Phonons, Magnons, and Excitons in the Noncentrosymmetric Magnetoelectric Antiferromagnet CuB2O4

In the last two decades copper metaborate CuB₂O₄ with a unique noncentrosymmetric crystal structure has become the subject of active research due to its unusual magnetic and optical properties. We consider the propagation and absorption of light in CuB₂O₄ based on the solution of Maxwell’s equations. We present an overview of the main results on the investigation of the phonon spectrum using infrared and Raman spectroscopy. Studies in the region of electronic transitions in Cu²⁺ ions in the crystal field have allowed the separation of contributions to the optical absorption from copper ions in inequivalent positions. A splitting of zero-phonon absorption lines in a magnetic field has been detected, and these results have received a theoretical explanation in terms of the exciton model. A rich structure of exciton–magnon states has been observed in the photoluminescence spectra. We have carried out a spectroscopic study of the optical second harmonic generation in the region of excitonic transitions, which has allowed the contribution of the toroidal moment and the Fano resonance to the observed signals to be revealed.