Electronic phases in an organic conductor α-(BEDT-TTF) ₂I ₃: Ultra narrow gap semiconductor, superconductor, metal, and charge-ordered insulator

We review the transport phenomena in an organic conductor α-(BEDT-TTF) ₂I ₃. It exhibits various types of transport depending on the circumstance in which it is placed. Under the ambient pressure, it is a charge-ordered insulator below 135 K. When high hydrostatic pressures are applied, it changes to a new type of narrow gap (or zero gap) semiconductor. The conductivity of this system is nearly constant between 300 and 1.5 K. In the same region, however, both the carrier (hole) density and the mobility change by about six orders of magnitude, in a manner so that the effects just cancel out giving rise to the temperature independent conductivity. The temperature (T) dependence of the carrier density n obey n ∞ T ² below 50 K. When it is compressed along the crystallographic a-axis, it changes from the charge ordered insulator to a narrow gap semiconductor. At the boundary between these phases, there appears a superconducting phase. On the other hand, when compressed in the b-axis, the system changes to a metal with a large Fermi surface. The effect of magnetic fields on samples in the narrow gap semiconductor phase was examined. Photo-induced transition from the charge ordered insulating state to a metallic state is also discussed.