First-order transition to a novel metallic state in [Na y+(CH) − y] x: In situ electron spin resonance during chemical and electrochemical doping

Abstract

The onset of the temperature-independent Pauli spin susceptibility takes place in [Na y +(CH) − y ] x as a sharp first-order phase transition at a dopant concentration of approximately 6 mole%. Using either chemical or electrochemical doping techniques, a single symmetric Lorentzian ESR line was observed for the full range of doping and at all temperatures, thereby ruling out the formation of metallic islands. Although the abrupt transition is easily smeared by disorder and/or non-uniform doping, the sharp onset of Pauli susceptibility is an intrinsic feature of the ordered system. The results are interpreted in terms of a first-order phase transition (at y c ∼= 0.06) from a soliton lattice (or a highly correlated soliton liquid) to a novel metallic state in which the finite density of states arises from a lattice of polaron-like distortions with half-filled polaron sub-band in the Peierls gap.