arXiv subjects

Zhigang Wang

Publications and source records attributed to Zhigang Wang.

At least 127 records · Page 7Linked to original sources

arXiv2014

Anisotropic Fabry-Pérot resonant states confined within nano-steps on the topological insulator surface

The peculiar nature of topological surface states, such as absence of backscattering, weak anti-localization, and quantum anomalous Hall effect, has been demonstrated mainly in bulk and film of topological insulator (TI), using surface sensitive probes and bulk transport probes. However, it is equally important and experimentally challenging to confine massless Dirac fermions with nano-steps on TI surfaces. This potential structure has similar ground with linearly-dispersed photons in Fabry-Pérot resonators, while reserving fundamental differences from well-studied Fabry-Pérot resonators and quantum corrals on noble metal surfaces. In this paper, we study the massless Dirac fermions confined within steps along the $x$ ($Γ\mathtt{-}$K) or $y$ ($Γ\mathtt{-}$M) direction on the TI surface, and the Fabry-Pérot-like resonances in the electronic local density of states (LDOS) between the steps are found. Due to the remarkable warping effect in the topological surface states, the LDOS confined in the step-well running along $Γ$-M direction exhibit anisotropic resonance patterns as compared to those in the step-well along $Γ$-K direction, which can be detected by scanning tunneling microscopy. The transmittance properties and spin orientation of Dirac fermion in both cases are also anisotropic in the presence of warping effect.

cond-mat.mes-hall↗

arXiv2013

Structural and electronic properties of uranium-encapsulated Au14 cage

The structural properties of the uranium-encapsulated nano-cage U@Au14 are predicted using density functional theory. The presence of the uranium atom makes the Au14 structure more stable than the empty Au14-cage, with a triplet ground electronic state for U@Au14. Analysis of the electronic structure shows that the two frontier single-occupied molecular orbital electrons of U@Au14 mainly originate from the 5f shell of the U atom after charge transfer. Meanwhile, the bonding orbitals have both the 5f and 6d components of the U atom, along with the 5d and 6s components of the Au atoms, indicating the covalent nature of the interaction between the U and Au atoms. Moreover, the charge population analysis shows that this nanostructure displays some unique electronic properties where the encapsulated atom gains electrons while the outer shell loses electrons. Therefore, this designed U@Au14 nano-cage structure is stabilized by ionocovalent interactions. The current findings provide theoretical basis for future syntheses and further study of actinide doped gold nanoclusters, which might subsequently facilitate applications of such structure in radio-labeling, nanodrug carrier and other biomedical applications.

physics.atm-clus↗

arXiv2013

Electron Delocalization Determined Anomalous Stability in Small Water Rings

Water clusters are known to form through hydrogen bonding. However, this study shows that the formation of very small water clusters significantly deviates from this mechanism and instead involves both hydrogen bonding and electron delocalization. Our density functional theory calculations show that small water rings (H2O)n of n=3 or 4 show strong electron delocalization originating from both the hydrogen and oxygen atoms and extending to the ring center. This is very different from larger rings. Further energy decomposition of rings with n=3-6 demonstrates an upward trend in the polarization component but an decrease in the electrostatic and exchange repulsion components, presenting a minimum and accounting for 33% of interaction energy at n=3. This significantly promotes stability of the small water rings. Our findings provide a comprehensive analysis and improve our understanding of the stability characteristics of water clusters.

physics.atm-clus↗

arXiv2013

Strong Adsorption between Uranium Dicarbide and Graphene Surface Induced by f Electrons

The interaction between gaseous uranium dicarbide and graphite is significant for the safety control and design of Gen-IV nuclear energy system. In this article, the interaction mechanism has been studied using a simplified model of adsorption of two typical UC2 isomers (linear CUC and symmetric triangular structures) on graphene based on density functional theory calculations. The results reveal strong chemisorption characteristics between the UC2 and graphene, which is found different from the conventional weak intermolecular interaction. Interestingly, although the CUC structure can induce a double sp3-hybridization at the graphene, the most stable adsorption structure is formed by the triangular UC2 adsorbed at the hollow site of the graphene. Further bonding analysis indicates that the U 5f orbitals of the triangular UC2 are more active than that in the CUC, providing a larger effective bonding area in the adsorption system. Our calculations are helpful for understanding the role of actinide compounds in adsorption on carbon nanomaterials surface, especially for elucidating the bonding properties of 5f electrons.

