Optical study of electronic structure and electron-phonon coupling inZrB12

We report the optical $(6\mathrm{meV}–4\mathrm{eV})$ properties of a boride superconductor $\mathrm{Zr}{\mathrm{B}}_{12}$ $(T_c=6\mathrm{K})$ in the normal state from $20\text{to}300\mathrm{K}$ measured on high-quality single crystals by a combination of reflectivity and ellipsometry. The Drude plasma frequency and interband optical conductivity calculated by the self-consistent, full-potential linear muffin-tin orbital method agree well with experimental data. The Eliashberg function ${\alpha }_{\mathrm{tr}}^{2}F\left(\omega \right)$ extracted from optical spectra features two peaks at about 25 and $80\mathrm{meV}$, in agreement with specific heat data. The total coupling constant is ${\lambda }_{\mathrm{tr}}=1.0\pm 0.35$. The low-energy peak presumably corresponds to the displacement mode of Zr inside $B_{24}$ cages, while the second one involves largely boron atoms. In addition to the usual narrowing of the Drude peak with cooling down, we observed an unexpected removal of about 10% of the Drude spectral weight, which was partially transferred to the region of the lowest-energy interband transition $(\sim 1\mathrm{eV})$. This effect may be caused by the delocalization of the metal ion from the center of the $B_{24}$ cluster.