Showing 1–50 of 1,074 results for author: Xu, X

Multi-state electromagnetic phase modulations in NiCo2O4 through cation disorder and hydrogenation

Authors: Xuanchi Zhou, Xiaohui Yao, Shuang Li, Xiaomei Qiao, Jiahui Ji, Guowei Zhou, Huihui Ji, Xiaohong Xu

Abstract: One focal challenge in engineering low-power and scalable all-oxide spintronic devices lies in exploring ferromagnetic oxide material with perpendicular magnetic anisotropy (PMA) and electronic conductivity while exhibiting tunable spin states. Targeting this need, spinel nickel cobaltite (NiCo2O4, NCO), featured by room-temperature ferrimagnetically metallic ground state with strong PMA, emerges… ▽ More One focal challenge in engineering low-power and scalable all-oxide spintronic devices lies in exploring ferromagnetic oxide material with perpendicular magnetic anisotropy (PMA) and electronic conductivity while exhibiting tunable spin states. Targeting this need, spinel nickel cobaltite (NiCo2O4, NCO), featured by room-temperature ferrimagnetically metallic ground state with strong PMA, emerges as a promising candidate in the field of oxide spintronics. The cation distribution disorder inherent to NCO renders competing electromagnetic states and abnormal sign reversal of anomalous Hall effect (AHE), introducing an additional freedom to adjust electromagnetic transports. Here, we unveil multi-state electromagnetic phase modulations in NCO system through controllable cation disorder and proton evolution, extensively expanding electromagnetic phase diagram. The cation disorder in NCO tunable by growth temperature is identified as a critical control parameter for kinetically adjusting the proton evolution, giving rise to intermediate hydrogenated states with chemical stability. Hydrogen incorporation reversibly drives structural transformation and electromagnetic state evolutions in NCO, with rich spin-dependent correlated physics uncovered by combining the AHE scaling relation and synchrotron-based spectroscopy. Our work not only establishes NCO as a versatile platform for discovering spin-dependent physical functionality but also extends the horizons in materials design for state-of-the-art spintronic devices harnessing magneto-ionic control and inherent cation disorder. △ Less

Submitted 23 December, 2025; originally announced December 2025.

Stoichiometry-Controlled Structural Order and Tunable Antiferromagnetism in $\mathrm{Fe}_{x}\mathrm{NbSe_2}$ ($0.05 \le x \le 0.38$)

Authors: Xiaotong Xu, Bei Jiang, Runze Wang, Zhibin Qiu, Shu Guo, Baiqing Lv, Ruidan Zhong

Abstract: Transition metal dichalcogenides (TMDs) enable magnetic property engineering via intercalation, but stoichiometry-structure-magnetism correlations remain poorly defined for Fe-intercalated $\mathrm{NbSe_2}$. Here, we report a systematic study of $\mathrm{Fe}_{x}\mathrm{NbSe_2}$ across an extended composition range $0.05 \le x \le 0.38$, synthesized via chemical vapor transport and verified by rigo… ▽ More Transition metal dichalcogenides (TMDs) enable magnetic property engineering via intercalation, but stoichiometry-structure-magnetism correlations remain poorly defined for Fe-intercalated $\mathrm{NbSe_2}$. Here, we report a systematic study of $\mathrm{Fe}_{x}\mathrm{NbSe_2}$ across an extended composition range $0.05 \le x \le 0.38$, synthesized via chemical vapor transport and verified by rigorous energy-dispersive X-ray spectroscopy (EDS) microanalysis. X-ray diffraction, magnetic, and transport measurements reveal an intrinsic correlation between Fe content, structural ordering, and magnetic ground states. With increasing $x$, the system undergoes a successive transition from paramagnetism to a spin-glass state, then to long-range antiferromagnetism (AFM), and ultimately to a reentrant spin-glass phase, with the transition temperatures exhibiting a non-monotonic dependence on Fe content. The maximum Néel temperature ($T_{\mathrm{N}}$ = $\mathrm{175K}$) and strongest AFM coupling occur at $x=0.25$, where Fe atoms form a well-ordered $2a_0 \times 2a_0 $ superlattice within van der Waals gaps. Beyond $x = 0.25$, the superlattice transforms or disorders, weakening Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions and reducing $T_{\mathrm{N}}$ significantly. Electrical transport exhibits distinct anomalies at magnetic transition temperatures, corroborating the magnetic state evolution. Our work extends the compositional boundary of Fe-intercalated $\mathrm{NbSe_2}$, establishes precise stoichiometry-structure-magnetism correlations, and identifies structural ordering as a key tuning parameter for AFM. These findings provide a quantitative framework for engineering altermagnetic or switchable antiferromagnetic states in van der Waals materials. △ Less

Submitted 21 December, 2025; originally announced December 2025.

Driving the field-free Josephson diode effect using Kagome Mott insulator barriers

Authors: Michiel P. Dubbelman, Heng Wu, Joost Aretz, Yaojia Wang, Chris M. Pasco, Yuzhou Zhao, Trent M. Kyrk, Jihui Yang, Xiaodong Xu, Tyrel M. McQueen, Malte Roesner, Mazhar N. Ali

Abstract: Josephson junctions (JJs), devices consisting of two superconductors separated by a barrier, are of great technological importance, being a cornerstone of quantum information processing. Classical understanding of superconductor-insulator-superconductor JJs is that conventional insulator's properties, other than magnetism, do not significantly influence the junction's behavior. However, recent wor… ▽ More Josephson junctions (JJs), devices consisting of two superconductors separated by a barrier, are of great technological importance, being a cornerstone of quantum information processing. Classical understanding of superconductor-insulator-superconductor JJs is that conventional insulator's properties, other than magnetism, do not significantly influence the junction's behavior. However, recent work on quantum material (QM) JJs - using Mott insulator Nb3Br8 - resulted in magnetic field-free non-reciprocal superconductivity, termed the Josephson diode effect (JDE), implying the QM's intrinsic properties can modulate superconductivity in non-trivial ways. To date, the underlying mechanism and dependence of the JDE on correlation strength (U/t) has not been elucidated. Here we fabricate QMJJs using correlated Kagome insulators with varying U/t, Nb3X8 (X=Cl, Br, I), observing a decreasing trend of the field-free JDE with Nb3Cl8 reaching ~48% efficiency, Nb3Br8 ~6%, and Nb3I8 having no discernible JDE, matching the trend of decreasing U/t from Cl to I and suggesting correlation in insulators drives the field-free JDE. △ Less

Submitted 18 December, 2025; originally announced December 2025.

Comments: 7 pages main text (15 pages in total), 4 main text figures (9 figures total)

Nonreciprocal Transport in chiral Mo3Al2C Near the Superconducting to Normal Transition

Authors: Jeongsoo Park, Sang-Wook Cheong, Xianghan Xu

Abstract: We investigate nonreciprocal electrical transport in bulk single-crystalline Mo3Al2C, a material known to host crystallographic chirality, a polar charge-density-wave instability, and a superconducting transition near 8 K. Using AC transport measurements to analyze the first-harmonic and second-harmonic resistance responses, we observe a distinct nonreciprocal second-harmonic signal that is signif… ▽ More We investigate nonreciprocal electrical transport in bulk single-crystalline Mo3Al2C, a material known to host crystallographic chirality, a polar charge-density-wave instability, and a superconducting transition near 8 K. Using AC transport measurements to analyze the first-harmonic and second-harmonic resistance responses, we observe a distinct nonreciprocal second-harmonic signal that is significantly enhanced near the boundary of the normal and superconducting phases. Phenomenologically, this response arises from direction-dependent coupling between the external magnetic field and the current-induced intrinsic magnetization within the chiral lattice. Furthermore, a persistent nonreciprocal response observed under perpendicular magnetic fields suggests a toroidal-induced effect linked to the electric polarization emerging from the charge-density-wave phase. These results demonstrate that bulk Mo3Al2C serves as an intrinsic platform for tunable nonreciprocal transport rooted in the interplay of chirality, polarity, and superconductivity. △ Less

Submitted 15 December, 2025; originally announced December 2025.

Comments: 9 pages, 4 main figures, 2 supplementary figures

Quantum-inspired Chemical Rule for Discovering Topological Materials

Authors: Xinyu Xu, Rajibul Islam, Ghulam Hussain, Yangming Huang, Xiaoguang Li, Pavlo O. Dral, Arif Ullah, Ming Yang

Abstract: Topological materials exhibit unique electronic structures that underpin both fundamental quantum phenomena and next-generation technologies, yet their discovery remains constrained by the high computational cost of first-principles calculations and the slow, resource-intensive nature of experimental synthesis. Recent machine-learning approaches, such as the heuristic topogivity rule, offer data-d… ▽ More Topological materials exhibit unique electronic structures that underpin both fundamental quantum phenomena and next-generation technologies, yet their discovery remains constrained by the high computational cost of first-principles calculations and the slow, resource-intensive nature of experimental synthesis. Recent machine-learning approaches, such as the heuristic topogivity rule, offer data-driven alternatives by quantifying each element's intrinsic tendency toward topological behavior. Here, we develop a quantum-classical hybrid artificial neural network (QANN) that extends this rule into a quantum-inspired formulation. Within this framework, the QANN maps compositional descriptors to quantum probability amplitudes, naturally introducing pairwise inter-element correlations inaccessible to classical heuristics. The physical validity of these correlations is substantiated by constructing an equivalent complex-valued neural network (CVNN), confirming both the consistency and interpretability of the formulation. Retaining the simplicity of chemical reasoning while embedding quantum-native features, our quantum-inspired rule enables efficient and generalizable topological classification. High-throughput screening combined with first-principles (DFT) validation reveals five previously unreported topological compounds, demonstrating the enhanced predictive power and physical insight afforded by quantum-inspired heuristics. △ Less

Submitted 15 December, 2025; originally announced December 2025.

Stable skyrmion bundles at room temperature and zero magnetic field in a chiral magnet

Authors: Yongsen Zhang, Jin Tang, Yaodong Wu, Meng Shi, Xitong Xu, Shouguo Wang, Mingliang Tian, Haifeng Du

Abstract: Topological spin textures are characterized by topological magnetic charges, Q, which govern their electromagnetic properties. Recent studies have achieved skyrmion bundles with arbitrary integer values of Q, opening possibilities for exploring topological spintronics based on Q. However, the realization of stable skyrmion bundles in chiral magnets at room temperature and zero magnetic field - the… ▽ More Topological spin textures are characterized by topological magnetic charges, Q, which govern their electromagnetic properties. Recent studies have achieved skyrmion bundles with arbitrary integer values of Q, opening possibilities for exploring topological spintronics based on Q. However, the realization of stable skyrmion bundles in chiral magnets at room temperature and zero magnetic field - the prerequisite for realistic device applications - has remained elusive. Here, through the combination of pulsed currents and reversed magnetic fields, we experimentally achieve skyrmion bundles with different integer Q values - reaching a maximum of 24 at above room temperature and zero magnetic field - in the chiral magnet Co8Zn10Mn2. We demonstrate the field-driven annihilation of high-Q bundles and present a phase diagram as a function of temperature and field. Our experimental findings are consistently corroborated by micromagnetic simulations, which reveal the nature of the skyrmion bundle as that of skyrmion tubes encircled by a fractional Hopfion. △ Less

Submitted 10 December, 2025; originally announced December 2025.

