Infrared spectroscopic study of CuO: Signatures of strong spin-phonon interaction and structural distortion

Optical properties of single-crystal monoclinic CuO in the range $70–6000\hspace{0.5em}{\mathrm{cm}}^{-1}$ were studied at temperatures from 7 to 300 K. Normal reflection spectra were obtained from the (001) and (010) crystal faces thus giving separate data for the $A_u$ and $B_u$ phonon modes excited in the purely transverse way (TO modes). Mode parameters, including polarizations of the $B_u$ modes not determined by the crystal symmetry, were extracted by the dispersion analysis of reflectivity curves as a function of temperature. Spectra of all the components of the optical conductivity tensor were obtained using the Kramers-Kronig method recently extended to the case of the low-symmetry crystals. The number of strong phonon modes is in agreement with the factor-group analysis for the crystal structure currently accepted for the CuO. However, several “extra” modes of minor intensity are detected; some of them are observed in the whole studied temperature range, while existence of others becomes evident at low temperatures. Comparison of frequencies of “extra” modes with the available phonon dispersion curves points to possible “diagonal” doubling of the unit cell $\{\mathbf{a},\mathbf{b},\mathbf{c}\overrightarrow{\}}\{\mathbf{a}+\mathbf{c},\mathbf{b},\mathbf{a}-\mathbf{c}\}$ and formation of the superlattice. The previously reported softening of the $A_u^3$ mode $\left(\sim 400\hspace{0.5em}{\mathrm{cm}}^{-1}\right)$ with cooling at $T_N$ is found to be $\sim 10\%$ for the TO mode. The mode is very broad at high temperatures and strongly narrows in the antiferromagnetic phase. We attribute this effect to strong resonance coupling of this mode to optical or acoustic bimagnons and reconstruction of the magnetic excitations spectrum at the Néel point. A significant anisotropy of ${\epsilon }^{\infty }$ is observed: it was found to be 5.9 along the $\mathbf{b}$ axis, 6.2 along the [101] chains, and 7.8 along the $\left[101¯\right]$ chains. The transverse effective charge $e_{\mathrm{T}}^{*}$ is more or less isotropic; its value is about two electrons.