Impedance measurements of superconducting transition-edge sensors (TESs) are a powerful tool to obtain information about the TES thermal and electrical properties. We apply this technique to a 20 μm × 20 μm Ti/Au TES, suitable for application in the optical and near-infrared range, and extend the measurements up to 250 kHz in order to obtain a complete frequency response in the complex plane. From these measurements we obtain important thermal and electrical device parameters such as heat capacity C, thermal conductance G and effective thermal time constant τ eff that will be compared with the corresponding values obtained from noise measurements.

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A theory for the voltage-current characteristic in high T C DC SQUIDs, which accounts for a second harmonic in the junction current-phase relation, is developed. The comparison with experiment is performed. It is shown that if the second harmonic is prevailed, the theory can explain the large deviations of the experimental voltage modulation from theoretical predictions and computer simulations based on conventional sinusoidal current-phase relation.PACS: 74.50.+r; 85.25.-j; 85.25.Dq IntroductionAs is well known, there exists a significant discrepancy between experimental results and numerical simulations of the voltage-to-flux transfer function of high T C DC SQUIDs [1,2]. This is one of the most important unsolved problems, which seriously hinders the optimization of high T C DC SQUIDs for applications.Inspite of extensive computer simulations [1,3] and theoretical studies [4, 5, 6] that have been performed in the attempt to predict reliably transfer function and energy resolution of high T C DC SQUIDs, a marked disagreement with experiment still exists: experimental transfer functions in many cases are much lower than the values predicted by theory and computer simulations; the white noise is about ten times higher than predicted.One of the possible reasons for these discrepancies could be attributed to the junction asymmetry of SQUID interferometer (unequal critical currents or (and) normal resistances ), which for grain boundary junctions is about 20%−30% due to on chip technological heterogeneity. However, the

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