Abstract
The characteristic frequencies of a system provide important information on the phenomena that govern its physical properties. In this framework, there has recently been renewed interest in cryogenic microwave characterization for condensed matter systems since it allows to probe energy scales of the order of a few μeV. However, broadband measurements of the absolute value of a sample response in this frequency range are extremely sensitive to its environment and require a careful calibration. In this paper, we present an in situ calibration method for cryogenic broadband microwave reflectometry experiments that is both simple to implement and through which the effect of the sample electromagnetic environment can be minimized. The calibration references are provided here by the sample itself, at three reference temperatures where its impedance is assumed or measured, and not by external standards as is usual. We compare the frequency-dependent complex impedance (0.1–2 GHz) of an a-Nb15Si85 superconducting thin film obtained through this Sample-Based Calibration (SBC) and through an Open-Short-Load Standard Calibration (SC) when working at a very low temperature (0.02–4 K) and show that the SBC allows us to obtain the absolute response of the sample. This method brings the calibration planes as close as possible to the sample, so that the environment electrodynamic response does not affect the measurement, provided it is temperature independent. This results in a heightened sensitivity, for a given experimental set–up.
Highlights
Measuring the frequency-dependent response of a system has long been a powerful mean to characterize it by determining its natural frequencies
We present an in situ calibration method for cryogenic broadband microwave reflectometry experiments that is both simple to implement and through which the effect of the sample electromagnetic environment can be minimized
We compare the frequency-dependent complex impedance (0.1–2 GHz) of an a-Nb15Si85 superconducting thin film obtained through this Sample-Based Calibration (SBC) and through an Open-Short-Load Standard Calibration (SC) when working at a very low temperature (0.02–4 K) and show that the SBC allows us to obtain the absolute response of the sample
Summary
Measuring the frequency-dependent response of a system has long been a powerful mean to characterize it by determining its natural frequencies. When a microwave is sent towards a sample, the reflected or transmitted signal depends on the sample impedance Z, the quantity of interest, and on the set-up itself This constraint is overcome when working at room temperature where three so-called “Standards” can be measured before determining the sample’s response. Translatable probes are difficult to implement at very low temperatures (
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