High-temperature spin dynamics in La2−δSrδCuO4

Abstract

Motivated by magnetic-resonance experiments for ${\mathrm{La}}_{2\mathrm{-}\mathrm{\delta }}$ ${\mathrm{Sr}}_{\mathrm{\delta }}$ ${\mathrm{CuO}}_4$ at temperatures up to 900 K, we calculate the spin-lattice relaxation rate 1/${\mathit{T}}_1$ for Cu nuclei within the Gaussian approximation at high temperatures (T≳J/2) for the S=1/2 Heisenberg antiferromagnet and at T=∞ 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/${\mathit{T}}_1$ 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/${\mathit{T}}_1$ 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/${\mathit{T}}_1$ nearly independent of δ at T≊J/2.