Ippei Danshita

Stability of Bose-Einstein condensates in a Kronig-Penney potential

We study the stability of Bose-Einstein condensates with superfluid currents in a one-dimensional periodic potential. By using the Kronig-Penney model, the condensate and Bogoliubov bands are analytically calculated and the stability of condensates in a periodic potential is discussed. The Landau and dynamical instabilities occur in a Kronig-Penney potential when the quasimomentum of the condensate exceeds certain critical values as in a sinusoidal potential. It is found that the onsets of the Landau and dynamical instabilities coincide with the point where the perfect transmission of low energy excitations through each potential barrier is forbidden. The Landau instability is caused by the excitations with small

Phase dependence of phonon tunnelling in bosonic superfluid–insulator–superfluid junctions

q

Collective Excitations of Bose–Einstein Condensates in a Double-Well Potential

and the dynamical instability is caused by the excitations with

Quantum phase transition of ultracold bosons in double-well optical lattices

q=\ensuremath{\pi}∕a

Instability of Bose-Einstein condensates moving in optical lattices

at their onsets, where

Landau and Dynamical Instabilities of Bose-Einstein Condensates in a Kronig-Penney Potential

q

Stability of Bose-Einstein condensates in a double-well potential

is the quasimomentum of excitation and

Quantum Phases of Bosons in Double-Well Optical Lattices

a

Landau damping: Instability mechanism of superfluid Bose gases moving in optical lattices

is the lattice constant. A swallow-tail energy loop appears at the edge of the first condensate band when the mean-field energy is sufficiently larger than the strength of the periodic potential. We find that the upper portion of the swallow-tail is always dynamically unstable, but the second Bogoliubov band has a phonon spectrum reflecting the positive effective mass.

Quasiresonant tunneling of phonons in bosonic superfluid-insulator- superfluid junctions

We consider the tunnelling of phonon excitations across a potential barrier spatially separating two condensates with different macroscopic phases. We analyse the relation between the phase difference of the two condensates and the transmission coefficient T by solving the Bogoliubov equations. It is found that T strongly depends on , and that the perfect transmission of low-energy excitations disappears when the phase difference reaches the critical value which gives the maximum supercurrent of the condensate. We also discuss the feasibility of observing the phase differences in experiments.