Theory of phonon-assisted "forbidden" optical transitions in spin-gapped systems

TL;DR

This paper develops a theoretical framework explaining how phonons enable forbidden optical transitions involving magnetic excitations in spin-gapped systems, aligning with experimental observations.

Contribution

It introduces a new theory showing phonon-assisted electric dipole transitions in spin-gapped materials, explaining previously puzzling polarized optical experiments.

Findings

The theory accounts for polarization dependence in CuGeO3 and SrCu2(BO3)2.

Virtual phonons can break inversion symmetry, enabling forbidden transitions.

The effective operator derived predicts transition intensities consistent with experiments.

Abstract

We consider the absorption of light with emission of one S(tot)=1 magnetic excitation in systems with a spin gap induced by quantum fluctuations. We argue that an electric dipole transition is allowed on the condition that a virtual phonon instantaneously breaks the inversion symmetry. We derive an effective operator for the transition and argue that the proposed theory explains the polarized experiments in CuGeO(3) and SrCu(2)[BO(3)](2).