Dielectric measurements on insulating materials at cryogenic temperatures can be challenging, depending on the frequency and temperature ranges of interest. We present a technique to study the dielectric properties of bulk dielectrics at GHz frequencies. A superconducting coplanar Nb resonator is deposited directly on the material of interest, and this resonator is then probed in distant-flip-chip geometry with a microwave feedline on a separate chip. Evaluating several harmonics of the resonator gives access to various probing frequencies in the present studies up to 20 GHz. We demonstrate the technique on three different materials (MgO, LaAlO3, and TiO2), at temperatures between 1.4 K and 7 K.
REFERENCES
The bare feedline chip was measured in the same sample holder as the case with the superconducting resonator, but with the complete sample holder at room temperature. Therefore, the coaxial lines of the sample holder and the coplanar feedline here have higher losses than for the 1.4 K measurement of the resonator.
The kinetic inductance Lk of a superconductor is proportional to the inverse of the superfluid density and hence proportional to λ2.71 Our modeling via Eq. (3) thus describes the temperature-dependent Lk of our resonator, and the rather small difference between measured and corrected data in Fig. 4 indicates that Lk of our transmission is much smaller than its geometric inductance.46