NUCLEAR-SPIN-LATTICE RELAXATION AND ANTIFERROMAGNETIC SPIN CORRELATIONS IN SUPERCONDUCTING THIOSPINEL CU1.5CO1.5S4

The antiferromagnetic and superconducting properties of a thiospinel Cu1.5Co1.5S4 (TN=19.0 K, Tc=2.3 K) have been investigated with Cu63 NMR (75 MHz), Co59 NMR (75 MHz), and Co59 pure quadrupole resonance between T=1.23 and 150 K. The linear dependence of negative Cu63 Knight shift (-0.013% at 4.2 K) on the Curie-Weiss-type susceptibility (T) and nearly independent Co59 Knight shift (+1.43%) indicate that the d hole band of Cu at the tetrahedral A site is mainly responsible for the spin paramagnetism. The spin-lattice relaxation rates (T1T)-1 of both Cu63 and Co59 are significantly enhanced with lowering temperature below 100 K, similar to those observed in high-Tc copper oxygen perovskite superconductors, which are associated with the growth of antiferromagntic spin correlations at low temperatures. Below Tc, T1-1 of the fractional Co at the octahedral B site, which increases from 0% at 0.65Tc to 30% at 0.5Tc, shows a rapid decrease, indicating a partial formation of the superconducting energy gap. On the other hand, T1-1 of the dominant part of Co follows a Korringa-like relation down to 0.5Tc, suggesting it is in a gapless superconducting state, probably due to strong antiferromagnetic spin correlations. © 1995 The American Physical Society.