Time-Resolved Photoresponse Measurements of the Electrical Conductivity of the Quasi-Two-Dimensional Organic Superconductor β-(BEDT-TTF)2I3 Using a Nanosecond Laser Pulse

Time-resolved photoresponses in resistance have been measured following the nanosecond laser pulse excitation for the quasi-two-dimensional organic superconductors of hydrogenated and deuterated beta-(BEDT-TTF)(2)I-3 [BEDT-TTF = bis-(ethylenedithio)tetrathiafulvalene], which show two different superconducting states with high-T-c and low-T-c at temperatures near the critical temperatures. A transient increase of the resistance is induced by photoirradiation at all the temperatures, but a marked temperature dependence of the decay time is observed at temperatures close to the high-T-c phase transition temperature; the decay rate becomes faster and then becomes constant in both compounds, as the temperature decreases across the high-T-c phase transition temperature. The temperature dependence of the photoresponse intensity is different from the one expected from the bolometric effects, indicating the presence of the nonbolometric photoresponse. A possible mechanism explaining the photoresponse of the conductivity is discussed, based on the isotope effect on the photoresponse. A comparison is also made between beta-(BEDT-TTF)(2)I-3 and kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br for the transient photoresponse in resistance at temperatures across the metal-superconductor phase transition temperature.