physics.atm-clus↗

arXiv2013

Antifferomagnetic FeSe monolayer on SiTiO$_{3}$: The charge doping and electric field effects

We present theoretically the electronic structure of antiferromagnetic (AFM) FeSe monolayer on TiO$_{2}$ terminated SrTiO$_{3}$(001) surface. It is revealed that the striking disappearance of the Fermi surface around the Brillouin zone (BZ) center can be well explained by the antiferromatnetic (AFM) phase. We show that the system has a considerable charge transfer from SrTiO$_{3}$(001) substrate to FeSe monolayer, and so has a self-constructed electric field. The FeSe monolayer band structure near the BZ center is sensitive to charge doping, and the spin-resolved energy bands at BZ corner are distorted to be flattened by the perpendicular electric field. We propose a tight-binding model Hamiltonian to take these key factors into account. We also show that this composite structure is an ideal electron-hole bilayer system, with electrons and holes respectively formed in FeSe monolayer and TiO$_{2}$ surface layer.

cond-mat.mtrl-sci↗

arXiv2013

The effect of the warping term on the fractional quantum Hall states in topological insulators

The warping effect on the fractional quantum Hall (FQH) states in topological insulators is studied theoretically. Based on the perturbed wavefunctions, which include contributions from the warping term, analytical expressions for Haldane's pseudopotentials are obtained. We show that the warping term does not break the symmetry of the pseudopotentials for $n$=$\pm1$ Landau levels (LLs). With increasing the warping strength of the Fermi surface, our results indicate that the stability of the FQH states for LL $n=0$ (LLs $n$=$\pm1$) becomes stronger (weaker), and the excitation gap at $ν=1/3$ FQH state for LL $n$=0 also increases while the gaps for LLs $n$=$\pm1$ are unchanged.

cond-mat.mes-hall↗

arXiv2013

Fractional quantum Hall effect of topological surface states under a strong tilted magnetic field

The fractional quantum Hall effect (FQHE) of topological surface-state particles under a tilted strong magnetic field is theoretically studied by using the exact diagonalization method. The Haldane's pseudopotentials for the Coulomb interaction are analytically obtained. The results show that by increasing the in-plane component of the tilted magnetic field, the FQHE state at $n$=0 Landau level (LL) becomes more stable, while the stabilities of $n$=$\pm1$ LLs become weaker. Moreover, we find that the excitation gaps of the $ν=1/3$ FQHE states increase as the tilt angle is increased.

cond-mat.mes-hall↗

arXiv2012

Fractional quantum Hall effect in topological insulators: The role of Zeeman effect

We study the role of Zeeman effect in fractional quantum Hall effect (FQHE) on the surface of topological insulators (TIs). We show that the effective pseudopotentials of the Coulomb interaction are reformed due to Zeeman effect, which are quite different from those in graphene. By exactly diagonalizing the many-body Hamiltonian in the sphere geometry, we find that the ground state energies and the excitation gaps at $ν$=1/3 FQHE between the $n$=$\pm1$ Landau levels (LLs) render asymmetry, and the FQHE state at the $n$=1 LL is more robust than that at $n$=-1 LL since the excitation gap at $n$=1 LL is larger than that at $n$=-1 LL.

cond-mat.str-el↗

arXiv2012

Theory of multiple magnetic scattering for quasiparticles on a gapless topological insulator surface

