Mechanisms of nuclear spin relaxation in (BEDT-TTF)-based organic superconductors

The proton spin-lattice relaxation rate T1-1 in the organic superconductor (BEDT-TTF)2Cu(NCS)2 has been measured for temperature 4.5≤T≤290 K and resonance frequency 33≤ω0/2π≤90 MHz. The results give evidence for a phase transition near 50 K accompanied by a change in the density of electron states at the Fermi level. The comparative anaysis of the normal-state T1-1 data in different (BEDT-TTF)-based superconductors allows one to divide them into two groups. For compounds with I3 and Cu(NCS)2 anions T1-1 shows the normal metallic (Korringa) behavior, while for compounds with Hg2.89Br8 and Hg2.78Cl8 anions the relaxation rate appears to be dominated by antiferromagnetic spin fluctuations. (orig.)