An organic ‘metal’?

Abstract

LITTLE1 has proposed a mechanism by which an electron–electron (e–e) interaction in certain organic polymers and macromolecules might result in a high superconducting transition temperature. This e–e mechanism requires a combination of two systems consisting of a conducting or semiconducting polymeric compound forming a spine, with highly polarisable side chains attached to the spine. According to Little, the addition of such side chains can increase the e–e attraction in the spine to the point where it becomes energetically favourable to enter the superconducting state. This e–e mechanism of superconductivity has led to lively discussions, primarily concerned with superconductivity in quasi one and two-dimensional systems2,3, and the magnitude of the e–e attraction4,5. It has been shown theoretically3 that, depending on, the nature of the interaction between the electrons, a quasi one-dimensional system may exist in different states: metallic, dielectric, antiferromagnetic and superconducting. It has also been shown experimentally that the organic charge transfer salt N-methyl-pehnazinium tetracyanoqui-nodimethan has ‘metallic’ properties at temperatures above 200 K with a continuous transition to a small-band-gap insulator below 200 K (ref. 6).