Electronic Structure and Lattice Dynamics of CuB₂O₄ and Cu¹¹B₂O₄ Single Crystals
Rea Divina Mero1*, Chun-Hao Lai2, Chao-Hung Du2, Hsiang-Lin Liu1
1Physics Department, National Taiwan Normal University, Taipei, 11677, Taiwan
2Physics Department, Tamkang University, Tamsui, New Taipei City, 25137, Taiwan
* Presenter:Rea Divina Mero, email:readivina@gmail.com
We investigated the electronic structure and lattice dynamics of copper metaborate CuB₂O₄ (nature boron) and Cu¹¹B₂O₄ (isotope boron). Copper metaborate is known to have a unique crystal structure and exhibits interesting magnetic transitions at low temperatures. Canted antiferromagnetic ordering occurs below TN = 21 K followed by a commensurate to incommensurate magnetic structure at T = 10 K. Optical absorption spectrum at room temperature reveals electronic transitions at 4.4 and 5.6 eV for CuB₂O₄ and at 4.2 and 5.2 eV for Cu¹¹B₂O₄ . These are related to charge transfer of electrons from the 3d states of the Cu2+ ions to the 2p states of O ions. Temperature dependence of the absorption spectrum shows that the transition energies shifts to higher energies and narrower linewidths as temperature decreases. Enhancement of the energy peak intensity was observed below 21 K along with anomalies in energy and linewidth behavior. Room temperature Raman scattering spectrum of CuB₂O₄ shows similar peaks with Cu¹¹B₂O₄ which is slightly red shifted from the CuB₂O₄ spectra. A more pronounced red shift is observed in the higher phonon peaks above 800 cm-1. These peaks correspond to the modes directly related to the boron sites. As temperature decreases, phonon frequencies shift to higher wavenumber and the linewidth decreases. Both compounds exhibit anomalous softening in the Raman peaks below 21 K. These results suggest a complex nature of spin-charge-phonon interactions in CuB₂O₄.

Keywords: strongly correlated electrons, magnetic phase transitions, lattice dynamics, electronic structure, copper metaborate