Complex permittivity of some ultralow loss dielectric crystals at cryogenic temperatures

Whispering gallery modes were used for very accurate permittivity and diele ctric loss measurements of ultralow loss isotropic and uniaxially anisotrop ic single crystals. Several materials including sapphire, YAG, quartz, and SrLaAlO4 were measured. The total absolute uncertainty in the real part of permittivity tensor components was estimated to be +/-0.1%, limited princip ally by the uncertainty in sample dimensions. Imaginary parts of permittivi ties were measured with uncertainties of about 10%, limited by the accuracy of e-factor measurements of whispering gallery modes. It has been observed that, for most crystals, dielectric losses can be approximated by a power function of absolute temperature only in limited temperature ranges. At tem peratures between 4-50 K, losses are often affected by impurities, which ar e always present in real crystals.