Comments: Published in Nature Communications

Quantum Coulomb drag signatures of Majorana bound states

Authors: Zi-Wei Li, Jiaojiao Chen, Wei Xiong, Xiao Xue, Zeng-Zhao Li

Abstract: Majorana bound states (MBSs), with their non-Abelian statistics and topological protection, are key candidates for fault-tolerant quantum computation. However, their unambiguous identification in solid-state systems remains a fundamental challenge. Here, we present a theoretical study demonstrating that drag transport in a capacitively coupled double quantum dot system offers a robust and nonlocal… ▽ More Majorana bound states (MBSs), with their non-Abelian statistics and topological protection, are key candidates for fault-tolerant quantum computation. However, their unambiguous identification in solid-state systems remains a fundamental challenge. Here, we present a theoretical study demonstrating that drag transport in a capacitively coupled double quantum dot system offers a robust and nonlocal probe of weakly coupled MBSs. Using the master equation approach, we investigate both steady-state and transient dynamics and uncover a distinctive signature of MBSs, i.e., the emergence of pronounced split peaks in the drag transconductance, directly linked to inter-MBS coupling. We further show that the dynamics of quantum coherence exhibit an inverse correlation with the emergence and enhancement of MBS-induced split peaks in the drag transconductance as the inter-MBS coupling increases. A comparative analysis with Andreev bound states (ABSs) reveals key differences, that is, MBS-induced transconductance peaks are symmetric and robust, while ABS features are asymmetric and sensitive to perturbations. These findings establish clear experimental criteria for distinguishing MBSs and provide a practical framework for probing Majorana physics through nonlocal transport. △ Less

Submitted 1 December, 2025; originally announced December 2025.

Comments: 11 pages, 4 figures

$J_1-J_2$ Triangular Lattice Antiferromagnet in a Magnetic Field

Authors: Anna Keselman, Xinyuan Xu, Hao Zhang, Cristian D. Batista, Oleg A. Starykh

Abstract: We investigate the spin-1/2 $J_1-J_2$ triangular-lattice Heisenberg antiferromagnet in a magnetic field by combining large-scale density matrix renormalization group (DMRG) simulations with self-consistent spin-wave theory. The resulting field-coupling phase diagram reveals that quantum fluctuations stabilize coplanar order across the entire parameter range, giving rise to a characteristic sequenc… ▽ More We investigate the spin-1/2 $J_1-J_2$ triangular-lattice Heisenberg antiferromagnet in a magnetic field by combining large-scale density matrix renormalization group (DMRG) simulations with self-consistent spin-wave theory. The resulting field-coupling phase diagram reveals that quantum fluctuations stabilize coplanar order across the entire parameter range, giving rise to a characteristic sequence of magnetization plateaux. Near the quantum-spin-liquid window $0.06 \lesssim J_2/J_1 \lesssim 0.14$, which extends to magnetic field $B \sim J_1$, we identify overlapping $m = 1/3$ and $m = 1/2$ plateaux - a distinctive hallmark of the system's proximity to the low-field spin-liquid regime. The excellent quantitative agreement between DMRG and self-consistent one-loop spin-wave calculations demonstrates that semiclassical approaches can reliably capture and parameterize the plateau phases of triangular quantum antiferromagnets. △ Less

Submitted 1 December, 2025; originally announced December 2025.

Equivalence of residual entropy of hexagonal and cubic ices from tensor network methods

Authors: Xia-Ze Xu, Tong-Yu Lin, Guang-Ming Zhang

Abstract: The long-standing question of whether the residual entropy of hexagonal ice ($S_h$) equals that of cubic ice ($S_c$) remains unresolved despite decades of research on ice-type models. While analytical studies have established the inequality $S_h \geq S_c$, numerical investigations suggest that the two values are very close. In this work, we revisit this problem using high-precision tensor-network… ▽ More The long-standing question of whether the residual entropy of hexagonal ice ($S_h$) equals that of cubic ice ($S_c$) remains unresolved despite decades of research on ice-type models. While analytical studies have established the inequality $S_h \geq S_c$, numerical investigations suggest that the two values are very close. In this work, we revisit this problem using high-precision tensor-network methods. In Monte Carlo approaches the residual entropy cannot be directly obtained by sampling the ground-state degeneracy space, however, the tensor-network framework enables an explicit encoding of the "ice rule'' into local tensors, and then the residual entropy is transformed into finding the largest eigenvalue of a transfer operator in the form of a projected entangled-pair operator, which allows high-accuracy numerical evaluation. Meanwhile, we propose a new perspective based on analyzing the normality of the transfer operator, and demonstrate that if the operator is normal, the equality $S_h = S_c$ follows directly. Then the variational tensor network methods are employed to numerically verify this normality. Finally both residual entropies are directly computed by using our recently developed split corner transfer matrix renormalization group algorithm, providing a rigorous evidence supporting the equality between $S_h$ and $S_c$. △ Less

Submitted 27 November, 2025; originally announced November 2025.

Comments: 10 pages, 6 figures, two tables

Phase Field Study of Exchange Coupling of Hard/Soft Ferrite on Magnetic Permeability

Authors: Xinyu Xu, Wenqin Yue, Yueli Yu, Yongke Yan, Liwei D. Geng

Abstract: Effective modulation of magnetic permeability plays a vital role in the development of high-performance inductors. Here, phase-field simulations of hard/soft ferrite composites (BaM/NiZn) clarify how exchange coupling and microstructure impact magnetic permeability. We show that particle size, volume fraction, and orientation of the hard phase can effectively control the transition from collinear… ▽ More Effective modulation of magnetic permeability plays a vital role in the development of high-performance inductors. Here, phase-field simulations of hard/soft ferrite composites (BaM/NiZn) clarify how exchange coupling and microstructure impact magnetic permeability. We show that particle size, volume fraction, and orientation of the hard phase can effectively control the transition from collinear to non-collinear coupling, with a critical exchange size r_cr approximately 12 nm. Increasing the hard-phase fraction deepens the anisotropy energy well and monotonically suppresses permeability. In contrast, rotating the BaM easy axis to 90 degrees relative to the applied field produces a strong enhancement: at a 10 nm radius and eta = 0.1 volume fraction, the effective permeability can be more than 30 times larger than in the parallel configuration and then saturates for larger particles. This study establishes a microstructure-permeability-based physical framework for designing hard/soft magnetic composite systems. △ Less

Submitted 25 November, 2025; originally announced November 2025.

A Full Minimal Coupling GW-BSE Framework for Circular Dichroism in Solids: Applications to Chiral 2D Perovskites

Authors: Xian Xu, Diana Y. Qiu

Abstract: Circular dichroism (CD) and other chiroptical responses are a key probe of both chirality and momentum-space geometry in solids, but first-principles calculations are still challenging in periodic systems with strong exciton effects. Here, we develop a gauge-invariant first-principles framework for CD including exciton effects based on full minimal coupling (FMC) within the GW plus Bethe-Salpeter… ▽ More Circular dichroism (CD) and other chiroptical responses are a key probe of both chirality and momentum-space geometry in solids, but first-principles calculations are still challenging in periodic systems with strong exciton effects. Here, we develop a gauge-invariant first-principles framework for CD including exciton effects based on full minimal coupling (FMC) within the GW plus Bethe-Salpeter equation (GW-BSE) formalism. In contrast to standard multipole expansion and sum-over-states (SOS) approaches, which require careful gauge-fixing, converge slowly, and suffer origin ambiguities, FMC evaluates optical matrix elements directly at finite photon wavevector, naturally including intraband and near-degenerate transitions while placing electric-dipole (ED), magnetic-dipole (MD), and electric-quadrupole (EQ) contributions on equal footing. Applied to two prototypical two-dimensional chiral hybrid perovskites, (S-NEA)2PbBr4 and (S-MBA)2PbI4, our calculations reveal that MD and EQ channels contribute equally to the CD signal. Crucially, intraband and quasi-degenerate transitions only captured within FMC can significantly modify CD spectra, especially in systems with dense band degeneracies. The FMC framework, therefore, offers a computationally efficient and numerically robust way for predicting chiral optoelectronic phenomena in complex solids. △ Less

Submitted 24 November, 2025; originally announced November 2025.

Comments: 8 pages, 3 figures

Neural optimization of the most probable paths of 3D active Brownian particles

Authors: Bin Zheng, Zhongqiang Xiong, Changhao Li, Zhanglin Hou, Ziluo Zhang, Xinpeng Xu, Li-Shing Lin, Kenta Ishimoto, Kento Yasuda, Shigeyuki Komura

Abstract: We develop a variational neural-network framework to determine the most probable path (MPP) of a 3D active Brownian particle (ABP) by directly minimizing the Onsager-Machlup integral (OMI). To obtain the OMI, we use the Onsager-Machlup variational principle for active systems and construct the Rayleighian of the ABP by including its active power. This approach reveals geometric transitions of the… ▽ More We develop a variational neural-network framework to determine the most probable path (MPP) of a 3D active Brownian particle (ABP) by directly minimizing the Onsager-Machlup integral (OMI). To obtain the OMI, we use the Onsager-Machlup variational principle for active systems and construct the Rayleighian of the ABP by including its active power. This approach reveals geometric transitions of the MPP from in-plane I- and U-shaped paths to 3D helical paths as the final time and net displacement are varied. We also demonstrate that the initial and final boundary conditions have a significant impact on the MPPs. Our results show that neural optimization combined with the Onsager-Machlup variational principle provides an efficient and versatile framework for exploring optimal transition pathways in active and nonequilibrium systems. △ Less

Submitted 20 November, 2025; v1 submitted 20 November, 2025; originally announced November 2025.

Comments: 6 pages, 3 figures

Tunable Luttinger liquid and correlated insulating states in one-dimensional moiré superlattices

Authors: Jiajun Chen, Bosai Lyu, Liguo Wang, Shuo Lou, Xianliang Zhou, Tongyao Wu, Jingxu Xie, Yi Chen, Cheng Hu, Kenji Watanabe, Takashi Taniguchi, Guibai Xie, Mengzhou Liao, Wei Yang, Guangyu Zhang, Binbin Wei, Xiaoqun Wang, Qi Liang, Guohua Wang, Jie Ma, Dong Qian, Guorui Chen, Tingxin Li, Mingpu Qin, Xiao Yan Xu , et al. (1 additional authors not shown)

Abstract: Two-dimensional moiré superlattices have been extensively studied, and a variety of correlated phenomena have been observed. However, their lower-dimensional counterpart, one-dimensional (1D) moiré superlattices, remain largely unexplored. Electrons in 1D are generally described by Luttinger liquid theory, with universal scaling relations depending only on the Luttinger parameter g. In particular,… ▽ More Two-dimensional moiré superlattices have been extensively studied, and a variety of correlated phenomena have been observed. However, their lower-dimensional counterpart, one-dimensional (1D) moiré superlattices, remain largely unexplored. Electrons in 1D are generally described by Luttinger liquid theory, with universal scaling relations depending only on the Luttinger parameter g. In particular, at half-filling, Umklapp scattering plays a crucial role, as it can significantly change the conductance-temperature scaling relation and lead to Mott insulators. However, this prediction has never been observed since doping an empty band to half-filling was extremely difficult. Here, we show that the marriage of moiré superlattices and 1D electrons makes it possible to study the Luttinger liquid in an exceptionally wide filling region simply by electrical gating. We perform transport measurements on 1D moiré superlattices of carbon nanotubes on hexagonal boron nitride (hBN) substrates, and observe correlated insulating states at 1/4 and 1/2 fillings of the superlattice mini-band, where Umklapp scattering becomes dominant. We also observe a T-linear conductance at these commensurate fillings over a range of temperatures. Strikingly, the T-linear conductance leads to a strongly suppressed Luttinger parameter, suggesting a state of extreme correlation. △ Less

Submitted 16 November, 2025; originally announced November 2025.

