due to odd-frequency Cooper pairing correlations

Authors: Takumi Sato, Satoru Hayami, Shingo Kobayashi, Yasuhiro Asano

Abstract: This paper discusses the origin of a nonuniform superconducting state in which Cooper pairs have a small but finite center-of-mass momentum. We analyze the instability of the normal state to such finite-momentum states using the pole of the pair fluctuation propagator in weak-coupling superconductors. The finite-momentum superconducting state is realized when the odd-frequency pairing correlations… ▽ More This paper discusses the origin of a nonuniform superconducting state in which Cooper pairs have a small but finite center-of-mass momentum. We analyze the instability of the normal state to such finite-momentum states using the pole of the pair fluctuation propagator in weak-coupling superconductors. The finite-momentum superconducting state is realized when the odd-frequency pairing correlations in the uniform superconducting state are expected to have sufficiently large amplitudes. We provide a perspective for a comprehensive understanding of inhomogeneous superconductivity and related phenomena. △ Less

Submitted 22 May, 2026; v1 submitted 3 March, 2025; originally announced March 2025.

Universal whirling magnetic orders in non-Heisenberg Tsai-type quasicrystal approximants

Authors: Farid Labib, Kazuhiro Nawa, Yusuke Nambu, Hiroyuki Takakura, Yoichi Ikeda, Kazuhiko Deguchi, Masato Matsuura, Asuka Ishikawa, Ryoichi Kajimoto, Kazuhiko Ikeuchi, Taku J. Sato, Ryuji Tamura

Abstract: Magnetic orders of non-Heisenberg Tsai-type 1/1 approximant crystals (ACs) in the Au-Ga-Dy system were studied through bulk magnetization, neutron diffraction, and inelastic neutron scattering techniques. The results uncovered noncoplanar, ferromagnetic (FM) and antiferromagnetic (AFM) spin configurations whirling along [111] crystallographic axis, which is analogous to those observed in the Tb- a… ▽ More Magnetic orders of non-Heisenberg Tsai-type 1/1 approximant crystals (ACs) in the Au-Ga-Dy system were studied through bulk magnetization, neutron diffraction, and inelastic neutron scattering techniques. The results uncovered noncoplanar, ferromagnetic (FM) and antiferromagnetic (AFM) spin configurations whirling along [111] crystallographic axis, which is analogous to those observed in the Tb- and Ho-contained counterparts. The crystal electric field excitations similar to those in the Tb-based counterpart are also observed indicating the strong Ising-like magnetic anisotropy. These comprehensive experiments and analyses have revealed the existence of a universal mechanism that stabilizes noncoplanar FM and AFM structures in non-Heisenberg Tsai-type ACs, independent of the rare-earth species (Tb, Dy, Ho); FM intra-cluster interactions and strong Ising-like anisotropy. △ Less

Submitted 13 January, 2026; v1 submitted 3 March, 2025; originally announced March 2025.

Anomalous enhancement of large-momentum scattering by electron-electron interaction in moiré superlattices

Authors: Taiki Sato, Hiroaki Ishizuka

Abstract: Using a microscopic model, we show that the electron-electron interaction of flat bands deviates significantly from the Coulomb interaction. In particular, we find that large-momentum scattering is enhanced at $θ\lesssim4^\circ$, with a non-monotonic momentum dependence appearing near the magic angle. For $θ\gtrsim 1.2^\circ$, the enhanced large-momentum scattering can be attributed to the compact… ▽ More Using a microscopic model, we show that the electron-electron interaction of flat bands deviates significantly from the Coulomb interaction. In particular, we find that large-momentum scattering is enhanced at $θ\lesssim4^\circ$, with a non-monotonic momentum dependence appearing near the magic angle. For $θ\gtrsim 1.2^\circ$, the enhanced large-momentum scattering can be attributed to the compact Wannier function. On the other hand, for $θ\lesssim1.2^\circ$, the nonmonotonic momentum dependence of the interaction matrix cannot be explained by a simple Wannier orbital, indicating a nontrivial modification to the el-el interaction. Notably, the range of angles $θ$ where the large-momentum scattering is enhanced differs from the magic angles at which nearly-flat bands emerge, suggesting that the angle dependence of material properties provides information about the effect of interaction. The results highlight unusual features of the interaction in moiré graphene. △ Less

Submitted 20 June, 2025; v1 submitted 23 February, 2025; originally announced February 2025.

