Phase diagram of organic superconductor κ-(ET) 2Cu[N(CN) 2]Br as it is seen from isothermal resistance relaxation

Abstract

Phase diagram of κ-(ET) 2Cu[N(CN) 2]Br superconductor was studied by resistance measurements of structural relaxation kinetics and properties of nonequiliblium phases after isochronous annealing. We show, that (1) the activation energy of the lattice relaxation time below 100 K corresponds well to the potential barrier heights for ET molecule conformation; (2) the shape of the diagram corresponds to that of the ethylene ordering phenomena with superstructure formation; (3) the known anomalies of the electronic properties of the salt, namely resistance hump at around 100 K and pseudogap behavior at 60 K correspond to upper and lower hysteresis branches of the ethylene transformation, while the main second order phase transition proceeds at 80 K; (4) the state that comes our of the transformation is not an ordered state, but rather contains superstructure interacting with the Fermi surface and causing its partial destruction. We discuss these findings, together with high superconducting T c and negative isotope effect as indications of crucial soft lattice contribution to the superconducting properties in organics.