Nonvacuum pseudoparticles, quantum tunneling and metastability

It is shown that nonvacuum pseudoparticles can account for quantum tunneling and metastability. In particular the saddle-point nature of the pseudoparticles is demonstrated, and the evaluation of path-integrals in their neighbourhood. Finally the relation between instantons and bounces is used to derive a result conjectured by Bogomolny and Fateyev.Comment: Latex, 16 pages, no figure

Quantum Phase Interference for Quantum Tunneling in Spin Systems

The point-particle-like Hamiltonian of a biaxial spin particle with external magnetic field along the hard axis is obtained in terms of the potential field description of spin systems with exact spin-coordinate correspondence. The Zeeman energy term turns out to be an effective gauge potential which leads to a nonintegrable pha se of the Euclidean Feynman propagator. The phase interference between clockwise and anticlockwise under barrier propagations is recognized explicitly as the Aharonov-Bohm effect. An additional phase which is significant for quantum phase interference is discovered with the quantum theory of spin systems besides the known phase obtained with the semiclassical treatment of spin. We also show the energ y dependence of the effect and obtain the tunneling splitting at excited states with the help of periodic instantons.Comment: 19 pages, no figure, to appear in PR

Winding number transitions at finite temperature in the Abelian-Higgs model

Following our earlier investigations we examine the quantum-classical winding number transition in the Abelian-Higgs system. It is demonstrated that the sphaleron transition in this system is of the smooth second order type in the full range of parameter space. Comparison of the action of classical vortices with that of the sphaleron supports our finding.Comment: final version, to appear in J. Phys.

Vacuum induced Berry phases in single-mode Jaynes-Cummings models

Motivated by the work [Phys. Rev. Lett. 89, 220404 (2002)] for detecting the vacuum-induced Berry phases with two-mode Jaynes-Cummings models (JCMs), we show here that, for a parameter-dependent single-mode JCM, certain atom-field states also acquire the photon-number-dependent Berry phases after the parameter slowly changed and eventually returned to its initial value. This geometric effect related to the field quantization still exists, even the filed is kept in its vacuum state. Specifically, a feasible Ramsey interference experiment with cavity quantum electrodynamics (QED) system is designed to detect the vacuum-induced Berry phase.Comment: 10 pages, 4 figures

Enhancement of Quantum Tunneling for Excited States in Ferromagnetic Particles

A formula suitable for a quantitative evaluation of the tunneling effect in a ferromagnetic particle is derived with the help of the instanton method. The tunneling between n-th degenerate states of neighboring wells is dominated by a periodic pseudoparticle configuration. The low-lying level-splitting previously obtained with the LSZ method in field theory in which the tunneling is viewed as the transition of n bosons induced by the usual (vacuum) instanton is recovered. The observation made with our new result is that the tunneling effect increases at excited states. The results should be useful in analyzing results of experimental tests of macroscopic quantum coherence in ferromagnetic particles.Comment: 18 pages, LaTex, 1 figur

Application of Instantons: Quenching of Macroscopic Quantum Coherence and Macroscopic Fermi-Particle Configurations

Starting from the coherent state representation of the evolution operator with the help of the path-integral, we derive a formula for the low-lying levels $E = \epsilon_0 - 2\triangle\epsilon cos (s+\xi)\pi$ of a quantum spin system. The quenching of macroscopic quantum coherence is understood as the vanishing of $cos (s+\xi)\pi$ in disagreement with the suppression of tunneling (i.e. $\triangle\epsilon = 0$) as claimed in the literature. A new configuration called the macroscopic Fermi-particle is suggested by the character of its wave function. The tunneling rate ($(2\triangle\epsilon)/(\pi)$) does not vanish, not for integer spin s nor for a half-integer value of s, and is calculated explicitly (for the position dependent mass) up to the one-loop approximation.Comment: 13 pages, LaTex, no figure

Phase diagram of two-species Bose-Einstein condensates in an optical lattice

The exact macroscopic wave functions of two-species Bose-Einstein condensates in an optical lattice beyond the tight-binding approximation are studied by solving the coupled nonlinear Schrodinger equations. The phase diagram for superfluid and insulator phases of the condensates is determined analytically according to the macroscopic wave functions of the condensates, which are seen to be traveling matter waves.Comment: 13 pages, 2 figure

Aharonov-Casher phase and persistent current in a polyacetylene ring

We investigate a polyacetylene ring in an axially symmetric, static electric field with a modified SSH Hamiltonian of a polyacetylene chain. An effective gauge potential of the single electron Hamiltonian due to spin-field interaction is obtained and it results in a Fr\"{o}hlich's type of superconductivity equivalent to the effect of travelling lattice wave. The total energy as well as the persistent current density are shown to be a periodic function of the flux of the gauge field embraced by the polyacetylene ring.Comment: 12 pages, 5 figure

Electronic States and Magnetism of Mn Impurities and Dimers in Narrow-Gap and Wide-Gap III-V Semiconductors

Electronic states and magnetic properties of single $Mn$ impurity and dimer doped in narrow-gap and wide-gap $III$-$V$ semiconductors have been studied systematically. It has been found that in the ground state for single $Mn$ impurity, $Mn$-$As(N)$ complex is antiferromagnetic (AFM) coupling when $p$-$d$ hybridization $V_{pd}$ is large and both the hole level $E_{v}$ and the impurity level $E_{d}$ are close to the midgap; or very weak ferromagnetic (FM) when $V_{pd}$ is small and both $E_{v}$ and $E_d$ are deep in the valence band. In $Mn$ dimer situation, the $Mn$ spins are AFM coupling for half-filled or full-filled $p$ orbits; on the contrast, the Mn spins are double-exchange-like FM coupling for any $p$-orbits away from half-filling. We propose the strong {\it p-d} hybridized double exchange mechanism is responsible for the FM order in diluted $III$-$V$ semiconductors