Anomalous boron isotope effects on electronic structure and lattice dynamics of CuB2O4

Abstract

Copper metaborate had a unique crystal structure and exhibited noteworthy magnetic phase transitions at 21 and 10 K. The electronic structure and lattice dynamics of copper metaborate Cu¹¹B₂O₄ single crystals were investigated and compared with the optical properties of CuB₂O₄, to assess the boron isotope effect. The optical absorption spectrum at room temperature revealed two charge-transfer bands at approximately 4.30 and 5.21 eV with an extrapolated direct optical band gap of 3.16 ± 0.07 eV. Compared with the data on CuB₂O₄, the electronic transitions were shifted to lower energies upon the replacement of a heavier boron isotope. The band gap was also determined to be lower in Cu¹¹B₂O₄. Anomalies in the temperature dependence of the optical band gap were observed below 21 K. Furthermore, 38 Raman-active phonon modes were identified in the room-temperature Raman scattering spectrum of Cu¹¹B₂O₄, which were also observed in CuB₂O₄ with a shift to lower frequencies. No broadening caused by isotopic changes was observed. As the temperature decreased, phonon frequencies shifted to higher wavenumbers and the linewidth decreased. Anomalous softening in the Raman peaks below 21 K was also revealed.

Fig. 1. The temperature dependence of magnetic susceptibility measured in an applied magnetic field of 50 Oe for H ⊥[001] of single-crystal Cu¹¹B₂O₄ and CuB₂O₄.

Fig. 2. Optical absorption spectra of Cu¹¹B₂O₄ and CuB₂O₄ at room temperature. The dashed lines illustrate the best fit with the Lorentz function. Inset illustrates the direct band gap analysis of Cu¹¹B₂O₄ and CuB₂O₄ at 300 K.

Fig. 3. Temperature-dependent optical absorption spectra of Cu¹¹B₂O₄. Inset illustrates the temperature-dependent band gap energy. The vertical dashed lines denote the magnetic phase transition temperatures at 10 and 21 K.

Fig. 4. Temperature dependence of the peak energy, linewidth, and normalized intensity of the (a) 4.30 and (b) 5.21 eV optical transitions. The vertical dashed lines denote the magnetic phase transition temperatures at 10 and 21 K.

Fig. 5. Unpolarized room-temperature Raman scattering spectrum of Cu¹¹B₂O₄ and CuB₂O₄. Inset illustrates the polarized Raman scattering spectra of Cu¹¹B₂O₄.

Fig. 6. Temperature-dependent unpolarized Raman scattering spectra of Cu¹¹B₂O₄. Inset illustrates the results of fitting the spectrum obtained at 10 K using the Lorentzian model.

Fig. 7. Temperature dependence of the frequency, linewidth, and normalized intensity of (a) 333 and (b) 443 cm⁻¹ phonon modes. Thin solid lines are the fitting results obtained with the anharmonic model. The vertical dashed lines denote the magnetic phase transition temperatures at 10 and 21 K.

Fig. 8. Temperature dependence of the shift in the phonon frequency of (a) 333 and (b) 443 cm⁻¹ modes plotted against the normalized square of the magnetic susceptibility. The vertical dashed line denotes the magnetic phase transition temperatures at 21 K.