Wonho Jhe

Periodically modulated systems form one of the most important classes of nonequilibrium systems, both conceptually and in terms of applications. They have discrete time-translation symmetry: they are invariant with respect to time translation by modulation period tF. Nevertheless, they have stable vibrational states with periods of 2tF, that is period doubling. Interestingly, in a many-body system, dynamical period doubling in itself does not break the time translation symmetry, a consequence of fluctuations. However, if as a result of the interaction the state populations become different, the symmetry is broken, an Ising-class phase transition. We show that an atomic system in a periodically modulated optical trap displays an ideal mean-field symmetry-breaking transition, which is a critical phenomenon as demonstrated by experimental measurement of critical exponents. We also discuss the dynamic phase transition as well as kinetic phase transition observed in the modulated atom trap.