Low-frequency dynamics of κ-(BEDT-TTF)2Cu(NCS)2 observed by C13 NMR

Abstract

κ-(BEDT-TTF)2Cu(NCS)2 [BEDT-TTF: bis-(ethylenedithio)-tetrathiafulvalene] behaves as a semiconductor at high temperatures, whereas it behaves as a Fermi liquid just above the superconducting transition temperature. To reveal the cause of this behavior, we experimented on κ-(BEDT-TTF)2Cu(NCS)2, in which one side of the central C=C in the BEDT-TTF molecules is substituted with C13 nuclei. We perfomed C13-nuclear magnetic resonance (NMR) spectroscopy on this salt and measured the temperature dependence of its spectral linewidths and its spin-spin relaxation time T2. We found anomalies in its linewidths and T2, which we connected to the ethylene motion within the salt. Compared with the C13-NMR measurements of κ-(BEDT-TTF-d8) 2Cu(NCS)2, we obtained the experimental evidence of the connection between the ethylene motion and the conduction electrons. Considering this connection, we examined the semiconductive behavior of κ-(BEDT-TTF)2Cu(NCS)2 at high temperatures. The contribution of ethylene motion to the electronic state is thought to be a common feature of BEDT-TTF salts. © 2011 American Physical Society.