13C spin-lattice relaxation study on the electronic properties of K 3C 60

Abstract

13C spin-lattice relaxation was measured for a superconducting fulleride K 3C 60. It is found that non-single exponential relaxation (NSER) data at low temperatures ( T < 55 K), which have a T-independent shape above and below T c, are well reproduced by only the anisotropy parameter ( α spin) for the hyperfine coupling that was derived from a line-shape analysis. From the simulation for various values of α spin, the shape of the NSER, as well as the asymmetric line shapes, is found to be caused by the anisotropy of an electronic single-site rather than electronic multi-sites. By extracting the isotropic part of relaxation time, ( T 1) iso , from the NSER, it is found that an extended Korringa relation holds up to 300 K with K( α) = 7.4, and that the T dependence of [( T 1) iso T] −1 resultss from a decrease in the density of the states. Below T c, a broadened Hebel-Slichter coherence peak is observed, which means s-wave Cooper pairing. The T dependence of [( T 1) iso T] −1 below T c is well reproduced by a theoretical prediction with the superconducting gap 2 Δ(0)/ k B T c = 4.3. The origin of the isotropic hyperfine coupling is also discussed.