arXiv subjects

Shiyuan Gao

Publications and source records attributed to Shiyuan Gao.

15 recordsLinked to original sources

arXiv2026

Observation of a structurally driven, reversible topological phase transition in a distorted square net material

Topological materials hold immense promise for exhibiting exotic quantum phenomena, yet achieving controllable topological phase transitions remains challenging. Here, we demonstrate a structurally driven, reversible topological phase transition in the distorted square net material GdPS, induced via in situ potassium dosing. Using angle-resolved photoemission spectroscopy and first principles calculations, we demonstrate a cascade of topological phases in the sub-surface P layer: from a large, topologically trivial band gap to a gapless Dirac cone state with a 2 eV dispersion, and finally to a two-dimensional topological insulator as inferred from theory. This evolution is driven by subtle structural distortions in the first P layer caused by potassium adsorption, which in turn contribute to the band gap closure and topological phase transition. Furthermore, the ability to manipulate the topology of a sub-surface layer in GdPS offers a unique route for exploring and controlling topological states in bulk materials.

cond-mat.mtrl-sci↗

arXiv2024

Quasi-two-dimensional Antiferromagnetic Spin Fluctuations in the Spin-triplet Superconductor Candidate CeRh$_2$As$_2$

The tetragonal heavy-fermion superconductor CeRh$_2$As$_2$ ($T_{\rm c}=0.3$ K) exhibits an exceptionally high critical field of 14 T for $\textbf{B} \parallel \textbf{c}$. It undergoes a field-driven first-order phase transition between superconducting (SC) states, potentially transitioning from spin-singlet to spin-triplet superconductivity. To elucidate the underlying pairing mechanism, we probe spin fluctuations in CeRh$_2$As$_2$ using neutron scattering. We find dynamic $(π,π)$ antiferromagnetic spin correlations with an anisotropic quasi-two-dimensional correlation volume. Our data place an upper limit of 0.31 $μ_{\rm B}$ on the staggered magnetization of corresponding Néel orders at $T=0.08$ K. Density functional theory (DFT) calculations, treating Ce $4f$ electrons as core states, show that the AFM wave vector connects significant areas of the Fermi surface. Our findings show the dominant excitations in CeRh$_2$As$_2$ for $\hbarω< 1.2$~meV are magnetic and indicate superconductivity in CeRh$_2$As$_2$ is mediated by AFM spin fluctuations associated with a proximate quantum critical point.

cond-mat.str-el↗

arXiv2024

First-principles electron-phonon interactions and electronic transport in large-angle twisted bilayer graphene

Twisted bilayer graphene (tBLG) has emerged as an exciting platform for novel condensed matter physics. However, electron-phonon ($e$-ph) interactions in tBLG and their effects on electronic transport are not completely understood. Here we show first-principles calculations of $e$-ph interactions and resistivity in commensurate tBLG with large twist angles of 13.2 and 21.8 degrees. These calculations overcome key technical barriers, including large unit cells of up to 76 atoms, Brillouin-zone folding of the $e$-ph interactions, and unstable lattice vibrations due to the AA-stacked domains. We show that $e$-ph interactions due to layer-breathing (LB) phonons enhance intervalley scattering in large-angle tBLG. This interaction effectively couples the two layers, which are otherwise electronically decoupled at such large twist angles. As a result, the phonon-limited resistivity in tBLG deviates from the temperature-linear trend characteristic of monolayer graphene and tBLG near the magic angle. Taken together, our work quantifies $e$-ph interactions and scattering mechanisms in tBLG, revealing subtle interlayer coupling effects at large twist angles.

