were directly pressed to one another without any spacer in between the two windows in order to avoid internal reflections in the time-domain signal. The cryostat cold finger was moved vertically using a motorised linear stage to switch between sample and reference at each measurement temperature. For each temperature, the sample and the reference were measured at the centre position, with 1000 waveforms co-averaged for each acquisition, resulting in a measurement time of approximately 1 minute for each sample.