Journal ref: Phys. Rev. B 112, 075410 (2025)

Finite temperature fermion Monte Carlo simulations of frustrated spin-Peierls systems

Authors: João C. Inácio, Jeroen van den Brink, Fakher F. Assaad, Toshihiro Sato

Abstract: The Abrikosov fermion representation of the spin-1/2 degree of freedom allows for auxiliary-field quantum Monte Carlo simulations of frustrated spin systems. This approach provides a manifold of equivalent actions over which the negative sign problem can be optimised. As a result, we can reach temperature scales well below the magnetic scale. Here, we show how to generalise this algorithm to spin-… ▽ More The Abrikosov fermion representation of the spin-1/2 degree of freedom allows for auxiliary-field quantum Monte Carlo simulations of frustrated spin systems. This approach provides a manifold of equivalent actions over which the negative sign problem can be optimised. As a result, we can reach temperature scales well below the magnetic scale. Here, we show how to generalise this algorithm to spin-Peierls systems. In contrast to exact diagonalisation approaches, Monte Carlo methods are not Hilbert space bound such that the computational effort per sweep remains invariant when adding phonons. However, the computational effort required to generate independent configurations increases in the presence of phonons. We also show that, for the specific case of the Kitaev-Heisenberg model, the inclusion of phonons does not render the negative sign problem more severe. This new algorithm hence allows us to investigate the interplay between phonon degrees of freedom and magnetic frustration. We present results for frustrated and non-frustrated spin systems. △ Less

Submitted 10 February, 2025; originally announced February 2025.

Comments: 10 pages, 8 figures, article

Direct observation of the exciton polaron by serial femtosecond crystallography on single CsPbBr$_3$ quantum dots

Authors: Zhou Shen, Margarita Samoli, Onur Erdem, Johan Bielecki, Amit Kumar Samanta, Juncheng E, Armando Estillore, Chan Kim, Yoonhee Kim, Jayanath Koliyadu, Romain Letrun, Federico Locardi, Jannik Lübke, Abhishek Mall, Diogo Melo, Grant Mills, Safi Rafie-Zinedine, Adam Round, Tokushi Sato, Raphael de Wijn, Tamme Wollweber, Lena Worbs, Yulong Zhuang, Adrian P. Mancuso, Richard Bean , et al. (6 additional authors not shown)

Abstract: The outstanding opto-electronic properties of lead halide perovskites have been related to the formation of polarons. Nevertheless, the observation of the atomistic deformation brought about by one electron-hole pair in these materials has remained elusive. Here, we measure the diffraction patterns of single CsPbBr$_3$ quantum dots (QDs) with and without resonant excitation in the single exciton l… ▽ More The outstanding opto-electronic properties of lead halide perovskites have been related to the formation of polarons. Nevertheless, the observation of the atomistic deformation brought about by one electron-hole pair in these materials has remained elusive. Here, we measure the diffraction patterns of single CsPbBr$_3$ quantum dots (QDs) with and without resonant excitation in the single exciton limit using serial femtosecond crystallography (SFX). By reconstructing the 3D differential diffraction pattern, we observe small shifts of the Bragg peaks indicative of a crystal-wide deformation field. Building on DFT calculations, we show that these shifts are consistent with the lattice distortion induced by a delocalized electron and a localized hole, forming a mixed large/small exciton polaron. This result creates a clear picture of the polaronic deformation in CsPbBr$_3$ QDs, highlights the exceptional sensitivity of SFX to lattice distortions in few-nanometer crystallites, and establishes an experimental platform for future studies of electron-lattice interactions. △ Less

Submitted 4 February, 2025; originally announced February 2025.

Comments: Main: 12 pages, 5 figures; Supplemental: 21 pages, 11 figures

Unusual temperature dependence of the band structure associated with local atomic distortion in monolayer 1T'-WTe2

Authors: Ryuichi Ando, Katsuaki Sugawara, Tappei Kawakami, Koki Yanagizawa, Ken Yaegashi, Takashi Takahashi, Takafumi Sato

