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 α-(BEDT-TTF)₂I₃ 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 77 K and 2 K. In this region, the carrier density (n) depends on temperature (T) as n∝T² 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/e² in terms of the quantum resistance h/e ²=25.8 kΩ, where g is a parameter that depends weakly on temperature.