arXiv subjects

Zhenyu Zhang

Publications and source records attributed to Zhenyu Zhang.

341 records · Page 19Linked to original sources

arXiv2010

Half-Heusler Compounds as a New Class of Three-Dimensional Topological Insulators

Using first-principles calculations within density functional theory, we explore the feasibility of converting ternary half-Heusler compounds into a new class of three-dimensional topological insulators (3DTI). We demonstrate that the electronic structure of unstrained LaPtBi as a prototype system exhibits distinct band-inversion feature. The 3DTI phase is realized by applying a uniaxial strain along the [001] direction, which opens a bandgap while preserving the inverted band order. A definitive proof of the strained LaPtBi as a 3DTI is provided by directly calculating the topological Z2 invariants in systems without inversion symmetry. We discuss the implications of the present study to other half-Heusler compounds as 3DTI, which, together with the magnetic and superconducting properties of these materials, may provide a rich platform for novel quantum phenomena.

cond-mat.mtrl-sci↗

arXiv2010

The finite-width Laplace sum rules for $0^{++}$ scalar glueball in instanton liquid model

In the framework of a semi-classical expansion for quantum chromodynamics in the instanton liquid background, the correlation function of the $0^{++}$ scalar glueball current is given. Besides the pure classical and quantum contributions, the contributions arising from the interactions between the classical instanton fields and quantum gluons are taken into account as well. Instead of the usual zero-width approximation for the resonance, the Brite-Wigner form for the spectral function of the finite-width resonance is adopted. The family of the Laplace sum rules for the scalar glueball in quantum chromodynamics with and without light quarks are studed. A consistency between the subtracted and unsubtracted sum rules are very well justified, and the values of the mass, decay width, and the coupling to the corresponding current for the $0^{++}$ resonance in which the glueball fraction is dominant, are obtained.

hep-ph↗

arXiv2010

Atomistic mechanisms and diameter selection during nanorod growth

We study in this paper the atomic mechanisms of nanorod growth and propose the way of diameter selection of nanorod. A characteristic radius is demonstrated to be crucial in nanorod growth, which increases proportional to one fifth power of the ratio of the interlayer hopping rate of adatoms across the monolayer steps to the deposition rate. When the radius of the initial island is larger than this characteristic radius, the growth morphology evolves from a taper-like structure to a nanorod with radius equal to the characteristic radius after some transient layers. Otherwise the nanorod morphology can be maintained during the growth, with stable radius being limited by both the radius of the initial island and the three-dimensional Ehrlich-Schwoebel barrier. Therefore different growth modes and diameter of nanorod can be selected by tuning the characteristic radius. The theoretical predictions are in good agreement with experimental observations of ZnO growth.

cond-mat.mes-hall↗

arXiv2009

Contrasting Behavior of Carbon Nucleation in the Initial Stages of Graphene Epitaxial Growth on Stepped Metal Surfaces

Using first-principles calculations within density functional theory, we study the energetics and kinetics of carbon nucleation in the early stages of epitaxial graphene growth on three representative stepped metal surfaces: Ir(111), Ru(0001), and Cu(111). We find that on the flat surfaces of Ir(111) and Ru(0001), two carbon atoms repel each other, while they prefer to form a dimer on Cu(111). Moreover, the step edges on Ir and Ru surfaces cannot serve as effective trapping centers for single carbon adatoms, but can readily facilitate the formation of carbon dimers. These contrasting behaviors are attributed to the delicate competition between C-C bonding and C-metal bonding, and a simple generic principle is proposed to predict the nucleation sites of C adatoms on many other metal substrates with the C-metal bond strengths as the minimal inputs.

cond-mat.mes-hall↗

arXiv2009

Adsorbate-induced Restructuring of Pb mesas Grown on Vicinal Si(111) in the Quantum Regime

Using scanning tunneling microscopy and spectroscopy, we demonstrate that the adsorption of a minute amount of Cs on a Pb mesa grown in the quantum regime can induce dramatic morphological changes of the mesa, characterized by the appearance of populous monatomic-layer-high Pb nano-islands on top of the mesa. The edges of the Pb nano-islands are decorated with Cs adatoms, and the nano-islands preferentially nucleate and grow on the quantum mechanically unstable regions of the mesa. Furthermore, first-principles calculations within density functional theory show that the Pb atoms forming these nano-islands were expelled by the adsorbed Cs atoms via a kinetically accessible place exchange process when the Cs atoms alloyed into the top layer of the Pb mesa.

