D. L. Lin

Critical behavior in magnetic superlattices

Abstract Based on the effective field theory with correlations, the critical behavior of a magnetic superlattice consisting of two different ferromagnets is examined. A simple cubic Ising model with nearest-neighbor coupling is assumed. It is found that there exists a critical value of the interface exchange coupling above which the interface magnetism appears. The critical temperature of the superlattice is studied as a function of the thickness of the constituents in a unit cell and of the exchange coupling energies in each material. A phase diagram is also given.

Equation of motion for nonequilibrium Green functions

The equation of motion for nonequilibrium Green functions is derived within the framework of the Schwinger and Keldysh formalism of perturbation expansion. For nonequilibrium distribution Green functions, the equation of motion derived from quantum mechanics contains undefined singularities, whose explicit form depends on the specific initial or boundary condition. In the present work, the exact expression of singular terms is found in the equation of motion from the time-looped perturbation theory in which the adiabatic initial condition is implied. Unlike the usual Dyson perturbation formalism or the well known Kadanoff-Baym equation of motion, our resulting equation can be adopted directly for calculations without the graphical analysis, which depends on the specific form of the Hamiltonian. On the basis of this equation of motion, the procedure of a nonperturbative solution is outlined and potential applications are briefly discussed.

Internal explosion in laser ablation of superconducting targets

The temperature profile inside a superconducting target in laser ablation is calculated for laser pulses of various shapes. The calculation is based on the equation of heat conduction. All parameters characterizing the target material are assumed to be temperature dependent and are determined empirically by extrapolating experimental data to the melting point. The receding velocity of the vapor-solid interface is determined by the dynamical balance of energy. The calculation shows that, in general, there exist subsurface overheating spots at different instants as long as the laser pulse intensity is strong enough. The dependence of their occurrence on the pulse shape is analyzed, and conditions to avoid them without jeopardizing the deposition process are discussed.

Interface phonons in semiconductor double heterostructures

Abstract Within the framework of the continuum model, the equation of motion for the polarization vector in a semiconductor double heterostructure is solved exactly for the interface phonon modes. Both the eigenvectors and dispersion relations are obtained analytically. It is shown that the slab modes observed in right-angle Raman scattering in a GaAs quantum can be understood in terms of the interface modes found in this paper.

Decay rate and resonance fluorescence spectrum of a molecule near a composite material surface

Abstract The spontaneous decay rate and resonance fl calculated in the effective medium approximation. Two substrates are considered: One is a semiconductor containing randomly distributed small dielectric particles and the other a dielectric with metallic particles. Results are analyzed in terms of energy transfer from the molecule to the substrate as functions of the volume fraction of impurity particles.

The spectrum of third-order nonlinear susceptibility of trans-polyacetylene

Abstract : By considering the finite lifetime of excitation and the band structure of a finite polymer chain, the two-peak structure in the spectrum of the third-order nonlinear susceptibility of trans-polyacetylene is calculated, with results in agreement with experiments.

Squeezing of cavity fields in cascade multiphoton processes

For the system of a multilevel atom interacting with a single-mode cavity field, squeezing of the field is studied numerically. It is found that the squeezing effect becomes stronger when the atomic level number increases, but tends to saturate when the number exceeds 20.

Polaronic effects on donor states in III-V and II-VI quantum wells under electric fields

Polaronic effects on the ground state of a donor in and quantum wells under the influence of an external electric field are investigated for the impurity atom doped at various positions. The confined electron interacts with the interface as well as confined phonon modes that exist in such structures. It is found that such effects in II-VI compounds are in general much more significant than in III-V compounds, reflecting the stronger electron-phonon coupling in more ionic II-VI materials. The ground state energy of the impurity is calculated by means of the Lee, Low and Pines transformation. Contributions from confined and interface phonon modes are considered separately and results calculated for various well widths, field strengths and different impurity positions are presented and discussed.

Effects of electron-phonon interaction on impurity states in ZnSeZnCdSe quantum wells under electric fields

Abstract Effects of electron-phonon interaction on the ground state of a donor in ZnSe ZnCdSe quantum wells under external electric fields are studied theoretically for the impurity atom doped at various positions. Both the interface and confined phonon modes that exist in such structures are included to interact with the electron. The ground state energy is calculated by means of the Lee, Low and Pines transformation. The polaronic effects are generally much more significant than those in III–V compounds we have found in the past due to stronger electron-phonon interactions. Contributions from different phonon modes are computed separately and results obtained for various cases are presented and discussed.

Surface-induced optical bistability in coupled exciton-phonon systems

Abstract Nonlinear optical responses of exciton-phonon coupling systems near a reflecting surface are investigated. For a polydiacetylene chain lying parallel to the surface, optical bistability induced by the surface is discovered. It is also found that the threshold and contrast of the bistability can be controlled by adjusting the distance of the system from the surface, and that vacuum fluctuations are reduced whenever the bistability occurs. The nature of the bistability and its origin are discussed.