Nonmetal to metal crossover and ethylene ordering in the organic superconductorκ−(BEDT−TTF)2Cu[N(CN)2]Br

Abstract

Phase transformations in organic superconductor $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F})}_{2}{\mathrm{Cu}[\mathrm{N}(\mathrm{CN})}_{2}]\mathrm{Br}$ and its deuterated analog have been studied by resistance measurements of structural relaxation kinetics under isothermal conditions. The nonmetallic to metallic resistance temperature dependence crossover observed in the salts and anomaly of spin susceptibility temperature dependence at 60 K are shown to be due to first-order phase transitions caused by ordering of the terminal ethylene groups. The experimentally determined transformation sequence is in good agreement with the microscopic mean field model of ethylene ordering, taking into account two possible configurations of ethylene groups and their interactions with anions.