Temperature independent conductivity in alpha-(BEDT-TTF)(2)I-3 - A novel mechanism

The electric conductivity of alpha-(BEDT-TTF)(2)I-3 under a high pressure is almost independent of temperature. On the other hand, the Hall coefficient (R-H) increases with lowering temperature; R-H at 4 K is 10(4) times as large as that at room temperature, indicating an extraordinarily low carrier density at low temperatures. We have found that the carrier density plays a key role in the physics behind this peculiar behavior. The carrier density at 4 K is estimated to be 10(-4) holes per unit cell, whereas that at room temperature amounts to 1-2 holes per unit cell. The situation is entirely modified, when the thermal effect is taken into account. The carrier can spread in the k-space over the region with energy of similar to epsilon(F) +/- k(B)T/2. Thus, at high temperatures the hole and electron pockets are expected to fuse into an obscured hole-like orbit, which is large enough to afford 1-2 holes per unit cell. This crossover from a semi-metal with low carrier density and high mobility to a metal with an obscured ''Fermi surface'' is novel and unique.