Second-order magnetoelectric susceptibility in the optical region: The case of boraciteCu3B7O13Br

Abstract

In the cubic phase of boracite ${\mathrm{Cu}}_{3}{\mathrm{B}}_{7}{\mathrm{O}}_{13}\mathrm{Br}$ (symmetry class ${T}_{d})$ the anisotropy and dispersion of nonreciprocal linear dichroism (ND) and nonreciprocal linear birefringence (NB) have been studied. The two magnetic-field-induced effects belong to the class of spatial dispersion phenomena. They have been observed near to the two strong absorption bands at the photon energies ${E}_{01}=1.5\mathrm{eV}$ and ${E}_{02}=1.2\mathrm{eV}.$ ${E}_{01}$ and ${E}_{02}$ are assigned to electronic transitions from the singlet ground state ${}^{2}{\mathrm{B}}_{2}$ to states of the doublet ${}^{2}E.$ The analysis of dispersion of ND and NB is based on the crystal-field theory whereby spin-orbit coupling and Zeeman interaction have been included. An adequate description is obtainable only if the symmetry of the crystal field at ${\mathrm{Cu}}^{2+}$ ions is lower than tetragonal. Two mechanisms are discussed as the origin of symmetry reduction. At first, a structural disorder of halogen and/or metal ions in the boracite crystal lattice produces low-symmetry components of the crystal field and these components split the excited state ${}^{2}E.$ Second, symmetry is reduced by the dynamic Jahn-Teller effect.