Intrinsic structure of relaxor ferroelectrics from first principles

Authors: Xinyu Xu, Kehan Cai, Pinchen Xie

Abstract: We hybridize the swap Monte Carlo and geometric relaxation methods to determine the intrinsic compositional structure (CS) of the lead magnesium niobate (PMN) relaxor. We verify the stability of a Nb-rich sublattice in PMN, as prescribed by the prevailing random-site model. However, ions in the complementary sublattice are not randomly mixed. Most Nb ions collapse into a single percolating cluster… ▽ More We hybridize the swap Monte Carlo and geometric relaxation methods to determine the intrinsic compositional structure (CS) of the lead magnesium niobate (PMN) relaxor. We verify the stability of a Nb-rich sublattice in PMN, as prescribed by the prevailing random-site model. However, ions in the complementary sublattice are not randomly mixed. Most Nb ions collapse into a single percolating cluster with a mesh-like structure. This specific geometry serves to prevent large space charges, and this behavior differs from typical phase separation in metallic alloys. Subsequent molecular dynamics simulations predict a pair distribution function that is consistent with neutron scattering experiments. Analysis of dipolar structures in the Nb cluster sheds light on the unique dielectric properties of PMN. △ Less

Submitted 20 November, 2025; v1 submitted 14 November, 2025; originally announced November 2025.

Effect of Concentration Fluctuations on Material Properties of Disordered Alloys

Authors: Han-Pu Liang, Chuan-Nan Li, Xin-Ru Tang, Xun Xu, Chen Qiu, Qiu-Shi Huang, Su-Huai Wei

Abstract: Alloying compound AX with another compound BX is widely used to tune material properties. For disordered alloys, due to the lack of periodicity, it has been challenging to calculate and study their material properties. Special quasi-random structure (SQS) method has been developed and widely used to treat this issue by matching averaged atomic correlation functions to those of ideal random alloys,… ▽ More Alloying compound AX with another compound BX is widely used to tune material properties. For disordered alloys, due to the lack of periodicity, it has been challenging to calculate and study their material properties. Special quasi-random structure (SQS) method has been developed and widely used to treat this issue by matching averaged atomic correlation functions to those of ideal random alloys, enabling accurate predictions of macroscopic material properties such as total energy and volume. However, in AxB1-x alloys, statistically allowed local concentration fluctuations can give rise to defect-like minority configurations, such as bulk-like AX or BX regions in the extreme, which could strongly affect calculation of some of the material properties such as semiconductor bandgap, if it is not defined properly, leading to significant discrepancies between theory and experiment. In this work, taking the bandgap as an example, we demonstrate that the calculated alloy bandgap can be significantly underestimated in standard SQS calculations when the SQS cell size is increased to improve the structural model and the bandgap is defined conventionally as the energy difference between the lowest unoccupied state and the highest occupied state, because the rare event motifs can lead to wavefunction localization and become the dominant factor in determining the "bandgap", contrary to experiment. To be consistent with experiment, we show that the bandgap of the alloy should be extracted from the majority configurations using a density-of-states fitting (DOSF) method. This DOSF approach resolves the long-standing issue of calculating electronic structure of disordered semiconductor alloys. Similar approaches should also be developed to treat material properties that depends on localized alloy wavefunctions. △ Less

Submitted 13 November, 2025; originally announced November 2025.

A Scalable Superconducting Circuit Framework for Emulating Physics in Hyperbolic Space

Authors: Xicheng Xu, Ahmed Adel Mahmoud, Noah Gorgichuk, Ronny Thomale, Steven Rayan, Matteo Mariantoni

Abstract: Theoretical studies and experiments in the last six years have revealed the potential for novel behaviours and functionalities in device physics through the synthetic engineering of negatively-curved spaces. For instance, recent developments in hyperbolic band theory have unveiled the emergence of higher-dimensional eigenstates -- features fundamentally absent in conventional Euclidean systems. At… ▽ More Theoretical studies and experiments in the last six years have revealed the potential for novel behaviours and functionalities in device physics through the synthetic engineering of negatively-curved spaces. For instance, recent developments in hyperbolic band theory have unveiled the emergence of higher-dimensional eigenstates -- features fundamentally absent in conventional Euclidean systems. At the same time, superconducting quantum circuits have emerged as a leading platform for quantum analogue emulations and digital simulations in scalable architectures. Here, we introduce a scalable superconducting circuit framework for the analogue quantum emulation of tight-binding models on hyperbolic and kagome-like lattices. Using this approach, we experimentally realize three distinct lattices, including, for the first time to our knowledge, a hyperbolic lattice whose unit cell resides on a genus-3 Riemann surface. Our method encodes the hyperbolic metric directly into capacitive couplings between high-quality superconducting resonators, enabling tenable reproduction of spectral and localization properties while overcoming major scalability and spectral resolution limitations of previous designs. These results set the stage for large-scale experimental studies of hyperbolic materials in condensed matter physics and lay the groundwork for realizing hyperbolic quantum processors, with potential implications for both fundamental physics and quantum computing △ Less

Submitted 27 October, 2025; originally announced October 2025.

Comments: 18 pages, 12 figures (5 figures in main body and 7 figures in extended material)

iDART: Interferometric Dual-AC Resonance Tracking nano-electromechanical mapping

Authors: J. Bemis, F. Wunderwald, U. Schroeder, X. Xu, A. Gruverman, R. Proksch

Abstract: Piezoresponse force microscopy (PFM) has established itself as a very successful and reliable imaging and spectroscopic tool for measuring a wide variety of nanoscale electromechanical functionalities. Quantitative imaging of nanoscale electromechanical phenomena requires high sensitivity while avoiding artifacts induced by large drive biases. Conventional PFM often relies on high voltages to over… ▽ More Piezoresponse force microscopy (PFM) has established itself as a very successful and reliable imaging and spectroscopic tool for measuring a wide variety of nanoscale electromechanical functionalities. Quantitative imaging of nanoscale electromechanical phenomena requires high sensitivity while avoiding artifacts induced by large drive biases. Conventional PFM often relies on high voltages to overcome optical detection noise, leading to various non-ideal effects including electrostatic crosstalk, Joule heating, and tip-induced switching. To mitigate this situation, we introduce interferometrically detected, resonance-enhanced dual AC resonance tracking (iDART), which combines femtometer-scale displacement sensitivity of quadrature phase differential interferometry with contact resonance amplification. Through this combination, iDART achieves 10x or greater signal-to-noise improvement over current state of the art PFM approaches including both single frequency interferometric PFM or conventional, resonance enhanced PFM using optical beam detection. In this work, we demonstrate a >10x improvement of imaging sensitivity on PZT and Y-HfO. Switching spectroscopy shows similar improvements, where further demonstrates reliable hysteresis loops at small biases, mitigating nonlinearities and device failures that can occur at higher excitation amplitudes. These results position iDART as a powerful approach for probing conventional ferroelectrics with extremely high signal to noise down to weak piezoelectric systems, extending functional imaging capabilities to thin films, 2D ferroelectrics, beyond-CMOS technologies and bio-materials. △ Less

Submitted 28 October, 2025; v1 submitted 21 October, 2025; originally announced October 2025.

Laser-Induced Heating in Diamonds: Influence of Substrate Thermal Conductivity and Interfacial Polymer Layers

Authors: Md Shakhawath Hossain, Jiatong Xu, Thi Ngoc Anh Mai, Nhat Minh Nguyen, Trung Vuong Doan, Chaohao Chen, Qian Peter Su, Yongliang Chen, Evgeny Ekimov, Toan Dinh, Xiaoxue Xu, Toan Trong Tran

Abstract: Diamonds hosting color centers possess intrinsically high thermal conductivity; therefore, laser-induced heating has often received little attention. However, when placed on substrates with low thermal conductivity, localized heating of diamonds under laser excitation can become significant, and the presence of an interfacial polymer layer between substrate and diamond further amplifies this effec… ▽ More Diamonds hosting color centers possess intrinsically high thermal conductivity; therefore, laser-induced heating has often received little attention. However, when placed on substrates with low thermal conductivity, localized heating of diamonds under laser excitation can become significant, and the presence of an interfacial polymer layer between substrate and diamond further amplifies this effect. Yet, the relationship between substrate thermal conductivity, polymer thickness, and laser heating remains to be established. Here, a systematic investigation is presented on laser-induced heating of silicon-vacancy diamond on substrates with varying thermal conductivity and interfacial polymer thickness. Results reveal that even at a low excitation power of 737~$μ$W/$μ$m$^2$, thin amorphous holey carbon -- the lowest-conductivity substrate ($\sim$0.2~W~m$^{-1}$~K$^{-1}$) studied -- exhibits substantial heating, while glass ($\sim$1.4~W~m$^{-1}$~K$^{-1}$) and polydimethylsiloxane (PDMS, $\sim$0.35~W~m$^{-1}$~K$^{-1}$) show noticeable heating only above 2.95~mW/$μ$m$^2$. For polymer interlayers, a thickness of just 2.2~$μ$m induces significant heating at 2.95~mW/$μ$m$^2$ and above, highlighting strong influence of both substrate and polymer thickness on local heating response. Experimental findings are further validated using COMSOL Multiphysics simulations with a steady-state 3D heat transfer model. These results provide practical guidance for substrate selection and sample preparation, enabling optimization of conditions for optical thermometry and quantum sensing applications. △ Less

Submitted 16 October, 2025; originally announced October 2025.

Spinons, solitons and random singlets in the spin-chain compound copper benzoate

Authors: Ying Chen, Guijing Duan, Yuejiu Zhao, Ning Xi, Bingying Pan, Xiaoyu Xu, Zhanlong Wu, Kefan Du, Shuo Li, Ze Hu, Rui Bian, Xiaoqun Wang, Wei Li, Long Zhang, Yi Cui, Shiyan Li, Rong Yu, Weiqiang Yu

Abstract: The $S=1/2$ antiferromagnetic Heisenberg chain is a paradigmatic quantum system hosting exotic excitations such as spinons and solitons, and forming random singlet state in the presence of quenched disorder. Realizing and distinguishing these excitations in a single material remains a significant challenge. Using nuclear magnetic resonance (NMR) on a high-quality single crystal of copper benzoate,… ▽ More The $S=1/2$ antiferromagnetic Heisenberg chain is a paradigmatic quantum system hosting exotic excitations such as spinons and solitons, and forming random singlet state in the presence of quenched disorder. Realizing and distinguishing these excitations in a single material remains a significant challenge. Using nuclear magnetic resonance (NMR) on a high-quality single crystal of copper benzoate, we identify and characterize all three excitation types by tuning the magnetic field at ultra-low temperatures. At a low field of 0.2 T, a temperature-independent spin-lattice relaxation rate ($1/T_1$) over more than a decade confirms the presence of spinons. Below 0.4 K, an additional relaxation channel emerges, characterized by $1/T_1 \propto T$ and a spectral weight growing as $-\ln(T/T_0)$, signaling a random-singlet ground state induced by weak quenched disorder. At fields above 0.5 T, a field-induced spin gap $Δ\propto H^{2/3}$ observed in both $1/T_1$ and the Knight shift signifies soliton excitations. Our results establish copper benzoate as a unique experimental platform for studying one-dimensional quantum integrability and the interplay of disorder and correlations. △ Less

Submitted 13 October, 2025; originally announced October 2025.