cond-mat.mtrl-sci↗

arXiv2023

Fragile superconductivity in a Dirac metal

Studying superconductivity in Dirac semimetals is an important step in understanding quantum matter with topologically non-trivial order parameters. We report on the properties of the superconducting phase in single crystals of the Dirac material LaCuSb2 prepared by the self-flux method. We find that chemical and hydrostatic pressure drastically suppress the superconducting transition. Furthermore, due to large Fermi surface anisotropy, magnetization and muon spin relaxation measurements reveal Type-II superconductivity for applied magnetic fields along the $a$-axis, and Type-I superconductivity for fields along the $c$-axis. Specific heat confirms the bulk nature of the transition, and its deviation from single-gap $s$-wave BCS theory suggests multigap superconductivity. Our tight-binding model points to an anisotropic gap function arising from the spin-orbital texture near the Dirac nodes, providing an explanation for the appearance of an anomaly in specific heat well below $T_c$. Given the existence of superconductivity in a material harboring Dirac fermions, LaCuSb2 proves an interesting material candidate in the search for topological superconductivity.

cond-mat.supr-con↗

arXiv2023

Coexistence of bulk-nodal and surface-nodeless Cooper pairings in a superconducting Dirac semimetal

The interplay of nontrivial topology and superconductivity in condensed matter physics gives rise to exotic phenomena. However, materials are extremely rare where it is possible to explore the full details of the superconducting pairing. Here, we investigate the momentum dependence of the superconducting gap distribution in a novel Dirac material PdTe. Using high resolution, low temperature photoemission spectroscopy, we establish it as a spin-orbit coupled Dirac semimetal with the topological Fermi arc crossing the Fermi level on the (010) surface. This spin-textured surface state exhibits a fully gapped superconducting Cooper pairing structure below Tc~4.5K. Moreover, we find a node in the bulk near the Brillouin zone boundary, away from the topological Fermi arc.These observations not only demonstrate the band resolved electronic correlation between topological Fermi arc states and the way it induces Cooper pairing in PdTe, but also provide a rare case where surface and bulk states host a coexistence of nodeless and nodal gap structures enforced by spin-orbit coupling.

cond-mat.supr-con↗

arXiv2022

Quasiparticle band alignment and stacking-independent exciton in MA$_2$Z$_4$ (M = Mo, W, Ti; A= Si, Ge; Z = N, P, As)

Motivated by the recently synthesized two-dimensional semiconducting MoSi$_2$N$_4$, we systematically investigate the quasiparticle band alignment and exciton in monolayer MA$_2$Z$_4$ (M = Mo, W, Ti; A= Si, Ge; Z = N, P, As) using ab initio GW and Bethe-Salpeter equation calculations. Compared with the results from density functional theory (DFT), our GW calculations reveal substantially more significant band gaps and different absolute quasiparticle energy but predict the same types of band alignments.

cond-mat.mtrl-sci↗

arXiv2020

Radiative properties of quantum emitters in boron nitride from excited state calculations and Bayesian analysis

Point defects in hexagonal boron nitride (hBN) have attracted growing attention as bright single-photon emitters. However, understanding of their atomic structure and radiative properties remains incomplete. Here we study the excited states and radiative lifetimes of over 20 native defects and carbon or oxygen impurities in hBN using ab initio density functional theory and GW plus Bethe-Salpeter equation calculations, generating a large data set of their emission energy, polarization and lifetime. We find a wide variability across quantum emitters, with exciton energies ranging from 0.3 to 4 eV and radiative lifetimes from ns to ms for different defect structures. Through a Bayesian statistical analysis, we identify various high-likelihood defect emitters, among which the native $\mathrm{V_NN_B}$ defect is predicted to possess emission energy and radiative lifetime in agreement with experiments. Our work advances the microscopic understanding of hBN single-photon emitters and introduces a computational framework to characterize and identify quantum emitters in 2D materials.

cond-mat.mtrl-sci↗

arXiv2020

MPSUM: Entity Summarization with Predicate-based Matching

With the development of Semantic Web, entity summarization has become an emerging task to generate concrete summaries for real world entities. To solve this problem, we propose an approach named MPSUM that extends a probabilistic topic model by integrating the idea of predicate-uniqueness and object-importance for ranking triples. The approach aims at generating brief but representative summaries for entities. We compare our approach with the state-of-the-art methods using DBpedia and LinkedMDB datasets.The experimental results show that our work improves the quality of entity summarization.

