an improved electronic circuit has been used to measure tunneling conductance, dI/dV, and its derivative, d2I/dV2 as a function of applied voltage for superconducting junctions. The configuration described permits four-terminal measurements. A harmonic excitation voltage applied to the sample has low noise (<0.1 nV/Hz1/2) and low second harmonic contamination (<10−3%). One unique feature is the use of a digitally switched analog gate in a feedback circuit in order to maintain a constant excitation level over a wide range of dynamic resistance exhibited by typical samples. Furthermore, a balanced bridge circuit is not required and both dI/dV and d2I/dV2 can be measured simultaneously. Alternatively, one can measure dV/dI and d2V/dI2.
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Flesner et al. (1980) studied this question.
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