Loss investigations of high frequency lithium niobate Lamb wave resonators at ultralow temperatures

Authors: Wenbing Jiang, Xuankai Xu, Jiazhen Pan, Hancong Sun, Yu Guo, Huabing Wang, Libing Zhou, Tao Wu

Abstract: Lamb wave resonators (LWRs) operating at ultralow temperatures serve as promising acoustic platforms for implementing microwave-optical transduction and radio frequency (RF) front-ends in aerospace communications because of the exceptional electromechanical coupling (k^2) and frequency scalability. However, the properties of LWRs at cryogenic temperatures have not been well understood yet. Herein,… ▽ More Lamb wave resonators (LWRs) operating at ultralow temperatures serve as promising acoustic platforms for implementing microwave-optical transduction and radio frequency (RF) front-ends in aerospace communications because of the exceptional electromechanical coupling (k^2) and frequency scalability. However, the properties of LWRs at cryogenic temperatures have not been well understood yet. Herein, we experimentally investigate the temperature dependence of the quality factor and resonant frequency in higher order antisymmetric LWRs down to millikelvin temperatures. The high-frequency A1 and A3 mode resonators with spurious-free responses are comprehensively designed, fabricated, and characterized. The quality factors of A1 modes gradually increase upon cryogenic cooling and shows 4 times higher than the room temperature value, while A3 mode resonators exhibit a non-monotonic temperature dependence. Our findings provide new insights into loss mechanisms of cryogenic LWRs, paving the way to strong-coupling quantum acoustodynamics and next-generation satellite wireless communications. △ Less

Submitted 12 October, 2025; originally announced October 2025.

Comments: Accepted for publication in Applied Physics Letters

Phase-sensitive evidence for 2x2 pair density wave in a kagome superconductor

Authors: Xiao-Yu Yan, Guowei Liu, Hanbin Deng, Xitong Xu, Haiyang Ma, Hailang Qin, Jun-Yi Zhang, Yuanyuan Zhao, Haitian Zhao, Zhe Qu, Yigui Zhong, Kozo Okazaki, Xiquan Zheng, Yingying Peng, Zurab Guguchia, X. X. Wu, Qianghua Wang, X-H Fan, Wei Song, M-W Gao, Hendrik Hohmann, Matteo Durrnagel, Ronny Thomale, Jia-Xin Yin

Abstract: The pair-density-wave (PDW) exhibits periodic amplitude and sign modulations of the superconducting order parameter. Such a pairing state has been proposed to be sensitive to nonmagnetic scattering. In this work, we observe the nonmagnetic PDW-breaking effect in a kagome superconductor, using scanning tunneling microscopy. We observe 2x2 PDW induced by the coupling between charge order and superco… ▽ More The pair-density-wave (PDW) exhibits periodic amplitude and sign modulations of the superconducting order parameter. Such a pairing state has been proposed to be sensitive to nonmagnetic scattering. In this work, we observe the nonmagnetic PDW-breaking effect in a kagome superconductor, using scanning tunneling microscopy. We observe 2x2 PDW induced by the coupling between charge order and superconductivity. The global PDW is substantially suppressed upon doping the kagome lattice with dilute isovalent nonmagnetic impurities, whereas the charge order and uniform superconductivity remain robust. Spatial correlation analysis further confirms that PDW is distinctly suppressed near dopants. We attribute the PDW suppression to a nonmagnetic PDW breaking effect, arising from phase sign modulation of PDW in the kagome d-orbital hosting Bogoliubov Fermi states. △ Less

Submitted 12 October, 2025; originally announced October 2025.

Probing Phase Diagrams of Ordered Two-Dimensional Ice

Authors: Bingzheng Wu, Jianming Wu, Sai Duan, Xin Xu

Abstract: Water, a ubiquitous and fundamental substance, plays a critical role across a wide range of disciplines from physics and chemistry to biology and engineering. Despite theoretical predictions of several phases of two-dimensional (2D) ice confined between idealized hydrophobic walls, experimental validation has been limited to the square phase, whose structural origin remains controversial. Here, we… ▽ More Water, a ubiquitous and fundamental substance, plays a critical role across a wide range of disciplines from physics and chemistry to biology and engineering. Despite theoretical predictions of several phases of two-dimensional (2D) ice confined between idealized hydrophobic walls, experimental validation has been limited to the square phase, whose structural origin remains controversial. Here, we propose a realistic nanoconfinement setup using wide-bandgap hexagonal boron nitride (h-BN) as the capping layer and Cu(111) as the substrate. This protocol enables scanning tunneling microscope (STM) to resolve the atomic-scale arrangement of water molecules beneath the h-BN layer, overcoming the limitations of conventional techniques. Simulated STM images unambiguously identify all ordered flat 2D ice phases, as well as coexisting phases, and effectively distinguish them from potential contaminants. These findings establish a robust framework for experiment to systematically probe the phase structures of 2D ice, opening an avenue for studying nanoconfined water under ambient conditions. △ Less

Submitted 8 October, 2025; originally announced October 2025.

Comments: 29 pages, 7 figures

Understanding Polaronic Transport in Anatase TiO2 Films by Combining Precise Synthesis and First-Principles Many-Body Theory

Authors: F. Liu, Z. Yang, Y. Luo, S. Guo, C. Zhang, S. Choo, X. Xu, X. Wang, K. A. Mkhoyan, M. Bernardi, B. Jalan

Abstract: In complex oxides, charge carriers often couple strongly with lattice vibrations to form polarons-entangled electron-phonon quasiparticles whose transport properties remain difficult to characterize. Experimental access to intrinsic polaronic transport requires ultraclean samples, while theoretical descriptions demand methods beyond low-order perturbation theory. Here, we combine the growth of hig… ▽ More In complex oxides, charge carriers often couple strongly with lattice vibrations to form polarons-entangled electron-phonon quasiparticles whose transport properties remain difficult to characterize. Experimental access to intrinsic polaronic transport requires ultraclean samples, while theoretical descriptions demand methods beyond low-order perturbation theory. Here, we combine the growth of high-quality oxygen-vacancy-doped anatase TiO2 films by hybrid molecular beam epitaxy (MBE) with a first-principles electron-phonon diagrammatic Monte Carlo (FEP-DMC) framework recently developed for accurate polaron predictions. Our films exhibit record-high electron mobility for anatase TiO2, in excellent agreement with FEP-DMC calculations conducted prior to experiment, which predict a room-temperature mobility of 45 +/- 15 cm2V-1s-1 and a mobility-temperature scaling of mobility proportional to T^(-1.9 +/- 0.077). Microscopic analysis using scanning transmission electron microscopy and X-ray photoelectron spectroscopy reveals the role of oxygen vacancies in modulating transport at lower temperatures. FEP-DMC further provides quantitative insight into polaron formation energy, phonon cloud distribution, lattice distortion around the polaron, and the polaronic contribution to mobility. Together, these results establish a predictive theory-experiment workflow to characterize polarons and provide a microscopic understanding of large-polaron transport in anatase TiO2, with broader implications for complex oxides and other polaronic materials. △ Less

Submitted 8 October, 2025; originally announced October 2025.

Comments: 22 pages, 4 figures

Dispersion and the transport of exciton-polaritons in an optical conveyor belt

Authors: Xingran Xu, Chunyu Jia, Xin-Xin Yang

Abstract: The growing interest in exciton-polaritons has driven the need to manipulate their motion and engineer their band structures to the forefront of contemporary research. This study explores the band structures that emerge from a spatially modulated potential, ingeniously realized through the use of an optical conveyor belt. By leveraging Bloch theory and conducting a meticulous analysis of the time… ▽ More The growing interest in exciton-polaritons has driven the need to manipulate their motion and engineer their band structures to the forefront of contemporary research. This study explores the band structures that emerge from a spatially modulated potential, ingeniously realized through the use of an optical conveyor belt. By leveraging Bloch theory and conducting a meticulous analysis of the time evolution of polariton intensity in Fourier space, we have derived the energy dispersion relations both analytically and numerically within the context of a static lattice model. For time-dependent potentials, we employ the Lagrange variational method to elucidate the dynamics of polariton motion. Our results reveal that polaritons exhibit linear dispersion and follow linear trajectories with minor oscillations superimposed. This investigation not only deepens our fundamental understanding of exciton-polaritons but also provides a robust tool for advancing photonic devices and exerting precise control over current transport in quantum computing. Our findings pave the way for future innovations in high-speed and high-performance technologies. △ Less

Submitted 8 October, 2025; originally announced October 2025.

Electrotoroidicity: New Paradigm for Transverse Electromagnetic Responses

Authors: Kai Du, Daegeun Jo, Xianghan Xu, Fei-Ting Huang, Ming-Hao Lee, Ming-Wen Chu, Kefeng Wang, David Vanderbilt, Hyun-Woo Lee, Sang-Wook Cheong

Abstract: The exploration of transverse electromagnetic responses in solids with broken spatial-inversion (I) and/or time-reversal (T) symmetries has unveiled numerous captivating phenomena, including the (anomalous) Hall effect, Faraday rotations, non-reciprocal directional dichroism, and off-diagonal linear magnetoelectricity, all within the framework of magnetotoroidicity. Here, we introduce a novel clas… ▽ More The exploration of transverse electromagnetic responses in solids with broken spatial-inversion (I) and/or time-reversal (T) symmetries has unveiled numerous captivating phenomena, including the (anomalous) Hall effect, Faraday rotations, non-reciprocal directional dichroism, and off-diagonal linear magnetoelectricity, all within the framework of magnetotoroidicity. Here, we introduce a novel class of transverse electromagnetic responses originating from electrotoroidicity in ferro-rotational (FR) systems with preserved I and T symmetries, distinct from magnetotoroidicity. We discover a high-order off-diagonal magnetic susceptibility of FR domains and a reduced linear diagonal magnetic susceptibility at FR domain walls in doped ilmenite FeTiO3. The non-trivial "Hall-like" effect of the former corresponds to an anomalous transverse susceptibility in the presence of spontaneous electrotoroidal moments in FR materials. Our findings unveil an emergent type of transverse electromagnetic responses even in I and T symmetry-conserved conditions and illustrate new functionalities of abundant FR materials. △ Less

Submitted 30 September, 2025; originally announced October 2025.

Comments: Nature Physics (2025)

Machine Learning Based Optical Thermometry Using Photoluminescence and Raman Spectra of Diamonds Containing SiV Centers

Authors: Md Shakhawath Hossain, Dylan G. Stone, G. Landry, Xiaoxue Xu, Carlo Bradac, Toan Trong Tran

Abstract: Micro- and nanothermometry enable precise temperature monitoring and control at the micro- and nanoscale, and have become essential diagnostic tools in applications ranging from high-power microelectronics to biosensing and nanomedicine. Most existing techniques rely on secondary micro- and nanothermometers that require individual calibration of each sensor, ideally both off- and in-situ, before u… ▽ More Micro- and nanothermometry enable precise temperature monitoring and control at the micro- and nanoscale, and have become essential diagnostic tools in applications ranging from high-power microelectronics to biosensing and nanomedicine. Most existing techniques rely on secondary micro- and nanothermometers that require individual calibration of each sensor, ideally both off- and in-situ, before use. We present an alternative approach that overcomes this limitation by employing fluorescent diamonds containing silicon-vacancy centers, where the thermo-sensitive physical quantities are the centers' photoluminescence and the diamond host's Raman signals. The photoluminescence and Raman data are analyzed using two multi-feature regression algorithms that leverage a minimal number of calibration diamonds and temperature set points to predict the temperature of previously unseen diamonds. Using this approach, the models achieve accuracies as low as 0.7 K, resolutions down to 0.6 K Hz$^{-1/2}$, and sensitivity as high as 0.04 K$^{-1}$. These correspond to improvements of roughly 70 percent (over threefold) in accuracy, 50 percent (twofold) in resolution, and 567 percent (sevenfold) in sensitivity compared with traditional single-feature models. Our approach is particularly suited to applications where pre-deployment calibration of every thermosensor is impractical, and it is generalizable to any thermometry platform with two or more simultaneously measurable temperature-dependent observables. △ Less

Submitted 24 October, 2025; v1 submitted 26 September, 2025; originally announced September 2025.

