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Design and operation of stable rf-biased superconducting point-contact quantum device, and a note on the properties of perfectly clean metal contacts

Je Zimmermen

Published Jan 1, 1970• 0 citations
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Design and Operation of Stable rf-Biased Superconducting Point-Contact Quantum Devices, and a Note on the Properties of Perfectly Clean Metal Contacts

James Edward Zimmerman, Paul Thiene, John T. Harding

Point-contact devices are described which have exhibited long-term stability and reliability. A particular mode of operation of the devices as rf-biased low-frequency detectors is described in detail, including a description of how the device drives a resonant circuit to which it is coupled. In this mode of operation the sensitivity of the devices to low-frequency fields is of the order of 10−9 G per root cycle bandwidth. Perfectly clean contacts were made by breaking a notched niobium member at 4.2°K and then remaking contact at the break. Quantum behavior of such contacts is identical to that of ordinary contacts, but effects of microscopic multiple connections at the contact were not observed.

Journal of Applied Physics•Mar 15, 1970•394 citations

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Current-phase relation and phase-dependent conductance of superconducting point contacts from rf impedance measurements

Robert Rifkin, Bascom S. Deaver

The rf impedance of superconducting point contacts has been measured as a function of the quantum-mechanical phase difference $\ensuremath{\varphi}$ across the point contact. By representing the point contact as a parallel combination of an inductor $\mathfrak{L}(\ensuremath{\varphi})=(\frac{\ensuremath{\hbar}}{2e}){(\frac{\ensuremath{\partial}{I}_{p}}{\ensuremath{\partial}\ensuremath{\varphi}})}^{\ensuremath{-}1}$ and a resistor $R\ensuremath{\equiv}\frac{1}{G(\ensuremath{\varphi})}$ the current-phase relation ${I}_{p}(\ensuremath{\varphi})$ and the phase-dependent conductance $G(\ensuremath{\varphi})$ have been determined from measurements at 30 MHz on phase-biased niobium point contacts. For point contacts with sufficiently small critical current ${I}_{c}$, the inductance was $\mathfrak{L}(\ensuremath{\varphi})=\frac{\ensuremath{\hbar}}{2e{I}_{c}cos\ensuremath{\varphi}}$ yielding the expected negative inductance branch for $\frac{\ensuremath{\pi}}{2}\ensuremath{\le}\ensuremath{\varphi}\ensuremath{\le}\ensuremath{\pi}$ and a sinusoidal current-phase relation. For larger critical currents there were departures from the sinusoidal form for the measured ${I}_{p}(\ensuremath{\varphi})$. There was a phase-dependent conductance that is an increasing function of $\ensuremath{\varphi}$ corresponding to a negative coefficient for the $cos\ensuremath{\varphi}$ term in the Josephson current.

Physical review. B, Solid state•May 1, 1976•89 citations

Engineering superconducting quantum circuits

Yasunobu Nakamura

International Conference on Solid State Devices and Materials (SSDM)•Sep 3, 2019•0 citations
Point-contact devices are described which have exhibited long-term stability and reliability. A particular mode of operation of the devices as rf-biased low-frequency detectors is described in detail, including a description of how the device drives a resonant circuit to which it is coupled. In this mode of operation the sensitivity of the devices to low-frequency fields is of the order of 10−9 G per root cycle bandwidth. Perfectly clean contacts were made by breaking a notched niobium member at 4.2°K and then remaking contact at the break. Quantum behavior of such contacts is identical to that of ordinary contacts, but effects of microscopic multiple connections at the contact were not observed.
The rf impedance of superconducting point contacts has been measured as a function of the quantum-mechanical phase difference $\ensuremath{\varphi}$ across the point contact. By representing the point contact as a parallel combination of an inductor $\mathfrak{L}(\ensuremath{\varphi})=(\frac{\ensuremath{\hbar}}{2e}){(\frac{\ensuremath{\partial}{I}_{p}}{\ensuremath{\partial}\ensuremath{\varphi}})}^{\ensuremath{-}1}$ and a resistor $R\ensuremath{\equiv}\frac{1}{G(\ensuremath{\varphi})}$ the current-phase relation ${I}_{p}(\ensuremath{\varphi})$ and the phase-dependent conductance $G(\ensuremath{\varphi})$ have been determined from measurements at 30 MHz on phase-biased niobium point contacts. For point contacts with sufficiently small critical current Ic{I}_{c}Ic​, the inductance was $\mathfrak{L}(\ensuremath{\varphi})=\frac{\ensuremath{\hbar}}{2e{I}_{c}cos\ensuremath{\varphi}}$ yielding the expected negative inductance branch for $\frac{\ensuremath{\pi}}{2}\ensuremath{\le}\ensuremath{\varphi}\ensuremath{\le}\ensuremath{\pi}$ and a sinusoidal current-phase relation. For larger critical currents there were departures from the sinusoidal form for the measured ${I}_{p}(\ensuremath{\varphi})$. There was a phase-dependent conductance that is an increasing function of $\ensuremath{\varphi}$ corresponding to a negative coefficient for the $cos\ensuremath{\varphi}$ term in the Josephson current.