Exciton and exciton-magnon photoluminescence in the antiferromagnet CuB2O4

Copper metaborate CuB2O4 crystallizes in a unique noncentrosymmetric structure, becomes antiferromagnetically ordered below T-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 Cu2+ (3d(9), S = 1/2) ions. Here we report on the detailed spectroscopic study of the photoluminescence originating from the Cu2+ ions. Our investigations are focused on understanding the energy-level scheme of the multiple excitations below the energetically lowest, crystal-field-split d-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.