arXiv subjects

Xin Zhou

Publications and source records attributed to Xin Zhou.

449 records · Page 25Linked to original sources

arXiv2008

Construction and Refinement of Coarse-Grained Models

A general scheme, which includes constructions of coarse-grained (CG) models, weighted ensemble dynamics (WED) simulations and cluster analyses (CA) of stable states, is presented to detect dynamical and thermodynamical properties in complex systems. In the scheme, CG models are efficiently and accurately optimized based on a directed distance from original to CG systems, which is estimated from ensemble means of lots of independent observable in two systems. Furthermore, WED independently generates multiple short molecular dynamics trajectories in original systems. The initial conformations of the trajectories are constructed from equilibrium conformations in CG models, and the weights of the trajectories can be estimated from the trajectories themselves in generating complete equilibrium samples in the original systems. CA calculates the directed distances among the trajectories and groups their initial conformations into some clusters, which correspond to stable states in the original systems, so that transition dynamics can be detected without requiring a priori knowledge of the states.

cond-mat.soft↗

arXiv2008

Fast Adaptive Flat-histogram Ensemble for Calculating Density of States and Enhanced Sampling in Large Systems

We presented an efficient algorithm, fast adaptive flat-histogram ensemble (FAFE), to estimate the density of states (DOS) and to enhance sampling in large systems. FAFE calculates the means of an arbitrary extensive variable $U$ in generalized ensembles to form points on the curve $β_{s}(U) \equiv \frac{\partial S(U)}{\partial U}$, the derivative of the logarithmic DOS. Unlike the popular Wang-Landau-like (WLL) methods, FAFE satisfies the detailed-balance condition through out the simulation and automatically generates non-uniform $(β_{i}, U_{i})$ data points to follow the real change rate of $β_{s}(U)$ in different $U$ regions and in different systems. Combined with a $U-$compression transformation, FAFE reduces the required simulation steps from $O(N^{3/2})$ in WLL to $O(N^{1/2})$, where $N$ is the system size. We demonstrate the efficiency of FAFE in Lennard-Jones liquids with several $N$ values. More importantly, we show its abilities in finding and identifying different macroscopic states including meta-stable states in phase co-existing regions.

cond-mat.stat-mech↗

arXiv2008

Discovery of molecular shells associated with supernova remnants. (II) Kesteven 75

The young composite supernova remnant (SNR) Kesteven 75, with a pulsar wind nebula at its center, has an unusual morphology with a bright southern half-shell structure in multiwavelengths. The distance to Kes 75 has long been uncertain. Aiming to address these issues, we have made millimeter spectroscopic observations of the molecular gas toward the remnant. The V_{LSR}~83--96 km/s molecular clouds (MCs) are found to overlap a large north-western region of the remnant and are suggested to be located in front of the SNR along the line of sight. Also in the remnant area, the V_{LSR}= 45--58 km/s MC shows a blue-shifted broadening in the 12CO (J=1-0) line profile and a perturbed position-velocity structure near the edge of the remnant, with the intensity centroid sitting in the northern area of the remnant. In particular, a cavity surrounded by a molecular shell is unveiled in the intensity map in the broadened blue wing (45--51 km/s), and the southern molecular shell follows the bright partial SNR shell seen in X-rays, mid-infrared, and radio continuum. These observational features provide effective evidences for the association of Kes 75 with the adjacent 54 km/s MC. This association leads to a determination of the kinematic distance at ~10.6 kpc to the remnant, which agrees with a location at the far side of the Sagittarius arm. The morphological coincidence of the shell seen in multiwavelengths is consistent with a scenario in which the SNR shock hits a pre-existing dense shell. This dense molecular shell is suggested to likely represent the debris of the cooled, clumpy shell of the progenitor's wind bubble proximately behind the 54 km/s cloud. The discovery of the association with MC provides a possible explanation for the gamma-ray excess of the remnant.

astro-ph↗

arXiv2008

Janossy densities for Unitary ensembles at the spectral edge

For a broad class of unitary ensembles of random matrices we demonstrate the universal nature of the Janossy densities of eigenvalues near the spectral edge, providing a different formulation of the probability distributions of the limiting second, third, etc. largest eigenvalues of the ensembles in question. The approach is based on a representation of the Janossy densities in terms of a system of orthogonal polynomials, plus the steepest descent method of Deift and Zhou for the asymptotic analysis of the associated Riemann-Hilbert problem.

math.PR↗

arXiv2006

Hyperdynamics for entropic systems: time-space compression and pair correlation function approximation