Abstract: The ground state of monolayer 1T'-WTe2 has been a target of intensive debate on whether or not it is a two-dimensional topological insulator (2D TI) associated with exciton formation. We investigated the band structure of an epitaxial monolayer 1T'-WTe2 film grown on graphene/SiC(0001) in a wide temperature range of T = 40 - 400 K by angle-resolved photoemission spectroscopy (ARPES). We observed a… ▽ More The ground state of monolayer 1T'-WTe2 has been a target of intensive debate on whether or not it is a two-dimensional topological insulator (2D TI) associated with exciton formation. We investigated the band structure of an epitaxial monolayer 1T'-WTe2 film grown on graphene/SiC(0001) in a wide temperature range of T = 40 - 400 K by angle-resolved photoemission spectroscopy (ARPES). We observed an electron band above the Fermi level (EF) slightly away from the Γ point, together with four hole bands below EF just at the Γ point. This signifies an indirect band gap exceeding 0.1 eV in support of the 2D-TI phase with the inverted band structure. We uncovered an unexpectedly large downward shift of valence bands upon cooling, accompanied with an upward shift of the conduction band. Comparison of the ARPES-derived band structure with first-principles band calculations suggests that the observed band shift is ascribed to the systematic local atomic distortion of tungsten atoms, which should be incorporated into the interpretation of unusual transport properties of 1T'-WTe2. △ Less

Submitted 29 January, 2025; originally announced January 2025.

Journal ref: Phys. Rev. Materials 9, L011001 (2025)

Selective Fabrication of Monolayer 1H- and 1T'-WTe2

Authors: Ryuichi Ando, Katsuaki Sugawara, Tappei Kawakami, Takashi Takahashi, Takafumi Sato

Abstract: We selectively fabricated monolayers of octahedral (1H) and distorted trigonal (1T') WTe2 on graphene/SiC(0001) by controlling the substrate temperature during epitaxy. Angle-resolved photoemission spectroscopy, combined with first-principles band-structure calculations, has revealed several drastic differences between these two polymorphs. The 1T' phase exhibits a semiconducting character with a… ▽ More We selectively fabricated monolayers of octahedral (1H) and distorted trigonal (1T') WTe2 on graphene/SiC(0001) by controlling the substrate temperature during epitaxy. Angle-resolved photoemission spectroscopy, combined with first-principles band-structure calculations, has revealed several drastic differences between these two polymorphs. The 1T' phase exhibits a semiconducting character with a nearly-zero energy gap, while the 1H phase shows a large band gap and the band splitting at the K/K' point. The present results pave a pathway toward developing nanoelectronic devices based with WTe2. △ Less

Submitted 29 January, 2025; originally announced January 2025.

Journal ref: J. Phys. Soc. Jpn. 93, 085002 (2024)

A sturdy spin-momentum locking in a chiral organic superconductor

Authors: Takuro Sato, Hiroshi Goto, Hiroshi M. Yamamoto

Abstract: Among noncentrosymmetric structures, chirality has recently been recognized as a novel source of asymmetrical charge/spin transports as exemplified by electrical magnetochiral anisotropy (EMChA) and chirality-induced spin selectivity. Although similar bulk-charge rectification and Rashba-Edelstein effect in polar systems are quantitively reproducible by theory based on the electronic band structur… ▽ More Among noncentrosymmetric structures, chirality has recently been recognized as a novel source of asymmetrical charge/spin transports as exemplified by electrical magnetochiral anisotropy (EMChA) and chirality-induced spin selectivity. Although similar bulk-charge rectification and Rashba-Edelstein effect in polar systems are quantitively reproducible by theory based on the electronic band structures, the relevance of band parameters in chiral effects remains elusive. Here, by working with a chiral organic superconductor, we experimentally demonstrate a gigantic EMChA and large superconducting diode effect, both of which are difficult to be explained solely by its band parameters. A two-critical-current signature and an enhanced critical field suggested triplet-mixed Cooper pairs with anomalously enhanced spin-orbit coupling above atomic limit. Our results clearly highlight a unique spin-momentum locking with large stiffness beyond the expectation, suggesting an unknown driving force for spin polarization inherent to chirality. △ Less

Submitted 28 January, 2025; originally announced January 2025.

Comments: 8 pages, 4 figures

Anomalous temperature-dependent magnetization in the nearly collinear antiferromagnet Y$_2$Co$_3$

Authors: Yunshu Shi, Huibo Cao, Hung-Cheng Wu, Li Yin, Neil Harrison, David S. Parker, Tushar Bhowmick, Tessa McNamee, Fatemeh Safari, Sergey L. Budko, James C. Fettinger, Susan M. Kauzlarich, Peter Klavins, Dmitry Popov, Ravhi Kumar, Russell J. Hemley, Shanti Deemyad, Taku J. Sato, Paul. C. Canfield, Valentin Taufour

