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IEEE Transactions on Magnetics

Observation of macroscopic quantum effects in high T/sub c/ weak link superconducting rings

Yin Huang, Hai Jiang, A. Widom, C. Vittoria

Published Sep 1, 1992• 3 citations
Publisher

Abstract

Thin-film weak link rings were constructed using high-T/sub c/ YBaCuO superconducting thin films. The thickness of the films was 5000 AA. The weak links were biased with an AC induced current. The resulting electron pair charges on the weak links were monitored by an output voltage. The charge stored on the link (as a function of the input bias voltage) is consistent with that of a macroscopic quantum pendulum.>

DOI
10.1109/20.179451

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We report here numerical calculations on the properties of superconducting weak links as mixers with external local oscillators (LO). Our calculations are based on the resistively shunted-junction model. We assume that the LO and the signal are ideal current sources of amplitudes ILO and IS and of frequencies ωLO and ωs, respectively. We calculate the IF voltage across the weak link as a function of the bias current, the LO current amplitude ILO, and the normalized signal frequency Ωs = ħωs/(2 e Ic R), where Ic is the critical current and R is the shunt resistance. If ωIF/ωLO and Is/Ic are small, we find a linear IF response. Using this method we find the frequency dependence of the IF response, estimate the conversion efficiency for the matched case in both the absence and the presence of noise, and calculate the required LO power. We have fabricated thin-film constrictions by using electron lithography. We have made constrictions of indium films with thicknesses of ~ 1 200 A with widths as narrow as 2 000 A and lengths as short as 1 000 A and have studied their I-V characteristics in the presence of RF radiation and measured their X-band mixing properties. We have measured an NEP of 10-13 W/Hz1/2 and a dynamic range of 23 dB (3 dB deviation from linearity) and estimate that, with optimum matching, these would become 3 × 10-18 W/Hz1/2 and 74 dB, respectively.

Revue de Physique Appliquée•Jan 1, 1974•23 citations
Thin-film weak link rings were constructed using high-T/sub c/ YBaCuO superconducting thin films. The thickness of the films was 5000 AA. The weak links were biased with an AC induced current. The resulting electron pair charges on the weak links were monitored by an output voltage. The charge stored on the link (as a function of the input bias voltage) is consistent with that of a macroscopic quantum pendulum.>
We fabricated weak link rings using YBCO thin films deposited on the single crystal MgO substrate by laser ablation. The dimensions of the microbridges in the rings were between 0.5–1 μ in length and 500 Å–0.5 μ in width which were made by focused ion beam milling. We biased the ring by Faraday induction voltage while monitoring the voltage across the microbridge. The frequency of the input signal was varied between 10 kHz and 5 MHz. When we superimposed a microwave signal (2–18 GHz) to the ring, below the superconducting transition temperature (Tc), we observed strong nonlinearities in the voltage measured across the microbridge. The microwave frequency was chosen to be close to the ring natural electromagnetic resonance.
When a thin-film ring with supercurrent is irradiated by coherent microwave field, the field induces coherent oscillations of all the Cooper pairs involved in the supercurrent. The oscillating charged condensate will emit photons with energies determined by the difference of quantized energy levels of the superconducting ring. This effect can be observed only in the rings, the thickness of which is less than both the field skin-depth and the London penetration depth. The probability of the microwave field-induced single-photon and two-photon emission is calculated. The energy - angle distributions of the emitted photons as a function of both the ring sizes and the fluxoid number trapped in the ring, are explored.
We have fabricated a YBa2Cu3O7 (YBCO) thin-film dc superconducting quantum interference device (SQUID) with lithographically defined deep-submicron weak links. At 77 K the voltage response to dc flux is sinusoidal and nonhysteretic, with maximum peak-to-peak amplitude of 5 μV. The maximum response is 8 μV at 70 K, where 2LI0/Φ0≊1. At lower temperatures, the maximum response oscillates in the range 4–6 μV. Random telegraph noise was observed near 30–40 K. Well-behaved Shapiro steps were observable at all measured temperatures below Tc.
Voltage biasing of a single superconducting weak link results in coherent electromagnetic oscillation at the frequency $\ensuremath{\nu}=\frac{V}{{\ensuremath{\Phi}}_{0}}$; simultaneous voltage biasing of two weak links in a superconducting ring produces two strongly coupled oscillators at the same frequency. The electromagnetic phase difference is the difference in the gauge-invariant quantum phases across the two contacts. Because of the quantization of the superconducting ring, this phase difference is $\frac{2\ensuremath{\pi}〈\ensuremath{\Phi}〉}{{\ensuremath{\Phi}}_{0}}$, where $〈\ensuremath{\Phi}〉$ is the time-averaged magnetic flux in the superconducting circuit and ${\ensuremath{\Phi}}_{0}$ is the flux quantum. We can characterize the coupled system in terms of normal modes, and we have experimentally observed these modes of oscillation at 30 MHz. This provides a means of understanding the earlier observations of quantum interference in the resistive state.
A thin-film dc superconducting quantum interference device (SQUID) consisting of quasiparticle-injected superconducting weak links was fabricated and successfully operated. This SQUID was made on a silicon substrate and it consists of a ∼100-nm-thick lead alloy with two injector junctions. The characteristics of this dc SQUID are controllable by varying the injecting rate of quasiparticles; therefore, it can be operated at arbitrary temperatures below the transition temperature of the superconducting material.
We report here numerical calculations on the properties of superconducting weak links as mixers with external local oscillators (LO). Our calculations are based on the resistively shunted-junction model. We assume that the LO and the signal are ideal current sources of amplitudes ILO and IS and of frequencies ωLO and ωs, respectively. We calculate the IF voltage across the weak link as a function of the bias current, the LO current amplitude ILO, and the normalized signal frequency Ωs = ħωs/(2 e Ic R), where Ic is the critical current and R is the shunt resistance. If ωIF/ωLO and Is/Ic are small, we find a linear IF response. Using this method we find the frequency dependence of the IF response, estimate the conversion efficiency for the matched case in both the absence and the presence of noise, and calculate the required LO power. We have fabricated thin-film constrictions by using electron lithography. We have made constrictions of indium films with thicknesses of ~ 1 200 A with widths as narrow as 2 000 A and lengths as short as 1 000 A and have studied their I-V characteristics in the presence of RF radiation and measured their X-band mixing properties. We have measured an NEP of 10-13 W/Hz1/2 and a dynamic range of 23 dB (3 dB deviation from linearity) and estimate that, with optimum matching, these would become 3 × 10-18 W/Hz1/2 and 74 dB, respectively.