Optical absorption of CuO3 antiferromagnetic chains at finite temperatures

We use a high-statistic quantum Monte Carlo and maximum entropy regularizat ion method to compute the dynamical energy correlation function (DECF) of t he one-dimensional (1D) S = 1/2 antiferromagnetic Heisenberg model at finit e temperatures. We also present a finite-temperature analytical ansatz for the DE;CT; that is in very good agreement with the numerical data in the co nsidered temperature range. From these results, and from a finite-temperatu re generalization of the mechanism proposed by Lorenzana and Sawatsky [Phys . Rev. Lett. 74, 1867 (1995)]. we compute the line shape for the optical ab sorption spectra of multimagnon excitations assisted by phonons for quasi-1 D compounds. The line shape has two contributions analogous to the Stokes a nd anti-Stokes process of Raman scattering. Our low-temperature data are in good agreement with optical absorption experiments of CuO3 chains in Sr2Cu O3. Out finite-temperature results provide a nontrivial prediction on the d ynamics of the Heisenberg model at finite temperatures that is easy to veri fy experimentally.