arXiv subjects

Xingqiang Shi

Publications and source records attributed to Xingqiang Shi.

10 recordsLinked to original sources

arXiv2020

Dynamic fingerprint of fractionalized excitations in single-crystalline Cu$_3$Zn(OH)$_6$FBr

Quantum spin liquid (QSL) represents a new class of condensed matter states characterized by the long-range many-body entanglement of topological orders. The most prominent feature of the elusive QSL state is the existence of fractionalized spin excitations. Subject to the strong quantum fluctuations, the spin-1/2 antiferromagnetic system on a kagome lattice is the promising candidate for hosting a QSL ground state, but the structurally ideal realization is rare. Here, we report Raman scattering on the single crystalline Cu$_3$Zn(OH)$_6$FBr, and confirm that the ideal kagome structure remains down to low temperatures without any lattice distortion by the angle-resolved polarized Raman responses and second-harmonic-generation measurements. Furthermore, at low temperatures the Raman scattering reveals a continuum of the spin excitations in Cu$_3$Zn(OH)$_6$FBr, in contrast to the sharp magnon peak in the ordered kagome antiferromagnet EuCu$_3$(OH)$_6$Cl$_3$. Such magnetic Raman continuum, in particular, the substantial low-energy one-pair spinon excitation serves as strong evidence for fractionalized spin excitations in Cu$_3$Zn(OH)$_6$FBr.

cond-mat.str-el↗

arXiv2019

Effective Hamiltonian for superconducting Ni oxides Nd$_{1-x}$Sr$_x$NiO$_2$

We derive the effective single-band Hamiltonian in the flat NiO$_2$ planes for nickelate compounds Nd$_{1-x}$Sr$_x$NiO$_2$. We first implement the first-principles calculation to study electronic structures of nickelates using the Heyd-Scuseria-Ernzerhof hybrid density functional and derive a three-band Hubbard model for Ni-O $pdσ$ bands of Ni$^+$ $3d_{x^2-y^2}$ and O$^{2-}$ $2p_{x/y}$ orbitals in the NiO$_2$ planes. To obtain the effective one-band $t$-$t'$-$J$ model Hamiltonian, we perform the exact diagonalization of the three-band Hubbard model for the Ni$_5$O$_{16}$ cluster and map the low-energy spectra onto the effective one-band models. We find that the undoped NiO$_2$ plane is a Hubbard Mott insulator, and the doped holes primarily locate on Ni sites. The physics of the NiO$_2$ plane is a doped Mott insulator, described by the one-band $t$-$t'$-$J$ model with $t=265$~meV, $t'=-21$~meV and $J=28.6$~meV. We also discuss the electronic structure for the "self-doping" effect and heavy fermion behavior of electron pockets of Nd$^{3+}$ $5d$ character in Nd$_{1-x}$Sr$_x$NiO$_2$.

cond-mat.supr-con↗

arXiv2019

Fermi-level depinning in metal-2D multilayered semiconductor junctions

Thicknesses-dependent performance of metal-two-dimensional (2D) semiconductor junctions (MSJ) in electronics/optoelectronics have attracted increasing attention, but till present, people have little knowledge about the micro-mechanism of the thicknesses (or layer-number) dependence. Here, by first-principles calculations based on density functional theory, we show that the Fermi-level pinning (FLP) factor of MSJ depends sensitively on the layer-number of few-layer 2D semiconductors, and, an extended FLP theory is proposed for metal-2D multilayered semiconductor junctions (MmSJ). Taking multilayered MoS2 as a typical example for van der Waals (vdW) semiconductor in MmSJ, the extended FLP theory has the following character: strong pinning right at the metal-1st-layer semiconductor interface while depinning occurs between MoS2 layers. This depinning effect between vdW layers has several important consequences: 1) the overall pinning in MmSJ is greatly weakened; 2) p-type contact, rarely obtained in metal-monolayer MoS2 junctions, becomes favored in MmSJ, which is important for CMOS logical devices; 3) depinning between MoS2 layers result in type II band alignment in MoS2 'homojunction' supported on metals, which is useful for optoelectronics. Moreover, our extended FLP theory sheds light on the recent controversial experimental observation and paves a new and universal route to type II band alignment in vdW 'homojunctions'.

cond-mat.mtrl-sci↗

arXiv2019

Charge distribution at metal-multilayered semiconductor interfaces

Thicknesses-dependent performances of metal-multilayered semiconductor junctions have attracted increasing attention, but till present, the mechanism of interaction and the resulting charge distribution at interfaces which control the Schottky barrier and band offset between the semiconductor layers have not been systematically studied. Based on first-principles calculations, the nature and strength of the non-bonding interactions at Metal-MoS2 (M-S) and MoS2-MoS2 (S-S) interfaces in meta-multilayered MoS2 are investigated. We show that the charge distribution at M-S interfaces depends sensitively on the dimensionality and work function of metal substrates: 1) push back effect and metal induced gap states play a main role at 3D metal-MoS2 interfaces; 2) charge transfer occurs in Mo2C(OH)2 (or Mo2CO2)-MoS2 interfaces which means electron distribution is determined by the band alignment of metal and MoS2; 3) covalent-like feature appears at Mo2CF2-MoS2 interface. The S-S interface inherit the charge redistribution at M-S interface for 2D metal-2L MoS2 junction, and have a depinning effect for M-S interface in 3D metal-2L MoS2 junction. We are trying to start drawing general conclusions and developing new concepts to understand metal-multilayered semiconductor interfaces in the strong interaction limit, where charge-transfer effects must be taken into consideration in this paper.

cond-mat.mtrl-sci↗

arXiv2016

Giant atomic magnetocrystalline anisotropy from degenerate orbitals around Fermi level

