arXiv subjects

Ping Zhang

Publications and source records attributed to Ping Zhang.

At least 487 records · Page 27Linked to original sources

arXiv2015

First-Principles Calculation of Principal Hugoniot and K-Shell X-ray Absorption Spectra for Warm Dense KCl

Principal Hugoniot and K-shell X-ray absorption spectra of warm dense KCl are calculated using the first-principles molecular dynamics method. Evolution of electronic structures as well as the influence of the approximate description of ionization on pressure (caused by the underestimation of the energy gap between conduction bands and valence bands) in the first-principles method are illustrated by the calculation. Pressure ionization and thermal smearing are shown as the major factors to prevent the deviation of pressure from global accumulation along the Hugoniot. In addition, cancellation between electronic kinetic pressure and virial pressure further reduces the deviation. The calculation of X-ray absorption spectra shows that the band gap of KCl persists after the pressure ionization of the $3p$ electrons of Cl and K taking place at lower energy, which provides a detailed understanding to the evolution of electronic structures of warm dense matter.

physics.plasm-ph↗

arXiv2015

Link between K-absorption edges and thermodynamic properties of warm-dense plasmas established by improved first-principles method

A precise calculation that translates shifts of X-ray K-absorption edges to variations of thermodynamic properties allows quantitative characterization of interior thermodynamic properties of warm dense plasmas by X-ray absorption techniques, which provides essential information for inertial confinement fusion and other astrophysical applications. We show that this interpretation can be achieved through an improved first-principles method. Our calculation shows that the shift of K-edges exhibits selective sensitivity to thermal parameters and thus would be a suitable temperature index to warm dense plasmas. We also show with a simple model that the shift of K-edges can be used to detect inhomogeneity inside warm dense plasmas when combined with other experimental tools.

physics.plasm-ph↗

arXiv2015

Power vs. Spectrum 2-D Sensing in Energy Harvesting Cognitive Radio Networks

Energy harvester based cognitive radio is a promising solution to address the shortage of both spectrum and energy. Since the spectrum access and power consumption patterns are interdependent, and the power value harvested from certain environmental sources are spatially correlated, the new power dimension could provide additional information to enhance the spectrum sensing accuracy. In this paper, the Markovian behavior of the primary users is considered, based on which we adopt a hidden input Markov model to specify the primary vs. secondary dynamics in the system. Accordingly, we propose a 2-D spectrum and power (harvested) sensing scheme to improve the primary user detection performance, which is also capable of estimating the primary transmit power level. Theoretical and simulated results demonstrate the effectiveness of the proposed scheme, in term of the performance gain achieved by considering the new power dimension. To the best of our knowledge, this is the first work to jointly consider the spectrum and power dimensions for the cognitive primary user detection problem.

cs.IT↗

arXiv2015

Consistence beats causality in recommender systems

The explosive growth of information challenges people's capability in finding out items fitting to their own interests. Recommender systems provide an efficient solution by automatically push possibly relevant items to users according to their past preferences. Recommendation algorithms usually embody the causality from what having been collected to what should be recommended. In this article, we argue that in many cases, a user's interests are stable, and thus the previous and future preferences are highly consistent. The temporal order of collections then does not necessarily imply a causality relationship. We further propose a consistence-based algorithm that outperforms the state-of-the-art recommendation algorithms in disparate real data sets, including \textit{Netflix}, \textit{MovieLens}, \textit{Amazon} and \textit{Rate Your Music}.

cs.IR↗

arXiv2014

Statistical problem of ideal gas in general 2-dimensional regions

In this paper, based on the conformal mapping method and the perturbation theory, we develop a method to solve the statistical problem within general 2-dimensional regions. We consider some examples and the numerical results and fitting results are given. We also give the thermodynamic quantities of the general 2-dimensional regions, and compare the thermodynamic quantities of the different regions.

physics.gen-ph↗

arXiv2014

Quantum Unfolding: A program for unfolding electronic energy bands of materials

We present Quantum Unfolding, a Fortran90 program for unfolding first-principles electronic energy bands. It unfolds energy bands accurately by handling the Fourier components of Bloch wavefunctions, which are reconstructed from Wannier functions from Wannier90. Due to the wide application of Wannier90 package and the possibility of focusing only on the most important energy bands, the present code works very conveniently.

cond-mat.mtrl-sci↗

arXiv2014

Global smooth axisymmetric solutions of 3-D Inhomogenenous incompressible Navier-Stokes system

In this paper, we investigate the global regularity to 3-D inhomogeneous incompressible Navier-Stokes system with axisymmetric initial data which does not have swirl component for the initial velocity. We first prove that the $L^\infty$ norm to the quotient of the inhomogeneity by $r,$ namely $a/r\eqdefa\bigl(1/\r-1\bigr)\bigl/r,$ controls the regularity of the solutions. Then we prove the global regularity of such solutions provided that the $L^\infty$ norm of $a_0/r$ is sufficiently small. Finally, with additional assumption that the initial velocity belongs to $L^p$ for some $p\in [1,2),$ we prove that the velocity field decays to zero with exactly the same rate as the classical Navier-Stokes system.