Signature of chiral superconducting order parameter evidenced in mesoscopic superconductors

Authors: Xiaoying Xu, Wei Qin, Yuelin Shen, Zixuan Huang, Zhuoya Zhou, Zirao Wang, Yufan Li

Abstract: Chiral superconductivity is a novel superconducting phase characterized by order parameters that break the time-reversal symmetry, endowing the state with a definite handedness. Unlike conventional superconductors, the Cooper pairs in a chiral superconductor carry nonzero orbital angular momentum. Through coupling with an external magnetic field, the finite angular momentum of the Cooper pair modu… ▽ More Chiral superconductivity is a novel superconducting phase characterized by order parameters that break the time-reversal symmetry, endowing the state with a definite handedness. Unlike conventional superconductors, the Cooper pairs in a chiral superconductor carry nonzero orbital angular momentum. Through coupling with an external magnetic field, the finite angular momentum of the Cooper pair modulates the temperature-magnetic field phase boundary in a distinctive way, which could serve as an experimental signature of the chiral superconducting state. Here we demonstrate that the chiral signature can be detected in mesoscopic superconducting rings of $β$-Bi$_2$Pd, manifesting as a linear-in-field modulation of the critical temperature in the Little-Parks effect. Our findings establish a new experimental method for detecting the chiral superconductivity. △ Less

Submitted 23 September, 2025; originally announced September 2025.

Large Anomalous and Topological Hall Effect and Nernst Effect in a Dirac Kagome Magnet Fe3Ge

Authors: Chunqiang Xu, Shuvankar Gupta, Hengxin Tan, Hyeonhu Bae, Olajumoke Oluwatobiloba Emmanuel, Mingyu Xu, Yan Wu, Xiaofeng Xu, Pengpeng Zhang, Weiwei Xie, Binghai Yan, Xianglin Ke

Abstract: The search for kagome magnets with unconventional magnetic and electronic properties has gained significant attention in recent years. We report the magnetic, electronic, and thermoelectric properties of Fe3Ge single crystals, where the Fe atoms form a slightly distorted kagome lattice. Fe3Ge exhibits a large anomalous Hall effect and anomalous Nernst effect. The anomalous transverse thermoelectri… ▽ More The search for kagome magnets with unconventional magnetic and electronic properties has gained significant attention in recent years. We report the magnetic, electronic, and thermoelectric properties of Fe3Ge single crystals, where the Fe atoms form a slightly distorted kagome lattice. Fe3Ge exhibits a large anomalous Hall effect and anomalous Nernst effect. The anomalous transverse thermoelectric conductivity reaches about 4.6 A m^-1 K^-1, exceeding values reported for conventional ferromagnets and most topological ferromagnets. First-principles calculations indicate that these transport responses are primarily governed by intrinsic mechanisms, highlighting the dominant role of Berry curvature arising from massive Dirac gaps in momentum space. In addition, we observe a topological Hall resistivity of about 0.9 microOhm cm and a topological Nernst coefficient of 1.2 microvolt K^-1, which are attributed to the Berry phase associated with field-induced scalar spin chirality. These findings demonstrate the combined influence of Berry phases in both momentum and real space, establishing Fe3Ge as a promising candidate for room-temperature transverse thermoelectric applications. △ Less

Submitted 22 September, 2025; originally announced September 2025.

Comments: Accepted in Advanced Functional Materials

Magnetically Enhanced Thermoelectric Effect Driven by Martensitic Transformation in the Weak Itinerant Ferromagnet Co$_2$NbSn

Authors: Takumi Kihara, Xiao Xu, Yuki Ogi, Yoshiya Adachi, Tufan Roy, Ryuji Matsuura, Takeshi Kanomata

Abstract: We investigated the magnetic and thermoelectric properties of the full Heusler alloy Co$_2$NbSn, which exhibits a martensitic transformation at 240 K. Magnetization measurements reveal weak itinerant ferromagnetism in the martensitic phase, which is well described by Takahashi's spin fluctuation theory. The characteristic spin fluctuation parameters were estimated to be T_0 = 1.0$\times$10^3 K and… ▽ More We investigated the magnetic and thermoelectric properties of the full Heusler alloy Co$_2$NbSn, which exhibits a martensitic transformation at 240 K. Magnetization measurements reveal weak itinerant ferromagnetism in the martensitic phase, which is well described by Takahashi's spin fluctuation theory. The characteristic spin fluctuation parameters were estimated to be T_0 = 1.0$\times$10^3 K and T_A = 7.2$\times$10^3 K. Seebeck coefficient measurements under magnetic fields up to 9 T show complex temperature and field dependence, which we decomposed into electron diffusion, spin fluctuation drag, and magnon drag components. A significant magnon-drag contribution was identified in both austenite and martensitic phases. Remarkably, this contribution is strongly enhanced in the martensitic phase compared to the austenite phase, despite a smaller magnetic moment. These findings provide evidence for robust low-energy spin excitations and highlight the potential of martensitic transformation in enhancing the thermoelectric performance of itinerant ferromagnetic alloys. △ Less

Submitted 22 September, 2025; originally announced September 2025.

Comments: 18 pages, 8 figures

Journal ref: Jounal of the Physical Society of Japan, 94, 113801 (2025)

Axial Hall Effect in Altermagnetic Lieb Lattices

Authors: Xilong Xu, Haonan Wang, Li Yang

Abstract: We predict a so-called axial Hall effect, a Berry-curvature-driven anomalous Hall response, in Lieb-lattice altermagnets. By constructing a tight-binding model, we identify the axial direction as a hidden topological degree of freedom. Breaking the double degeneracy of axial symmetry generates substantial Berry curvature and induces a pronounced anomalous Hall conductivity. First-principles calcul… ▽ More We predict a so-called axial Hall effect, a Berry-curvature-driven anomalous Hall response, in Lieb-lattice altermagnets. By constructing a tight-binding model, we identify the axial direction as a hidden topological degree of freedom. Breaking the double degeneracy of axial symmetry generates substantial Berry curvature and induces a pronounced anomalous Hall conductivity. First-principles calculations further confirm the emergence of this effect in strained altermagnets, particularly in ternary transition-metal dichalcogenides. We take Mn2WS4 as an example to reveal that the axial Hall effect originates from the interplay between Dresselhaus spin-orbit coupling and the intrinsic piezomagnetic response of Lieb-lattice altermagnets, leading to highly localized and enhanced Berry curvature. Remarkably, the magnitude of the axial Hall effect is significant and remains unchanged when varying the strain, highlighting the topological nature of the axial degree of freedom. Finally, in multilayer systems, the effect manifests as a distinctive thickness-dependent modulation of both anomalous and spin Hall responses. These findings emphasize the critical role of spin-orbit coupling and noncollinear spin textures in altermagnets, an area that has received limited attention, and open new pathways for exploring intrinsic Hall phenomena in topological magnetic systems. △ Less

Submitted 16 September, 2025; originally announced September 2025.

Comments: 19 pages with 4 figures and 2 tables

Valley-Selective Linear Dichroism and Excitonic Effects in Lieb-Lattice Altermagnets

Authors: Haonan Wang, Xilong Xu, Du Li, Li Yang

Abstract: Altermagnets have recently been recognized as a distinct class of magnetic materials characterized by alternative spin-split electronic structures without net magnetization. Despite intensive studies on their single-particle spintronic and valleytronic properties, many-electron interactions and optical responses of altermagnets remain less explored. In this work, we employ many-body perturbation t… ▽ More Altermagnets have recently been recognized as a distinct class of magnetic materials characterized by alternative spin-split electronic structures without net magnetization. Despite intensive studies on their single-particle spintronic and valleytronic properties, many-electron interactions and optical responses of altermagnets remain less explored. In this work, we employ many-body perturbation theory to investigate excited states and their strain tunability. Using monolayer Mn2WS4 as a representative candidate, we uncover a novel spin valley-dependent excitonic selection rule in two-dimensional altermagnetic Lieb lattices. In addition to strongly bound excitons, we find that linearly polarized light selectively excites valley spin-polarized excitons. Moreover, due to the interplay between altermagnetic spin symmetry and electronic orbital character, we predict that applying uniaxial strain can lift valley degeneracy and enable the selective excitation of spin-polarized excitons, an effect not achievable in previously studied transition-metal dichalcogenides. These spin-valley-locked excitonic states and their strain tunability offer a robust mechanism for four-fold symmetric altermagnets to encode, store, and read valley/spin information. △ Less

Submitted 16 September, 2025; originally announced September 2025.

Comments: 20 pages with 4 figures

Persistent Interfacial Topological Hall Effect Demonstrating Electrical Readout of Topological Spin Structures in Insulators

Authors: Jing Li, Huilin Lai, Andrew H. Comstock, Aeron McConnell, Bharat Giri, Yu Yun, Tianhao Zhao, Xiao Wang, Yongseong Choi, Xuemei Cheng, Jian Shen, Zhigang Jiang, Dali Sun, Wenbin Wang, Xiaoshan Xu

Abstract: Conventional topological Hall effects (THE) require conducting magnets, leaving insulating systems largely inaccessible. Here we introduce the interfacial topological Hall effect (ITHE), where the noncoplanar spin textures of insulating magnets are imprinted onto an adjacent heavy metal via the magnetic proximity effect (MPE) and detected electrically. In Pt/h-LuFeO3 bilayers, h-LuFeO3 hosts a top… ▽ More Conventional topological Hall effects (THE) require conducting magnets, leaving insulating systems largely inaccessible. Here we introduce the interfacial topological Hall effect (ITHE), where the noncoplanar spin textures of insulating magnets are imprinted onto an adjacent heavy metal via the magnetic proximity effect (MPE) and detected electrically. In Pt/h-LuFeO3 bilayers, h-LuFeO3 hosts a topological spin structure robust against high magnetic fields, arising from a 120° triangular spin lattice with small spin canting that yields nontrivial topology but minimal magnetization. This generates a giant Hall response in Pt up to 0.5% of the longitudinal resistivity and a Hall-conductivity/magnetization ratio above 2 V^{-1}, clearly distinguishable from the spin Hall Hanle effect background. Field- and temperature-dependent analysis further reveals that Pt nanoclusters inherit topological textures from h-LuFeO3 via MPE. Unlike the conventional THE narrow peak-and-dip features, ITHE in Pt/h-LuFeO3 persists across a broad magnetic field range up to 14 T, demonstrating the exceptional stability of the underlying topological spin structure. This establishes ITHE as a powerful and sensitive probe for topological magnetism in ultrathin insulating films and paves the way for new spintronic applications. △ Less

Submitted 16 September, 2025; originally announced September 2025.

