arXiv subjects

Yufei Zhao

Publications and source records attributed to Yufei Zhao.

At least 19 recordsLinked to original sources

arXiv2026

Discovery of New Zintl Films and Nanowires Grown by Topotaxy Conversion of III-V Semiconductors

Zintl phases draw broad interest for their diverse structural, magnetic, thermoelectric, topological and optical properties. Recently, Zintl Eu$_3$In$_2$As$_4$ and Eu$_5$In$_2$As$_6$ nanowires with axion magneto-topology have been synthesized by molecular beam epitaxy via topotactic conversion of InAs wurtzite and zincblende nanowires. Here we extend this methodology, demonstrating that topotaxial mutual-exchange growth applies not only to a broader set of III-V semiconductors beyond InAs but also to three-dimensional substrates whose surfaces are converted into Zintl thin films, as well as to nanowires. We report the growth of two new compounds: Eu$_5$Ga$_2$As$_6$ thin films converted from GaAs substrates, and Eu$_5$Al$_2$As$_6$ thin films from AlAs films. We also convert GaAs nanowires of both wurtzite and zincblende structures into Eu$_5$Ga$_2$As$_6$ nanowires. Though the stoichiometry is the same as in the previously reported Eu$_5$In$_2$As$_6$ case, microscopy and diffraction reveal a single-phase Pnma symmetry group rather than Pbam, highlighting symmetry-guided topotactic pathways to new Zintl frameworks. The compounds host an intricate magnetic phase diagram with three magnetic transitions, including two distinct antiferromagnetic orders and a canted antiferromagnetic phase that evolves into another antiferromagnetic phase under applied field through a spin-flop transition. Ab initio calculations predict that both Zintls are semiconductors with gaps of 0.79 eV in Eu$_5$Ga$_2$As$_6$ and 0.90 eV in Eu$_5$Al$_2$As$_6$. The lower symmetry and increased structural complexity suggest suppressed lattice thermal conductivity, pointing to thermoelectric potential alongside prospects in spintronics and detector technologies. These results give an epitaxy-compatible route for discovering and integrating magnetic Zintl thin films and nanowires directly from relevant III-V semiconductors.

cond-mat.mtrl-sci↗

arXiv2026

Joint Beamforming and Phase Shifts Design for RIS-Enabled RSMA-ISAC Systems

This paper investigates the sensing-centric design of reconfigurable intelligent surface (RIS)-enabled rate-splitting multiple access-integrated sensing and communication (RSMA-ISAC) systems. Specifically, we propose a new beam-gain approximation method to enhance the sensing beam gain while satisfying communication quality-of-service (QoS) constraints.Since the joint optimization of the beamforming vectors and RIS phase shifts is highly coupled and non-convex, existing methods typically rely on generic optimization solvers involving substantial computational complexity. To address this issue, we propose an efficient constraints-separation-based alternating optimization algorithm (CS-AO). Our proposed algorithm effectively decouples the optimization variables and yields closed-form solutions for all subproblems, thereby significantly reducing the computational burden. Simulation results show that the proposed algorithm achieves sensing beam-gain performance comparable to successive convex approximation (SCA) and semidefinite relaxation (SDR) benchmarks, while achieving more than 120-fold and 50-fold runtime reductions. In addition, compared with conventional space-division multiple access (SDMA) schemes, the proposed design exhibits substantial sensing beam gain.

cs.IT↗

arXiv2026

Structured-Sparsity-Aware Joint User Activity Detection and Channel Estimation for OTFS-Based Grant-Free Random Access

Grant-free random access (GFRA) is a promising solution for massive machine-type communications (mMTC) in future wireless networks. However, reliable user activity detection and channel estimation are critical challenges, particularly when orthogonal time-frequency space (OTFS) modulation is integrated with GFRA to address doubly selective channels induced by high mobility. In this paper, we propose an OTFS-based GFRA framework that exploits the inherent structured sparsity of delay-Doppler channels. By adopting a basis expansion model (BEM), we formulate joint user activity detection and channel estimation as a structured compressive sensing problem. A bi-level sparsity structure is identified, consisting of common sparsity across multiple receive antennas and activation sparsity across mMTC users. To effectively leverage this structure, we construct a two-layer factor graph and develop a structured sparsity expectation propagation (SS-EP) algorithm for efficient Bayesian inference. Simulation results demonstrate that the proposed scheme significantly outperforms existing benchmarks.

cs.IT↗

arXiv2026

Interface-Confined Superconductivity with Thickness-Independent Superfluid Stiffness in (Pb,Sn)Te/FeTe Bilayers