We develop a generalized hyperdynamics method, which is able to simulate slow dynamics in atomistic general (both energy and entropy-dominated) systems. We show that a few functionals of the pair correlation function, involving two-body entropy, form a low-dimensional collective space, which is a good approximation that is able to distinguish stable and transitional conformations. A bias potential, which raises the energy in stable regions, is constructed on the fly. We examine the slowly nucleation processes of a Lennard-Jones gas and show that our new method can generate correct long time dynamics without a prior knowledge.

cond-mat.stat-mech↗

arXiv2004

Enhancement of solid-state proton NMR via SPINOE with laser-polarized xenon

We have first successfully transferred the 129Xe polarization of natural isotopic composition to the proton of solid-state 1HCl via Spin Polarization-Induced Nuclear Overhauser Effect (SPINOE), by mixing the hyperpolarized 129Xe gas and the 1HCl gas and then cooling them to their condensated state in a flow system. The solid-state enhanced factor of the NMR signal of 6 for 1H was observed, and the equation of solid-state polarization enhancement via cross relaxation has also been theoretically deduced. Using this equation, the theoretically calculated enhancement is in agreement with the measured value within error. Also, this technique is maybe useful to establish a solid state NMR quantum computer.

cond-mat.soft↗

arXiv2003

Experiment and Dynamic Simulations of Radiation Damping of Laser-polarized liquid 129Xe at low magnetic field in a flow system

Radiation damping is generally observed when the sample with high spin concentration and high gyro-magnetic ratio is placed in a high magnetic field. However, we firstly observed liquid state 129Xe radiation damping using laser-enhanced nuclear polarization at low magnetic field in a flow system in which the polarization enhancement factor for the liquid state 129Xe was estimated to be 5000, and furthermore theoretically simulated the envelopes of the 129Xe FID and spectral lineshape in the presence of both relaxation and radiation damping with different pulse flip angles and ratios of T2*/Trd. The radiation damping time constant Trd of 5 ms was derived based on the simulations. The reasons of depolarization and the further possible improvements were also discussed.

cond-mat.soft↗

arXiv2003

Orientational orders in binary mixtures of hard HGO molecules

studied liquid crystal phases of binary mixtures of non-spherical molecules. The components of the mixtures are two kinds of hard Gaussian overlap (HGO) molecules, one kind of molecules with a small molecular-elongation parameter (small HGO molecules) cannot form stable liquid crystal phase in bulk, and other with a large elongation parameter (large HGO molecules) can form liquid crystal phase easily. In the mixtures, like the large HGO molecules, the small HGO molecules can also form an orientation-ordered phase, which is because that the large HGO molecules can form complex confining surfaces to induce the alignment of the small molecules and generate an isotropic-anisotropic phase transition in the whole binary mixtures. We also study the transition on different mixtures composed of small and large HGO molecules with different elongations and different concentrations of the large molecules. The obtained result implies that small anisotropic molecules might show liquid crystal behavior in confinement.

cond-mat.soft↗

arXiv2003

Orientational orders of small anisotropic molecules confined in slit pores

We have studied phase behavior of hard gaussian overlap molecules with small anisotropic parameter confined in two plane parallel structureless hard walls. Our investigation based on standard constant-NPT Monte Carlo molecular simulation led us to some interesting findings. For small anisotropic molecules the nematic phase is instable in bulk, while, if the distance between the walls is small enough, an orientation-ordered phase can form. This result indicates that the required molecular elongation forming liquid-crystal phases is smaller in confinement than that in bulk. Considering the value of the elongation of molecules, the computed result inplies that small molecule liquid crystals may exist in confinement.

cond-mat.soft↗

arXiv2003

Heat Conduction and Long-Range Spatial Correlation in 1D Models

Heat conduction in one-dimensional (1D) systems is studied based on an analytical S-matrix method, which is developed in the mesoscopic electronic transport theory and molecular dynamic (MD) simulations. It is found that heat conduction in these systems is related to spatial correlation of particle motions. Randomizations of scatterers is found to break the correlation, hence results in normal thermal conduction. Our MD simulations are in agreement with the theoretical expectations. The results are useful for an understanding of the relation between heat conduction and dynamic instablities or other random behavior in 1D systems.

cond-mat.stat-mech↗

arXiv2002

Equilibrium and Kinetics: Water Confined in Carbon Nanotube as 1D Lattice Gas

A simple 1D lattice gas model is presented, which very well describes the equilibrium and kinetic behaviors of water confined in a thin carbon nanotube found in an atomistic molecular dynamics(MD) simulation {[} Nature {\bf 414}, 188 (2001) {]}. The model parameters are corresponding to various physical interactions and can be calculated or estimated in statistic mechanics. The roles of every interaction in the water filling, emptying and transporting processes are clearly understood. Our results indicate that the physical picture of the single-file kinetics is very simple.

cond-mat.stat-mech↗

arXiv2001

Optimal tail estimates for directed last passage site percolation with geometric random variables

In this paper, we obtain optimal uniform lower tail estimates for the probability distribution of the properly scaled length of the longest up/right path of the last passage site percolation model considered by Johansson in [12]. The estimates are used to prove a lower tail moderate deviation result for the model. The estimates also imply the convergence of moments, and also provide a verification of the universal scaling law relating the longitudinal and the transversal fluctuations of the model.

math.PR↗

arXiv2001

Can Electric Field Induced Energy Gaps In Metallic Carbon Nanotubes?

