PARAMAGNETIC-SUSCEPTIBILITY AND PERMITTIVITY MEASUREMENTS AT MICROWAVE-FREQUENCIES IN CRYOGENIC SAPPHIRE RESONATORS

We present high-resolution measurements of the temperature-dependence of mode frequency for three monocrystalline microwave sapphire dielect ric resonators at liquid helium temperatures. In two resonators the su sceptibility is independent of frequency and displays a non-Curie law behaviour. This can be interpreted as arising from a van Vleck tempera ture-independent contribution to the paramagnetic susceptibility of th e Mo3+ and Ti3+ ions involved. We also report measurements of the comb ined dependence of thermal expansion and permittivity on temperature i n the range 2-20 K and show that this is about 50% larger in all three resonators than the Value calculated from the presently accepted ther mal expansion coefficient of sapphire.