We review the transport phenomena in an organic conductor-(BEDT-TTF) 2 I 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. KEYWORDS:-(BEDT-TTF) 2 I 3 , transport phenomenon, ultra narrow gap semiconductor, hydrostatic pressure, uniaxial compression, photo-induced insulator-metal transition

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We report on the experimental results of interlayer magnetoresistance in multilayer massless Dirac fermion system α-(BEDT-TTF)2I3 under hydrostatic pressure and its interpretation. We succeeded in detecting the zero-mode Landau level (n=0 Landau level) that is epected to appear at the contact points of Dirac cones in the magnetic field normal to the two-dimensional plane. The characteristic feature of zero-mode Landau carriers including the Zeeman effect is clearly seen in the interlayer magnetoresistance.

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