Interface-induced superconductivity in FeTe-based heterostructures provides a promising route toward topological superconductivity, yet the roles of the neighboring layers topology, symmetry, and electronic structure remain unresolved. In this work, we employ molecular beam epitaxy to grow Pb1-xSnxTe/FeTe bilayers and use angle-resolved photoemission spectroscopy to track the evolution of the Pb1-xSnxTe layer from a trivial insulator to a topological crystalline insulator hosting multiple Dirac surface states. Electrical transport measurements reveal robust superconductivity throughout the entire composition range, with a nearly constant superconducting transition temperature of ~12 K despite substantial changes in the electronic structure and topology of Pb1-xSnxTe. Double-coil mutual-inductance measurements further reveal comparable superfluid stiffness across the topological phase transition and nearly thickness-independent superfluid stiffness despite large variations in the constituent-layer thicknesses, demonstrating that superconductivity is confined near the interface. These results establish that superconductivity in FeTe-based heterostructures is largely insensitive to the topology, crystal symmetry, and detailed electronic structure of the neighboring layer, supporting a primary origin in modifications to the FeTe layer. The coexistence of interface-confined superconductivity and tunable multiple Dirac surface states in Pb1-xSnxTe/FeTe bilayers provides a versatile platform for exploring topological superconductivity and interactions among multiple Majorana zero modes.

cond-mat.supr-con↗

arXiv2026

Residual orbital magnetization governs the anomalous Hall effect in altermagnets

In altermagnets that exhibit anomalous Hall effect, the small remanent magnetization exists but has been treated as too small to be relevant to the Hall response. In this work, we point out that this dismissal is incomplete because the generalized St\v{r}eda relation ties the intrinsic anomalous Hall conductivity ($\sigma_{xy}$) to the orbital magnetization ($M_z$, the topological component from the modern orbital magnetization) by $\sigma_{xy}=-e\frac{\partial M_z}{\partial \mu}$. We reveal a microscopic mechanism to generate net orbital moment from the interplay of local crystal field and spin-orbit coupling for MnTe-type altermagnets, in which the magnetic anisotropy generates weak net magnetization without invoking exchange between neighboring spins (e.g., Dzyaloshinskii-Moriya interaction). Our work indicates that residual orbital and spin magnetization is an intrinsic thermodynamic property that governs anomalous transport in unconventional antiferromagnets, including altermagnets and noncollinear antiferromagnets.

cond-mat.mes-hall↗

arXiv2026

Evaluating the Structural Basis for Polar Altermagnet Candidate Ca$_{3}$(Ru,Ti)$_{2}$O$_{7}$

The interplay between polar and altermagnetic orders remains largely unexplored in the broader landscape of correlated electron systems. Ca$_{3}$Ru$_{2}$O$_{7}$ has been proposed by density functional theory (DFT) as a polar altermagnet, reliant on the transformation of experimentally reported $Bb2_{1}m$ phase to a lower symmetry $Pn2_{1}a$ structure. Here, we perform a targeted search for the $Pn2_{1}a$ phase using synchrotron X-ray diffraction on single crystals of Ca$_{3}$Ru$_{2}$O$_{7}$ and Ca$_{3}$(Ru$_{0.99}$Ti$_{0.01}$)$_{2}$O$_{7}$. No diffraction signature of the $Pn2_{1}a$ structure is detected down to 20 K within experimental limits of $\sim$60-200 fm atomic displacements, significantly smaller than the DFT prediction of $\sim$1 pm. Combined with recent nonlinear transport measurements, our structural study suggests Ca$_{3}$Ru$_{2}$O$_{7}$ as a unique system where strong electron correlations drive an electronic phase transition without any measurable lattice symmetry change. With Ti substitution exceeding $\sim$3%, a chemically tunable altermagnetic phase with $Bb2_{1}m$ structure emerges. The study highlights the importance of sub-picometer metrology towards de-convolving structural versus electronic origins of altermagnets.

cond-mat.str-el↗

arXiv2026

Designing Strong and Broadband Nonreciprocal Thermal Radiation in Magnetic Topological Materials

