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

Controllable Persistent Atom Current of Bose-Einstein Condensates in an Optical Lattice Ring

In this paper the macroscopic quantum states of Bose-Einstein condensates in optical lattices is studied by solving the periodic Gross-Pitaevskii equation in one-dimensional geometry. It is shown that an exact solution seen to be a travelling wave of excited macroscopic quantum states resultes in a persistent atom current which can be controlled by adjusting of the barrier height of the optical periodic potential. A critical condition to generate the travelling wave is demonstrated and we moreover propose a practical experiment to realize the persistent atom current in a toroidal atom waveguide.Comment: 9 pages, 1 figure

EPR spectrum via entangled states for an Exchange-Coupled Dimer of Single-Molecule Magnets

Multi-high-frequency electron paramagnetic resonance(EPR) spectrum for a supermolecular dimer $[ Mn_4]_2$ of single-molecule magnets recently reported [S. Hill, R. S. Edwards, N. Aliaga-Alcalde and G. Christou(HEAC), Science 302, 1015 (2003)] is studied in terms of the perturbation method in which the high-order corrections to the level splittings of degenerate states are included. It is shown that the corresponding eigenvectors are composed of entangled states of two molecules. The EPR-peak positions are calculated in terms of the eigenstates at various frequencies. From the best fit of theoretical level splittings with the measured values we obtain the anisotropy constant and exchange coupling which are in agreement with the corresponding values of experimental observation. Our study confirms the prediction of HEAC that the two $Mn_4$ units within the dimer are coupled quantum mechanically by the antiferromagnetic exchange interaction and the supermolecular dimer behaviors in analogy with artificially fabricated quantum dots.Comment: 16 pages,2 figures, 2 table

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.

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

Spin-polarized quantum transport through a T-shape quantum dot-array: a model of spin splitter

We in this paper study theoretically the spin-polarized quantum transport through a T-shape quantum dot-array by means of transfer-matrix method along with the Green^{,}s function technique. Multi-magnetic fields are used to produce the spin-polarized transmission probabilities and therefore the spin currents, which are shown to be tunable in a wide range by adjusting the energy, and the direction-angle of magnetic fields as well. Particularly the opposite- spin- polarization currents separately flowing out to two electrodes can be generated and thus the system acts as a spin splitter.Comment: 8 pages, 8 figure