Optical and magneto-optical properties and electronic structures of single-crystallineRAl2(R=Y, La, Ce, Pr, and Lu)

Abstract

The diagonal optical conductivity spectra of single crystals of $R{\mathrm{Al}}_{2}$ $(R=\mathrm{Y},$ La, Ce, Pr, and Lu) were measured at room temperature by spectroscopic ellipsometry in the $1.5--5.6\ensuremath{-}\mathrm{eV}$ range. All the compounds exhibit two strong interband absorption peaks at about 1.8 and 3.6 eV for ${\mathrm{YAl}}_{2}$ and ${\mathrm{LuAl}}_{2},$ and at about 2.0 and 3.0 eV for ${\mathrm{LaAl}}_{2},$ ${\mathrm{CeAl}}_{2},$ and ${\mathrm{PrAl}}_{2}.$ Such differences in the second peak position appear in the theoretical optical conductivity spectra calculated from their band structures obtained by the tight-binding linear-muffin-tin-orbitals method. Most of the contributions to the two peaks in ${\mathrm{LaAl}}_{2}$ are from the p and d states, i.e., $\stackrel{\ensuremath{\rightarrow}}{p}d$ and $\stackrel{\ensuremath{\rightarrow}}{d}p$ transitions, while those involving f states are negligible. These results suggest that f character near ${E}_{F}$ for ${\mathrm{LaAl}}_{2},$ ${\mathrm{CeAl}}_{2},$ and ${\mathrm{PrAl}}_{2}$ distorts their conduction bands significantly through hybridization, leading to different optical spectra compared to those of ${\mathrm{YAl}}_{2}$ and ${\mathrm{LuAl}}_{2}.$ The magneto-optical properties of ${\mathrm{CeAl}}_{2}$ and ${\mathrm{PrAl}}_{2}$ were measured at low temperatures. The Kerr rotation $({\ensuremath{\Theta}}_{K})$ and ellipticity $({\ensuremath{\epsilon}}_{K})$ for both compounds show similar spectral variations with maximum ${\ensuremath{\Theta}}_{K}$ of $0.35\ifmmode^\circ\else\textdegree\fi{}$ and $0.55\ifmmode^\circ\else\textdegree\fi{}$ at about 2.1 eV for ${\mathrm{CeAl}}_{2}$ and ${\mathrm{PrAl}}_{2},$ respectively. The evaluated off-diagonal conductivity spectra of the two compounds are also similar, with two structures at about 2.1 and 3.8 eV for ${\mathrm{CeAl}}_{2}$ and 2.1 and 3.4 eV for ${\mathrm{PrAl}}_{2}.$ The energy difference in the second structures is interpreted as due to the different conduction-band structures of the two compounds caused by different hybridization strengths of their f states with conduction bands, because of the difference in their degree of localization.