Ultrastable cryogenic sapphire dielectric microwave resonators

Measurements of the properties of cryogenic sapphire resonators with various concentrations of paramagnetic ions are reported, and a new way of achieving self-compensation of temperature fluctuations for such resonators mounted inside conducting shields is described. It is shown that frequency-temperature self-compensation, in which the T/sup 4/ temperature dependence of the sapphire dielectric constant compensates the effect of the T/sup -1/ Curie law of paramagnetic impurities, can be achieved at convenient operating temperatures around 6 K. Samples with a few ppm of chromium and iron impurity ions display strong influence on mode frequency and Q of nearby spin resonances at 11.45 GHz (Cr/sup 3+/) and 12.05 GHz (Fe/sup 3+/) and fail to compensate above the spin resonances. In samples with much reduced chromium and iron concentrations, an ion with a very high frequency paramagnetic resonance (such as Ti/sup 3+/) has been exploited to obtain essentially frequency-independent self-compensation.<
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