arXiv subjects

Yongping Du

Publications and source records attributed to Yongping Du.

At least 19 recordsLinked to original sources

arXiv2025

Symmetry-Breaking Magneto-Optical Effects in Altermagnets

The recently discovered altermagnets (AMs), hosting momentum-dependent spin splitting and vanishing net magnetization, have attracted intensive attention for their promising application in novel spintronics. However, limited by facility and material constraints, experimentally distinguishing them from conventional antiferromagnets (AFMs) remains a challenge, which hinders the high-throughput screening for AM candidates. Here, we predict strain-mediated magneto-optical responses in AMs, which can serve as a universal and experimentally accessible strategy for efficient identification of AMs. Symmetry analysis reveals that uniaxial strain can selectively breaks rotation or mirror symmetries in AMs while preserving $PT$ symmetry in AFMs, thereby activating distinct linear magneto-optical responses (e.g., optical absorption and Kerr rotation) unique to AMs. First-principles calculations across prototypical systems -- including semiconducting V$_2$Se$_2$O monolayer and metallic CrSb bulk -- show that the strain-induced optical signatures are significant enough for conventional optical measurements. Our work establishes a rapid, non-invasive characterization methodology for altermagnetism across material platforms, accelerating its exploration for spin-based technologies.

cond-mat.mtrl-sci↗

arXiv2025

Optical Controllable Spin-Polarization in Two Dimensional Altermagnets via Robust Spin-Momentum Locking Excitons

Spin-momentum locking (SML) excitons in two-dimensional semiconductors are appealing to programmable optical control of spin-polarized carriers in ultrafast spintronics. To address the current thirsty for long-lived excitons with zero-external-field stability and room-temperature spin-polarization, we hereby predict the existence of intrinsically SML excitons in altermagnetic V$_2 X_2$O ($X=$ S, Se) driven by giant non-relativistic spin-splittings ($>$ 1.2 eV). First-principles calculations reveal SML excitons with binding energies exceeding 1400 meV in monolayers and 430 meV in their van der Waals heterobilayers, along with stacking-dependent optical selection rules for tunable interlayer excitons. These remarkable physical properties, combined with their long radiative lifetimes, strongly suggest the feasibility of SML excitons with robust spin-polarization at room temperature. Our work provides a new paradigm for SML exciton physics via the novel altermagnetism, opening up new possibilities for all-optical manipulation in advanced opto-spintronics.

cond-mat.mes-hall↗

arXiv2022

Contextual embedding and model weighting by fusing domain knowledge on Biomedical Question Answering

Biomedical Question Answering aims to obtain an answer to the given question from the biomedical domain. Due to its high requirement of biomedical domain knowledge, it is difficult for the model to learn domain knowledge from limited training data. We propose a contextual embedding method that combines open-domain QA model \aoa and \biobert model pre-trained on biomedical domain data. We adopt unsupervised pre-training on large biomedical corpus and supervised fine-tuning on biomedical question answering dataset. Additionally, we adopt an MLP-based model weighting layer to automatically exploit the advantages of two models to provide the correct answer. The public dataset \biomrc constructed from PubMed corpus is used to evaluate our method. Experimental results show that our model outperforms state-of-the-art system by a large margin.

cs.CL↗

arXiv2021

Controllable Data Synthesis Method for Grammatical Error Correction

Due to the lack of parallel data in current Grammatical Error Correction (GEC) task, models based on Sequence to Sequence framework cannot be adequately trained to obtain higher performance. We propose two data synthesis methods which can control the error rate and the ratio of error types on synthetic data. The first approach is to corrupt each word in the monolingual corpus with a fixed probability, including replacement, insertion and deletion. Another approach is to train error generation models and further filtering the decoding results of the models. The experiments on different synthetic data show that the error rate is 40% and the ratio of error types is the same can improve the model performance better. Finally, we synthesize about 100 million data and achieve comparable performance as the state of the art, which uses twice as much data as we use.

cs.CL↗

arXiv2021

Room-Temperature Anisotropic Plasma Mirror and Polarization-Controlled Optical Switch Based on Type-II Weyl Semimetal WP2

Anisotropy in electronic structures may ignite intriguing anisotropic optical responses, as has been well demonstrated in various systems including superconductors, semiconductors, and even topological Weyl semimetals. Meanwhile, it is well established in metal optics that the metal reflectance declines from one to zero when the photon frequency is above the plasma frequency ωp , behaving as a plasma mirror. However, the exploration of anisotropic plasma mirrors and corresponding applications remains elusive, especially at room temperature. Here, we discover a pronounced anisotropic plasma reflectance edge in the type-II Weyl semimetal WP2, with an anisotropy ratio of ωp up to 1.5. Such anisotropic plasma mirror behavior and its robustness against temperature promise optical device applications over a wide temperature range. For example, the high sensitivity of polarization-resolved plasma reflectance edge renders WP2 an inherent polarization detector. We further achieve a room-temperature WP2-based optical switch, effectively controlled by simply tuning the light polarization. These findings extend the frontiers of metal optics as a discipline and promise the design of multifunctional devices combining both topological and optical features.