We develop a general low-energy multiple-scattering partial-wave theory for gapless topological insulator (TI) surfaces in the presence of magnetic impurities. As applications, we discuss the differential cross section (CS) $dΛ/dφ$, the total CS $Λ_{tot}$, the Hall component of resistivity $Ω$, and inverse momentum relaxation time $Γ_{M}$ for single- and two-centered magnetic scattering. We show that differing from the nonmagnetic impurity scattering, $s\mathtt{-}$wave approximation is not advisable and convergent in the present case. The symmetry of CS is reduced and the backscattering occurs and becomes stronger with increasing the effective magnetic moment $M$ of single magnetic impurity. We show a non-zero perpendicular resistivity component $Ω$, which may be useful for tuning the Hall voltage of the sample. Consistent with the analysis of $dΛ/dφ$, by comparing $Γ_{M}$ with $Λ_{tot}$, we can determine different weights of backscattering and forward scattering. Similar to CS, $Ω$ and $Γ_{M}$ also exhibit oscillating behavior for multiple magnetic scattering centers due to interference effect.

cond-mat.mes-hall↗

arXiv2012

Excitonic condensation for the surface states of topological insulator bilayers

We propose a generic topological insulator bilayer (TIB) system to study the excitonic condensation with self-consistent mean-field (SCMF) theory. We show that the TIB system presents the crossover behavior from the Bardeen-Cooper-Schrieffer (BCS) limit to Bose-Einstein condensation (BEC) limit. Moreover, by comparison with traditional semiconductor systems, we find that for the present system the superfluid property in the BEC phase is more sensitive to electron-hole density imbalance and the BCS phase is more robust. Applying this TIB model into Bi$_{2}$Se$_{3}$-family material, we find that the BEC phase is most probable to be observed in experiment. We also calculate the critical temperature for Bi$_{2}$Se$_{3}$-family TIB system, which is $\mathtt{\sim}100$ K. More interestingly, we can expect this relative high-temperature excitonic condensation since our calculated SCMF critical temperature is approximately equal to the Kosterlitz-Thouless transition temperature.

cond-mat.mes-hall↗

arXiv2012

Pseudo-magnetoexcitons in strained graphene bilayers without external magnetic fields

The structural and electronic properties of graphene leads its charge carriers to behave like relativistic particles, which is described by a Dirac-like Hamiltonian. Since graphene is a monolayer of carbon atoms, the strain due to elastic deformations will give rise to so-called `pseudomagnetic fields (PMF)' in graphene sheet, and that has been realized experimentally in strained graphene sample. Here we propose a realistic strained graphene bilayer (SGB) device to detect the pseudo-magnetoexcitons (PME) in the absence of external magnetic field. The carriers in each graphene layer suffer different strong PMFs due to strain engineering, which give rise to Landau quantization. The pseudo-Landau levels (PLLs) of electron-hole pair under inhomogeneous PMFs in SGB are analytically obtained in the absence of Coulomb interactions. Based on the general analytical optical absorption selection rule for PME, we show that the optical absorption spectrums can interpret the corresponding formation of Dirac-type PME. We also predict that in the presence of inhomogeneous PMFs, the superfluidity-normal phase transition temperature of PME is greater than that under homogeneous PMFs.}

cond-mat.mes-hall↗

arXiv2012

Superfluidity and effective mass of magnetoexcitons in topological insulator bilayers: Effect of inter-Landau-level Coulomb interaction

The effective mass and superfluidity-normal phase transition temperature of magnetoexcitons in topological insulator bilayers are theoretically investigated. The intra-Landau-level Coulomb interaction is treated perturbatively, from which the effective magnetoexciton mass is analytically discussed. The inclusion of inter-Landau-level Coulomb interaction by more exact numerical diagonalization of the Hamiltonian brings out important modifications to magnetoexciton properties, which are specially characterized by prominent reduction in the magnetoexciton effective mass and promotion in the superfluidity-normal phase transition temperature at a wide range of external parameters.

cond-mat.mes-hall↗

arXiv2012

Magnetoexcitons and optical absorption of bilayer-structured topological insulators

Optical absorption properties of magnetoexcitons in topological insulator bilayers under a strong magnetic field are theoretically studied. A general analytical formula of optical absorption selection rule is obtained in the noninteracting as well as Coulomb intra-Landau-level interacting cases, which remarkably helps to interpret the resonant peaks in absorption spectroscopy and the corresponding formation of Dirac-type magnetoexcitons. We also discuss the optical absorption spectroscopy of magnetoexcitons in the presence of inter-Landau-level Coulomb interaction, which becomes more complex. Our results could be detected in the magneto-optical experiments.

