Transport properties of massless Dirac fermions in an organic conductor α-(BEDT-TTF)2I3 under pressure

A zero-gap state with the Dirac cone-type energy dispersion was found in an organic conductor alpha-(BEDT-TTF)(2)I(3) under high hydrostatic pressures. This is the first two-dimensional zero-gap state discovered in bulk crystals with layered structures. In contrast to the case of graphene, the Dirac cone in this system is highly anisotropic. The present system, therefore, provides a new type of massless Dirac fermions with anisotropic Fermi velocity. From the galvano-magnetic measurements, the density and mobilities of electrons and holes were determined in the temperature region between 77K and 2K. In this region, the carrier density (n) depends on temperature (T) as n alpha T(2) and decreases by about four orders of magnitude. On the other hand, the sheet resistance per BEDT-TTF layer (R(S)) stays almost constant in the region. The value is written as R(S) = gh/e(2) in terms of the quantum resistance h/e(2)= 25.8 k Omega, where g is a parameter that depends weakly on temperature. Copyright (c) EPLA, 2007.