Proton spin-lattice relaxation in the organic superconductor (BEDT-TTF)2Cu(NCS)2 : evidence for relaxation by localized paramagnetic centres

Abstract

The spin-lattice relaxation of the protons in the compound was investigated at nu L=13.5 MHz and 270 MHz for 4.2 K<or=T<or=100 K. The field corresponding to nu L=13.5 MHz, B0=0.31 T, is below the upper critical field and allows NMR measurements right in the superconducting state. Two kinds of samples were prepared: coarse grain crystals and finely powdered crystallites. In the finely powdered samples the proton relaxation at nu L=13.5 MHz follows an exponential-square-root law. This law is traced back to proton relaxation caused by localized paramagnetic centres. At nu L=270 MHz the relaxation follows a superposition of an exponential and an exponential-square-root law. The temperature dependence of the exponential contribution obeys the Korringa relation. This contribution is ascribed to conduction electrons. In coarse grain crystals the proton relaxation is exponential at nu L=270 MHz for T>10 K and becomes non-exponential for T<10 K. At nu L=13.5 MHz the transition from exponential to noticeably non-exponential relaxation occurs already at T approximately=25 K. The cause for the non-exponential proton relaxation in the coarse grains is finite penetration of the RF-field (skin-effect) into the electrically conducting crystals. Powdering the crystals suppresses the skin-effect; this procedure, however, generates relaxation sinks in the form of localized paramagnetic centres. There is reason to believe that the skin-effect in coarse grains and generation of relaxation sinks by a powdering procedure complicate proton relaxation studies as well in other organic superconductors.