Resonant two-magnon Raman scattering in parent compounds of high-T$_c$ superconductors.

TL;DR

This paper develops a theoretical model for two-magnon Raman scattering in parent compounds of high-Tc superconductors, explaining experimental features through resonant and nonresonant processes using spin density wave formalism.

Contribution

It introduces a comprehensive theory incorporating resonant effects and triple resonance phenomena to explain two-magnon Raman scattering in high-Tc parent compounds.

Findings

Identifies the dominant diagram in the resonant regime responsible for Raman intensity.

Explains the maximum of the two-magnon peak intensity at the optical data edge.

Provides a theoretical basis for the unusual features observed in two-magnon Raman spectra.

Abstract

We propose a theory of two-magnon Raman scattering from the insulating parent compounds of high-T$_c$ superconductors, which contains information not only on magnetism, but also on the electronic properties in these materials. We use spin density wave formalism for the Hubbard model, and study diagrammatically the profile of the two-magnon scattering and its intensity dependence on the incoming photon frequency $\omega_i$ both for $\omega_i \ll U$ and in the resonant regime, in which the energy of the incident photon is close to the gap between conduction and valence bands. In the nonresonant case, we identify the diagrams which contribute to the conventional Loudon-Fleury Hamiltonian. In the resonant regime, where most of the experiments have been done, we find that the dominant contribution to Raman intensity comes from a different diagram, one which allows for a simultaneous vanishing of all three of its denominators (i.e., a triple resonance). We study this diagram in detail and show that the triple resonance, combined with the spin-density-wave dispersion relation for the carriers, explains the unusual features found in the two-magnon profile and in the two-magnon peak intensity dependence on the incoming photon frequency. In particular, our theory predicts a maximum of the two-magnon peak intensity right at the upper edge of the features in the optical data, which has been one of the key experimental puzzles.