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Ping Zhang

Publications and source records attributed to Ping Zhang.

At least 631 records · Page 35Linked to original sources
arXiv2007

First-Principles calculation of atomic hydrogen adsorption on Be(10\={1}0) thin films

We present a first-principles study of the atomic hydrogen adsorption onto the Be(10\={1}0) thin film. There are two types of Be(10\={1}0) surfaces according to the interlayer spacing between the surface and its nearest-neighbor layer. We show that the H adsorption features on these two kinds of surfaces are remarkably different. The work function, averaged electrostatic potential, and the local charge density consistently show that the charge is transferred from H to Be for L-type (see the text below) surfaces, while the transfer process is inverted for S-type surfaces.

cond-mat.mtrl-sci↗
arXiv2007

Mean-field potential calculations of high-pressure equation of state for shock-compressed BeO

A systematic study of the Hugoniot equation of state, phase transition, and the other thermodynamic properties including the Hugoniot temperature, the electronic and ionic heat capacities, and the Grüneisen parameter for shock-compressed BeO, is presented by calculating the total free energy. The method of calculations combines first-principles treatment for 0-K and finite-T electronic contribution and the mean-field-potential approach for the vibrational contribution of the lattice ion to the total energy. Our calculated Hugoniot shows good agreement with the experimental data.

cond-mat.mtrl-sci↗
arXiv2007

First-principles calculations of Cu(001) thin films: quantum size effect in surface energetics and surface chemical reactivities

First-principles calculations of Cu(001) free-standing thin films have been performed to investigate the oscillatory quantum size effects exhibited in surface energy, work function, atomic relaxation, and adsorption energy of the cesium adsorbate. The quantum well states have been shown and clarified at particular $k$-points corresponding to the stationary extrema in bulk Brillouin zone, which are in good agreement with experimental observations. The calculated surface energetics and geometry relaxations are clearly featured by quantum oscillations as a function of the film thickness of the film with oscillation periods characterized by a superposition of long and short length scales. Furthermore, we have investigated Cs adsorption onto Cu(001) thin films as a function of the film thickness. Our systematic calculated results clearly show the large-amplitude quantum oscillations in adsorption energetics, which may be used to tailor catalysis, chemical reactions and other surface processes in nanostructured materials.

cond-mat.mes-hall↗
arXiv2007

Theory of conserved spin current and its application to two dimensional hole gas

We present a detailed microscopic theory of the conserved spin current which is introduced by us [Phys. Rev. Lett. \textbf{96}, 196602 (2006)] and satisfies the spin continuity equation even for spin-orbit coupled systems. The spin transport coefficients $σ_{μν}^{s}$ as a response to the electric field are shown to consist of two parts, i.e., the conventional part $σ_{μν}^{s0}$ and the spin torque dipole correction $σ_{μν}^{sτ}$. As one key result, an Onsager relation between $σ_{μν}^{s}$ and other kinds of transport coefficients are shown. The expression for $σ_{μν}^{s}$ in terms of single-particle Bloch states are derived, by use of which we study the conserved spin Hall conductivity in the two dimensional hole gas modeled by a combined Luttinger and SIA Rashba spin-orbit coupling. It is shown that the two components in spin Hall conductivity usually have the opposite contributions. While in the absence of Rashba spin splitting, the spin Hall transport is dominated by the conventional contribution, the presence of Rashba spin splitting stirs up a large enhancement of the spin torque dipole correction, leading to an overall sign change for the total spin Hall conductivity. Furthermore, an approximate two-band calculation and the subsequent comparison with the exact four-band results are given, which reveals that the coupling between the heavy hole and light hole bands should be taken into account for strong Rashba spin splitting.

cond-mat.mes-hall↗
arXiv2006

First-Principles Calculation of Mg(0001) Thin Films: Quantum Size Effect and Adsorption of Atomic Hydrogen

We have carried out first-principles calculation of Mg(0001) free-standing thin films to study the oscillatory quantum size effect exhibited in the surface energy, work function, interlayer relaxation, and adsorption energy of the atomic hydrogen adsorbate. The quantum well states have been shown. The calculated energetics and interlayer relaxation of clean and H-adsorbed Mg films are clearly featured by quantum oscillations as a function of the thickness of the film, with oscillation period of about 8 monolayers, consistent with recent experiments. The calculated quantum size effect in H adsorption can be verified by observing the dependence of H coverage on the thickness of Mg(0001) thin films gown on Si(111) or W(110) substrate which has been experimentally accessible.

