Fluctuating superconductivity in kappa-type organic compounds probed by polarized time-resolved spectroscopy

In this work, polarized time-resolved spectroscopy was carried out to investigate a superconducting (SC) state of the organic compounds kappa-(BEDT-TTF)(2)X (X = Cu(NCS)(2) (kappa-NCS, T-c similar to 9 K) and Cu[N(CN)(2)]Br (kappa-Br, T-c similar to 12 K)), which have different effective electron correlations. We found that the SC response, which was characterized by probe polarization anisotropy with slow relaxation time, decreases steeply at approximately T-c in is kappa-NCS with increasing temperature, corresponding to the closing of the SC gap. On the other hand, in is kappa-Br the SC response persisted significantly above T-c. The results indicate that fluctuating superconductivity (FSC) emerges in kappa-Br but not in kappa-NCS, suggesting that strong electron correlation can play a critical role for FSC