Far infrared optical properties of the pyrochlore spin ice compound Dy2Ti2O7

Near normal incidence far infrared reflectivity spectra of [111] dysprosium titanate (Dy2Ti2O7) single crystal have been measured at different temperatures. Seven phonon modes (eight at low temperature) are identified at frequencies below 1000 cm-1. Optical conductivity spectra are obtained by fitting all the reflectivity spectra with the factorized form of the dielectric function. Both the Born effective charges and the static optical permittivity are found to increase with decreasing temperature. Moreover, phonon linewidth narrowing and a phonon mode shift with decreasing temperature are also observed, which may result from enhanced charge localization. The redshift of several low frequency modes is attributed to the spin-phonon coupling. All observed optical properties can be explained within the framework of the nearest neighbour ferromagnetic (FM) spin ice model