Nano-structures with giant magnetocrystalline anisotropy energies (MAE) are desired in designing miniaturized magnetic storage and quantum computing devices. Through ab initio and model calculations, we propose that special p-element dimers and single-adatom on symmetry-matched substrates possess giant atomic MAE of 72-200 meV with room temperature structural stability. The huge MAE originates from degenerate orbitals around Fermi level. More importantly, we developed a simplified quantum mechanical model to understand the principle on how to obtain giant MAE for supported magnetic structures. These discoveries and mechanisms provide a paradigm to design giant atomic MAE in nanostructures.

cond-mat.mtrl-sci↗

arXiv2016

Fullerene antiferromagnetic reconstructed spinterface subsurface layer dominates multi-orbitals spin-splitting and large magnetic moment in C60

The interfaces between organic molecules and metal surfaces with layered antiferromagnetic order have gained increasing interests in the field of antiferromagnetic spintronics. The C60 layered AFM spinterfaces have been studied for C60 bonded only to the outermost ferromagnetic layer. Using density functional theory calculations, here we demonstrate that C60 adsorption can reconstruct the layered AFM Cr(001) surface so that C60 bonds to the top two Cr layers with opposite spin direction. Surface reconstruction drastically changes C60 s spintronic properties 1 the spin-split p-d hybridization involve multi-orbitals of C60 and metal double layers, 2 the subsurface layer dominates the C60 spin properties, and 3) reconstruction induces a large magnetic moment in C60 of 0.58 B, which is a synergetic effect of the top two layers as a result of a magnetic direct-exchange interaction. Understanding these complex spinterfaces phenomena is a crucial step for their device applications. The surface reconstruction can be realized by annealing at above room temperature in experiments.

physics.comp-ph↗

arXiv2015

Stability of Two-Dimensional Iron-Carbides Suspended across Graphene Pores: First-principles Particle Swarm Optimization

Inspired by recent experimental realizations of two-dimensional (2D) metals and alloys, we theoretically investigate the stability and electronic properties of monolayer (ML) Fe-C compounds and pure Fe. According to our and others theoretical results, the experiment [Science 343, 1228 (2014)] proposed ML pure Fe square-lattices embedded in graphene (Gr) pores are energetically unstable compared to that of the Fe triangular-lattices in Gr. To solve the above contradiction, we search for the stable structures of ML Fe-C with various Fe to C ratios (as a generalization of ML Fe in Gr) using ab initio particle swarm optimization technique. A Fe1C1 square-lattice embedded in Gr is found. We propose and demonstrate that the square-lattices observed in the experiment were iron-carbides (Fe-C) but not pure Fe from the square-lattice shape, Fe-Fe lattice constant and energetic considerations. Note that the coexistence of C with Fe cannot be excluded from the experiment. More importantly, we find a lowest energy and dynamically stable structure, ML Fe2C2 with Fe atoms form distorted square lattices. High spin polarization around the Fermi level is predicted for different 2D Fe-C structures due to significant orbital hybridization between C and Fe.

cond-mat.mtrl-sci↗

arXiv2015

Lithium Intercalation in Graphene/MoS2 Composites: First-Principles Insights

As a storage material for Li-ion batteries, graphene/molybdenum disulfide (Gr/MoS2) composites have been intensively studied in experiments. But the relevant theoretical works from first-principles are lacking. In the current work, van-der-Waals-corrected density functional theory calculations are performed to investigate the interaction of Li in Gr/MoS2 composites. Three interesting features are revealed for the intercalated Gr/Li(n)/MoS2 composites (n = 1 to 9). One is the reason for large Li storage capacity of Gr/MoS2: due to the binding energies per Li atom increase with the increasing number of intercalated Li atoms. Secondly, the band gap opening of Gr is found, and the band gap is enlarged with the increasing number of intercalated Li atoms, up to 160 meV with nine Li; hence these results suggest an efficient way to tune the band gap of graphene. Thirdly, the Dirac cone of Gr always preserve for different number of ionic bonded Li atoms.

physics.comp-ph↗

arXiv2015

Molecular Precursors-Induced Surface Reconstruction at Graphene/Pt(111) Interfaces

Inspired by experimental observations of Pt(111) surfaces reconstruction at the Pt/graphene (Gr) interfaces with ordered vacancy networks in the outermost Pt layer, the mechanism of the surface reconstruction is investigated by van-der-Waals-corrected density functional theory in combination with particle-swarm optimization algorithm and ab initio atomistic thermodynamics. Our global structural search finds a more stable reconstructed (Rec) structure than that was reported before. With correction for vacancy formation energy, we demonstrate that the experimental observed surface reconstruction occurred at the earlier stages of graphene formation: 1) reconstruction occurred when C60 adsorption (before decomposition to form graphene) for C60 as a molecular precursor, or 2) reconstruction occurred when there were (partial) hydrogens retain in the adsorbed carbon structures for C2H4 and C60H30 as precursors. The reason can be attributed to that the energy gain, from the strengthened Pt-C bonding for C of C60 or for C with partial H, compensates the energy cost of formation surface vacancies and makes the reconstruction feasible, especially at elevated temperatures. In the Rec structure, two Pt-C covalent bonds are formed per unit cell, which have a great impact on the adsorbed Gr electronic structures.

physics.chem-ph↗

arXiv2006

Conductance oscillation and quantization in monoatomic Al wires

We present first-principles calculations for the transport properties of monoatomic Al wires sandwiched between Al(100) electrodes. The conductance of the monoatomic Al wires oscillates with the number of the constituent atoms as a function of the wire length, either with a period of four-atom for wires with the typical interatomic spacing or a period of six-atom with the interatomic spacing of the bulk fcc aluminum, indicating a dependence of the period of conductance oscillation on the interatomic distance of the monoatomic Al wires.

cond-mat.mes-hall↗