cs.IR↗

arXiv2020

Raman Response and Transport Properties of One-Dimensional van der Waals Tellurium Nanowires

Tellurium can form nanowires of helical atomic chains. Given their unique one-dimensional van der Waals structure, these nanowires are expected to show remarkably different physical and electronic properties than bulk tellurium. Here we show that few-chain and single-chain van der Waals tellurium nanowires can be isolated using carbon nanotube and boron nitride nanotube encapsulation. With the approach, the number of atomic chains can be controlled by the inner diameter of the nanotube. The Raman response of the structures suggests that the interaction between a single-atomic tellurium chain and a carbon nanotube is weak, and that the inter-chain interaction becomes stronger as the number of chains increases. Compared with bare tellurium nanowires on SiO2, nanowires encapsulated in boron nitride nanotubes exhibit a dramatically enhanced current-carrying capacity, with a current density of 1.5*10^8 A cm-2, which exceeds that of most semiconducting nanowires. We also use our tellurium nanowires encapsulated in boron nitride nanotubes to create field-effect transistors that have a diameter of only 2 nm.

cond-mat.mes-hall↗

arXiv2019

Emerging Photoluminescence from the Dark-Exciton Phonon Replica in Monolayer WSe2

Tungsten-based monolayer transition metal dichalcogenides host a long-lived dark exciton, an electron-hole pair in a spin-triplet configuration. The long lifetime and unique spin properties of the dark exciton provide exciting opportunities to explore light-matter interactions beyond electric dipole transitions. Here we demonstrate that the coupling of the dark exciton and an optically silent chiral phonon enables the intrinsic photoluminescence of the dark-exciton replica in monolayer WSe2. Gate and magnetic-field dependent PL measurements unveil a circularly-polarized replica peak located below the dark exciton by 21.6 meV, equal to E" phonon energy from Se vibrations. First-principles calculations show that the exciton-phonon interaction selectively couples the spin-forbidden dark exciton to the intravalley spin-allowed bright exciton, permitting the simultaneous emission of a chiral phonon and a circularly-polarized photon. Our discovery and understanding of the phonon replica reveals a chirality dictated emission channel of the phonons and photons, unveiling a new route of manipulating valley-spin.

cond-mat.mes-hall↗

arXiv2018

Edge-Insensitive Magnetism and Half Metallicity in Graphene Nanoribbons

Realizing magnetism in graphene/carbon nanostructures is a decade-long challenge. The magnetic edge state and half metallicity in zigzag graphene nanoribbons are particularly promising [Y.-W. Son, et al., Nature 444, 347 (2006)]. However, its experimental realization has been hindered by the stringent requirement of the mono-hydrogenated zigzag edge. Using first-principle calculations, we predict that free-carrier doping can overcome this challenge and realize ferromagnetism and half-metallicity in narrow graphene nanoribbons of general types of edge structures. This magnetism exists within the density range of gate-doping experiments (~1013 cm-2) and has large spin polarization energy up to 17 meV per carrier, which induces a Zeeman splitting equivalent to an external magnetic field of a few hundred Tesla. Finally, we trace the formation mechanics of this edge-insensitive magnetism to the quantum confinement of the electronic state near the band edge and reveal the scaling law of magnetism versus the ribbon width. Our findings suggest that combining doping with quantum confinement could be a general tool to realize transition-metal-free magnetism in light-element nanostructures.

cond-mat.mes-hall↗

arXiv2017

Interlayer coupling and gate-tunable excitons in transition metal dichalcogenide heterostructures

