RAMAN-SPECTRA OF 2-DIMENSIONAL SPIN-1/2 HEISENBERG ANTIFERROMAGNETS

The Raman spectrum of two-dimensional spin-1/2 Heisenberg antiferromagnets is calculated by exactly diagonalizing clusters of up to 26 sites. The obtained spectra are compared to experimental results for various high-T(c) precursors, such as La2CuO4 and YBa2Cu3O6.2. In spite of good agreement in the position of the main excitation in the B1g channel, i.e, the two-magnon peak around 0.4 eV, an additional mechanism has to be invoked to account for the broad and asymmetric shape of the overall spectrum. Here, we consider the phonon-magnon interaction which, in a quasistatic approximation, renormalizes the Heisenberg exchange integral. This mechanism is motivated in part by recent experimental observations that the Raman linewidth broadens with increasing temperature. Our results are in good agreement with Raman scattering experiments performed by various groups; in particular, the calculations reproduce the broad line shape of the two-magnon peak, the asymmetry about its maximum, the existence of spectral weight at high energies, and the observation of nominally forbidden A1g scattering.