Optical study of electronic structure and electron-phonon coupling inZrB12

Abstract

We report the optical $(6\phantom{\rule{0.3em}{0ex}}\mathrm{meV}--4\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ properties of a boride superconductor $\mathrm{Zr}{\mathrm{B}}_{12}$ $({T}_{c}=6\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ in the normal state from $20\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 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 ${\ensuremath{\alpha}}_{\mathrm{tr}}^{2}F(\ensuremath{\omega})$ extracted from optical spectra features two peaks at about 25 and $80\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, in agreement with specific heat data. The total coupling constant is ${\ensuremath{\lambda}}_{\mathrm{tr}}=1.0\ifmmode\pm\else\textpm\fi{}0.35$. 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 $(\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$. This effect may be caused by the delocalization of the metal ion from the center of the ${B}_{24}$ cluster.