7,132 research outputs found
A condition for first order phase transitions in quantum mechanical tunneling models
A criterion is derived for the determination of parameter domains of first
order phase transitions in quantum mechanical tunneling models. The criterion
is tested by application to various models, in particular to some which have
been used recently to explore spin tunneling in macroscopic particles. In each
case agreement is found with previously heuristically determined domains.Comment: 13 pages, 5 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
A Note on Tachyons in the System
The periodic bounce of Born-Infeld theory of -branes is derived, and the
BPS limit of infinite period is discussed as an example of tachyon
condensation. The explicit bounce solution to the Born--Infeld action is
interpreted as an unstable fundamental string stretched between the brane and
its antibrane.Comment: 10 pages, 2 figures. v2: minor changes, acknowledgement added; v3:
explanations and references added. Final version to appear in Mod. Phys.
Lett.
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
Periodic Bounce for Nucleation Rate at Finite Temperature in Minisuperspace Models
The periodic bounce configurations responsible for quantum tunneling are
obtained explicitly and are extended to the finite energy case for
minisuperspace models of the Universe. As a common feature of the tunneling
models at finite energy considered here we observe that the period of the
bounce increases with energy monotonically. The periodic bounces do not have
bifurcations and make no contribution to the nucleation rate except the one
with zero energy. The sharp first order phase transition from quantum tunneling
to thermal activation is verified with the general criterions.Comment: 17 pages, 5 postscript figures include
Periodic instanton method and macroscopic quantum tunneling between two weakly-linked Bose-Einstein condensates
A new method is used to investigate the tunneling between two weakly-linked
Bose-Einstein condensates confined in double-well potential traps. The
nonlinear interaction between the atoms in each well contributes to a finite
chemical potential, which, with consideration of periodic instantons, leads to
a remarkably high tunneling frequency. This result can be used to interpret the
newly found Macroscopic Quantum Self Trapping (MQST) effect. Also a new kind of
first-order crossover between different regions is predicted.Comment: 4 pages, 2 eps figures, final version to appear in Phys. Rev.
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