THE NUCLEAR-SPIN-LATTICE RELAXATION OF AN AMORPHOUS HUBBARD-MODEL

We study the temperature dependence of the nuclear spin-lattice relaxation rate, (1/T-1), of a structurally disordered Hubbard model for different electronic concentration n(e), and for various interaction strength U. The relaxation process is assumed to be caused by the hyperfine Fermi contact interaction. The required electronic spin correlation function is studied up to the fourth order in hopping, and a Gaussian approximation is constructed. Two types of transfer integral have been examined: (1) quasiexponential, and (2) Gaussian. At half-filling (n(e) = 1), the relaxation rate shows a strong U dependence at low temperatures. For large U, (1/T-1) drops markedly when holes are introduced into the system at low temperatures, and its temperature dependence becomes weak when n(e) reaches 0.8. We also calculate the spin susceptibility and examine the n(e) dependence of the Korringa relation. We compare the relaxation rates with those for the two-dimensional Hubbard model with nearest-neighbor hopping on a square lattice, and contact is made with the recent NMR study of Sr-doped La2CuO4 at high temperatures.