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 less-than-or-equal-to T less-than-or-equal-to 290 K and resonance frequency 33 less-than-or-equal-to omega(0)/2-pi less-than-or-equal-to 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 analysis 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.