Physics – Condensed Matter – Other Condensed Matter
Scientific paper
2006-07-07
J. Low Temp. Phys. 148, 337 (2007)
Physics
Condensed Matter
Other Condensed Matter
6 pages, 2 figures, Proceedings of the International Symposium on Quantum Fluids and Solids (QFS2006)
Scientific paper
10.1007/s10909-007-9394-x
We study the elementary excitations of Bose-Einstein condensates in a one-dimensional periodic potential and discuss the stability of superfluid flow based on the Kronig-Penney model. We analytically solve the Bogoliubov equations and calculate the excitation spectrum. The Landau and dynamical instabilities occur in the first condensate band when the superfluid velocity exceeds certain critical values, which agrees with the result of condensates in a sinusoidal potential. It is found that the onset of the Landau instability coincides with the point where the perfect transmission of low-energy excitations is forbidden, while the dynamical instability occurs when the effective mass is negative. It is well known that the condensate band has a peculiar structure called swallowtail when the periodic potential is shallow compared to the mean field energy. We find that the upper side of the swallowtail is dynamically unstable although the excitations have the linear dispersion reflecting the positive effective mass.
Danshita Ippei
Tsuchiya Shunji
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