cond-mat.mtrl-sci↗

arXiv2009

Spin continuity equation and definition of spin current

For an electron, a spin-1/2 particle, the spin charge $\mathbf{s}$, a real pseudovector with constant length, could determine the spin polarization properties in quantum mechanics. Since spin density $ρ_{\mathbf{s}}$ could be expressed as the product of probability density $ρ$ and the spin charge $\mathbf{s}$, the spin continuity equation could be derived from fundamental principles of quantum mechanics and the definition of spin current arise naturally. Only in some specific conditions the conventional definition of spin current coincides with ours. The equilibrium spin currents would vanish automatically in two-dimensional electron gas with Rashba spin-orbit interaction by using our definition of spin current.

cond-mat.mes-hall↗

arXiv2009

Optimization of Mn Doping in Group-IV-based Dilute Magnetic Semiconductors by Electronic Co-dopants

The percentage of substitutional doping of magnetic atoms (Mn) in group-IV-based dilute magnetic semiconductors (DMS) can be increased by co-doping with another conventional electronic dopant (e-dopant) [Zhu et al., Phys. Rev. Lett. 100, 027205 (2008)]. Here, we report extensive theoretical investigations of the kinetic and thermodynamic characteristics of several co-doped systems including bulk Si and Ge as hosts and various group-III and group-V e-dopants. The main findings are as follows: The n-p pairing of n-type e-dopants with p-type substitutional Mn is energetically stable in bulk Ge and Si. Mn atoms move from interstitial sites to substitutional sites easier in the presence of a neighboring n-type e-dopant. Magnetic coupling between two Mn atoms in bulk Ge oscillates between positive (ferromagnetic) and negative (antiferromagnetic) values with increasing Mn-Mn distance, but in Mn/As co-doped Ge the coupling parameter remains positive at all distances beyond nearest-neighbors and this qualitative difference does not change with the doping level. For Mn doped Si, all coupling values except for the nearest neighbor one are positive and do not change much upon co-doping. We find an unconventional magnetic anisotropy in the co-doped system, that is, the dependence of magnetic coupling on the relative positions of the magnetic ions and their neighboring e-dopants. We employ Monte Carlo simulations to estimate the Curie temperature (Tc). We find that in Mn doped Ge no ferromagnetic order exists for Mn concentrations ranging from 3.13% to 6%. Instead, a spin-glass phase transition occurs at ~5K at 5% Mn doping. For Mn/As co-doped Ge, Tc increases nearly linearly with the Mn concentration and reaches 264K at 5% Mn doping.

cond-mat.mtrl-sci↗

arXiv2008

Persistent spin currents in electron systems with spin-orbit interaction

We investigate the persistent spin currents in one- and two-dimensional electron systems with spin-orbit interaction in thermodynamics equilibrium at absolute zero temperature. The persistent spin current is the intrinsic one which is connected with the Berry phases in the configuration spaces of an electron system and winding numbers in the field configurations of electrons. When the topological space of the configuration of a system has the nontrivial first homotopy groups, the persistent spin currents in the system could be nonzero and not easily destroyed by impurity scattering in ballistic limit. The non-vanishing background spin currents in infinite two-dimensional electron system found by Rashba could be realized by the transport persistent spin currents in a finite torus electron system with spin-orbit interaction. In this sense, we meet the challenge proposed by Rashba.

cond-mat.mes-hall↗

arXiv2007

Dynamics Analysis Of Plasmon Resonance Modes In Nanoparticles

A novel theoretical approach to the dynamics analysis of excitation of plasmon modes in nanoparticles is presented. This approach is based on the biorthogonal plasmon mode expansion and it leads to the predictions of time-dynamics of excitation of specific plasmon modes as well as their steady state amplitude and their decay. Temporal characteristics of plasmon modes in nanoparticles are expressed in terms of their shapes, permittivity dispersion relations and excitation conditions. In the case of the Drude model, analytical expressions for time-dynamics of plasmon modes are obtained.

cond-mat.mtrl-sci↗

arXiv2005

Multiple Bosonic Mode Coupling in Electron Self-Energy of (La_2-xSr_x)CuO_4

High resolution angle-resolved photoemission spectroscopy data along the (0,0)-($π$,$π$) nodal direction with significantly improved statistics reveal fine structure in the electron self-energy of the underdoped (La$_{2-x}$Sr$_x$)CuO$_4$ samples in the normal state. Fine structure at energies of (40$\sim$46) meV and (58$\sim$63)meV, and possible fine structure at energies of (23$\sim$29)meV and (75$\sim$85)meV, have been identified. These observations indicate that, in LSCO, more than one bosonic modes are involved in the coupling with electrons.

