Broadband microwave spectroscopy in Corbino geometry for temperatures down to 1.7 K

We present a broadband microwave spectrometer covering the range from 45 MHz up to 20 GHz (in some cases up to 40 GHz) which employs the Corbino geometry, meaning that the flat sample terminates the end of a coaxial transmission line. This setup is optimized for low-temperature performance (temperature range 1.7-300 K) and for the study of highly conductive samples. The actual sensitivity in reflection coefficient can be as low as 0.001, leading to a resolution of 10% in absolute values of the impedance or complex conductivity. For optimum accuracy a full low-temperature calibration is necessary; therefore up to three calibration measurements (open, short, and load) are performed at the same temperature as the sample measurement. This procedure requires excellent reproducibility of the cryogenic conditions. We compare further calibration schemes based on just a single low-temperature calibration measurement or employing a superconducting sample as a calibration standard for its normal state, and we document the capability of the instrument with test measurements on metallic thin films. Finally we apply the spectrometer to thin films of a heavy-fermion compound as an example for a strongly correlated electron system. (c) 2005 American Institute of Physics.