Breaking reciprocity in thermal radiation opens opportunities for energy harvesting, sensing, and thermal management. Traditional nonreciprocal radiative semiconductor devices need external magnetic field. In this work, we predict a series of magnetic topological materials for magnetic-field-free nonreciprocal thermal radiation in the infrared regime, by combining first-principles calculations with Maxwell electrodynamics. We find strong and broadband nonreciprocity in magnetic Weyl semimetals (e.g., Co$_3$Sn$_2$S$_2$), outperforming the conventional semiconductor such as InAs. Furthermore, we propose universal material design recipes: strong nonreciprocity requires a large anomalous Hall response relative to the optical loss, whereas the broadband response favors large optical loss and small dielectric dispersion. Our work establishes a predictive materials-discovery framework and quantitative design rules for next-generation magnet-free nonreciprocal thermal devices.

cond-mat.mtrl-sci↗

arXiv2026

Stochastically evolving ellipsoids with symmetries

We prove that there is a universal constant $c > 0$ such that, along an infinite sequence of dimensions $N$, there are lattice sphere packings in $\mathbb{R}^N$ of density at least $c N^2 \log\log N \, 2^{-N}$, improving the previous best bound due to Klartag by a $\log\log N$ factor. The proof follows Klartag's stochastic ellipsoid evolution process, subject to the cyclotomic symmetries introduced by Venkatesh.

math.MG↗

arXiv2026

Imaging Surface Magnetization in Altermagnetic MnTe Films

Altermagnets with pronounced spin-splitting band structure, unconventional magnetic and crystal symmetries, and exotic magneto-transport properties have received immense interest in cutting-edge spintronics, materials science, and condensed matter physics research. Microscopic imaging of spontaneous magnetic domains and phases in altermagnets constitutes an important step for investigating their underlying material properties, mechanisms, and spin behaviors. Taking advantage of scanning-probe quantum microscopy, here we report nanoscale quantum sensing of a prototypical altermagnet candidate $\alpha$-MnTe. We visualize evanescent magnetization and the associated magnetic domains in epitaxial MnTe films, which allows external magnetic fields to control the intrinsic altermagnetic order and configurations. By evaluating a series of MnTe films with different thicknesses down to the atomic scale, we further present evidence for the interfacial origin of the observed weak magnetization and show its correlation with the anomalous Hall effect in MnTe film. Our results advance the current understanding of emergent altermagnetism, providing insights into future material design of altermagnet-integrated spintronic devices.

cond-mat.mtrl-sci↗

arXiv2026

SwiftI2V: Efficient High-Resolution Image-to-Video Generation via Conditional Segment-wise Generation

High-resolution image-to-video (I2V) generation aims to synthesize realistic temporal dynamics while preserving fine-grained appearance details of the input image. At 2K resolution, it becomes extremely challenging, and existing solutions suffer from various weaknesses: 1) end-to-end models are often prohibitively expensive in memory and latency; 2) cascading low-resolution generation with a generic video super-resolution tends to hallucinate details and drift from input-specific local structures, since the super-resolution stage is not explicitly conditioned on the input image. To this end, we propose SwiftI2V, an efficient framework tailored for high-resolution I2V. Following the widely used two-stage design, it addresses the efficiency--fidelity dilemma by first generating a low-resolution motion reference to reduce token costs and ease the modeling burden, then performing a strongly image-conditioned 2K synthesis guided by the motion to recover input-faithful details with controlled overhead. Specifically, to make generation more scalable, SwiftI2V introduces Conditional Segment-wise Generation (CSG) to synthesize videos segment-by-segment with a bounded per-step token budget, and adopts bidirectional contextual interaction within each segment to improve cross-segment coherence and input fidelity. On VBench-I2V at 2K resolution, SwiftI2V achieves performance comparable to end-to-end baselines while reducing total GPU-time by 202x. Particularly, it enables practical 2K I2V generation on a single datacenter GPU (e.g., H800) or consumer GPU (e.g., RTX 4090).

cs.CV↗

arXiv2026

DRL-Based Antenna Position Optimization For MA-Assisted OTFS System Under Imperfect CSI

In this paper, we introduce movable antenna (MA) technology into orthogonal time frequency space (OTFS) systems to enable wavelength-level antenna position optimization under imperfect channel state information (CSI), thereby mitigating deep fading. To accurately acquire CSI, we develop a sparse Bayesian learning method with variational inference (SBLVI) method. Based on estimated CSI, we formulate an MA position optimization problem with the objective of maximizing channel gain. Due to the highly non-convex character of the problem, we further develop a deep reinforcement learning (DRL) strategy to intelligently optimize MA positions. Simulation results show that the proposed SBLVI method significantly improves channel estimation accuracy over benchmark methods, and MA position optimization based on estimated CSI achieves substantially higher channel gains than the fixed-position antenna (FPA), demonstrating the effectiveness of the proposed MA-assisted OTFS system.

