6,700 research outputs found
Effect of the Zero-Mode on the Response of a Trapped Bose-Condensed Gas
The dynamical response of a trapped Bose-Einstein condensate (BEC) is
formulated consistently with quantum field theory and is numerically evaluated.
We regard the BEC as a manifestation of the breaking of the global phase
symmetry. Then, the Goldstone theorem implies the existence of a zero energy
excitation mode (the zero-mode). We calculate the effect of the zero-mode on
the response frequency and show that the contribution of the zero-mode to the
first excitation mode is not so important in the parameter set realized in the
existing experiment. This is the reason that experimental results can be
described using the Bogoliubov prescription, although it breaks the consistency
of the description in quantum field theory.Comment: 18 pages, 3 figure
Structural Transition of Li2RuO3 Induced by Molecular-Orbit Formation
A pseudo honeycomb system Li2RuO3 exhibits a second-order-like transition at
temperature T=Tc=540 K to a low-T nonmagnetic phase with a significant lattice
distortion forming Ru-Ru pairs. For this system, we have calculated the band
structure, using the generalized gradient approximation (GGA) in both the high-
and low- T phases, and found that the results of the calculation can naturally
explain the insulating behavior observed in the low-T phase. The detailed
characters of the Ru 4d t2g bands obtained by the tight-binding fit to the
calculated dispersion curves show clear evidence that the structural transition
is driven by the formation of the Ru-Ru molecular-orbits, as proposed in our
previous experimental studies.Comment: 5 pages, 5 figures, 4 tables, submitted to J. Phys. Soc. Jp
Condition for emergence of complex eigenvalues in the Bogoliubov-de Gennes equations
The condition for the appearance of dynamical instability of the
Bose-condensed system, characterized by the emergence of complex eigenvalues in
the Bogoliubov-de Gennes equations, is studied analytically. We perturbatively
expand both the Gross-Pitaevskii and Bogoliubov-de Gennes equations with
respect to the coupling constant. It is concluded that the degeneracy between a
positive-norm eigenmode and a negative-norm one is essential for the emergence
of complex modes. Based on the conclusion, we justify the two-mode
approximation applied in our previous work [E. Fukuyama \textit{et al}., Phys.
Rev. A {\bf 76}, 043608 (2007)], in which we analytically studied the condition
for the existence of complex modes when the condensate has a highly quantized
vortex.Comment: 7page
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