HIGH-TEMPERATURE SPIN DYNAMICS IN LA2-DELTA-SR-DELTA-CUO4

Motivated by magnetic-resonance experiments for La2-deltaSrdeltaCuO4 at temperatures up to 900 K, we calculate the spin-lattice relaxation rate 1/T1 for Cu nuclei within the Gaussian approximation at high temperatures (T greater than or similar to J/2) for the S = 1/2 Heisenberg antiferromagnet and at T = infinity for the t-J model, taking into account the transferred as well as the direct hyperfine coupling. For the Heisenberg model, including the transferred coupling weakens the temperature dependence of 1/T1 and leads to better agreement with the experimental data than the calculation that only includes direct hyperfine coupling. For the t-J model, the infinite-temperature 1/T1 within the Gaussian approximation decreases markedly upon doping. We propose a non-Gaussian form for the spectral density that is consistent with the exact moments and more in accord with the experiments, which find 1/T1 nearly independent of delta at T almost-equal-to J/2.