cs.IT↗

arXiv2026

Millimeter-Scale, Atomically Controlled 2D Topological Insulators Revealed by Multimodal Spectroscopy

Quantum spin Hall insulators, or synonymously known as 2D topological insulators, are crucial 2D systems hosting topologically protected edge states. The working temperature of this topological quantum phase is dictated by the inverted bandgap. However, the previously identified large-gap 2D topological insulators are either extremely chemically unstable, or cannot be made with atomistic precision over macroscopic scales. Here, we establish two-quintuple-layer Bi2Te3 and MnBi2Te4/Bi2Te3 heterostructures as atomically controlled, millimeter-scale 2D topological insulators, enabled by precision layer-by-layer growth that yields a carpet-like morphology extending coherently over macroscopic distances. This carpet-like growth mode renders the films amenable to mechanical exfoliation and subsequent wet or dry transfer. Multimodal spectroscopies and microscopies reveal the integer-layer tuned electronic structure of (Bi2Te3)n with excellent agreement to theory. Photon-energy-dependent photoemission and time-resolved photoemission identify band inversion and band dynamics, respectively, while scanning tunneling spectroscopy resolves topological edge states, characteristic of the 2D topological insulator phase. Thickness- and photon-energy-dependent photoemission further validates MnBi2Te4/Bi2Te3 as a robust 2D topological insulator. The large inverted gaps of ~100 meV in (Bi2Te3)2 and ~150 meV in MnBi2Te4/Bi2Te3 suggest operation near ambient temperature. These results define a scalable materials platform for next-generation, low-loss quantum and energy-efficient devices.

cond-mat.mtrl-sci↗

arXiv2026

Emergent Anomalous Hall Effect from Surface States in the Altermagnet MnTe Thin Films

Transport measurements on thin films of the prototypical altermagnet MnTe have reported conflicting phenomena of anomalous Hall effects (AHE), including opposite signs and thickness-independent resistivity. Here we resolve these discrepancies by separating bulk and surface contributions to the AHE for different crystal terminations. Using first-principles calculations and symmetry-based effective models, we show that although the bulk hosts a characteristic $g$-wave Fermi surface, surface states within the bulk gap acquire a ferromagnet-like spin polarization and dominate the AHE at experimentally relevant Fermi energies. While the surface magnetization follows the surface spin sublattice, the resulting AHE is uniquely determined by the bulk N\'eel order for a given termination. Both bulk and surface contributions are closely linked to a small but finite out-of-plane orbital magnetization. Incorporating realistic interfacial chemistry further reveals that a Te capping layer can reverse the surface AHE sign relative to that on an InP substrate. Our results establish a microscopic framework for interpreting and engineering AHE responses in altermagnetic thin films through interface design.

cond-mat.mtrl-sci↗

arXiv2026

Surface-State-Driven Anomalous Hall Effect in Altermagnetic MnTe Films

Altermagnets have recently emerged as a new class of magnetic materials that combine compensated magnetic order with spin-split electronic band structures. In this work, we employ molecular beam epitaxy to grow MnTe thin films with controlled thickness on InP(111)A substrates. By performing angle-resolved photoemission spectroscopy measurements, we observe a large spin splitting of ~230 meV for bulk bands well below the Fermi level and identify surface states that cross the Fermi level. Electrical transport measurements reveal that a robust anomalous Hall (AH) effect persists down to 2 K and an AH sign reversal occurs near 175 K. By systematically tuning film thickness, growth conditions, and interfacial structure, we demonstrate that the AH response in MnTe films originates from the Berry curvature of surface states rather than from bulk bands. Our first-principles calculations reveal that this surface-state-driven AH effect is imprinted by the bulk altermagnetic order and remains unchanged for terminations with opposite Mn magnetic orientations. Our results establish a unique surface transport probe of bulk altermagnetism, demonstrate interface engineering as an effective route to generate and control the AH effect in altermagnets, and provide a unified understanding of the AH response in altermagentic MnTe films.

cond-mat.mtrl-sci↗

arXiv2026

Two-Stage Coded-Sliding Beam Training and QoS-Constrained Sum-Rate Maximization for SIM-Assisted Wireless Communications

