arXiv subjects

Fangping Ouyang

Publications and source records attributed to Fangping Ouyang.

14 recordsLinked to original sources

arXiv2026

Orthogonal Attosecond Control of Solid-State Harmonics by Optical Waveforms and Quantum Geometry Engineering

High-harmonic generation (HHG) in two-dimensional materials offers a compelling route toward compact extreme ultraviolet sources and probing electron dynamics on the attosecond scale. However, achieving precise control over the emission and disentangling the complex interplay between intraband and interband quantum pathways remains a central challenge. Here, we demonstrate through first-principles simulations that HHG in monolayer WS2 can be subjected to precise, complementary control by combining all-optical two-color laser fields with mechanical strain engineering. This dual-mode strategy provides distinct, orthogonal control over harmonic yield, polarization, and spectral features. We reveal that sculpting the two-color field's relative phase provides a sub-femtosecond switch for the quantum coherence of electron-hole pairs, thereby optimizing harmonic emission. Crucially, we uncover that tensile strain modulates the total harmonic yield and specifically amplifies the perpendicular harmonic component by nearly a factor of two. This enhancement arises through a dual mechanism - while strain-modified band dispersion enhances the intraband current, a significant reshaping of the Berry curvature (BC) substantially increases the anomalous velocity contribution to the interband response. This quantum geometric effect manifests as a robust, monotonic dependence of the harmonic yield on strain and a significant amplification of the perpendicularly polarized harmonics, providing a clear experimental signature for probing quantum geometric effects. Our findings establish a versatile framework for optimizing solid-state HHG and introduce a powerful all-optical method to map strain and quantum geometric properties of materials, positioning monolayer WS2 as a model system for exploring attosecond physics at the nexus of bulk and atomic scales.

physics.comp-ph↗

arXiv2026

Two-Dimensional Spin-Antiferroelectric Altermagnets with Giant Spin Splitting: From Model to Material Realization

The realization of multiferroic altermagnets featuring giant intrinsic spin splitting, hold great promise for next-generation spintronics. In this work, based on the recently proposed concept of spin-antiferroelectric (spin-AFE), we construct a class of two-dimensional (2D) multiferroic altermagnets, termed 2D spin-antiferroelectric altermagnets (2D spin-AFEAMs), enabling electrical control of spin polarization via a gate field. Furthermore, we propose a general design strategy for constructing 2D spin-AFEAMs with large intrinsic spin splitting. Guided by this strategy, we predict monolayer $(\mathrm{CoCl})_2\mathrm{Te}$ and its family materials as potential candidates of 2D spin-AFEAM. We uncover a highly tunable transport regime in monolayer $(\mathrm{CoCl})_2\mathrm{Te}$, where the spin current can be switched via the in-plane electric field angle when hole-doped, and via the gate field polarity when electron-doped. Our work enriches the family of 2D multiferroics and provides a blueprint for realizing high-performance, electrically switchable altermagnetic spintronic devices.

cond-mat.mtrl-sci↗

arXiv2026

Data-knowledge dual-driven intelligent framework for full-chain, experiment-efficient synthesis of 2D dendrites

Exemplified by the chemical vapor deposition growth of two-dimensional dendrites, which has potential applications in catalysis and presents a parameter-intensive, data-scarce and reaction process-complex model problem, we devise a machine intelligence-empowered framework for the full chain support of material synthesis, encompassing rapid process optimization, accurate customized synthesis, and comprehensive mechanism deciphering.First, active learning is integrated into the experimental workflow, identifying an optimal recipe for the growth of highly-branched, electrocatalytically-active ReSe2 dendrites through 60 experiments (4 iterations), which account for less than 1.3% of the numerous possible parameter combinations.Then, a prediction accuracy-guided data augmentation strategy is developed combined with a tree-based machine learning (ML) algorithm, unveiling a non-linear correlation between 5 process variables and fractal dimension (DF) of ReSe2 dendrites with only 9 experiment additions, which guides the synthesis of various user-defined DF. Finally, we construct a data-knowledge dual-driven mechanism model by integration of cross-scale characterizations, interpretable ML models, and domain knowledge in thermodynamics and kinetics, unraveling synergistic contributions of multiple process parameters to the product morphology. This work demonstrates the ML potential to transform the research paradigm and is adaptable to broader material synthesis.

