Optimum DC current resolution of a ferromagnetic-core flux transformer coupled SQUID instrument

This paper describes theoretical and reproducible experimental results for a ferromagnetic-core (VITROVAC) flux transformer coupled SQUID instrument with equivalent noise temperatures < 1 K for 10-kΩ source resistance. The optimum dc current resolution of the superconducting transformer with and without a ferromagnetic core will be discussed. The high resolution of the SQUID instrument is demonstrated by the noise current plot and by noise temperature data of two-dimensional electron gases in GaAs-Gax Al1−xAs heterostructures.