47,398 research outputs found
Quantum Tunneling of Spin Particles in Periodic Potentials with Asymmetric Twin Barriers
The tunneling effect of a periodic potential with an asymmetric twin barrier
per period is calculated using the instanton method. The model is derived from
the Hamiltonian of a small ferromagnetic particle in an external magnetic field
using the spin-coherent-state path integral. The instantons in two neighbouring
barriers differ and lead to different level shifts . We derive with Bloch theory the energy spectrum which has
formally the structure of an energy band. The spectrum depends on both level
shifts. The removal of Kramer's degeneracy by an external magnetic field is
discussed. In addition we find a new kind of quenching of macroscopic quantum
coherence which is irrelevant to Kramer's degeneracy.Comment: 18 pages, LaTex, one figur
Calculation of Spin Tunneling Effects in the Presence of an Applied Magnetic Field
The tunneling splitting of the energy levels of a ferromagnetic particle in
the presence of an applied magnetic field - previously derived only for the
ground state with the path integral method - is obtained in a simple way from
Schroedinger theory. The origin of the factors entering the result is clearly
understood, in particular the effect of the asymmetry of the barriers of the
potential. The method should appeal particularly to experimentalists searching
for evidence of macroscopic spin tunneling.Comment: 10 pages, LaTex, 4 figures included using graphic
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
Quantum Phase Transition in Finite-Size Lipkin-Meshkov-Glick Model
Lipkin model of arbitrary particle-number N is studied in terms of exact
differential-operator representation of spin-operators from which we obtain the
low-lying energy spectrum with the instanton method of quantum tunneling. Our
new observation is that the well known quantum phase transition can also occur
in the finite-N model only if N is an odd-number. We furthermore demonstrate a
new type of quantum phase transition characterized by level-crossing which is
induced by the geometric phase interference and is marvelously periodic with
respect to the coupling parameter. Finally the conventional quantum phase
transition is understood intuitively from the tunneling formulation in the
thermodynamic limit.Comment: 4 figure
Holographic Dark Energy Characterized by the Total Comoving Horizon and Insights to Cosmological Constant and Coincidence Problem
The observed acceleration of the present universe is shown to be well
explained by the holographic dark energy characterized by the total comoving
horizon of the universe (HDE). It is of interest to notice that the very
large primordial part of the comoving horizon generated by the inflation of
early universe makes the HDE behave like a cosmological constant. As a
consequence, both the fine-tuning problem and the coincidence problem can
reasonably be understood with the inflationary universe and holographical
principle. We present a systematic analysis and obtain a consistent
cosmological constraint on the HDE model based on the recent cosmological
observations. It is found that the HDE model gives the best-fit result
() and the minimal
which is compatible with for the CDM model.Comment: 17 pages, 4 figures, two eqs. (26)(27) added for the consistent
approximate solution of dark energy in early universe, references added,
published version in PR
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An agent-based DDM for high level architecture
The Data Distribution Management (DDM) service is one of the six services provided in the Runtime Infrastructure (RTI) of High Level Architecture (HLA). Its purpose is to perform data filtering and reduce irrelevant data communicated between federates. The two DDM schemes proposed for RTI, region based and grid based DDM, are oriented to send as little irrelevant data to subscribers as possible, but only manage to filter part of this information and some irrelevant data is still being communicated. Previously (G. Tan et al., 2000), we employed intelligent agents to perform data filtering in HLA, implemented an agent based DDM in RTI (ARTI) and compared it with the other two filtering mechanisms. The paper reports on additional experiments, results and analysis using two scenarios: the AWACS sensing aircraft simulation and the air traffic control simulation scenario. Experimental results show that compared with other mechanisms, the agent based approach communicates only relevant data and minimizes network communication, and is also comparable in terms of time efficiency. Some guidelines on when the agent based scheme can be used are also give
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