cond-mat.mtrl-sci↗

arXiv2026

Nonvolatile Control of Nonlinear Hall and Circular Photogalvanic Effects via Berry Curvature Dipole in Multiferroic Monolayer CrNBr2

The Berry curvature dipole induced by symmetry breaking play a pivotal role in electronic transport properties and nonlinear responses, such as the nonlinear Hall effect and circular photogalvanic effect. The study of the Berry curvature dipole, often explored in time-reversal symmetric systems, but it should not be limited to such materials. Here, we predicted that the ferroelectricity in monolayer CrNBr2 produces Berry curvature dipole, leading to the nonlinear Hall effect and circular photogalvanic current. The linear anomalous Hall effect and circularly polarized optical absorption, governed by spin-orbit coupling, are independent of ferroelectric polarization and exhibit extremely small conductance. In contrast, multiferroic monolayer CrNBr2 achieves a large nonlinear Hall conductivity and circular photogalvanic current, despite its suppression at high temperatures from phonon scattering. The coupling between the ferroelectric polarization and the Berry curvature dipoles allows for nonvolatile switching of these effects, presenting substantial promise for nanoelectronic and optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2026

A Dual-Gate Altermagnetic Tunnel Junction Based on Bilayer Cr$_{2}$SeO

Altermagnets demonstrate significant potential in spintronics due to their unique non-relativistic spin-splitting properties, yet altermagnetic devices still face challenges in efficiently switching logic states. Here, we report electrostatically controllable spin-momentum locking in bilayer Cr$_{2}$SeO and design a dual-gate altermagnetic tunnel junction (AMTJ), which can switch between high and low resistance states without switching the Néel vector. First-principles calculations demonstrate that vertical electric field can induce significant spin splitting in bilayer Cr$_{2}$SeO. Reversing the electric field direction can alter the spin-momentum locking in bilayer Cr$_{2}$SeO. Leveraging this electric-field-tunable spin splitting, the dual-gate AMTJ exhibits an ultrahigh tunneling magnetoresistance (TMR) ratio of $10^{7}$. This work provides theoretical support for the design of fully electrically controlled AMTJs and demonstrates their great potential for applications in spintronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Distortion-Driven Carrier Decoupling in Doped LiMgPO4

The interplay between lattice distortions and charge carriers governs the properties of many functional oxides. In alkali-doped LiMgPO4, a significant enhancement in dosimetric response is observed, but its microscopic origin is not understood. Using non-adiabatic molecular dynamics, we reveal a fundamental mechanism of carrier decoupling driven by a hierarchy of lattice distortions. We show that electrons localize into stable small polarons on an ultrafast timescale, trapped by the strong local potential induced by the dopant, while holes form more delocalized polarons that migrate efficiently through a lattice smoothed by global strain. The stark contrast between the dynamics of trapped electrons and mobile holes explains the suppressed recombination and enhanced energy storage. These results present a clear physical picture of how multiscale lattice distortions can independently control electron and hole transport, offering new insights into the physics of polarons in complex materials.

cond-mat.mtrl-sci↗

arXiv2025

Spin-chirality-dependent modulation of topological gap, Chern number, and valley-polarization in monolayer kagome lattice Cr3Se4

Kagome materials exhibit unique electronic properties, such as the quantum anomalous Hall effect. The control of Chern numbers is critical for quantum device manipulation, but existing research has mainly focused on collinear magnetization while neglecting chiral spin textures. Through first-principles calculations and tight-binding modeling of monolayer Cr3Se4, this study reveals spin-chirality-dependent control of topological gaps, Chern numbers, and valley polarization in kagome materials. The results demonstrate that the azimuthal angle has no observable effect. For collinear magnetization (\k{appa} = 0) or spin-chirality \k{appa} = -1, the topological bandgap decreases as the spin orientation approaches the in-plane direction. Conversely, increasing the polar angle enhances the bandgap for \k{appa} = 1. In the breathing kagome lattice, the degeneracy between K and K' valleys is lifted. As the gap undergoes sequential closure and reopening in the two valleys, the structural asymmetry and spin-chirality allow for controlled tuning of the topological gap, Chern number, and valley polarization. Moreover, the emergence of a topological Hall effect is is also demonstrated. These findings provide strategies for controlling topological states and advancing applications in quantum devices and valleytronic systems.

