Electronic Phases in an Organic Conductor α-(BEDT-TTF)₂I₃: Ultra Narrow Gap Semiconductor, Superconductor, Metal, and Charge-Ordered Insulator

Abstract

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

Theoretical study of the zero-gap organic conductor α-(BEDT-TTF)2I3

“…The Hall coefficient at low temperatures become 10 5 -10 6 times larger than those at room temperature [19,[22][23][24].…”

Paired electron crystal: Order from frustration in the quarter-filled band

“…What is interesting though is that the proposed scenario can give a unified approach to SC in all organic CTS, irrespective of whether the insulating state proximate to SC is AFM 102 , CO 67,103 or VBS 82 . Recall that within existing mean field theories the AFM-to-SC transition is driven by spin fluctuations [13][14][15][16][17][18][19][20][21][22][23] , while the CO-to-SC transition is driven by charge fluctuations 65 .…”

Effect of the Zero-Mode Landau Level on Interlayer Magnetoresistance in Multilayer Massless Dirac Fermion Systems

“…However, graphene is not a bulk crystal but a single layer of graphite. The bulk crystal zero-gap system was realized in the organic conductor α-(BEDT-TTF) 2 I 3 under high pressure [3,4,5,6,7,8]. The existence of the massless Dirac fermions state in bulk crystal has already been discovered in graphite [9,10] and Cd 1−x Hg x Te [11].…”

“…Carrier density written as n ∝ T 2 is a characteristic feature of 2D zero-gap conductors with the Fermi energy located at the contact point [4,6]. Carrier mobility, on the other hand, is determined as follows.…”

“…As the temperature is decreased, the mean free path becomes long because the wavelength (λ) becomes long with decreasing energy of carriers. As a result, the mobility (µ) of carriers increases as µ ∝ T −2 in the 2D zero-gap system [4,6]. Consequently, the resistivity per layer (sheet resistance R S ) is given as R S = h/e 2 , which is independent of temperature [6].…”