Controlled growth of polar altermagnets via chemical vapor transport

Authors: Hiraka Haruhiro, Raktim Datta, Poonam Yadav, Anzar Ali, Suheon Lee, Matthias J. Gutmann, Duhee Yoon, Dirk Wulferding, Xianghan Xu, Moon-Ho Jo, Sang-Wook Cheong, Sungkyun Choi

Abstract: Altermagnetic properties have been recently proposed in polar magnetic oxides, M$_{2}$Mo$_{3}$O$_{8}$ (M = Mn, Fe, Co, Ni), where improved characteristics of stronger magnetoelectric coupling and higher magnetic transition temperatures were observed. Thus, understanding their microscopic origins is of fundamental and technological importance. However, the difficulty in growing large single crystal… ▽ More Altermagnetic properties have been recently proposed in polar magnetic oxides, M$_{2}$Mo$_{3}$O$_{8}$ (M = Mn, Fe, Co, Ni), where improved characteristics of stronger magnetoelectric coupling and higher magnetic transition temperatures were observed. Thus, understanding their microscopic origins is of fundamental and technological importance. However, the difficulty in growing large single crystals hinders detailed experimental studies. Here, we report the successful growth of large single crystals of the pyroelectric antiferromagnet using two representative compounds, Fe$_{2}$Mo$_{3}$O$_{8}$ and NiZnMo$_{3}$O$_{8}$. Growth was optimized using various parameters, finding the transport agent density as a primary factor, which depends strongly on the position of the pellet, the starting powder form, and the volume of the ampule. We demonstrated a controlled growth method by manipulating the convection and diffusion kinetics. High-quality crystals were characterized by using single-crystal X-ray diffraction, Laue diffraction, magnetic susceptibility, and Raman spectroscopy. Manipulation of magnetic properties through nonmagnetic Zn doping was shown in NiZnMo$_{3}$O$_{8}$. Our results enable the detailed investigation and manipulation of their unconventional altermagnetic and multiferroic properties. This study provides crucial insight into the controlled growth of other functional quantum materials. △ Less

Submitted 15 September, 2025; originally announced September 2025.

Comments: 12 pages, 8 figures, 1 Table

Journal ref: Cryst. Growth Des. 25, 4991 (2025)

Visualizing Electronic Structure of Twisted Bilayer MoTe2 in Devices

Authors: Cheng Chen, William Holtzmann, Xiao-Wei Zhang, Eric Anderson, Shanmei He, Yuzhou Zhao, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Kenji Watanabe, Takashi Taniguchi, Ting Cao, Di Xiao, Xiaodong Xu, Yulin Chen

Abstract: The pursuit of emergent quantum phenomena lies at the forefront of modern condensed matter physics. A recent breakthrough in this arena is the discovery of the fractional quantum anomalous Hall effect (FQAHE) in twisted bilayer MoTe2 (tbMoTe2), marking a paradigm shift and establishing a versatile platform for exploring the intricate interplay among topology, magnetism, and electron correlations.… ▽ More The pursuit of emergent quantum phenomena lies at the forefront of modern condensed matter physics. A recent breakthrough in this arena is the discovery of the fractional quantum anomalous Hall effect (FQAHE) in twisted bilayer MoTe2 (tbMoTe2), marking a paradigm shift and establishing a versatile platform for exploring the intricate interplay among topology, magnetism, and electron correlations. While significant progress has been made through both optical and electrical transport measurements, direct experimental insights into the electronic structure - crucial for understanding and modeling this system - have remained elusive. Here, using spatially and angle-resolved photoemission spectroscopy (μ-ARPES), we directly map the electronic band structure of tbMoTe2. We identify the valence band maximum, whose partial filling underlies the FQAHE, at the K points, situated approximately 150 meV above the Γ valley. By fine-tuning the doping level via in-situ alkali metal deposition, we also resolve the conduction band minimum at the K point, providing direct evidence that tbMoTe2 exhibits a direct band gap - distinct from all previously known moire bilayer transition metal dichalcogenide systems. These results offer critical insights for theoretical modeling and advance our understanding of fractionalized excitations and correlated topological phases in this emergent quantum material. △ Less

Submitted 10 September, 2025; originally announced September 2025.

Comments: 13 pages, 4 figures

Non-monotonic band flattening near the magic angle of twisted bilayer MoTe$_2$

Authors: Yujun Deng, William Holtzmann, Ziyan Zhu, Timothy Zaklama, Paulina Majchrzak, Takashi Taniguchi, Kenji Watanabe, Makoto Hashimoto, Donghui Lu, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Liang Fu, Thomas P. Devereaux, Xiaodong Xu, Zhi-Xun Shen

Abstract: Twisted bilayer MoTe$_2$ (tMoTe$_2$) is an emergent platform for exploring exotic quantum phases driven by the interplay between nontrivial band topology and strong electron correlations. Direct experimental access to its momentum-resolved electronic structure is essential for uncovering the microscopic origins of the correlated topological phases therein. Here, we report angle-resolved photoemiss… ▽ More Twisted bilayer MoTe$_2$ (tMoTe$_2$) is an emergent platform for exploring exotic quantum phases driven by the interplay between nontrivial band topology and strong electron correlations. Direct experimental access to its momentum-resolved electronic structure is essential for uncovering the microscopic origins of the correlated topological phases therein. Here, we report angle-resolved photoemission spectroscopy (ARPES) measurements of tMoTe$_2$, revealing pronounced twist-angle-dependent band reconstruction shaped by orbital character, interlayer coupling, and moiré potential modulation. Density functional theory (DFT) captures the qualitative evolution, yet underestimates key energy scales across twist angles, highlighting the importance of electronic correlations. Notably, the hole effective mass at the K point exhibits a non-monotonic dependence on twist angle, peaking near 2°, consistent with band flattening at the magic angle predicted by continuum models. Via electrostatic gating and surface dosing, we further visualize the evolution of electronic structure versus doping, enabling direct observation of the conduction band minimum and confirm tMoTe$_2$ as a direct band gap semiconductor. These results establish a spectroscopic foundation for modeling and engineering emergent quantum phases in this moiré platform. △ Less

Submitted 10 September, 2025; originally announced September 2025.

Comments: 11 pages, 4 figures

Silicon-Compatible Ionic Control over Multi-State Magnetoelectric Phase Transformations in Correlated Oxide System

Authors: Xuanchi Zhou, Jiahui Ji, Wentian Lu, Huihui Ji, Chunwei Yao, Xiaohui Yao, Xiaomei Qiao, Guowei Zhou, Xiaohong Xu

Abstract: Realizing room-temperature ferromagnetic insulators, critical enablers for low-power spintronics, is fundamentally challenged by the long-standing trade-off between ferromagnetic ordering and indirect exchange interactions in insulators. Ionic evolution offers tempting opportunities for accessing exotic magnetoelectric states and physical functionality beyond conventional doping paradigm via tailo… ▽ More Realizing room-temperature ferromagnetic insulators, critical enablers for low-power spintronics, is fundamentally challenged by the long-standing trade-off between ferromagnetic ordering and indirect exchange interactions in insulators. Ionic evolution offers tempting opportunities for accessing exotic magnetoelectric states and physical functionality beyond conventional doping paradigm via tailoring the charge-lattice-orbital-spin interactions. Here, we showcase the precise magneto-ionic control over magnetoelectric states in LSMO system, delicately delivering silicon-compatible weakly ferromagnetic insulator state above room temperature. Of particular note is the decoupling of ion-charge-spin interplay in correlated LSMO system, a primary obstacle in clarifying underlying physical origin, with this process concurrently giving rise to an emergent intermediate state characterized by a weakly ferromagnetic half-metallic state. Benefiting from the SrTiO3 buffer layer as epitaxial template to promote interfacial heterogeneous nucleation, hydrogenation enables diverse magnetoelectric states in LSMO integrated on silicon, fully compatible with traditional semiconductor processing. Assisted by theoretical calculations and spectroscopic techniques, hydrogen-induced magnetoelectric transitions in LSMO are driven by band-filling control and suppression in double exchange interaction. Our work not only defines a novel design paradigm for exploring exotic quantum states in correlated system, with transformative potential for spintronics, but also fundamentally unveils the physical origin behind ionic evolution via disentangling the ion-charge-spin coupling. △ Less

Submitted 8 September, 2025; originally announced September 2025.

Direct spatiotemporal imaging of a long-lived bulk photovoltaic effect in $BiFeO_{3}$

Authors: Saptam Ganguly, Sebin Varghese, Aaron M. Schankler, Xianfei Xu, Kazuki Morita, Michel Viret, Andrew M. Rappe, Gustau Catalan, Klaas-Jan Tielrooij

Abstract: The bulk photovoltaic effect (BPVE), a manifestation of broken centrosymmetry, has attracted interest as a probe of the symmetry and quantum geometry of materials, and for use in novel optoelectronic devices. Despite its bulk nature, the BPVE is typically measured with interfaces and metal contacts, raising concerns as to whether the observed signals are genuinely of bulk origin. Here, we use a co… ▽ More The bulk photovoltaic effect (BPVE), a manifestation of broken centrosymmetry, has attracted interest as a probe of the symmetry and quantum geometry of materials, and for use in novel optoelectronic devices. Despite its bulk nature, the BPVE is typically measured with interfaces and metal contacts, raising concerns as to whether the observed signals are genuinely of bulk origin. Here, we use a contactless pump-probe microscopy method to observe the space- and time-resolved dynamics of photoexcited carriers in single-crystal, monodomain $BiFeO_{3}$. We observe asymmetric transport of carriers along the polar axis, confirming the intrinsic bulk and symmetry-driven nature of BPVE. This asymmetric transport persists for several nanoseconds after photoexcitation, which cannot be explained by the shift or phonon ballistic current BPVE mechanisms. Monte Carlo simulations show that asymmetric momentum scattering by defects under non-equilibrium conditions explains the long-lived carrier drift, while first principles calculations confirm that oxygen vacancies have an asymmetric electronic state that can cause such asymmetric scattering. Our findings highlight the critical role of defects in long-lived photoresponses. △ Less

Submitted 1 September, 2025; originally announced September 2025.

Optical control over topological Chern number in moiré materials

Authors: Olivier Huber, Kilian Kuhlbrodt, Eric Anderson, Weijie Li, Kenji Watanabe, Takashi Taniguchi, Martin Kroner, Xiaodong Xu, Atac Imamoglu, Tomasz Smolenski

Abstract: Controlling quantum matter with light offers a promising route to dynamically tune its many-body properties, ranging from band topology to superconductivity. However, achieving such optical control for strongly correlated electron systems in the steady-state has remained elusive. Here, we demonstrate all-optical switching of the spin-valley degree of freedom of itinerant ferromagnets in twisted Mo… ▽ More Controlling quantum matter with light offers a promising route to dynamically tune its many-body properties, ranging from band topology to superconductivity. However, achieving such optical control for strongly correlated electron systems in the steady-state has remained elusive. Here, we demonstrate all-optical switching of the spin-valley degree of freedom of itinerant ferromagnets in twisted MoTe2 homobilayers. This system uniquely features flat valley-contrasting Chern bands and exhibits a range of strongly correlated phases at various moiré lattice fillings, including Chern insulators and ferromagnetic metals. We show that the spin-valley orientation of all of these phases can be dynamically reversed by resonantly exciting the attractive polaron transition with circularly-polarized light. These findings not only constitute the first direct evidence for non-thermal switching of a ferromagnetic spin state at zero magnetic field, but also demonstrate the possibility of dynamical control over topological order parameter, paving the way for all-optical generation of chiral edge modes and topological quantum circuits. △ Less

Submitted 26 August, 2025; originally announced August 2025.