cond-mat.mtrl-sci↗

arXiv2020

Moiré effects in graphene--hBN heterostructures

Encapsulating graphene in hexagonal Boron Nitride has several advantages: the highest mobilities reported to date are achieved in this way, and precise nanostructuring of graphene becomes feasible through the protective hBN layers. Nevertheless, subtle effects may arise due to the differing lattice constants of graphene and hBN, and due to the twist angle between the graphene and hBN lattices. Here, we use a recently developed model which allows us to perform band structure and magnetotransport calculations of such structures, and show that with a proper account of the moiré physics an excellent agreement with experiments can be achieved, even for complicated structures such as disordered graphene, or antidot lattices on a monolayer hBN with a relative twist angle. Calculations of this kind are essential to a quantitative modeling of twistronic devices.

cond-mat.mes-hall↗

arXiv2020

Butterfly-like anisotropic magnetoresistance and angle-dependent Berry phase in Type-II Weyl semimetal WP2

Weyl semimetal emerges as a new topologically nontrivial phase of matter, hosting low-energy excitations of massless Weyl fermions. Here, we present a comprehensive study of the type-II Weyl semimetal WP2. Transport studies show a butterfly-like magnetoresistance at low temperature, reflecting the anisotropy of the electron Fermi surfaces. The four-lobed feature gradually evolves into a two-lobed one upon increasing temperature, mainly due to the reduced relative contribution of electron Fermi surfaces compared to hole Fermi surfaces for the magnetoresistance. Moreover, angle-dependent Berry phase is further discovered from the quantum oscillations, which is ascribed to the effective manipulation of the extremal Fermi orbits by the magnetic field to feel the nearby topological singularities in the momentum space. The revealed topological characters and anisotropic Fermi surfaces of WP2 substantially enrich the physical properties of Weyl semimetals and hold great promises in topological electronic and Fermitronic device applications.

cond-mat.mtrl-sci↗

arXiv2020

Improving Interpretability of Word Embeddings by Generating Definition and Usage

Word embeddings are substantially successful in capturing semantic relations among words. However, these lexical semantics are difficult to be interpreted. Definition modeling provides a more intuitive way to evaluate embeddings by utilizing them to generate natural language definitions of corresponding words. This task is of great significance for practical application and in-depth understanding of word representations. We propose a novel framework for definition modeling, which can generate reasonable and understandable context-dependent definitions. Moreover, we introduce usage modeling and study whether it is possible to utilize embeddings to generate example sentences of words. These ways are a more direct and explicit expression of embedding's semantics for better interpretability. We extend the single task model to multi-task setting and investigate several joint multi-task models to combine usage modeling and definition modeling together. Experimental results on existing Oxford dataset and a new collected Oxford-2019 dataset show that our single-task model achieves the state-of-the-art result in definition modeling and the multi-task learning methods are helpful for two tasks to improve the performance.

cs.CL↗

arXiv2018

Turning Copper Metal into Weyl Semimetal

A search for new topological quantum systems is challenging due to the requirement of non{trivial band connectivity that leads to protected surface states of electrons. A progress in this field was primarily due to a realization of band inversion mechanism between even and odd parity states that was proven to be very useful in both predicting many of such systems and our understanding their topological properties. Despite many proposed materials assume the band inversion between s and p (or p/d, d/f) electrons, here, we explore a different mechanism where the occupied d states subjected to a tetrahedral crystal field produce an active t2g manifold behaving as a state with an effective orbital momentum equal to negative one, and pushing jeff = 1=2 doublet at a higher energy. Via hybridization with nearest neighbor orbitals realizable, e.g., in a zincblende structural environment, this allows a formation of odd parity state whose subsequent band inversion with an unoccupied s band becomes possible, prompting us to look for the compounds with Cu+1 ionic state. Chemical valence arguments coupled to a search in materials database lead us to systematically investigate electronic structures and topological properties of CuY (Y=F, Cl, Br, I) and CuXO (X=Li, Na, K, Rb) families of compounds. Our theoretical results show that CuF displays a behavior characteristic of an ideal Weyl semimetal with 24 Weyl nodes at the bulk Brillouin Zone. We also find that another compounds CuNaO and CuLiO are the s-d inversion type topological insulators. Results for their electronic structures and corresponding surfaces states are presented and discussed in the context of their topological properties.

