Title: Nonequilibrium Nonlinear Dynamics and Phase Transition in a Periodically Modulated Atomic Trap
Abstract: Mean-field approach has been instrumental for developing an insight into critical phenomena. However, close to the phase transition point the mean field approximation usually breaks down because of enhanced spatial fluctuation. Here we report an observation of an ideal mean-field symmetry breaking transition and the associated critical behavior. The transition occurs in an atomic cloud confined in a periodically modulated magneto-optical trap once the number of atoms reaches a critical value. In the symmetric phase, the atoms equally occupy two parametrically excited vibrational states with opposite phase. As a result of the transition, the state occupations become different. This is symmetry breaking in time rather than in space. It is due to the interatomic interaction and is mediated by nonequilibrium fluctuations. The measured critical exponents show that this transition is the ideal mean-field transition which has been considered physically not feasible. The results demonstrate that an ideal mean-field transition can occur in a many-body system and moreover that the notion of discrete symmetry breaking transitions can be extended to the time domain.