Pseudo gap and superconducting gap in an organic superconductor kappa-(BEDT-TTF)(2)[Cu(SCN)(2)]

The low energy electronic structure of kappa-(BEDT-TTF)(2)Cu(SCN)(2) has be en investigated by means of polarized reflectance over 20-5000 cm(-1) (2.5- 620 meV). The Kramers-Kronig-derived conductivity clearly demonstrates a ps eudo gap of similar to 20 meV developed at 50 K for both the E\\b and E\\c polarizations. The sum rule on sigma (1)(omega) shows that the number of ch arge carriers is 33% larger at low temperatures (T < 100 K) than that at ro om temperature (298 K). These optical results consistently involve, the obs erved structural and transport anomalies around 100 K as well as the Mott-i nsulator fluctuation around 50 K.