Dynamics of electronic transport

We developed a microwave calibration system at low temperatures (1.2 K). The microwave reflectometry measurements must be calibrated at the sample level using 3 known impedances (usually an open circuit (open), a short circuit (short) and a load resistor 50 Ohms (load). This procedure is very tricky at low temperatures because it requires several thermal cycling that deteriorate the quality of the calibration. We realized a device allowing this calibration textit {in situ} in a 4He cryostat pumped in a frequency range dc - 2 GHz (Fig. a) Its operating principle is based on a rotary switch that allows to successively connect the calibration impedances and the sample while keeping an identical line geometry (Fig. c). The set of curves represented in the figure ref {fig_cal} - textbf {c} corresponds to calibration points (open-short-load) in reflection and the calibrated measurement of the impedance of a sample of Nb_ {0.25} Si_ {0.75} to 4.2 K} $. The different curves correspond to successive measurements. The sample (composition and geometry) was chosen so that its resistance is close to $ 50 , Omega $.