Energy Scale in Organic Conductors and the Problem of Superconductivity in the Bechgaard Salts

Abstract

There are various electronic energy or frequency scales in all solids. The high energy limit is set by core atomic orbitals with energies typically 103 eV corresponding to temperatures of 107 K (I shall use the temperature T scale from now on equivalently for energies or frequencies thereby implying that kB = ħ=1). The atomic or molecular valence orbitals which are more relevant to the chemical processes have energies in the range 104 − 105 K. The conduction processes, which are our prime concern, occur within the bands resulting from single-electron tunneling processes between orbitals of neighboring molecules. The energy scale for this motion is set by the tunneling amplitudes t" (along the stacks) and t⊥i. (in both perpendicular directions). One has t"~103 K, 1–31 , t⊥1, can be 10 K in the highly anisotropic TTF-TCNQ salts for instance, 100 K in the Bechgaard salts2 (TMTSF2 − X), or close to 103 K in the BEDT-TTF superconductors3 which are not very anisotropic, and t⊥2 is a few K1–3. The phonon energies ⊖ are typically 100 K. Finally the strength of the Coulomb interaction, which I shall bluntly characterize by a single garameter U (the Hubbard contact interaction), is of the order of 104 K4. The scale is shown in Fig. 1.KeywordsCoulomb InteractionRandom Phase ApproximationOrganic ConductorSynthetic MetalPeierls TransitionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.