257 research outputs found
Multiphonon decay of strong mode in quantum lattice
A nonperturbative theory of multiphonon anharmonic decay of strongly excited
local mode is developed whereby the mode is considered classically and phonons,
quantum mechanically. The decay rate of the mode is expressed via the negative
frequency parts of the phonon pair correlation functions. In the case of
two-phonon decay the later satisfy the linear integral equations while in the
case of two- and more-phonon decay they satisfy the nonlinear integral
equations. As a result, the processes mentioned differently depend on the mode
amplitude A: two-phonon processes smoothly deminish if A tends to infinity
while three- and more-phonon processes are fully switched-off at large
amplitudes and they abruptly switch-on if the amplitude approaches the critical
value. At that the decay rate gets rather high value (of the order of the mode
quantum per period). The final stage of the relaxation is well described by the
perturbation theory.Comment: 13 pages. submitted to Zeitschrift fur Physik B, to appear Vol.104,
Dec. 199
Pressure dependence of two-level systems in disordered atomic chain
The dependence of two-level systems in disordered atomic chain on pressure,
both positive and negative was studied numerically. The disorder was produced
through the use of interatomic pair potentials having more than one energy
minimum. It was found that there exists a correlation between the energy
separation of the minima of two-level systems Delta and the variation of this
separation with pressure. The correlation may have either positive or negative
sign, implying that the asymmetry of two-level systems may in average increase
or decrease with pressure depending on the interplay of different interactions
between atoms in disordered state. The values of Delta depend on the sign of
pressure.Comment: 5 pages, 5 figure
Structure and time-dependence of quantum breathers
Quantum states of a discrete breather are studied in two ways. One method
involves numerical diagonalization of the Hamiltonian, the other uses the path
integral to examine correlations in the eigenstates. In both cases only the
central nonlinearity is retained. To reduce truncation effects in the numerical
diagonalization, a basis is used that involves a quadratic local mode. A
similar device is used in the path integral method for deducing localization.
Both approaches lead to the conclusion that aside from quantum tunneling the
quantized discrete breather is stable.Comment: 33 pages, 20 figures, to appear in J. Chem. Phy
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