Exciton and exciton-magnon photoluminescence in the antiferromagnet CuB2O4

Copper metaborate ${\mathrm{CuB}}_{2}{\mathrm{O}}_{4}$ crystallizes in a unique noncentrosymmetric structure, becomes antiferromagnetically ordered below ${T}_{\text{N1}}=20$ K, and exhibits a great diversity in magnetic, optical, and magneto-optical properties. In particular, it shows strong photoluminescence rarely observed before in other magnetically ordered copper oxides in which magnetic properties are defined by magnetic ${\mathrm{Cu}}^{2+}$ ($3{d}^{9}$, $S=1/2$) ions. Here we report on the detailed spectroscopic study of the photoluminescence originating from the ${\mathrm{Cu}}^{2+}$ ions. Our investigations are focused on understanding the energy-level scheme of the multiple excitations below the energetically lowest, crystal-field-split $d\text{\ensuremath{-}}d$ electronic transition at 1.405 eV. We identify multiple emission lines, and among them we distinguish three sets of lines, each composed of an exciton line and a satellite attributed to magnon-assisted exciton recombination. The emission intensity of the three sets changes strongly in the temperature range $1.7--40$ K, showing pronounced correlations with the magnetic phase transitions between the commensurate and incommensurate phases. Photoluminescence excitation spectra and time-resolved emission dynamics give closer insight into the energy relaxation channels populating the exciton-magnon sets.