Abstract: Y$_2$Co$_3$ is a newly discovered antiferromagnetic (AFM) compound with distorted kagome layers. Previous investigations via bulk magnetization measurements suggested a complex noncollinear magnetic behavior, with magnetic moments primarily anti-aligned along the $b$ axis and some canting towards the $ac$ plane. In this study, we report the magnetic structure of Y$_2$Co$_3$ to be an A-type AFM str… ▽ More Y$_2$Co$_3$ is a newly discovered antiferromagnetic (AFM) compound with distorted kagome layers. Previous investigations via bulk magnetization measurements suggested a complex noncollinear magnetic behavior, with magnetic moments primarily anti-aligned along the $b$ axis and some canting towards the $ac$ plane. In this study, we report the magnetic structure of Y$_2$Co$_3$ to be an A-type AFM structure with ferromagnetic (FM) interactions within the distorted kagome plane and an interplane antiferromagnetic interaction, as determined by single-crystal neutron diffraction. The magnetic moments align along the $b$ axis, with minimal canting towards the $c$ axis, at odds with the previous interpretation of bulk magnetization measurements. The magnetic moments on the two distinct Co sites are [0, -0.68(3), 0] $μ_B$ and [0, 1.25(4), 0.07(1)] $μ_B$. We attribute the previously reported "noncollinear" behavior to the considerable temperature dependence of itinerant AFM exchange interactions, induced by thermal contraction along the $b$ axis. Additionally, our examination of lattice constants through pressure studies reveals compensating effects on FM and AFM interactions, resulting in negligible pressure dependence of $T_\textrm{N}$. △ Less

Submitted 26 January, 2025; originally announced January 2025.

Journal ref: Physical Review B 110,235159 (2024)

Spin-degenerate bulk bands and topological surface states associated with Dirac nodal lines in RuO2

Authors: T. Osumi, K. Yamauchi, S. Souma, S. Paul, A. Honma, K. Nakayama, K. Ozawa, M. Kitamura, K. Horiba, H. Kumigashira, C. Bigi, F. Bertran, T. Oguchi, T. Takahashi, Y. Maeno, T. Sato

Abstract: Altermagnets are a novel platform to realize exotic electromagnetic properties distinct from those of conventional ferromagnets and antiferromagnets. We report results of micro-focused angle-resolved photoemission spectroscopy (ARPES) on RuO2, in which its altermagnetic nature has been under fierce debate in connection with crystal-orientation-dependent spintronic functionalities. By elucidating t… ▽ More Altermagnets are a novel platform to realize exotic electromagnetic properties distinct from those of conventional ferromagnets and antiferromagnets. We report results of micro-focused angle-resolved photoemission spectroscopy (ARPES) on RuO2, in which its altermagnetic nature has been under fierce debate in connection with crystal-orientation-dependent spintronic functionalities. By elucidating the band structure of the (100), (110) and (101) surfaces of a bulk single crystal using micro-ARPES, we found that, irrespective of the surface orientation, the experimental band structures show a good agreement with the bulk-band calculations for the nonmagnetic phase, but display a severe disagreement with those for the antiferromagnetic phase. Moreover, spin-resolved ARPES signifies a negligible spin polarization in the bulk bands, suggesting the absence of antiferromagnetism and altermagnetic spin splitting. In addition, we identified a nearly flat surface band and a dispersive one near the Fermi level at the (100)/(110) and (101) surfaces, respectively. Our first-principles calculations and analysis of Berry phase attribute these states to the topological surface bands emerging from the bulk Dirac nodal lines around the Fermi level. Our results indicate that such topological surface/interface states must be considered to understand the spintronic functionalities of RuO2, and may provide new insights into its catalytic characteristics. △ Less

Submitted 14 January, 2026; v1 submitted 17 January, 2025; originally announced January 2025.

Comments: 44 pages, 15 figures, accepted in Phy. Rev. B

Journal ref: Phys. Rev. B 113, 085116 (2026)

Picosecond expansion in LaAlO3 resonantly driven by infrared-active phonons

Authors: Jakob Gollwitzer, Jeffrey Z. Kaaret, Y. Eren Suyolcu, Guru Khalsa, Rylan C. Fernandes, Oleg Gorobtsov, Sören Buchenau, ChanJu You, Jayanti Higgins, Ryan S. Russell, Ziming Shao, Yorick A. Birkhölzer, Takahiro Sato, Matthieu Chollet, Giacomo Coslovich, Mario Brützam, Christo Guguschev, John W. Harter, Ankit S. Disa, Darrell G. Schlom, Nicole A. Benedek, Andrej Singer

