The dependence of the superconducting transition temperatureT_con the quasi-two-dimensional carrier concentration in the organic superconductor κ-(BEDT-TTF)₂Cu(SCN)₂(BEDT-TTF ≡ bis(ethylene-dithio)tetrathiafulvalene)

Abstract

In this paper we discuss the effect of the quasi-two-dimensional carrier concentration, n Q2D on the superconducting transition temperature, T c , of the organic superconductor κ-(BEDT-TTF) 2 Cu(SCN) 2 .Recent ac susceptibility as well as magnetotransport measurements on the organic superconductor κ-(BEDT-TTF) 2 Cu(SCN) 2 under pressure with different pressure media are compared. The exact results of the individual pressure experiments are pressure media dependent, thus making the experimentally determined pressure an unsuitable parameter for describing the physical properties of κ-(BEDT-TTF) 2 Cu(SCN) 2 . The pressure media dependence is thought to originate from the difference in thermal contraction between the isotropic pressure medium and the highly anisotropic and very soft sample. However, comparison of pressure measurements with different media reveals a pressure medium independent correlation between the superconducting transition temperature, T c , and the size of the quasi-two-dimensional Fermi surface pocket and thus the quasi-two-dimensional carrier density in κ-(BEDT-TTF) 2 Cu(SCN) 2 . The observed pressure-induced increase in the quasi-two-dimensional carrier density can be interpreted as a transfer of carriers from quasi-one-dimensional Fermi surface sections, reminiscent of a mechanism in cuprate superconductors, where pressure is known to transfer carriers from the insulating charge reservoir layers into the conducting cuprate sheets. In further analogy with the cuprate superconductors, it is seen that the highest T c is achieved at carrier concentrations of ∼0.15 holes/dimer, which is very similar to the value found consistently for optimal doping in cuprate superconductors.