International Journal of Modern Physics B

We obtain the numerical solutions for a single electron energy level of a hydrogen impurity confined inside an off-center donor in a spherical quantum dot. The energy spectrum of single electron impurity for off-center donor atom subject to screened Coulomb's attractive potential barrier well is presented. Using B-spline finite element method, we generate a complete set of wavefunctions within the central unit cell, and through the perturbation theory, the whole energy spectrum of an impurity at low temperature can be explored. We find that the energy splitting and different related magnetic quantum number m_l, i.e., (n, l, m_l) varies with quantum dot size. This is due to the fact that quantum confinement effect lifts the degeneracy which comes from the Coulomb interaction. On the other hand, the phenomena of energy spectrum splitting under this model is an analogous case, considering that degeneracy of hydrogen energy levels in free space has been lifted by an external magnetic field (Zeeman effect).

This paper analyzes the security of a recent cryptosystem based on the ergodicity property of chaotic maps. It is shown how to obtain the secret key using a chosen-ciphertext attack. Some other design weaknesses are also shown.

The structure function of the vortex lattice of layered superconductor is calculated to one-loop order. Based on a phenomenological melting criterion concerning the Debye–Waller factor, we calculate the melting line of the vortex lattice, and compare our results to Monte Carlo simulation and experiment. We find that our results are in good agreement with the Monte Carlo results. Moreover, our analytic calculation of the melting line of BSCCO fits the experiment reasonably.

Magnetization measurements were carried out on crystalline and grain-oriented YBa₂Cu₃O_7-δ samples. We have measured the creep of magnetic flux lines along the a-axis (perpendicular to the c-axis) of the crystalline sample and perpendicular to the c-axis of a c-axis-grain-oriented polycrystalline sample. The measurements were done at 4.2 K and for magnetic fields in the range 1 kOe < H < 10 kOe. The magnetization M was found to decrease linearly with ln t in both samples for all the applied magnetic fields. With the initial increase of the applied magnetic field, the vortex pinning potential, U_p, was found to decrease sharply in the grain-oriented sample, followed by a slow (almost linear) rate of decrease. In the crystalline sample, the rate of decrease of U_p was found to be slow and linear for all applied magnetic fields.

We approach the case of two coupled oscillators where the first one may correspond to a photonic field, while the second one is damped and driven. We model the oscillator's damping via a bath and consider the relevant master equation. We use perturbation theory to handle it. We then path integrate over the effective Hamiltonian of the two oscillators and derive the path integrated density matrix. We suppose that initially both of the oscillators are in coherent states and study the quadrature squeezing effect of the second oscillator.

We report a theoretical study on the bias dependence of the current-induced torque in the noncollinear ferromagnet/barrier/ferromagnet (FM) tunnel junction. By using the Keldysh–Green function technique, we derived an analytic expression of the spin transfer torque (STT) based on a continuum model. It was found that for finite bias, the torque induced by current increases monotonically with bias and is nearly antisymmetric with regard to the current direction, which agrees with the experimental observation on the STT in the spin valve system.

We consider a large population of globally coupled subthreshold Morris-Lecar neurons. By varying the noise intensity D, we numerically investigate stochastic spiking coherence (i.e., collective coherence between noise-induced neural spikings). As D passes a lower threshold, a transition from an incoherent to a coherent state occurs because of a constructive role of noise to stimulate coherence between noise-induced spikings. However, when passing a higher threshold of D, another transition from a coherent to an incoherent state takes place due to a destructive role of noise to spoil the spiking coherence. Such an incoherence-coherence-incoherence transition is well-described in terms of the order parameter which is just the mean square deviation of the global potential. In the coherent regime, we also characterize the degree of stochastic spiking coherence by using a coherence measure which reflects the degree of "resemblance" of the global potential to the local potential. Thus, stochastic spiking coherence with large coherence measure is found to occur over a large range of intermediate noise intensity.

We have investigated the behavior of vortices in a polycrystalline YBa₂Cu₃O_7-δ disk sample using rotational magnetization-vector (RMV) measurements. The measurements were conducted in the field-cooled (FC) and the zero-field cooled (ZFC) states for several values of applied magnetic field parallel to the sample surface at 4.2 K and for various angles of rotation (θ_rot) up to 360°. The total magnetic flux density, B, in the sample at any rotational angle was considered to be composed of two types of vortices: weakly pinned vortices that rotate frictionally in the opposite direction of sample rotation and strongly pinned vortices that rotate rigidly in the same direction of sample rotation. From the rate of change of these two types of vortices and the rate of change of B, we were able to develop a tool to qualitatively describe the average strength of pinning centers in high-temperature superconductors.

The electronic band structure, density of states, structural phase transition, superconducting transition and Fermi surface cross section of titanium (Ti) under normal and high pressures are reported. The high pressure band structure exhibits significant deviations from the normal pressure band structure due to s → d transition. On the basis of band structure and total energy results obtained using tight-binding linear muffin-tin orbital method (TB LMTO), we predict a phase transformation sequence of α(hcp) → ω (hexagonal) → γ(distorted hcp) → β(bcc) in titanium under pressure. From our analysis, we predict a δ (distorted bcc) phase which is not stable at any high pressures. At ambient pressure, the superconducting transition occurs at 0.354 K. When the pressure is increased, it is predicted that, T_c increases at a rate of 3.123 K/Mbar in hcp–Ti. On further increase of pressure, T_c begins to decrease at a rate of 1.464 K/Mbar. The highest value of T_c(P) estimated is 5.043 K for hcp–Ti, 4.538 K for ω–Ti and 4.85 K for bcc–Ti. From this, it is inferred that the maximum value of T_c(P) is rather insensitive to the crystal structure of Ti. The nonlinearities in T_c(P) is explained by considering the destruction and creation of new parts of Fermi surface at high pressure. At normal pressure, the hardness of Ti is in the following order: ω-Ti > hcp-Ti > bcc-Ti > γ-Ti.

In this paper, the optimal velocity (OV) model is extended to take account of the effect that the driver's memory has on the car-following behavior. The stability condition of the proposed model is obtained by using linear stability theory. The modified Korteweg-de Vries (mKdV) equation is obtained and solved. Traffic flows in the headway-sensitivity space are classified into three types as stable, metastable and unstable. Both analytical and simulation results show that introduction of driver's memory in the acceleration can improve the stability of traffic flow. It is also found that the stable region will be enlarged with the increase of the past information considered. Finally, numerical tests show that properly considering driver's memory can improve the stability of traffic flow.

We study a classical fluid of nonspherical molecules. The components of the fluid are the ellipsoidal molecules interacting through the Gay–Berne potential model. A method is described, which allows the Percus–Yevick (PY) and hypernetted-chain (HNC) integral equation theories to be solved numerically for this fluid. Explicit results are given and comparisons are made with recent Monte Carlo (MC) simulations. It is found that, at lower cutoff l_max, the HNC and the PY closures give significantly different results. The HNC and PY (approximately) theories, at higher cutoff l_max, are superior in predicting the existence of the phase transition in a qualitative agreement with computer simulation.