cond-mat.mtrl-sci↗

arXiv2025

Multiple Topological Phases Controlled via Strain in Two-Dimensional Altermagnets

Altermagnets (AMs) are an emergent class of magnetic materials that combine properties of ferromagnets and antiferromagnets, exhibiting spin-polarized Fermi surfaces and zero net magnetic moment due to combined time-reversal and crystal symmetry. Here, we construct a Kondo-lattice model on a two-dimensional square Lieb lattice to investigate the topological properties of AMs. We identify a type-II quantum spin Hall state characterized by spin-polarized counterpropagating edge states. Breaking the $C_{4z}\mathcal{T}$ symmetry, which connects magnetic sublattices, induces a transition to a quantum anomalous Hall state. We further establish a strain-induced mechanism to control these topological phase transitions and present the corresponding phase diagram. Finally, we demonstrate the predicted transitions in monolayer CrO, a realistic altermagnetic candidate, using first-principles calculations. Our findings highlight the potential of 2D AMs as a versatile platform for topological spintronics, enabling strain-tunable helical and chiral edge states within a single system.

cond-mat.mes-hall↗

arXiv2024

Exploring structure diversity in atomic resolution microscopy with graph neural networks

The emergence of deep learning (DL) has provided great opportunities for the high-throughput analysis of atomic-resolution micrographs. However, the DL models trained by image patches in fixed size generally lack efficiency and flexibility when processing micrographs containing diversified atomic configurations. Herein, inspired by the similarity between the atomic structures and graphs, we describe a few-shot learning framework based on an equivariant graph neural network (EGNN) to analyze a library of atomic structures (e.g., vacancies, phases, grain boundaries, doping, etc.), showing significantly promoted robustness and three orders of magnitude reduced computing parameters compared to the image-driven DL models, which is especially evident for those aggregated vacancy lines with flexible lattice distortion. Besides, the intuitiveness of graphs enables quantitative and straightforward extraction of the atomic-scale structural features in batches, thus statistically unveiling the self-assembly dynamics of vacancy lines under electron beam irradiation. A versatile model toolkit is established by integrating EGNN sub-models for single structure recognition to process images involving varied configurations in the form of a task chain, leading to the discovery of novel doping configurations with superior electrocatalytic properties for hydrogen evolution reactions. This work provides a powerful tool to explore structure diversity in a fast, accurate, and intelligent manner.

cond-mat.mtrl-sci↗

arXiv2024

Auto-resolving the atomic structure at van der Waals interfaces using a generative model

The high-resolution visualization of atomic structures is significant for understanding the relationship between the microscopic configurations and macroscopic properties of materials. However, a rapid, accurate, and robust approach to automatically resolve complex patterns in atomic-resolution microscopy remains difficult to implement. Here, we present a Trident strategy-enhanced disentangled representation learning method (a generative model), which utilizes a few unlabelled experimental images with abundant low-cost simulated images to generate a large corpus of annotated simulation data that closely resembles experimental results, producing a high-quality large-volume training dataset. A structural inference model is then trained via a residual neural network which can directly deduce the interlayer slip and rotation of diversified and complicated stacking patterns at van der Waals interfaces with picometer-scale accuracy across various materials (e.g. MoS2, WS2, ReS2, ReSe2, and 1T'-MoTe2) with different layer numbers , demonstrating robustness to defects, imaging quality, and surface contaminations. The framework can also identify pattern transition interfaces, quantify subtle motif variations, and discriminate moire patterns that are difficult to distinguish in frequency domains. Finally, the high-throughput processing ability of our method provides insights into a van der Waals epitaxy mode where various thermodynamically favorable slip stackings can coexist.

cond-mat.mtrl-sci↗

arXiv2023

Janus monolayer TaNF: a new ferrovalley material with large valley splitting and tunable magnetic properties

