Theoretical study on the Quantum Statistical Aspects of BEC

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The A4 group consisted of theoreticians working on the physics of BEC. (1) Ueda's group studied on the physics of BEC under attractive interaction. The state of the attractive BEC is at best a metastable state and eventually collapses. They studied the mechanism of the BEC collapsing and found that a BEC near the critical temperature grows macroscopic quantum tunneling, and then the BEC collapses. The found also the quantization of circulation that was guaranteed in a repulsive BEC was only partially satisfied in an attractive BEC. The quantization of circulation is know as a signature of superfluidity. The breakdown of the quantization in an attractive BEC adds new insight on the fundamental properties of BEC. They studied also the dynamics of BEC when the interaction is switched from repulsive to attractive. Their findings are (i) when a large BEC is switched from repulsive to attractive interaction, various nonlinear pattern (shell structure, layered structure,....) is expected to appear. (ii) They studied the dynamics of vortex generated when the interaction is repulsive. They found various dynamical instabilities that was a function of the interaction strength. The instabilities cause fracture of the vortex, and the vortex collapses. (iii) They showed in an attractive BEC two-dimensional bright is stabilized if the sign and amplitude of the interaction potential is modulated. Ohmi and Machida studied on me phase separation of multi-state BEC, dynamics of the phase transition, a new method of generating vortices, and on the stability of vortices.