Abstract: We investigate the ultrafast structural dynamics of LaAlO3 thin films driven by short mid-infrared laser pulses at 20 THz. Time-resolved X-ray diffraction reveals an immediate lattice expansion and an acoustic breathing mode of the film. First-principles theory and a spring-mass model identify the direct coupling between coherently driven infrared-active phonons and strain as the underlying mechan… ▽ More We investigate the ultrafast structural dynamics of LaAlO3 thin films driven by short mid-infrared laser pulses at 20 THz. Time-resolved X-ray diffraction reveals an immediate lattice expansion and an acoustic breathing mode of the film. First-principles theory and a spring-mass model identify the direct coupling between coherently driven infrared-active phonons and strain as the underlying mechanism. Time-resolved optical birefringence measurements confirm that the amplitude of this acoustic mode scales linearly with the pump fluence, which agrees with the theory. Furthermore, time-resolved X-ray diffuse scattering indicates that THz excitation enhances crystallinity by inducing a non-thermal increase in structural symmetry originating from preexisting defects. These findings highlight the potential of a multimodal approach-combining time-resolved X-ray and optical measurements and first-principles theory-to elucidate and control structural dynamics in nanoscale materials. △ Less

Submitted 22 December, 2024; originally announced December 2024.

Multiple metamagnetic transitions in helical antiferromagnet CeVGe$_3$

Authors: Hanshang Jin, Eun Sang Choi, Hung-Cheng Wu, N. J. Curro, K. Nawa, T. J. Sato, R. Kiyanagi, T. Ohhara, Peter Klavins, Valentin Taufour

Abstract: We report on neutron diffraction, magnetoresistance, magnetization, and magnetic torque measurements under high magnetic field in the helical antiferromagnet CeVGe$_3$. This compound exhibits Kondo lattice coherence and helical antiferromagnetic (AFM) ordering at ambient pressure, similar to the well-studied CeRhIn$_5$. Our measurements reveal that CeVGe$_3$ undergoes a magnetic transition from an… ▽ More We report on neutron diffraction, magnetoresistance, magnetization, and magnetic torque measurements under high magnetic field in the helical antiferromagnet CeVGe$_3$. This compound exhibits Kondo lattice coherence and helical antiferromagnetic (AFM) ordering at ambient pressure, similar to the well-studied CeRhIn$_5$. Our measurements reveal that CeVGe$_3$ undergoes a magnetic transition from an incommensurate (ICM) AFM state to an up-up-down-down commensurate (CM) AFM structure, followed by a transition to a novel phase at higher fields. A quantum phase transition occurs around 21.3 T. This rich magnetic field phase diagram closely resembles that of CeRhIn$_5$. Furthermore, angle-dependent magnetoresistance measurements reveal that all transitions in CeVGe$_3$ occur from the field component along the $ab$ plane. These findings highlight the intricate interplay among exchange interactions, crystal field effects, ground state properties, and crystalline symmetries. △ Less

Submitted 11 December, 2024; originally announced December 2024.

Comments: 8 pages, 9 figures, accepted by Physics Review B

Thermoelectric effect in a superconductor with Bogoliubov Fermi surfaces

Authors: Tomoya Sano, Takumi Sato, Akihiro Sasaki, Satoshi Ikegaya, Shingo Kobayashi, Yasuhiro Asano

Abstract: We study theoretically the thermoelectric effect in a superconducting state having the Bogoliubov-Fermi surfaces which stays in a thin superconducting layer between a conventional superconductor and an insulator. The thermoelectric coefficients calculated based on the linear response theory show the remarkable anisotropy in real space, which are explained well by the anisotropic shape of the Bogol… ▽ More We study theoretically the thermoelectric effect in a superconducting state having the Bogoliubov-Fermi surfaces which stays in a thin superconducting layer between a conventional superconductor and an insulator. The thermoelectric coefficients calculated based on the linear response theory show the remarkable anisotropy in real space, which are explained well by the anisotropic shape of the Bogoliubov-Fermi surface in momentum space. Our results indicate a way to check the existence of the Bogoliubov-Fermi surfaces in a stable superconducting state because the anisotropy is controlled by the direction of an applied magnetic field. △ Less

Submitted 16 March, 2025; v1 submitted 10 November, 2024; originally announced November 2024.

Comments: 11 pages, 10 figures,

Journal ref: Phys. Rev. B 111, 174503 (2025)