Temperature independent conductivity in α-(BEDT-TTF)2I3 —A novel mechanism

The electric conductivity of α-(BEDT-TTF)2I3 under a high pressure is almost independent of temperature. On the other hand, the Hall coefficient (RH) increases with lowering temperature; RH at 4 K is 104 times as large as that at room temperature, indicating an extraordinary low carrier density at low temperatures. e 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 ∼ εF ± κBT/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.