The low-energy electronic structure of metallic single-walled carbon nanotube (SWNT) in an external electric field perpendicular to the tube axis is investigated. Based on tight-binding approximation, a field-induced energy gap is found in all (n, n) SWNTs, and the gap shows strong dependence on the electric field and the size of the tubes. We numerically find a universal scaling that the gap is a function of the electric field and the radius of SWNTs, and the results are testified by the second-order perturbation theory in weak field limit. Our calculation shows the field required to induce a 0.1 ${\rm eV}$ gap in metallic SWNTs can be easily reached under the current experimental conditions. It indicates a kind of possibility to apply nanotubes to electric signal-controlled nanoscale switching devices.

cond-mat.mes-hall↗

arXiv2001

Large deformation of spherical vesicle studied by perturbation theory and Surface evolver

With tangent angle perturbation approach the axial symmetry deformation of a spherical vesicle in large under the pressure changes is studied by the elasticity theory of Helfrich spontaneous curvature model.Three main results in axial symmetry shape: biconcave shape, peanut shape, and one type of myelin are obtained. These axial symmetry morphology deformations are in agreement with those observed in lipsome experiments by dark-field light microscopy [Hotani, J. Mol. Biol. 178, (1984) 113] and in the red blood cell with two thin filaments (myelin) observed in living state (see, Bessis, Living Blood Cells and Their Ultrastructure, Springer-Verlag, 1973). Furthermore, the biconcave shape and peanut shape can be simulated with the help of a powerful software, Surface Evolver [Brakke, Exp. Math. 1, 141 (1992) 141], in which the spontaneous curvature can be easy taken into account.

physics.bio-ph↗

arXiv2001

The structure relaxation of carbon nanotube

A simple macroscopic continuum elasticity theory (CET) is used to calculate the structure relaxation of single-wall carbon nanotube (SWNT), an analytic formula is obtained. We also expand an atomic scale three-parameter empirical model [ T. Lenosky {\emph et al.} Nature 355, 333(1992)] in order to correctly describe the bond-length change effects. The structure relaxation of SWNT expected by the model is good in agreement with our CET results, and very well consistent with the previous calculation from a first principles local density function approximation. Using the expanded Lenosky model, we calculate the strain energy of bending tube. The obtained results are good in agreement with the previous theoretical expectation. It shows the model may be a good simple replacement of some more sophisticated methods on determining carbon networks deformations.

cond-mat.mtrl-sci↗

arXiv2001

The Polarizability and Electric Field-Induced Energy Gaps In Carbon Nanotubes

A simple method to calculate the static electric polarization of single-walled carbon nanotube (SWNT) is obtained within the second-order perturbation approximation. The results are in agreement with the previous calculation within the random-phase approximation. We also find that the low energy electronic band structure of SWNT can be affected by an external electric field perpendicular to the axis of the tube. The field-induced energy gap shows strong dependence on the electric field and the size of the tubes, and a metal-insulator transition is predicted for all $(n, n)$ tubes. Universal scaling is found for the gap as function of the electric field and the radius of SWNTs. The fact that the external field required to induce a 0.1 ${\rm eV}$ gap in SWNTs can be reached under the currently available experimental conditions indicates a possibility of further applying nanotubes to electric signal-controlled nanoscale switching devices.

cond-mat.mes-hall↗

arXiv1992

A steepest descent method for oscillatory Riemann-Hilbert problems

In this announcement we present a general and new approach to analyzing the asymptotics of oscillatory Riemann-Hilbert problems. Such problems arise, in particular, in evaluating the long-time behavior of nonlinear wave equations solvable by the inverse scattering method. We will restrict ourselves here exclusively to the modified Korteweg de Vries (MKdV) equation, $$y_t-6y^2y_x+y_{xxx}=0,\qquad -\infty<x<\infty,\ t\ge0, y(x,t=0)=y_0(x),$$ but it will be clear immediately to the reader with some experience in the field, that the method extends naturally and easily to the general class of wave equations solvable by the inverse scattering method, such as the KdV, nonlinear Schrödinger (NLS), and Boussinesq equations, etc., and also to ``integrable'' ordinary differential equations such as the Painlevé transcendents.

math.AP↗