The effects of an air gap on the measurement of the dielectric constant of at cryogenic temperatures

A resonator technique has been employed to measure the relative permittivity of single crystal squares of strontium titanate in the temperature range 20-300 K at microwave frequencies. We found that the method of metallization of the strontium titanate crucially affects the effective measured dielectric constant, with an air gap as small as giving a 50% error in the inferred value of at 20 K; as the gap size increases the corresponding error increases dramatically. This has important implications when using strontium titanate, or indeed any high-permittivity crystals, at cryogenic temperatures.