Electronic and vibrational structure of one-dimensional conductors and superconductors

Abstract

An attempt is made to treat molecular wires by quantum chemical methods. What is the electronic difference between systems (molecules, stacks of molecules, or polymers) that are conducting and almost identical systems that are insulating? At 50% band filling a one-dimensional crystal undergoes a Peierls transition and becomes an insulator (without doping). In the case of 75% band filling, on the other hand, two phases are possible: charge density wave (CDW) and spin-density wave (SDW). The transition between these two insulating phases should be connected to a high conductivity. If CDW and SDW are energetically possible at zero T, vibrational coupling leads to stabilization of a superconducting (SC) ground state and the formation of an energy gap. This hypothesis is exemplified on (SN)_x and (TMTSF)₂X (X = ClO₄, PF₆).