Terahertz Spectroscopy of Sr$_{2-x}$Ca$_{x}$CoOsO$_{6}$ Double Perovskites*

The osmate double perovskite family of materials Sr$_{2-x}$Ca$_{x}$CoOsO$_{6}$ displays a rich phase diagram as a function of temperature and Ca substitution. Using time-domain terahertz (THz) spectroscopy, we observe that a phonon centered at a frequency of 1.4 THz disappears between x=0 and x=0.1. However, this phonon seems to be decoupled from the magnetic and structural phase transitions that occur as a function of temperature. In addition, the THz conductivities of the compacted powders below 1 THz can be interpreted either as variable range hopping conductivity, or understood within the framework of universality of disordered systems (Jonscher Law). We will also report a study of absorptions and spin-phonon coupling in the infrared using Fourier transform infrared spectroscopy.