TODA Group

Photoinduced phase transition

We consider the electron dynamics of ordered electron systems with a narrow gap. In the weak pump regime, we can assume the energy gap (electron order) remains unchanged in the non-equilibrium condition. In this case, the relaxation of the photoexcited electrons occurs by the relaxation down to states near the gap and the relaxation across the gap. On the other hand, in the case of the excitation with intense pump pulse, the relaxation involves the phase ordering dynamics.
Figure shows nonequilibrium quasiparticle (QP) dynamics of a high-T_C superconductor (SC; T_C=78K). In the saturation condition of the SC state, where the SC condensate is fully destroyed within the photoexcited volume, a delay of the exponential decay associated with a transient normal state is observed at TC. The subsequent exponential decay thus reflects the SC state recovery. Moreover, the presence of a psuedogap (PG) is a critical character of high-T_C superconductivity. In the transient data, the relaxation associated with the PG is clearly visible in the range of T_C*. The QP dynamics in the saturation condition of the SC also shows a contribution of the PG component, whose magnitude at the start of the SC state recovery is independent of the temperature, suggesting a critical level of PG order before the SC condensate can recover.