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Physica B Condensed Matter

Observation of quantum susceptance in superconducting tunnel junctions

Qing Hu, C. A. Mears, P. L. Richards, Frances L. Lloyd

Published Aug 1, 1990• 2 citations
Publisher
DOI
10.1016/s0921-4526(09)80379-7

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Superconductor Science and Technology•Mar 3, 2010•1 citation
We have made the first direct measurement of the quantum susceptance that arises from the nondissipative part of quasiparticle tunneling in a superconductor-insulator-superconductor tunnel junction. The junction is coupled to an antenna and a superconducting microstrip stub to form a resonator; the resonant frequency is determined from the response of the junction to broadband radiation from a Fourier-transform spectrometer. A 19% shift of the resonant frequency, from 73 to 87 GHz, is observed, which arises from the change of the quantum susceptance of the junction with dc bias voltage. This shift is in excellent agreement with calculations based on the Werthamer-Tucker theory, which includes the quantum susceptance. We also demonstrate that it is essential to include the quantum susceptance in our theoretical computation to explain the photon-assisted-tunneling steps, which have negative dynamic conductance. Such steps are observed when the junction is pumped at slightly below the resonant frequency of the capacitor and the stub. The quantum susceptance should exist in all tunnel devices whose nonlinear I-V characteristics are due to elastic tunneling.
In this paper, we present an investigation of the contribution of quantum susceptance in capacitance measurements of superconductor-insulator-superconductor tunnel junctions. We directly measured the one-port S-parameter S11by utilizing a 4-K probe station in combination with a network analyzer. We observed a bias voltage dependence of the measured junction susceptance which demonstrates the contribution of quantum susceptance. Calculation results performed, based on a simple equivalent circuit model, were in excellent agreement with the measurements. Experiment and analysis methods suggested here enable the quantitative verification of the contribution of quantum susceptance and are an effective way of extracting the junction's geometric capacitance.
Here we report normal-state conductance measurements of three different types of superconducting tunnel junctions that are being used or proposed for quantum computing applications: p-Al/a-AlO/p-Al, e-Re/e-AlO/p-Al, and e-V/e-MgO/p-V, where p stands for polycrystalline, e for epitaxial, and a for amorphous. All three junctions exhibited significant deviations from the parabolic behavior predicted by the WKB approximation models. In the p-Al/a-AlO/p-Al junction, we observed enhancement of tunneling conductances at voltages matching harmonics of Al–O stretching modes. On the other hand, such Al–O vibration modes were missing in the epitaxial e-Re/e-AlO/p-Al junction. This suggests that absence or existence of the Al–O stretching mode might be related to the crystallinity of the AlO tunnel barrier and the interface between the electrode and the barrier. In the e-V/e-MgO/p-V junction, which is one of the candidate systems for future superconducting qubits, we observed suppression of the density of states at zero bias. This implies that the interface is electronically disordered, presumably due to oxidation of the vanadium surface underneath the MgO barrier, even if the interface was structurally well ordered, suggesting that the e-V/e-MgO/p-V junction will not be suitable for qubit applications in its present form. This also demonstrates that the normal-state conductance measurement can be effectively used to screen out low quality samples in the search for better superconducting tunnel junctions.