cond-mat.str-el↗

arXiv2004

Direct extraction of the Eliashberg function for electron-phonon coupling: A case study of Be(1010)

We propose a systematic procedure to directly extract the Eliashberg function for electron-phonon coupling from high-resolution angle-resolved photoemission data. The procedure is successfully applied to the Be(1010) surface, providing new insights to electron-phonon coupling at this surface. The method is shown to be robust against imperfections in experimental data and suitable for wider applications.

cond-mat.str-el↗

arXiv2003

Surface Phase Transitions Induced by Electron Mediated Adatom-Adatom Interaction

We propose that the indirect adatom-adatom interaction mediated by the conduction electrons of a metallic surface is responsible for the $\sqrt{3}\times \sqrt{3}\Leftrightarrow 3\times 3$ structural phase transitions observed in Sn/Ge (111) and Pb/Ge (111). When the indirect interaction overwhelms the local stress field imposed by the substrate registry, the system suffers a phonon instability, resulting in a structural phase transition in the adlayer. Our theory is capable of explaining all the salient features of the $\sqrt{3}\times \sqrt{3}\Leftrightarrow 3\times 3$ transitions observed in Sn/Ge (111) and Pb/Ge (111), and is in principle applicable to a wide class of systems whose surfaces are metallic before the transition.

cond-mat.mtrl-sci↗

arXiv2002

Self-assembly of Nanometer-scale Magnetic Dots with Narrow Size Distributions on an Insulating Substrate

The self-assembly of iron dots on the insulating surface of NaCl(001) is investigated experimentally and theoretically. Under proper growth conditions, nanometer-scale magnetic iron dots with remarkably narrow size distributions can be achieved in the absence of a wetting layer Furthermore, both the vertical and lateral sizes of the dots can be tuned with the iron dosage without introducing apparent size broadening, even though the clustering is clearly in the strong coarsening regime. These observations are interpreted using a phenomenological mean-field theory, in which a coverage-dependent optimal dot size is selected by strain-mediated dot-dot interactions.

cond-mat.mtrl-sci↗

arXiv2000

Dynamical Scaling Properties of Electrons in Quantum Systems with Multifractal Eigenstates

We study the intricate relationships between the dynamical scaling properties of electron wave packets and the multifractality of the eigenstates in quantum systems. Numerical simulations for the Harper model and the Fibonacci chain indicate that the root mean square displacement displays the scaling behavior $r(t)\sim t^β$ with $β=D_2^ψ$, where $D_2^ψ$ is the correlation dimension of the multifractal eigenstates. The equality can be generalized to $d$-dimensional systems as $β=D_2^ψ/d$, as long as the electron motion is ballistic in the effective $D_2^ψ$-dimensional space. This equality should be replaced by $β<D_2^ψ/d$ if the motion is non-ballistic, as supported by all known results.

cond-mat.dis-nn↗

arXiv2000

Morphological Symmetry Breaking During Epitaxial Growth at Grazing Incidence

It is shown that, in submonolayer growth at off-normal incidence, even much less than one percent of transfer from the condensation energy of the deposited atoms into adatom motion is sufficient to induce a net adatom current from the illuminated edge of a two-dimensional island to the other edges, thereby breaking the island symmetry. Such a symmetry breaking phenomenon is most pronounced for deposition at grazing incidence. Comparison between our theoretical predictions and existing experimental results confirms the general validity of the model.

cond-mat.mtrl-sci↗

arXiv1999

Two-dimensional pattern formation in surfactant-mediated epitaxial growth

The effects of a surfactant on two-dimensional pattern formation in epitaxial growth are explored theoretically using a simple model, in which an adatom becomes immobile only after overcoming a large energy barrier as it exchanges positions with a surfactant atom, and subsequent growth from such a seed is further shielded. Within this model, a fractal-to-compact island shape transition can be induced by either decreasing the growth temperature or increasing the deposition flux. This and other intriguing findings are in excellent qualitative agreement with recent experiments.

cond-mat↗

arXiv1996

Structural, Magnetic and Electronic Properties of Fe/Au Monatomic Multilayers

An Fe/Au monatomic multilayer, consisting of alternating single Fe and Au layers, has been studied by means of the self-consistent full-potential linearized augmented plane wave method. We show by total energy minimization that this artificial thin film is in the tetragonal $L1_{0}$ ordered structure with the ratio of the interlayer spacing to the intralayer lattice constant at 0.865. In this configuration, the magnetic moment in each monolayer, the spin-polarized electronic density of states, and the corresponding band structure are calculated. The results are discussed in connection with recent experiments.

cond-mat↗