cond-mat.mes-hall↗
arXiv2006

Low compressible noble metal carbides with rock-salt structure: ab initio total energy calculations of the elastic stability

We have systematically studied the mechanical stability of all noble metal carbides with the rock-salt structure by calculating their elastic constants within the density function theory scheme. It was found that only four carbides (RuC, PdC, AgC and PtC) are mechanically stable. In particular, we have shown that RuC, PdC, and PtC have very high bulk modulus, which has been remarkably observed by the most recent experiment for the case of PtC. From the calculated density of states, we can conclude that these compounds are metallic, like the conventional group IV and group V transition metal carbides.

cond-mat.mtrl-sci↗
arXiv2006

Loschmidt Echo and Berry phase of the quantum system coupled to the XY spin chain: Proximity to quantum phase transition

We study the Loschmidt echo (LE) of a coupled system consisting of a central spin and its surrounding environment described by a general XY spin-chain model. The quantum dynamics of the LE is shown to be remarkably influenced by the quantum criticality of the spin chain. In particular, the decaying behavior of the LE is found to be controlled by the anisotropy parameter of the spin chain. Furthermore, we show that due to the coupling to the spin chain, the ground-state Berry phase for the central spin becomes nonanalytical and its derivative with respect to the magnetic parameter $λ$ in spin chain diverges along the critical line $λ=1$, which suggests an alternative measurement of the quantum criticality of the spin chain.

quant-ph↗
arXiv2006

Potential super-hard Osmium di-nitride with fluorite structure: First-principles calculations

We have performed systematic first-principles calculations on di-carbide, -nitride, -oxide and -boride of platinum and osmium with the fluorite structure. It is found that only PtN$_{2}$, OsN$_{2}$ and OsO$_{2}$ are mechanically stable. In particular OsN$_{2}$ has the highest bulk modulus of 360.7 GPa. Both the band structure and density of states show that the new phase of OsN$_{2}$ is metallic. The high bulk modulus is owing to the strong covalent bonding between Os 5\textit{d} and N 2\textit{p} states and the dense packed fluorite structure.

cond-mat.mtrl-sci↗
arXiv2006

On A Proper Definition of Spin Current

The conventional definition of spin current is incomplete and unphysical in describing spin transport in systems with spin-orbit coupling. A proper and measurable spin current is established in this study, which fits well into the standard framework of near-equilibrium transport theory and has the desirable property to vanish in insulators with localized orbitals. Experimental implications of our theory are discussed.

cond-mat.mes-hall↗
arXiv2005

On Asymptotic Variational Wave Equations

We investigate the equation $(u_t + (f(u))_x)_x = f''(u) (u_x)^2/2$ where $f(u)$ is a given smooth function. Typically $f(u)= u^2/2$ or $u^3/3$. This equation models unidirectional and weakly nonlinear waves for the variational wave equation $u_{tt} - c(u) (c(u)u_x)_x =0$ which models some liquid crystals with a natural sinusoidal $c$. The equation itself is also the Euler-Lagrange equation of a variational problem. Two natural classes of solutions can be associated with this equation. A conservative solution will preserve its energy in time, while a dissipative weak solution loses energy at the time when singularities appear. Conservative solutions are globally defined, forward and backward in time, and preserve interesting geometric features, such as the Hamiltonian structure. On the other hand, dissipative solutions appear to be more natural from the physical point of view. We establish the well-posedness of the Cauchy problem within the class of conservative solutions, for initial data having finite energy and assuming that the flux function $f$ has Lipschitz continuous second-order derivative. In the case where $f$ is convex, the Cauchy problem is well-posed also within the class of dissipative solutions. However, when $f$ is not convex, we show that the dissipative solutions do not depend continuously on the initial data.

math.AP↗
arXiv2004

Charge-Hall effect driven by spin force: reciprocal of the spin-Hall effect

A new kind of charge-Hall effect is shown. Unlike in the usual Hall effect, the driving force in the longitudinal direction is a spin force, which may originate from the gradient of a Zeeman field or a spin-dependent chemical potential. The transverse force is provided by a Berry curvature in a mixed position-momentum space. We can establish an Onsager relation between this effect and the spin-Hall effect provided the spin current in the latter is modified by a torque dipole contribution. This remarkable relation leads to new ways for experimental detection of spin accumulation predicted by the spin Hall effect.

