Cao Yi-Gang

Numerical Study on the Two-Dimensional Vortex System with Random Pinning

We study numerically the dynamic behavior of the two-dimensional vortex lattice, moving in a random pinning potential under the action of an external driving force. Different densities of randomly distributed pointlike pinning centers and different vortex-vortex interacting strength are considered. Then the elastic and plastic regimes are found to be dependent on the relation between the vortex-vortex and the vortex-pin interacting potentials. The critical driving force increases with increasing density of pinning centers and with decreasing of the vortex-vortex interacting strength. The simulated results are consistent with the recent experiments, and some of our results fall outside the region of validity of collective pinning theory.

Numerical Study on Depinning Dynamics of Fluids Interacting with Disorder

We investigate the depinning of two-dimensional fluids interacting with quenched disorder, based on Langevin simulations. For weak disorder the fluids depin elastically and flow in an ordered state. A power-law scaling fit between velocity and driving force can be obtained for the onset of motion in the elastic regime. This is in good agreement with that of colloid, charge density wave, and superconducting vortex systems. With an increasing strength of the disorder, we find a sharp crossover to plastic depinning, accompanied by a substantial increase in the depinning force. The scaling fit obtained in the elastic regime becomes invalid when plastic flow occurs. In the plastic regime, the fluids flow in channels and the hexatic order decays exponentially with drives.

Radiative Decay in the Technicolor with a Massless Scalar Doublet Model

Applying perturbative QCD, we study the process in the technicolor with a massless scalar doublet model (TCMLSM). There are mainly two mechanisms contributing to the process. One proceeds through the short distance transition and the other through weak annihilation accompanied by a photon emission. We find that, compared with the standard model, the modification of from πp (the physical pions in the TCMLSM) is so small that can be neglected for the allowed mass of πp. The weak-annihilation contribution is found to be about one order larger than that of the electromagnetic penguin diagrams.

Mode-locking behaviour: ac response of a driven disordered vortex lattice

Using random pinning force simulations, we study numerically the alternating current (ac) response of the driven disordered vortex lattice. In the presence of a superimposed ac force, mode-locking steps are observed in the velocity–direct current (dc) force characteristics at high dc forces, where a smectic order occurs. The step width shows damped oscillations with the strength of interaction between vortices, in good agreement with recent experiments.

Depinning dynamics of driven colloids with random pinning: a numerical study

Using Langevin simulations, we have investigated numerically the depinning dynamics of driven two-dimensional colloids subject to the randomly distributed point-like pinning centres. With increasing strength of pinning, we find a crossover from elastic to plastic depinnings, accompanied by an order to disorder transition of state and a substantial increase in the depinning force. In the elastic regime, no peaks are found in the differential curves of the velocity?force dependence (VFD) and the transverse motion is almost none. In addition, the scaling relationship between velocity and force is found to be valid above depinning. However, when one enters the plastic regime, a peak appears in the differential curves of VFD and transverse diffusion occurs above depinning. Furthermore, history dependence is found in the plastic regime.

Alternating Current Response of Two-Dimensional Vortex Lattice with Random Pinning*

Using a model of long-range interactions between vortices, we investigate numerically the alternating current (ac) response of two-dimensional vortex lattice with randomly distributed point-like pinning centers. Mode-locking steps are observed in the simulated current-voltages characteristics, and the number of steps increases with the superimposed ac amplitude and frequency. Our results are in good agreement with recent experiments.

Dynamic Phase Transition of Two-Dimensional Disordered Colloids

Using Langevin simulations, we numerically study the influence of temperature to the dynamics of driven two-dimensional colloids on a disordered substrate. With decreasing temperature, the result shows a dynamic phase transition from the moving liquid to the moving smectic at high driving forces. A peak appears in the dynamically-critical driving force across the transition, accompanied by a clear cross of the curves of velocity–force dependence.

A δ-function-like peak in the specific heat of two-dimensional vortex lattice: Monte Carlo study

A repulsive vortex-vortex interaction model was used to numerically study the melting transition of the two-dimensional vortex system with Monte Carlo method. Then a δ-function-like peak in the specific heat was observed and the internal energy showed a sharp drop at the melting temperature, which indicated that there exists a first-order melting transition at finite temperatures. The Lindemann criterion was also investigated and valid, but different from previous simulation results.

Relaxation of Small Molecules: an ab initio Study

Using an ab initio total energy and force method, we have relaxed several group IV and group V elemental clusters, in detail the arsenic and antimony dimers, silicon, phosphorus, arsenic and antimony tetramers. The obtained bond lengths and cohesive energies are more accurate than other calculating methods, and in excellent agreement with the experimental results.

Stable Structure of the Sb Monolayer on a Strained Ge(111) Substrate

Using an ab initio total energy and force method, we have investigated the stability of different structures of Ge(111):Sb(1 ML) as a function of the lateral lattice constant. We find that the (2×1)-reconstruction of Ge(111):Sb, experimentally found to be stable at the equilibrium lattice constant of Ge, is also the stable structure for slightly dilated Ge films (< 1%), while for larger dilatations the (1×1)-structure becomes stable. For compressed Ge films the ((3)1/2×(3)1/2)T4-structure (found experimentally on Si(111):Sb) becomes competitive and it is stable for the lattice constants compressed by more than 5%. Furthermore, we find that for each structure, the equilibrium lattice constant is different from the bulk Ge crystal. Our results are helpful for the understanding of surfactant mediated island growth on strained films.