Spectroscopic Signature of Spin–Charge–Lattice Coupling in CuB₂O₄

CuB 2 O 4 has attracted considerable attention because of its unique chemical and physical properties and potential practical applications. In this paper, we investigated the optical properties of CuB 2 O 4 single crystals through spectroscopic ellipsometry and Raman scattering spectroscopy. The optical absorption spectrum at room temperature revealed a direct band gap at approximately 3.88 ± 0.01 eV and two bands near 4.49 and 5.90 eV. The observed bands were related to charge transfer of electrons from the 2p states of the oxygen ions to the 3d states of the copper ions. The band gap exhibited unusual redshift with a decrease in temperature. Upon cooling across 21 K, which is the canted antiferromagnetic ordering temperature, the band gap, peak energy, and normalized intensity of charge-transfer bands presented anomalies. Furthermore, 38 phonon modes were identified in the room-temperature Raman scattering spectrum of CuB 2 O 4 . The phonon modes at approximately 335, 393, 403, 445, 473, 598, 706, 787, and 900 cm –1 exhibited softening below 21 K. The spin–phonon coupling constants were estimated to be 0.02–0.03 cm –1 . These findings highlight a complex nature of spin–charge–lattice interactions in CuB 2 O 4 .