Comments: 7 pages, 4 figures

Optical Control of Integer and Fractional Chern Insulators

Authors: William Holtzmann, Weijie Li, Eric Anderson, Jiaqi Cai, Heonjoon Park, Chaowei Hu, Takashi Taniguchi, Kenji Watanabe, Jiun-Haw Chu, Di Xiao, Ting Cao, Xiaodong Xu

Abstract: Optical control of topology, particularly in the presence of electron correlations, is a fascinating topic with broad scientific and technological impact. Twisted MoTe$_2$ bilayer (tMoTe$_2$) is a newly discovered zero-field fractional Chern insulator (FCI), exhibiting the fractionally quantized anomalous Hall (FQAH) effect. Since the chirality of the edge states and sign of the Chern number are d… ▽ More Optical control of topology, particularly in the presence of electron correlations, is a fascinating topic with broad scientific and technological impact. Twisted MoTe$_2$ bilayer (tMoTe$_2$) is a newly discovered zero-field fractional Chern insulator (FCI), exhibiting the fractionally quantized anomalous Hall (FQAH) effect. Since the chirality of the edge states and sign of the Chern number are determined by the underlying ferromagnetic polarization, manipulation of ferromagnetism would realize control of the CI/FCI states. Here, we demonstrate control and switching of ferromagnetic polarization, and thus the CI and FCI states by circularly polarized optical pumping in tMoTe$_2$. At low optical excitation power, we achieve on-demand preparation of ferromagnetic polarization by optical training, i.e., electrically tuning the system from non-ferromagnetic to desirable ferromagnetic states accompanied with helicity-selective optical pumping. With increased excitation power, we further realize direct optical switching of ferromagnetic polarization at a temperature far below the Curie temperature. Both optical training and direct switching of ferromagnetism are most effective near CI/FCI states, which we attribute to a gap enhanced valley polarization of photo-injected holes. We show that the magnetization can be dynamically switched by modulating the helicity of optical excitation. Spatially resolved measurements further demonstrate optical writing of a ferromagnetic, and thus a CI (or FCI) domain. Our work realizes precise optical control of a topological quantum many-body system with potential applications in topological spintronics, quantum memories, and creation of exotic edge states by programmable patterning of integer and fractional QAH domains. △ Less

Submitted 25 August, 2025; originally announced August 2025.

Comments: First submitted version on April 24th, 2025

Ice-assisted soft-landing deposition for van der Waals integration

Authors: Xinyu Sun, Xiang Xu, BinBin Jin, Yihan Lu, Jichuang Shen, Wei Kong, Ding Zhao, Min Qiu

Abstract: Van der Waals integration enables the creation of electronic and optoelectronic devices with unprecedented performance and novel functionalities beyond the existing material limitations. However, it is typically realized using a physical pick-up-and-place process to minimize interfacial damages and is hardly integrated into conventional lithography and metallization procedures. Here we demonstrate… ▽ More Van der Waals integration enables the creation of electronic and optoelectronic devices with unprecedented performance and novel functionalities beyond the existing material limitations. However, it is typically realized using a physical pick-up-and-place process to minimize interfacial damages and is hardly integrated into conventional lithography and metallization procedures. Here we demonstrate a simple in situ transfer strategy for van der Waals integration, in which a thin film of amorphous water ice acts as a buffer layer to shield against the bombardment of energetic clusters during metallization. After ice sublimation, the deposited metal film can be gently and in situ placed onto underlying substrates, to form an atomically clean and damage-free metal-semiconductor interface. This strategy allows ultra-clean and non-destructive fabrication of high-quality contacts on monolayer MoS2, which is extremely beneficial to produce a high-performance 2D field-effect transistor with an ultra-high on/off ratio of 1010, mobility of 80 (cm2 V-1s-1), and also with reduced Fermi level pinning effect. We also demonstrate the batch production of CVD-grown MoS2 transistor arrays with uniform electrical characteristics. Such a gentle and ultra-clean fabrication approach has been further extended to materials with high reactivity, such as halide perovskites. Our method can be easily integrated with mature semiconductor manufacturing technology and may become a generic strategy for fabricating van der Waals contacted devices. △ Less

Submitted 25 August, 2025; originally announced August 2025.

Lattice distortions and non-sluggish diffusion in BCC refractory high entropy alloys

Authors: Jingfeng Zhang, Xiang Xu, Fritz Körmann, Wen Yin, Xi Zhang, Christian Gadelmeier, Uwe Glatzel, Blazej Grabowski, Runxia Li, Gang Liu, Biao Wang, Gerhard Wilde, Sergiy V. Divinski

Abstract: Refractory high-entropy alloys (RHEAs) have emerged as promising candidates for extreme high-temperature applications, for example, in next-generation turbines and nuclear reactors. In such applications, atomic diffusion critically governs essential properties including creep resistance and microstructural stability. The present study systematically investigates impurity diffusion of Co, Mn, and Z… ▽ More Refractory high-entropy alloys (RHEAs) have emerged as promising candidates for extreme high-temperature applications, for example, in next-generation turbines and nuclear reactors. In such applications, atomic diffusion critically governs essential properties including creep resistance and microstructural stability. The present study systematically investigates impurity diffusion of Co, Mn, and Zn in single phase (BCC solid solution) HfTiZrNbTa and HfTiZrNbV RHEAs applying the radiotracer technique. A neutron total scattering technique is used to evaluate the pair distribution functions and element-specific lattice distortions in these alloys. \textit{Ab initio}-based calculations give access to lattice distortions and solubilities of the impurities under investigation, including the impact of short-range order. The diffusion results are discussed in relation to calculated substitutional and interstitial solution energies, local lattice distortions, and short-range order effects. Co diffusion is found to be dominated by the interstitial mechanism, exhibiting fast diffusion. These findings reveal important structure-property relationships between local atomic environments and diffusion kinetics in BCC RHEAs, providing critical insights for designing alloys with enhanced high-temperature performance through targeted control of impurity diffusion processes. △ Less

Submitted 21 August, 2025; originally announced August 2025.

Comments: 18 pages, 8 figures

Journal ref: Acta Materialia 297 (2025) 121283

Accurate complex-stacking-fault Gibbs energy in Ni3Al at high temperatures

Authors: Xiang Xu, Xi Zhang, Andrei Ruban, Siegfried Schmauder, Blazej Grabowski

Abstract: To gain a deeper insight into the anomalous yield behavior of Ni3Al, it is essential to obtain temperature-dependent formation Gibbs energies of the relevant planar defects. Here, the Gibbs energy of the complex stacking fault (CSF) is evaluated using a recently proposed ab initio framework [Acta Materialia, 255 (2023) 118986], accounting for all thermal contributions - including anharmonicity and… ▽ More To gain a deeper insight into the anomalous yield behavior of Ni3Al, it is essential to obtain temperature-dependent formation Gibbs energies of the relevant planar defects. Here, the Gibbs energy of the complex stacking fault (CSF) is evaluated using a recently proposed ab initio framework [Acta Materialia, 255 (2023) 118986], accounting for all thermal contributions - including anharmonicity and paramagnetism - up to the melting point. The CSF energy shows a moderate decrease from 300K to about 1200 K, followed by a stronger drop. We demonstrate the necessity to carefully consider the individual thermal excitations. We also propose a way to analyze the origin of the significant anharmonic contribution to the CSF energy through atomic pair distributions at the CSF plane. With the newly available high-temperature CSF data, an increasing energy barrier for the cross-slip process in Ni3Al with increasing temperature is unveiled, necessitating the refinement of existing analytical models. △ Less

Submitted 21 August, 2025; originally announced August 2025.

Journal ref: Scripta Materialia 242 (2024) 115934

Unveiling the Puzzle of Brittleness in Single Crystal Iridium

Authors: Qing Cheng, Sergey V. Erohin, Konstantin V. Larionov, Bin Gan, Pavel B. Sorokin, Xiandong Xu

Abstract: Iridium is critical for extreme-environment applications due to its exceptional thermal stability and corrosion resistance, but its intrinsic brittleness remains a decades-old puzzle. Combining atomic-resolution scanning transmission electron microscopy, density first-principles calculations, and discrete dislocation dynamics simulations, we identify high-density, sessile Frank dislocation loops w… ▽ More Iridium is critical for extreme-environment applications due to its exceptional thermal stability and corrosion resistance, but its intrinsic brittleness remains a decades-old puzzle. Combining atomic-resolution scanning transmission electron microscopy, density first-principles calculations, and discrete dislocation dynamics simulations, we identify high-density, sessile Frank dislocation loops with zero-net Burgers vectors as the key mechanism. These loops form via an energetically favorable transformation from mixed perfect dislocations under stress, a process unique to iridium among face-centered cubic metals. The immobile loops act as potent barriers, drastically increasing yield strength and work hardening by impeding dislocation glide and consuming mobile dislocations. This dominance of these findings deepens the understanding of iridium's brittleness and offers a pathway for designing more ductile variants of this critical material. △ Less

Submitted 9 August, 2025; originally announced August 2025.

Comments: 36 pages, 16 figues

Electronic ordering driven by flat band nesting in a van der Waals magnet Fe5GeTe2

Authors: Qiang Gao, Gabriele Berruto, Khanh Duy Nguyen, Chaowei Hu, Haoran Lin, Beomjoon Goh, Bo Gyu Jang, Xiaodong Xu, Peter Littlewood, Jiun-Haw Chu, Shuolong Yang

Abstract: Solid-state systems with flat electronic bands have a theoretical propensity to form electronic orders such as superconductivity and charge-density waves. However, for many flat-band systems such as Kagome and Clover lattices, the flat bands do not naturally appear at the Fermi level, hence not driving the low-energy electronic ordering. Here we demonstrate the concurrent formation of flat bands a… ▽ More Solid-state systems with flat electronic bands have a theoretical propensity to form electronic orders such as superconductivity and charge-density waves. However, for many flat-band systems such as Kagome and Clover lattices, the flat bands do not naturally appear at the Fermi level, hence not driving the low-energy electronic ordering. Here we demonstrate the concurrent formation of flat bands at the Fermi level and a $\sqrt{3} \times \sqrt{3}\, R30^\circ$ charge order in a van der Waals magnet Fe5GeTe2 using high-resolution angle-resolved photoemission spectroscopy. This charge order is manifested by clear band structure folding below 100 K, yet the band folding is limited to 30 meV below the Fermi level where the flat bands reside. The nesting vector in the reciprocal space connects segments of Fermi surfaces where pronounced flat bands are discovered. Taken together with calculations of the Lindhard response function, our results establish Fe5GeTe2 as a model system where flat bands promote inter-band nesting and electronic ordering. The appearance of the flat band at the Fermi level is reminiscent of the Kondo lattice effect, yet we point out that the flat bands may originate from the abundance of vacancies in the Fe(1) sublattice, where the vacancies induce flat dispersions via destructive charge or spin interactions. △ Less

Submitted 5 August, 2025; originally announced August 2025.

Comments: 15 pages, 5 figures

Dichotomy of flat bands in the van der Waals ferromagnet Fe$_5$GeTe$_2$

Authors: Han Wu, Jianwei Huang, Chaowei Hu, Lei Chen, Yiqing Hao, Yue Shi, Paul Malinowski, Yucheng Guo, Bo Gyu Jang, Jian-Xin Zhu, Andrew F. May, Siqi Wang, Xiang Chen, Yaofeng Xie, Bin Gao, Yichen Zhang, Ziqin Yue, Zheng Ren, Makoto Hashimoto, Donghui Lu, Alexei Fedorov, Sung-Kwan Mo, Junichiro Kono, Yu He, Robert J. Birgeneau , et al. (6 additional authors not shown)

Abstract: Quantum materials with bands of narrow bandwidth near the Fermi level represent a promising platform for exploring a diverse range of fascinating physical phenomena, as the high density of states within the small energy window often enables the emergence of many-body physics. On one hand, flat bands can arise from strong Coulomb interactions that localize atomic orbitals. On the other hand, quantu… ▽ More Quantum materials with bands of narrow bandwidth near the Fermi level represent a promising platform for exploring a diverse range of fascinating physical phenomena, as the high density of states within the small energy window often enables the emergence of many-body physics. On one hand, flat bands can arise from strong Coulomb interactions that localize atomic orbitals. On the other hand, quantum destructive interference can quench the electronic kinetic energy. Although both have a narrow bandwidth, the two types of flat bands should exhibit very distinct spectral properties arising from their distinctive origins. So far, the two types of flat bands have only been realized in very different material settings and chemical environments, preventing a direct comparison. Here, we report the observation of the two types of flat bands within the same material system--an above-room-temperature van der Waals ferromagnet, Fe$_{5-x}$GeTe$_2$, distinguishable by a switchable iron site order. The contrasting nature of the flat bands is also identified by the remarkably distinctive temperature-evolution of the spectral features, indicating that one arises from electron correlations in the Fe(1) site-disordered phase, while the other geometrical frustration in the Fe(1) site-ordered phase. Our results therefore provide a direct juxtaposition of the distinct formation mechanism of flat bands in quantum materials, and an avenue for understanding the distinctive roles flat bands play in the presence of magnetism, topology, and lattice geometrical frustration, utilizing sublattice ordering as a key control parameter. △ Less

Submitted 6 August, 2025; v1 submitted 4 August, 2025; originally announced August 2025.