math.AP↗

arXiv2014

Long time well-posdness of Prandtl system with small and analytic initial data

In this paper, we investigate the long time existence and uniqueness of small solution to $d,$ for $d=2,3,$ dimensional Prandtl system with small initial data which is analytic in the horizontal variables. In particular, we prove that $d$ dimensional Prandtl system has a unique solution with the life-span of which is greater than $\e^{-\f43}$ if both the initial data and the value on the boundary of the tangential velocity of the outflow are of size $\e.$ We mention that the tool developed in \cite{Ch04, CGP} to make the analytical type estimates and the special structure of the nonlinear terms to this system play an essential role in the proof of this result.

math.AP↗

arXiv2014

Global well-posedness of 3-D inhomogeneous Navier-Stokes equations with ill-prepared initial data

In this paper, we investigate the global well-posedness of 3-D incompressible inhomogeneous Navier-Stokes equations with ill-prepared large initial data which are slowly varying in one space variable, that is, initial data of the form $\bigl(1+\e^{\be}a_0(x_{\rm h},\e x_3),(\ve^{1-\al} v^{\rm h}_0, \ve^{-\al}v_0^3)(x_{\rm h},\e x_3)\bigr)$ for any $\al\in ]0,1/3[,$ $\be>2\al,$ and $\ve$ being sufficiently small. We remark that initial data of this type do not satisfy the smallness conditions in \cite{c-p-z,HPZ3} no matter how small $\e$ is. In particular, this result greatly improves the global well-posedness result in \cite{PZZ3} with the so-called well-prepared initial data.

math.AP↗

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↗

arXiv2014

Decoherence of a quantum system coupled to an XY spin chain: Role of the initial state of the spin chain

We study the decoherence of a coupled quantum system consisting of a central spin and its correlated environment described by a general $XY$ spin-chain model. We make it clear that the evolution of the coherence factor sensitively depends on the initial states of the environment spin-chain. Specially, the dynamical evolution of the coherence factor of the central spin is numerically and analytically investigated in both weak and strong coupling cases for different initial states including thermal equilibrium state. In both weak and strong coupling regimes, the decay of the coherence factor can be approximated by a Gaussian and in the strong coupling regime the coherence factor oscillate rapidly under a Gaussian envelope. The width of the Gaussian decay (envelope) has been studied in details and we explained the origin of the so-called universal regime.

quant-ph↗

arXiv2014

Critical behavior of the energy gap and its relation with the Berry phase close to the excited state quantum phase transition in the Lipkin model

In our previous work [Phys. Rev. A \textbf{85}, 044102 (2012)], we have studied the Berry phase of the ground state and exited states in the Lipkin model. In this paper, using Hellmann-Feynman theorem, we derive the relation between the energy gap and the Berry phase close to the excited state quantum phase transition (ESQPT) in the Lipkin model. We find that the energy gap is linearly dependent upon the Berry phase close to the ESQPT. As a result, the critical behavior of the energy gap is similar to that of the Berry phase. In addition, we also perform a semiclassical qualitative analysis about the critical behavior of the energy gap.

cond-mat.str-el↗

arXiv2014

Direct observation of quantum confinement of massless Dirac fermions in a topological insulator

Since the discovery of topological insulators (TIs)1,2, the peculiar nature of their chiral surface states has been experimentally demonstrated both in bulk and in film materials with open boundaries3,4. Closed boundary on a TI surface may intrigue more interesting phenomena such as quantum confinement of massless Dirac fermions (DFs), which is analogous to the quantum corral (QC) for massive free electrons on a metal surface5-10. To date, it keeps a highly stringent challenge to realize a true Dirac QC due to the unusual transmitting power of a massless fermion. Through heteroepitaxially growing a Bi bilayer on the Bi2Te3 surface with appropriate coverage, here we demonstrate the realization of a true Dirac QC. Specifically, spectacular maps of quantum interference in equilateral triangle-shaped QCs surrounded by Bi bilayers are directly visualized by using a low-temperature scanning tunneling microscope. The present success is ascribed to a perfect orientation matching between the QC boundary and the stationary-phase scattering of massless DFs. In addition, the quasiparticle lifetime of the confined DFs is also systematically measured and analyzed.

cond-mat.mes-hall↗

arXiv2014

Ferromagnetism in a two-component Bose-Hubbard model with a synthetic spin-orbit coupling