cond-mat.mes-hall↗
arXiv2003

Spin Current Through a Magnetic-Oscillating Quantum Dot

Non-equilibrium spin transport through an interacting quantum dot is analyzed. The coherent spin oscillations in the dot provide a generating source for spin current. In the interacting regime, the Kondo effect is influenced in a significant way by the presence of the precessing magnetic field. In particular, when the precession frequency is tuned to resonance between spin up and spin down states of the dot, Kondo singularity for each spin splits into a superposition of two resonance peaks. The Kondo-type cotunneling contribution is manifested by a large enhancement of the pumped spin current in the strong coupling, low temperature regime.

cond-mat.mes-hall↗
arXiv2002

Quantum Entanglement of Excitons in Coupled Quantum Dots

Optically-controlled exciton dynamics in coupled quantum dots is studied. We show that the maximally entangled Bell states and Greenberger-Horne-Zeilinger (GHZ) states can be robustly generated by manipulating the system parameters to be at the avoided crossings in the eigenenergy spectrum. The analysis of population transfer is systematically carried out using a dressed-state picture. In addition to the quantum dot configuration that have been discussed by Quiroga and Johnson [Phys. Rev. Lett. \QTR{bf}{83}, 2270 (1999)], we show that the GHZ states also may be produced in a ray of three quantum dots with a shorter generation time.

cond-mat.mes-hall↗
arXiv2002

Quantum control of two interacting electrons in a coupled quantum dot

Quantum-state engineering, i.e., active manipulation over the coherent dynamics of suitable quantum-mechanical systems, has become a fascinating prospect of modern physics. Here we discuss the dynamics of two interacting electrons in a coupled quantum dot driven by external electric field. We show the two quantum dots can be used to prepare maximally entangled Bell state by varying the strength and duration of an oscillatory electric field. Different from suggestion given by Loss \QTR{it}{et al}.[Phys. Rev. A, \QTR{bf}{57} (1998) 120], the present entanglement involves the spatial degree of freedom for the two electrons. We also find that the coherent tunneling suppression discussed by Grossmann \QTR{it}{et al}.[Phys. Rev. Lett., \QTR{bf}{67} (1991) 516] persists in the two-particle case, i.e., two electrons initially localized in one dot can remain dynamically localized, although the strong Coulomb repulsion prevents them behaving so. Surprisingly, the interaction enhances the degree of localization to a larger extent compared to non-interacting case. We can call this phenomenon Coulomb-enhanced dynamical localization.

cond-mat.mes-hall↗
arXiv2002

Dynamics of a two-component atomic Bose-Einstein condensate

The dynamical population oscillations between two internal states of a Bose-Einstein condensate are investigated within the rotating wave approximation. Analytical expressions for the population imbalance in the number states and coherent states have been derived, which predict different revival periods. Thus the true quantum state of the condensates may be unambiguously determined by detecting the atom intensity evolution for one internal state.

cond-mat.mes-hall↗
arXiv2002

Coulomb-enhanced dynamic localization and Bell state generation in coupled quantum dots

We investigate the dynamics of two interacting electrons in coupled quantum dots driven by an AC field. We find that the two electrons can be trapped in one of the dots by the AC field, in spite of the strong Coulomb repulsion. In particular, we find that the interaction may enhance the localization effect. We also demonstrate the field excitation procedure to generate the maximally entangled Bell states. The generation time is determined by both analytic and numerical solutions of the time dependent Schrodinger equation.

cond-mat.mes-hall↗
arXiv2002

Dynamics of spin-2 Bose condensate driven by external magnetic fields

Dynamic response of the F=2 spinor Bose-Einstein condensate (BEC) under the influence of external magnetic fields is studied. A general formula is given for the oscillation period to describe population transfer from the initial polar state to other spin states. We show that when the frequency and the reduced amplitude of the longitudinal magnetic field are related in a specific manner, the population of the initial spin-0 state will be dynamically localized during time evolution. The effects of external noise and nonlinear spin exchange interaction on the dynamics of the spinor BEC are studied. We show that while the external noise may eventually destroy the Rabi oscillations and dynamic spin localization, these coherent phenomena are robust against the nonlinear atomic interaction.

cond-mat.mes-hall↗