Comments: The manuscript was submitted on June 12 2024

In-Plane Magnetic Anisotropy and Large topological Hall Effect in Self-Intercalated Ferromagnet Cr1.61Te2

Authors: Yalei Huang, Na Zuo, Zheyi Zhang, Xiangzhuo Xing, Xinyu Yao, Anlei Zhang, Haowei Ma, Chunqiang Xu, Wenhe Jiao, Wei Zhou, Raman Sankar, Dong Qian, Xiaofeng Xu

Abstract: Self-intercalated chromium tellurides Cr1+xTe2 have garnered growing attention due to their high-temperature ferromagnetism, tunable spin structures and air stability, all of which are vital for versatile applications in next-generation memory and information technology. Here, we report strong magnetic anisotropy and a large topological Hall effect (THE) in self-intercalated Cr1.61Te2 single cryst… ▽ More Self-intercalated chromium tellurides Cr1+xTe2 have garnered growing attention due to their high-temperature ferromagnetism, tunable spin structures and air stability, all of which are vital for versatile applications in next-generation memory and information technology. Here, we report strong magnetic anisotropy and a large topological Hall effect (THE) in self-intercalated Cr1.61Te2 single crystals, which are both highly desirable properties for future spintronic applications. Our results demonstrate that Cr1.61Te2 is a soft ferromagnet with strong in-plane magnetic anisotropy. Remarkably, distinct THE behaviors are observed in different temperature regimes, reflecting the intricate spin structures and competing exchange interactions. More interestingly, a large topological Hall resistivity, induced by microscopic non-coplanar spin structures, emerges in the temperature range 70-240 K, reaching a maximum value of 0.93 μΩ cm at 150 K. Moreover, a sign-reversed and weak THE is observed at low temperatures below ~70 K, indicating the emergence of an additional topological spin structure with opposite topological charges. This work not only offers valuable insights into the correlation between magnetocrystalline anisotropy and topological phenomena in Cr1+xTe2 systems, but also provides a robust platform for engineering the evolution of complex spin textures that can be leveraged in diverse spintronic device applications. △ Less

Submitted 30 July, 2025; originally announced July 2025.

Comments: 5 figures, 1 table

Journal ref: Adv. Funct. Mater. 2025, e10351

Universal Magnetic Phases in Twisted Bilayer MoTe$_2$

Authors: Weijie Li, Evgeny Redekop, Christiano Wang Beach, Canxun Zhang, Xiaowei Zhang, Xiaoyu Liu, Will Holtzmann, Chaowei Hu, Eric Anderson, Heonjoon Park, Takashi Taniguchi, Kenji Watanabe, Jiun-haw Chu, Liang Fu, Ting Cao, Di Xiao, Andrea F. Young, Xiaodong Xu

Abstract: Twisted bilayer MoTe$_2$ (tMoTe$_2$) has emerged as a robust platform for exploring correlated topological phases, notably supporting fractional Chern insulator (FCI) states at zero magnetic field across a wide range of twist angles. The evolution of magnetism and topology with twist angle remains an open question. Here, we systematically map the magnetic phase diagram of tMoTe$_2$ using local opt… ▽ More Twisted bilayer MoTe$_2$ (tMoTe$_2$) has emerged as a robust platform for exploring correlated topological phases, notably supporting fractional Chern insulator (FCI) states at zero magnetic field across a wide range of twist angles. The evolution of magnetism and topology with twist angle remains an open question. Here, we systematically map the magnetic phase diagram of tMoTe$_2$ using local optical spectroscopy and scanning nanoSQUID-on-tip (nSOT) magnetometry. We identify spontaneous ferromagnetism at moiré filling factors $ν= -1$ and $-3$ over a twist angle range from 2.1$^\circ$ to 3.7$^\circ$, revealing a universal, twist-angle-insensitive ferromagnetic phase. At 2.1$^\circ$, we further observe robust ferromagnetism at $ν= -5$, absent in the devices with larger twist angle -- a signature of the flattening of higher bands in this twist angle range. Temperature-dependent measurements reveal a contrasting twist-angle dependence of the Curie temperatures between $ν= -1$ and $ν= -3$, indicating distinct interplay between exchange interaction and bandwidth for the two Chern bands. Despite spontaneous time-reversal symmetry breaking, we find no evidence of a topological gap at $ν= -3$; however, fragile correlated topological phases could be obscured by the device disorder evident in our spatially resolved measurements. Our results establish a global framework for understanding and controlling magnetic order in tMoTe$_2$ and highlight its potential for accessing correlated topological phases in higher energy Chern band. △ Less

Submitted 29 July, 2025; originally announced July 2025.

Weak low-temperature ferromagnetism and linear magnetoresistance in Lu$_{0.75}$Fe$_6$Sn$_6$ with a disordered HfFe$_6$Ge$_6$-type structure

Authors: Chenfei Shi, Zhaodi Lin, Qiyuan Liu, Junai Lv, Xiaofan Xu, Baojuan Kang, Jin-Hu Yang, Yi Liu, Jian Zhang, Shixun Cao, Jin-Ke Bao

Abstract: We report the synthesis of Lu$_{0.75}$Fe$_6$Sn$_6$ single crystals with a Fe-kagome lattice using a self-flux method. The crystal structure, magnetic, thermodynamic and electrical transport properties were investigated. Structure refinement reveals that Lu$_{0.75}$Fe$_6$Sn$_6$ has a HfFe$_6$Ge$_6$-type structure as the major framework intergrown with a CoSn-type structure, leading to a vacancy of… ▽ More We report the synthesis of Lu$_{0.75}$Fe$_6$Sn$_6$ single crystals with a Fe-kagome lattice using a self-flux method. The crystal structure, magnetic, thermodynamic and electrical transport properties were investigated. Structure refinement reveals that Lu$_{0.75}$Fe$_6$Sn$_6$ has a HfFe$_6$Ge$_6$-type structure as the major framework intergrown with a CoSn-type structure, leading to a vacancy of 25% on the Lu-site and disorder on the Sn-site. It exhibits a significant magnetic anisotropy with weak ferromagnetism in the ab-plane below 40 K and antiferromagnetic behavior along the c-axis. The weak ferromagnetism is due to the canted antiferromagnetism with magnetic moment deviating from the $c$-axis to the ab-plane. Besides, an anisotropic non-saturated linear magnetoresistance is also observed in Lu$_{0.75}$Fe$_6$Sn$_6$, probably resulting from the structural disorder in the sample. △ Less

Submitted 14 July, 2025; originally announced July 2025.

Comments: 20 pages, 6 figures

Journal ref: Journal of Magnetism and Magnetic Materials 615, 172793 (2025)

Continuous-time parametrization of neural quantum states for quantum dynamics

Authors: Dingzu Wang, Wenxuan Zhang, Xiansong Xu, Dario Poletti

Abstract: Neural quantum states are a promising framework for simulating many-body quantum dynamics, as they can represent states with volume-law entanglement. As time evolves, the neural network parameters are typically optimized at discrete time steps to approximate the wave function at each point in time. Given the differentiability of the wave function stemming from the Schrödinger equation, here we imp… ▽ More Neural quantum states are a promising framework for simulating many-body quantum dynamics, as they can represent states with volume-law entanglement. As time evolves, the neural network parameters are typically optimized at discrete time steps to approximate the wave function at each point in time. Given the differentiability of the wave function stemming from the Schrödinger equation, here we impose a time-continuous and differentiable parameterization of the neural network by expressing its parameters as linear combinations of temporal basis functions with trainable, time-independent coefficients. We test this ansatz, referred to as the smooth neural quantum state (\textit{s}-NQS) with a loss function defined over an extended time interval, under a sudden quench of a non-integrable many-body quantum spin chain. We demonstrate accurate time evolution using a restricted Boltzmann machine as the instantaneous neural network architecture. We show that the parameterization enables accurate simulations with fewer variational parameters, independent of time-step resolution. Furthermore, the smooth neural quantum state also allows us to initialize and evaluate the wave function at times not included in the training set, both within and beyond the training interval. △ Less

Submitted 7 December, 2025; v1 submitted 11 July, 2025; originally announced July 2025.

Comments: 13 pages, 5 figures

Purcell enhancement of photogalvanic currents in a van der Waals plasmonic self-cavity

Authors: Xinyu Li, Jesse Hagelstein, Gunda Kipp, Felix Sturm, Kateryna Kusyak, Yunfei Huang, Benedikt F. Schulte, Alexander M. Potts, Jonathan Stensberg, Victoria Quirós-Cordero, Chiara Trovatello, Zhi Hao Peng, Chaowei Hu, Jonathan M. DeStefano, Michael Fechner, Takashi Taniguchi, Kenji Watanabe, P. James Schuck, Xiaodong Xu, Jiun-Haw Chu, Xiaoyang Zhu, Angel Rubio, Marios H. Michael, Matthew W. Day, Hope M. Bretscher , et al. (1 additional authors not shown)

Abstract: Cavities provide a means to manipulate the optical and electronic responses of quantum materials by selectively enhancing light-matter interaction at specific frequencies and momenta. While cavities typically involve external structures, exfoliated flakes of van der Waals (vdW) materials can form intrinsic self-cavities due to their small finite dimensions, confining electromagnetic fields into pl… ▽ More Cavities provide a means to manipulate the optical and electronic responses of quantum materials by selectively enhancing light-matter interaction at specific frequencies and momenta. While cavities typically involve external structures, exfoliated flakes of van der Waals (vdW) materials can form intrinsic self-cavities due to their small finite dimensions, confining electromagnetic fields into plasmonic cavity modes, characterized by standing-wave current distributions. While cavity-enhanced phenomena are well-studied at optical frequencies, the impact of self-cavities on nonlinear electronic responses--such as photogalvanic currents--remains largely unexplored, particularly in the terahertz regime, critical for emerging ultrafast optoelectronic technologies. Here, we report a self-cavity-induced Purcell enhancement of photogalvanic currents in the vdW semimetal WTe$_2$. Using ultrafast optoelectronic circuitry, we measured coherent near-field THz emission resulting from nonlinear photocurrents excited at the sample edges. We observed enhanced emission at finite frequencies, tunable via excitation fluence and sample geometry, which we attribute to plasmonic interference effects controlled by the cavity boundaries. We developed an analytical theory that captures the cavity resonance conditions and spectral response across multiple devices. Our findings establish WTe$_2$ as a bias-free, geometry-tunable THz emitter and demonstrate the potential of self-cavity engineering for controlling nonlinear, nonequilibrium dynamics in quantum materials. △ Less

Submitted 10 July, 2025; originally announced July 2025.