Suppression of inhomogeneous electron localization in $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{Cu}}_{2}{(\mathrm{C}\mathrm{N})}_{3}$ under pressure

Suppression of inhomogeneous electron localization in $κ − {(BEDT − TTF)}_{2} {Cu}_{2} {(C N)}_{3}$ under pressure

Atsushi Kawamoto, Yosuke Honma, Ken-ichi Kumagai, Noriaki Matsunaga, and Kazushige Nomura

Phys. Rev. B 74, 212508 – Published 29 December 2006

Abstract

In order to investigate the nature of the insulator phase of $κ − {(BEDT − TTF)}_{2} {Cu}_{2} {(C N)}_{3}$ [where BEDT-TTF represents bis(ethylenedithio)terathiofulvalene] and why this salt is a superconductor under pressure but not under ambient pressure, we examined the ${}^{13}C$ NMR spectra and spin lattice relaxation rate of $κ − {(BEDT − TTF)}_{2} {Cu}_{2} {(C N)}_{3}$ under pressure. The spin susceptibility evaluated from the Knight shift decreased with increase in pressure, and line broadening was suppressed above a critical pressure. The temperature dependence of ${(T_{1} T)}^{- 1}$ for $κ − {(BEDT − TTF)}_{2} {Cu}_{2} {(C N)}_{3}$ above a critical pressure was that for $κ − {(BEDT − TTF)}_{2} X$ , where $X$ was $Cu [N {(C N)}_{2}] Br$ or $Cu {(N C S)}_{2}$ . These results suggest that the electronic structure of $κ − {(BEDT − TTF)}_{2} {Cu}_{2} {(C N)}_{3}$ under pressure is similar to that of other $κ − {(BEDT − TTF)}_{2} X$ salts. The results support our model that electron inhomogeneity and hopping conduction, which are common features of semiconductivity in $κ − {(BEDT − TTF)}_{2} X$ at high temperatures, are responsible for the insulator phase in $κ − {(BEDT − TTF)}_{2} {Cu}_{2} {(C N)}_{3}$ . The electron inhomogeneity suppresses the semiconductor-metal transformation and superconductivity observed in other $κ − {(BEDT − TTF)}_{2} X$ salts.