We study the effect of the synthetic spin-orbit coupling in a two-component Bose-Hubbard model in one dimension by employing the density-matrix renormalization group method. A ferromagnetic long-range order emerges in both Mott insulator and superfluid phases resulting from the spontaneous breaking of the $Z_2$ symmetry, when the spin-orbit coupling term becomes comparable to the hopping kinetic energy and the inter-component interaction is smaller than the intra-one as well. This novel effect is expected to be detectable with the present realization of the synthetic spin-orbit coupling in experiments.

cond-mat.str-el↗

arXiv2014

The evolution of magnetic structure driven by a synthetic spin-orbit coupling in two-component Bose-Hubbard model

We study the evolution of magnetic structure driven by a synthetic spin-orbit coupling in a one-dimensional two-component Bose-Hubbard model. In addition to the Mott insulator-superfluid transition, we found in Mott insulator phases a transition from a gapped ferromagnetic phase to a gapless chiral phase by increasing the strength of spin-orbit coupling. Further increasing the spin-orbit coupling drives a transition from the gapless chiral phase to a gapped antiferromagnetic phase. These magnetic structures persist in superfluid phases. In particular, in the chiral Mott insulator and chiral superfluid phases, incommensurability is observed in characteristic correlation functions. These unconventional Mott insulator phase and superfluid phase demonstrate the novel effects arising from the competition between the kinetic energy and the spin-orbit coupling.

cond-mat.str-el↗

arXiv2014

Remarks on the global solutions of 3-D Navier-Stokes system with one slow variable

By applying Wiegner' method in \cite{Wiegner}, we first prove the large time decay estimate for the global solutions of a 2.5 dimensional Navier-Stokes system, which is a sort of singular perturbed 2-D Navier-Stokes system in three space dimension. As an application of this decay estimate, we give a simplified proof for the global wellposedness result in \cite{cg3} for 3-D Navier-Stokes system with one slow variable. Let us also mention that compared with the assumptions for the initial data in \cite{cg3}, here the assumptions in Theorem \ref{slowvarsimplifie} are weaker.

math.AP↗

arXiv2014

Electrical and optical properties of fluid iron from compressed to expanded regime

Using quantum molecular dynamics simulations, we show that the electrical and optical properties of fluid iron change drastically from compressed to expanded regime. The simulation results reproduce the main trends of the electrical resistivity along isochores and are found to be in good agreement with experimental data. The transition of expanded fluid iron into a nonmetallic state takes place close to the density at which the constant volume derivative of the electrical resistivity on internal energy becomes negative. The study of the optical conductivity, absorption coefficient, and Rosseland mean opacity shows that, quantum molecular dynamics combined with the Kubo-Greenwood formulation provides a powerful tool to calculate and benchmark the electrical and optical properties of iron from expanded fluid to warm dense region.

physics.plasm-ph↗

arXiv2014

The different roles of Pu-oxide overlayers in the hydrogenation of Pu-metal: An ab initio molecular dynamics study based on vdW-DFT+U

Based on the van der Waals density functional theory (vdW-DFT)+U scheme, we carry out the ab initio molecular dynamics (AIMD) study of the interaction dynamics for H$_{2}$ impingement against the stoichiometric PuO$_{2}$(111), the reduced PuO$_{2}$(111), and the stoichiometric $α$-Pu$_{2}$O$_{3}$(111) surfaces. The hydrogen molecular physisorption states, which can not be captured by pure DFT+\textit{U} method, are obtained by employing the vdW-DFT+\textit{U} scheme. We show that except for the weak physisorption, PuO$_{2}$(111) surfaces are so difficult of access that almost all of the H$_{2}$ molecules will bounce back to the vacuum when their initial kinetic energies are not sufficient. Although the dissociative adsorption of H$_{2}$ on PuO$_{2}$(111) surfaces is found to be very exothermic, the collision-induced dissociation barriers of H$_{2}$ are calculated to be as high as $3.2$ eV and $2.0$ eV for stoichiometric and reduced PuO$_{2}$ surfaces, respectively. Unlike PuO$_{2}$, our AIMD study directly reveals that the hydrogen molecules can penetrate into $α$-Pu$_{2}$O$_{3}$(111) surface and diffuse easily due to the $25$\ native O vacancies located along the $\langle $111$\rangle $ diagonals of $α$-Pu$_{2}$O$_{3}$ matrix. By examining the temperature effect and the internal vibrational excitations of H$_{2}$, we provide a detailed insight into the interaction dynamics of H$_{2}$ in $α$-Pu$_{2}$O$_{3}$. The optimum pathways for hydrogen penetration and diffusion, the corresponding energy barriers ($1.0$ eV and $0.53$ eV, respectively) and rate constants are systematically calculated. Overall, our study fairly reveals the different interaction mechanisms between H$_{2}$ and Pu-oxide surfaces, which have strong implications to the interpretation of experimental observations.

cond-mat.mtrl-sci↗