Materials with large intrinsic valley splitting and high Curie temperature are a huge advantage for studying valleytronics and practical applications. In this work, using first-principles calculations, a new Janus TaNF monolayer is predicted to exhibit excellent piezoelectric properties and intrinsic valley splitting, resulting from the spontaneous spin polarization, the spatial inversion symmetry breaking and strong spin-orbit coupling (SOC). TaNF is also a potential two-dimensional (2D) magnetic material due to its high Curie temperature and huge magnetic anisotropy energy. The effective control of the band gap of TaNF can be achieved by biaxial strain, which can transform TaNF monolayer from semiconductor to semi-metal. The magnitude of valley splitting at the CBM can be effectively tuned by biaxial strain due to the changes of orbital composition at the valleys. The magnetic anisotropy energy (MAE) can be manipulated by changing the energy and occupation (unoccupation) states of d orbital compositions through biaxial strain. In addition, Curie temperature reaches 373 K under only -3% biaxial strain, indicating that Janus TaNF monolayer can be used at high temperatures for spintronic and valleytronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Enhancement of electron transport and bandgap opening in graphene induced by adsorbates

Impurities are unavoidable during the preparation of graphene samples and play an important role in graphene's electronic properties when they are adsorbed on graphene surface. In this work, we study the electronic structures and transport properties of a two-terminal zigzag graphene nanoribbon (ZGNR) device whose scattering region is covered by various adsorbates within the framework of the tight-binding approximation, by taking into account the coupling strength $γ$ between adsorbates and carbon atoms, the adsorbate concentration $n_i$, and the on-site energy disorder of adsorbates. Our results indicate that when the scattering region is fully covered by homogeneous adsorbates, i.e., $n_i=1$, a transmission gap opens around the Dirac point and its width is almost proportional to $γ^2$. In particular, two conductance plateaus of $G=2e^2/h$ appear in the vicinity of the electron energy $E=\pm γ$. When the scattering region is partially covered by homogeneous adsorbates ($0<n_i<1$), the transmission gap still survives around the Dirac point even at low $n_i$, and its width is firstly increased by $n_i$ and then declined by further increasing $n_i$; whereas the conductance decreases with $n_i$ in the regime of low $n_i$ and increases with $n_i$ in the regime of high $n_i$. While in the presence of disordered adsorbates whose on-site energies are random variables characterized by the disorder degree, the transmission gap disappears at low $n_i$ and reappears at relatively high $n_i$. Furthermore, the transmission ability of the ZGNR device can be enhanced by the adsorbate disorder when the disorder degree surpasses a critical value, contrary to the localization picture that the conduction of a nanowire becomes poorer with increasing the disorder degree. The physics underlying these transport characteristics is discussed. Our results are in good agreement with experiments.

cond-mat.mes-hall↗

arXiv2020

Isotropic All-electric Spin analyzer based on a quantum ring with spin-orbit coupling

Here we propose an isotropic all electrical spin analyzer in a quantum ring with spin-orbit coupling by analytically and numerically modeling how the charge transmission rates depend on the polarization of the incident spin. The formalism of spin transmission and polarization rates in an arbitrary direction is also developed by analyzing the Aharonov-Bohm and the Aharonov-Casher effects. The topological spin texture induced by the spin-orbit couplings essentially contributes to the dynamic phase and plays an important role in spin transport. The spin transport features derived analytically has been confirmed numerically. This interesting two-dimensional electron system can be designed as a spin filter, spin polarizer and general analyzer by simply tuning the spin-orbit couplings, which paves the way for realizing the tunable and integrable spintronics device.

cond-mat.mes-hall↗

arXiv2019

Spin-orbit Interaction driven Topological Features in a Quantum Ring

One-dimensional quantum rings with Rashba and Dresselhaus spin-orbit couplings are studied analytically and are in perfect agreement with the numerical results. The topological charge of the spin field defined by the winding number along the ring is also studied analytically and numerically in the presence of the spin-orbit interactions. We also demonstrate the cases where the one-dimensional model is invalid for a relatively large radius. However, the numerical results of the two-dimensional model always remain reliable. Just as many physical properties of the quantum rings are influenced by the Aharonov-Bohm effect, the topological charge is also found to vary periodically due to the step-like change of the angular momentum with an increase of the magnetic field. This is significantly different from the cases of quantum dots. We also study how the current is induced by the magnetic field and spin-orbit couplings, which is strong enough that it could to be detected. The magnetic induction lines induced by the spin field and the current are also analyzed which can be observed and could perhaps help identifying the topological features of the spin fields in a quantum ring.

cond-mat.mes-hall↗