Thin-film technology and dc-SQUID instrumentation for a superheated superconducting detector

We report on improvements of the dc-SQUID instrumentation developed to study superheating-supercooling transitions of thin-film indium dots. In previous measurements on ordered arrays of dots with diameters larger than 100µm the flip signal of an individual dot in perpendicular magnetic field has been detected. In order to use these dots as superheated superconducting detectors it is necessary to reduce the dot diameter to lower the energy threshold and to avoid intermediate states. For these reasons, our effort has been to develop small and thick dots with diameter down to10 µm and thickness up to3.5 µm. At the same time the coupling with the dc-SQUID readout was improved using a thin-film niobium pickup coil integrated on the same chip containing the dots. Phase transitions of individual dots were observed. The superheating phase transition spread in temperature of the samples was about 0.15 %