Ultrafast Carrier Dynamics in an Organic Superconductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br by Spectrally Resolved Pump-Probe Spectroscopy

In this work, spectrally resolved optical pump-probe measurements were carried out on strongly correlated organic superconductor.-(BEDT-TTF)2Cu[N(CN)2]Br. We find two different types of carrier relaxation dynamics, one of which appears at 1.82 eV and 1.94 eV, and another at 2.17 eV. The former is characterized by short decay time similar to 1-2 ps and appears below similar to 60 K. From the comparison with the previous studies, the dynamics arises from the photo-induced pseudogap (PG) formation of the partially appeared Mott phase. The latter has long relaxation time similar to 9 ps at 5 K and developed as temperature decreases below 30 K. The slow dynamics is consistent with appearance of superconducting (SC) gap. Such separate observation of PG and SC dynamics enables us to proceed further investigation of a relationship between them