Anomalous Spin Excitation in the Metallic State of La_{<b>2-<i><i>x</i></i></b>}Sr_{<i><i>x</i></i>}CuO_<b>4</b>Observed by Cu NQR

Abstract

We have measured the temperature dependence of the planar 63 Cu nuclear spin-lattice relaxation rate, 1/ T 1 , up to 700 K in underdoped La 2- x Sr x CuO 4 ( x =0.08, 0.10, 0.12). The observed 1/ T 1 T for those samples show Curie-Weiss like behaviour at high temperatures as previously reported. However, we can extract a characteristic temperature, T * , below which the data deviate from the high temperature Curie-Weiss law for each sample. The hole concentration dependence of T * is discussed from a viewpoint of the pseudo spin-gap in this single layer system. We have also found an anomalous behaviour in which the copper relaxation rate, 1/ T 1 , depends on the NQR frequency especially in under-doping region. In this system, it is important to recognize microscopic disorder of the CuO 2 planes which modifies the intrinsic spin dynamics.