Observation of nondissipative quasiparticle tunnel currents in superconducting tunnel junctions

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Abstract
We have made the first direct measurement of the quantum susceptance which arises from the nondissipative part of the quasiparticle tunneling in a superconductor-insulator-superconductor (SIS) junction. The resonant frequency of a resonator which contains an SIS junction depends on the dc bias due to the bias dependence of the quantum susceptance. The observed 19% shift of the resonant frequency, form 73 to 87 GHz, is in excellent agreement with the Werthamer-Tucker theory. This quantum susceptance should exist in other devices such as quantum-well structures.