Stacked intelligent metasurfaces (SIM) provide a cost-effective and scalable solution for large-scale antenna communications.However, efficient channel state information acquisition and phase shift optimization remain critical challenges. In this paper, we develop a unified framework of low-complexity algorithms for SIM-assisted communication systems to address these issues. Specifically, we propose a generalized two-step codebook construction (TSCC) method that leverages two-dimensional angular-domain decoupling to transform planar array beamformer design into two independent one-dimensional linear array beamformer design problems, efficiently solved via the Gerchberg-Saxton algorithm and our proposed majorization-minimization-based proximal distance (PDMM) algorithm. We further develop a two-stage coded-sliding beam training (TSCSBT) method for low-overhead and high-accuracy beam training, where error-correcting codes are embedded in the first-stage training to enhance robustness against noise, and sliding sampling is subsequently performed around the matched angular samples to improve angular resolution. The proposed framework is further extended to multi-path user channels. Finally, a variable decoupling-based block successive upper bound minimization (VD-BSUM) algorithm is proposed to directly solve the QoS-constrained sum-rate maximization problem through closed-form iterative updates with substantially reduced computational complexity. Simulation results demonstrate the effectiveness of the proposed methods in achieving precise beam pattern realization, improved beam training accuracy and angular resolution, and enhanced sum-rate performance.

cs.IT↗

arXiv2026

Cram\'er-Rao Bound Minimization for Flexible Intelligent Metasurface-Enabled ISAC Systems

Integrated sensing and communication (ISAC) have been widely recognized as a key enabler for future wireless networks, where the Cram\'er-Rao bound (CRB) plays a central role in quantifying sensing accuracy.In this paper, we present the first study on CRB minimization in flexible intelligent metasurface (FIM)-enabled ISAC systems.Specifically, we first derive an average CRB expression that explicitly depends on FIM surface shape and demonstrate that array reconfigurability can substantially reduce the CRB, thereby significantly enhancing sensing performance.Moreover, to tackle the challenging CRB minimization problem, we adopt average Fisher information maximization as a surrogate objective and use the Gauss-Hermite quadrature method to obtain an explicit approximation of the objective function.The resulting problem is then decoupled into three subproblem, i.e., beamforming optimization and transmit/receive FIM surface shape optimization.For beamforming optimization, we employ the Schur complement and penalty-based semi-definite relaxation (SDR) technique to solve it.Furthermore, we propose a fixed-point equation method and a projected gradient algorithm to optimize the surface shapes of the receive and transmit FIMs, respectively.Simulation results demonstrate that, compared to rigid arrays, surface shaping of both transmit and receive FIMs can significantly reduce the average sensing CRB while maintaining communication quality, and remains effective even in multi-target scenarios.

cs.IT↗

arXiv2026

Efficient Beamforming for Discrete SIM-Aided Multiuser Systems Under Statistical CSI

Stacked Intelligent Metasurfaces (SIM) have emerged as a revolutionary architecture for next-generation wireless communications, offering wave-domain signal processing capabilities with significantly reduced hardware complexity compared to conventional systems. However, most existing SIM research assumes continuous phase shifts and perfect instantaneous channel state information (CSI), which are impractical due to hardware discrete phase shift constraints and prohibitive pilot overhead. This paper presents a joint power allocation and discrete phase shift optimization framework for SIM-aided multiuser multiple-input single-output(MISO) downlink systems under statistical CSI. We formulate the achievable sum rate maximization problem considering practical discrete phase constraints and derive a closed-form expression for the average achievable rate under statistical CSI. To tackle the resulting non-convex optimization problem, we decouple the problem by using the weighted minimum mean square error (WMMSE) algorithm and alternating optimization (AO). Subsequently, we utilize the Lagrangian multiplier method and alternating direction method of multipliers (ADMM) to obtain closed-form iterative solutions. Our simulations demonstrate that the proposed algorithm reduces computational complexity by a factor of 50 compared to semi-definite relaxation (SDR) methods, , while maintaining over 85% of the continuous phase shift performance with only 1-bit quantization, highlighting its feasibility for low-cost hardware systems.

cs.IT↗

arXiv2025

QMBench: A Research Level Benchmark for Quantum Materials Research

We introduce QMBench, a comprehensive benchmark designed to evaluate the capability of large language model agents in quantum materials research. This specialized benchmark assesses the model's ability to apply condensed matter physics knowledge and computational techniques such as density functional theory to solve research problems in quantum materials science. QMBench encompasses different domains of the quantum material research, including structural properties, electronic properties, thermodynamic and other properties, symmetry principle and computational methodologies. By providing a standardized evaluation framework, QMBench aims to accelerate the development of an AI scientist capable of making creative contributions to quantum materials research. We expect QMBench to be developed and constantly improved by the research community.

cond-mat.mtrl-sci↗