cond-mat.mtrl-sci↗

arXiv2018

Discovery of Intrinsic Ferromagnetic Ferroelectricity in Transition Metal Halides Monolayer

The realization of multiferroics in nanostructures, combined with a large electric dipole and ferromagnetic ordering, could lead to new applications, such as high-density multi-state data storage. Although multiferroics have been broadly studied for decades, ferromagnetic ferroelectricity is rarely explored, especially in two-dimensional (2D) systems. Here we report the discovery of 2D ferromagnetic ferroelectricity in layered transition metal halide systems. On the basis of first-principles calculations, we reveal that charged CrBr3 monolayer exhibits in-plane multiferroicity, which is ensured by the combination of orbital and charge ordering as realized by the asymmetric Jahn-Teller distortions of octahedral Cr-Br6 units. As an example, we further show that (CrBr3)2Li is a ferromagnetic ferroelectric multiferroic. The explored phenomena and mechanism of multiferroics in this 2D system are not only useful for fundamental research in multiferroics but also enable a wide range of applications in nano-devices.

cond-mat.mtrl-sci↗

arXiv2017

Realization of Massive Relativistic Spin-3/2 Rarita-Schwinger Quasiparticle in Condensed Matter Systems

The spin-3/2 elementary particle, known as Rarita-Schwinger (RS) fermion, is described by a vector-spinor field ψ_{μα}, whose number of components is larger than its independent degrees of freedom (DOF). Thus the RS equations contain nontrivial constraints to eliminate the redundant DOF. Consequently the standard procedure adopted in realizing relativistic spin-1/2 quasi-particle is not capable of creating the RS fermion in condensed matter systems. In this work, we propose a generic method to construct a Hamiltonian which implicitly contains the RS constraints, thus includes the eigenstates and energy dispersions being exactly the same as those of RS equations. By implementing our 16X16 or 6X6 Hamiltonian, one can realize the 3 dimensional or 2 dimensional (2D) massive RS quasiparticles, respectively. In the non-relativistic limit, the 2D 6X6 Hamiltonian can be reduced to two 3X3 Hamiltonians which describe the positive and negative energy parts respectively. Due to the nontrivial constraints, this simplified 2D massive RS quasiparticle has an exotic property: it has vanishing orbital magnetic moment while its orbital magnetization is finite. Finally, we discuss the material realization of RS quasiparticle. Our study provides an opportunity to realize higher spin elementary fermions with constraints in condensed matter systems.

cond-mat.mes-hall↗

arXiv2017

Emergence of Topological Nodal Lines and Type II Weyl Nodes in Strong Spin--Orbit Coupling System InNbX2(X=S,Se)

Using first--principles density functional calculations, we systematically investigate electronic structures and topological properties of InNbX2 (X=S, Se). In the absence of spin--orbit coupling (SOC), both compounds show nodal lines protected by mirror symmetry. Including SOC, the Dirac rings in InNbS2 split into two Weyl rings. This unique property is distinguished from other dicovered nodal line materials which normally requires the absence of SOC. On the other hand, SOC breaks the nodal lines in InNbSe2 and the compound becomes a type II Weyl semimetal with 12 Weyl points in the Brillouin Zone. Using a supercell slab calculation we study the dispersion of Fermi arcs surface states in InNbSe2, we also utilize a coherent potential approximation to probe their tolernace to the surface disorder effects. The quasi two--dimensionality and the absence of toxic elements makes these two compounds an ideal experimental platform for investigating novel properties of topological semimetals.

cond-mat.mtrl-sci↗

arXiv2017

First Principles Study of the Giant Magnetic Anisotropy Energy in Bulk Na4IrO4

In 5d transition metal oxides, novel properties arise from the interplay of electron correlations and spin--orbit interactions. Na4IrO4, where 5d transition-metal Ir atom occupies the center of the square-planar coordination environment, is synthesized. Based on density functional theory, we calculate its electronic and magnetic properties. Our numerical results show that the Ir-5d bands are quite narrow, and the bands around the Fermi level are mainly contributed by d_{xy},d_{yz} and d_{zx} orbitals. The magnetic easy-axis is perpendicular to the IrO4 plane, and the magnetic anisotropy energy (MAE) of Na4IrO4 is found to be very giant. We estimate the magnetic parameters by mapping the calculated total energy for different spin configurations onto a spin model. The next nearest neighbor exchange interaction J2 is much larger than other intersite exchange interactions and results in the magnetic ground state configuration. Our study clearly demonstrates that the huge MAE comes from the single-ion anisotropy rather than the anisotropic interatomic spin exchange. This compound has a large spin gap but very narrow spin-wave dispersion, due to the large single-ion anisotropy and relatively small exchange couplings. Noticing this remarkable magnetic feature originated from its highly isolated IrO4 moiety, we also explore the possiblity to further enhance the MAE.

