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-(BEDT-TTF) 4 Hg 2.89 Br 8 is an ambient-pressure superconductor with a transition temperature (T c ) value of 4.3 K. This salt can be considered as an electron system that is naturally carrier doped into a Mott insulator by a slight excalation of the Hg ions. In order to obtain information about the nature of the metallic and superconducting states in this doping system, we performed electrical resistivity measurements as functions of temperature and pressure. We revealed that T c showed an anomalous two-peak structure along the pressure axis. In addition, we found that the power of the temperature in the low-temperature normal-state resistivity correlated with the T c diagram. We concluded that an observable critical boundary existed between the non-Fermi-liquid and Fermi-liquid states in this system.

We demonstrated high-pressure studies on two kinds of layered organics. One is a Mott insulator, β -(BEDT-TTF)2ICl2, with an exactly half-filled band. By applying extremely high pressure to it, we observed the highest-Tc superconductivity among organics. Tc amounts to 14.2 K at 8.2 GPa. The other is a doped-type conductor, κ-(BEDT-TTF)4Hg2.78Cl8, with the band-filling shifted from the half-filled state. We uncovered that it had an anomalous pressuretemperature phase diagram. Tc had a minimum structure within the superconducting phase along with pressure axis. In addition, we found that this superconducting phase was lying in between two nonmetallic phases.

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