cond-mat.mes-hall↗

arXiv2012

Quasiparticle states and quantum interference induce by magnetic impurities on a two-dimensional topological superconductor

We theoretically study the effect of localized magnetic impurities on two-dimensional topological superconductor (TSC). We show that the local density of states (LDOS) can be tuned by the effective exchange field $m$, the chemical potential $μ$ of TSC, and the distance $Δr$ as well as relative spin angle $α$ between two impurities. The changes in $Δr$ between two impurities alter the interference and result in significant modifications to the bonding and antibonding states. Furthermore, the bound-state spin LDOS induced by single and double magnetic impurity scattering, the quantum corrals, and the quantum mirages are also discussed. Finally, we briefly compare the impurities in TSC with those in topological insulators.

cond-mat.supr-con↗

arXiv2012

Quantum Corrals and Quantum Mirages on the Surface of a Topological Insulator

We study quantum corrals on the surface of a topological insulator (TI). Different resonance states induced by nonmagnetic (NM), antiferromagnetic (AFM), and ferromagnetic (FM) corrals are found. Intriguingly, the spin is clearly energy-resolved in a FM corral, which can be effectively used to operate surface carrier spins of TI. We also show that an observable quantum mirage of a magnetic impurity can be projected from the occupied into the empty focus of a FM elliptic corral, while in NM and AFM corrals the mirage signal becomes negligibly weak. In addition, the modulation of the interaction between two magnetic impurities in the quantum corrals is demonstrated. These prominent effects may be measured by spin-polarized STM experiments.

cond-mat.mes-hall↗

arXiv2011

Multiple scattering theory of quasiparticles on a topological insulator surface

A general partial-wave multiple scattering theory for scattering from cylindrically symmetric potentials on a topological insulator (TI) surface is developed. As an application, the cross sections for a single scatterer and two scatterers are discussed. We find that the symmetry of differential cross section is reduced and the backscattering is allowed for massive Dirac fermions on gapped TI surface. Remarkably, a sharp resonance peak at the band edge of the gapped TI is found in the total cross section $Λ_{tot}$, which may offer a useful way to determine the gap (as well as the effective mass of quasiparticles) on TI surface. We show that the interference effect is obvious in cross sections during the quasiparticle scattering between the scatterer pair, and additional resonance peaks are introduced in $Λ_{tot}$ when the higher partial waves are taken into account.

cond-mat.mes-hall↗

arXiv2011

Quantum spin Hall effect and spin-charge separation in a kagome lattice

A two-dimensional kagome lattice is theoretically investigated within a simple tight-binding model, which includes the nearest neighbor hopping term and the intrinsic spin-orbit interaction between the next nearest neighbors. By using the topological winding properties of the spin-edge states on the complex-energy Riemann surface, the spin Hall conductance is obtained to be quantized as $-e/2π$ ($e/2π$) in insulating phases. This result keeps consistent with the numerical linear-response calculation and the \textbf{Z}$_{2}$ topological invariance analysis. When the sample boundaries are connected in twist, by which two defects with $π$ flux are introduced, we obtain the spin-charge separated solitons at 1/3 (or 2/3) filling.

cond-mat.mes-hall↗

arXiv2011

Magnetic quantum oscillations of the topological insulator surface states

We study quantum oscillations of the magnetization in Bi$_{2}$Se$_{3}$(111) surface system in the presence of a perpendicular magnetic field. The combined spin-chiral Dirac cone and Landau quantization produce profound effects on the magnetization properties that are fundamentally different from those in the conventional semiconductor two-dimensional electron gas. In particular, we show that the oscillating center in the magnetization chooses to pick up positive or negative values depending on whether the zero-mode Landau level is occupied or empty. An intuitive analysis of these new features is given and the subsequent effects on the magnetic susceptibility and Hall conductance are also discussed.

cond-mat.mes-hall↗