cond-mat.mtrl-sci↗

arXiv2016

CaTe: a new topological node-line and Dirac semimetal

Topological semimetals recently stimulate intense research activities. Combining first-principles calculations and effective model analysis, we predict that CaTe is topological node-line semimetal when spin-orbit coupling (SOC) is ignored. We also obtain the nearly flat surface state which has the drumhead characteristic. When SOC is included, three node lines evolve into a pair of Dirac points along the $M-R$ line. These Dirac points are robust and protected by $C_{4}$ rotation symmetry. Once this crystal symmetry is broken, the Dirac points will be eliminated, and the system becomes a strong topological insulator.

cond-mat.mtrl-sci↗

arXiv2015

A new topological semimetal with iso-energetic Weyl fermions in TaAs under high pressure

TaAs as one of the experimentally discovered topological Weyl semimetal has attracted intense interests recently. The ambient TaAs has two types of Weyl nodes which are not on the same energy level. As an effective way to tune lattice parameters and electronic interactions, high pressure is becoming a significant tool to explore new materials as well as their exotic states. Therefore, it is highly interesting to investigate the behaviors of topological Weyl fermions and possible structural phase transitions in TaAs under pressure. Here, with a combination of ab initio calculations and crystal structure prediction techniques, a new hexagonal P-6m2 phase is predicted in TaAs at pressure around 14 GPa. Surprisingly, this new phase is a topological semimetal with only single set of Weyl nodes exactly on the same energy level. The phase transition pressure from the experimental measurements, including electrical transport measurements and Raman spectroscopy, agrees with our theoretical prediction reasonably. Moreover, the P-6m2 phase seems to be quenched recoverable to ambient pressure, which increases the possibilities of further study on the exotic behaviors of single set of Weyl fermions, such as the interplay between surface states and other properties.

cond-mat.mtrl-sci↗

arXiv2015

Pressure-induced semimetal to superconductor transition in a three-dimensional topological material ZrTe5

As a new type of topological materials, ZrTe5 shows many exotic properties under extreme conditions. Utilizing resistance and ac magnetic susceptibility measurements under high pressure, while the resistance anomaly near 128 K is completely suppressed at 6.2 GPa, a fully superconducting transition emerges surprisingly. The superconducting transition temperature Tc increases with applied pressure, and reaches a maximum of 4.0 K at 14.6 GPa, followed by a slight drop but remaining almost constant value up to 68.5 GPa. At pressures above 21.2 GPa, a second superconducting phase with the maximum Tc of about 6.0 K appears and coexists with the original one to the maximum pressure studied in this work. In situ high-pressure synchrotron X-ray diffraction and Raman spectroscopy combined with theoretical calculations indicate the observed two-stage superconducting behavior is correlated to the structural phase transition from ambient Cmcm phase to high-pressure C2/m phase around 6 GPa, and to a mixture of two high-pressure phases of C2/m and P-1 above 20 GPa. The combination of structure, transport measurement and theoretical calculations enable a complete understanding of the emerging exotic properties in three-dimensional topological materials happened under extreme environments.

cond-mat.supr-con↗

arXiv2015

Enhanced quantum coherence in graphene caused by Pd cluster deposition

We report on the unexpected increase in the dephasing lengths of a graphene sheet caused by the deposition of Pd nanoclusters, as demonstrated by weak localization measurements. The dephasing lengths reached saturated values at low temperatures. Theoretical calculations indicate the p-type charge transfer from the Pd clusters, which contributes more carriers. The saturated values of dephasing lengths often depend on both the carrier concentration and mean free path. Although some impurities are increased as revealed by decreased mobilities, the intense charge transfer leads to the improved saturated values and subsequent improved dephasing lengths.

cond-mat.mes-hall↗

arXiv2014

Dirac and Weyl Semimetal in XYBi (X=Ba, Eu; Y=Cu, Ag and Au)

Weyl and Dirac semimetals recently stimulate intense research activities due to their novel properties. Combining first-principles calculations and effective model analysis, we predict that nonmagnetic compounds BaYBi (Y=Au, Ag and Cu) are Dirac semimetals. As for the magnetic compound EuYBi, although the time reversal symmetry is broken, their long-range magnetic ordering cannot split the Dirac point into pairs of Weyl points. However, we propose that partially substitute Eu ions by Ba ions will realize the Weyl semimetal.

cond-mat.mtrl-sci↗