We investigate the complex conductivity of a highly disordered MoC superconducting film with kF l ≈ 1, where kF is the Fermi wavenumber and l is the mean free path, derived from experimental transmission characteristics of coplanar waveguide resonators in a wide temperature range below the superconducting transition temperature Tc. We find that the original Mattis-Bardeen model with a finite quasiparticle lifetime, τ , offers a perfect description of the experimentally observed complex conductivity. We show that τ is appreciably reduced by scattering effects. Characteristics of the scattering centers are independently found by the scanning tunneling spectroscopy and agree with those determined from the complex conductivity.
An ultra-low-noise one-stage SiGe heterojunction bipolar transistor amplifier was designed for cryogenic temperatures and a frequency range of 10 kHz-100 MHz. A noise temperature T(N) ≈ 1.4 K was measured at an ambient temperature of 4.2 K at frequencies between 100 kHz and 100 MHz for a source resistance of ~50 Ω. The voltage gain of the amplifier was 25 dB at a power consumption of 720 μW. The input voltage noise spectral density of the amplifier is about 35 pV/√Hz. The low noise resistance and power consumption makes the amplifier suitable for readout of resistively shunted DC SQUID magnetometers and amplifiers.
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