Bilayer van der Waals (vdW) heterostructures such as MoS2/WS2 and MoSe2/WSe2 have attracted much attention recently, particularly because of their type II band alignments and the formation of interlayer exciton as the lowest-energy excitonic state. In this work, we calculate the electronic and optical properties of such heterostructures with the first-principles GW+Bethe-Salpeter Equation (BSE) method and reveal the important role of interlayer coupling in deciding the excited-state properties, including the band alignment and excitonic properties. Our calculation shows that due to the interlayer coupling, the low energy excitons can be widely tunable by a vertical gate field. In particular, the dipole oscillator strength and radiative lifetime of the lowest energy exciton in these bilayer heterostructures is varied by over an order of magnitude within a practical external gate field. We also build a simple model that captures the essential physics behind this tunability and allows the extension of the ab initio results to a large range of electric fields. Our work clarifies the physical picture of interlayer excitons in bilayer vdW heterostructures and predicts a wide range of gate-tunable excited-state properties of 2D optoelectronic devices.

cond-mat.mes-hall↗

arXiv2017

Renormalization of quasiparticle band gap in doped two-dimensional materials from many-body calculations

Doped free carriers can substantially renormalize electronic self-energy and quasiparticle band gaps of two-dimensional (2D) materials. However, it is still challenging to quantitatively calculate this many-electron effect, particularly at the low doping density that is most relevant to realistic experiments and devices. Here we develop a first-principles-based effective-mass model within the GW approximation and show a dramatic band gap renormalization of a few hundred meV for typical 2D semiconductors. Moreover, we reveal the roles of different many-electron interactions: The Coulomb-hole contribution is dominant for low doping densities while the screened-exchange contribution is dominant for high doping densities. Three prototypical 2D materials are studied by this method, h-BN, MoS2, and black phosphorus, covering insulators to semiconductors. Especially, anisotropic black phosphorus exhibits a surprisingly large band gap renormalization because of its smaller density-of-state that enhances the screened-exchange interactions. Our work demonstrates an efficient way to accurately calculate band gap renormalization and provides quantitative understanding of doping-dependent many-electron physics of general 2D semiconductors.

cond-mat.mes-hall↗

arXiv2017

Ligand-field helical luminescence in a 2D ferromagnetic insulator

Bulk chromium triiodide (CrI$_3$) has long been known as a layered van der Waals ferromagnet. However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet, providing a new platform for investigating light-matter interactions and magneto-optical phenomena in the atomically thin limit. Here, we report spontaneous circularly polarized photoluminescence in monolayer CrI$_3$ under linearly polarized excitation, with helicity determined by the monolayer magnetization direction. In contrast, the bilayer CrI$_3$ photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in CrI$_3$ bilayers. Distinct from the Wannier-Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors, our absorption and layer-dependent photoluminescence measurements reveal the importance of ligand-field and charge-transfer transitions to the optoelectronic response of atomically thin CrI$_3$. We attribute the photoluminescence to a parity-forbidden d-d transition characteristic of Cr$^{3+}$ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized molecular orbital nature.

cond-mat.mes-hall↗

arXiv2015

Quasiparticle Band Gaps, Excitonic Effects, and Anisotropic Optical Properties of Monolayer Distorted 1-T Diamond-chain Structures

We report many-body perturbation theory calculations of excited-state properties of distorted 1-T diamond-chain monolayer rhenium disulfide (ReS2) and diselenide (ReSe2). Electronic self-energy substantially enhances their quasiparticle band gaps and, surprisingly, converts monolayer ReSe2 to a direct-gap semiconductor, which was, however, regarded to be an indirect one by density-functional-theory calculations. Their optical absorption spectra are dictated by strongly bound excitons. Unlike hexagonal structures, the lowest-energy bright exciton of distorted 1-T ReS2 exhibits a perfect figure-8 shape polarization dependence but those of ReSe2 only exhibit a partial polarization dependence, which results from two nearly-degenerated bright excitons whose polarization preferences are not aligned. Our first-principles calculations are in agreement with experiments and pave the way for optoelectronic applications.

cond-mat.mtrl-sci↗