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Japanese Journal of Applied Physics

The Josephson Effect in a Ceramic Bridge at Liquid Nitrogen Temperature

Peiheng Wu, Qilin Cheng, Shuzhen Yang, J. Chen, Y. Li, Zheng Ji, Jiming Song, Huanjun Lu, Xinliu Gao, Jianghua Wu, Xiuying Zhang

Published Oct 1, 1987• 13 citations
Publisher

Abstract

A bridge junction can be made by cutting a bulk ceramic superconductor into the right sizes. Using this sort of bridge, both dc and ac Josephson effects are confirmed in liquid nitrogen. We have observed microwave-induced steps at either nhf/2e or 2nhf/2e, where n is an integral, as well as the modulation of a critical supercurrent with the increase of microwave power or the externally applied magnetic field.

DOI
10.1143/jjap.26.l1579

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I. The phase-current relation at zero voltage in proximity effect bridges. II. The interaction of proximity effect bridges with superconducting microstrip resonators

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Experimental investigations on the proximity effect bridge (a Josephson device) at zero voltage and at finite voltages in the µV range are reported. The phase-super current relation at zero voltage was measured using an asymmetric superconducting quantum interferormeter circuit. The data are in agreement with the Josephson supercurrent-phase relation I_S=I_C sinδ with deviation less than 5% of the critical current I_c. The supercurrent density in the measured bridges reached as high as 50-100 µA/µm^2. Using microcircuitry techniques, proximity effect bridges were strongly coupled to superconducting microstrip resonators. Selfinduced steps in the I-V characteristics of bridges coupled to resonators were observed in the GHz region at voltages (frequencies) corresponding to the expected modes of the resonators. Two types of steps were seen depending on whether the resonator impedance on resonance was much higher or much smaller than the bridge resistance. A simple two fluid model of the bridge-resonator circuit was developed and the size and shape of self-induced steps were calculated for a generalized Josephson oscillator relation I_S = I_c(l-q + q sin∫ 2e/hV dt) where q = 1 corresponds to the original Josephson relation and q = 1/2 represents the phase slip regime. At low critical currents (I_c < 10 µA) and low voltages (V < 3µV) the size and shape of experimentally observed self-induced steps agree with the q = 1 model. At higher voltages and/or critical currents the step size increasingly deviates from the q = 1 model towards q = 1/2. These observations are interpreted to indicate a progressive reduction of the amplitude of the oscillating Josephson supercurrent in proximity effect bridges from I_c towards I_c /2 as the critical current and/or voltage are increased.

•Jan 1, 1976•0 citations

Electromagnetic properties of the Dayem bridge

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The first measurements on the Dayem bridge showed that the behaviour in an applied microwave field was quite different from that of the Josephson tunnel junction. Measurements on bridges one magnitude smaller later showed this difference to result from the bridge size. A very simple equivalent circuit was furthermore shown to characterize these small bridges. Analog computer calculations on this circuit will be compared to experimental results. At high temperatures a rounding of steps and supercurrent presumably due to noise is observed. A comparison with the theory of Ambegaokar and Halperin will be presented. At low temperatures a hysteresis develops. This may result from the bridge capacitance which also may give rise to subharmonic steps. However, the magnitude (1 pF) inferred from the hysteresis is too small to account for the subharmonic steps actually observed. In contrast to other types of junctions the bridge is mechanically very robust. We have therefore been interested in the applicability of the bridge. An upper limit to the sensitivity to 10 GHz radiation has been found in terms of NEP to 9 × 10-14 W/√Hz. Furthermore, we have demonstrated that mixing via the Josephson effect is possible. The power dependence of the sum and difference steps is in fair agreement with analog computer calculations on the equivalent circuit. It has been debated whether the bridge radiates at all. By the observation of a cavity induced step we have now established that bridges do radiate.

Revue de Physique Appliquée•Jan 1, 1974•29 citations
A bridge junction can be made by cutting a bulk ceramic superconductor into the right sizes. Using this sort of bridge, both dc and ac Josephson effects are confirmed in liquid nitrogen. We have observed microwave-induced steps at either nhf/2e or 2nhf/2e, where n is an integral, as well as the modulation of a critical supercurrent with the increase of microwave power or the externally applied magnetic field.
Microwave and millimeterwave dependences of ac Josephson effect were investigated in YBaCuO ceramic superconductors. We observed very clear constant-voltage steps (Shapiro steps) on the current-voltage curve of YBaCuO/Nb junctions at signals of 14.1,25.5 and 35.0 GHz. The observed step voltages were frequency dependent in accordance with the (h/2e)f rule. This strongly suggests that pair-electrons serve as the superconducting particle even in ceramic materials. In addition, the ac Josephson effect was also observed in point-contact ErBaCuO ceramics containing a magnetic element Er.
Experimental investigations on the proximity effect bridge (a Josephson device) at zero voltage and at finite voltages in the µV range are reported. The phase-super current relation at zero voltage was measured using an asymmetric superconducting quantum interferormeter circuit. The data are in agreement with the Josephson supercurrent-phase relation I_S=I_C sinδ with deviation less than 5% of the critical current I_c. The supercurrent density in the measured bridges reached as high as 50-100 µA/µm^2. Using microcircuitry techniques, proximity effect bridges were strongly coupled to superconducting microstrip resonators. Selfinduced steps in the I-V characteristics of bridges coupled to resonators were observed in the GHz region at voltages (frequencies) corresponding to the expected modes of the resonators. Two types of steps were seen depending on whether the resonator impedance on resonance was much higher or much smaller than the bridge resistance. A simple two fluid model of the bridge-resonator circuit was developed and the size and shape of self-induced steps were calculated for a generalized Josephson oscillator relation I_S = I_c(l-q + q sin∫ 2e/hV dt) where q = 1 corresponds to the original Josephson relation and q = 1/2 represents the phase slip regime. At low critical currents (I_c < 10 µA) and low voltages (V < 3µV) the size and shape of experimentally observed self-induced steps agree with the q = 1 model. At higher voltages and/or critical currents the step size increasingly deviates from the q = 1 model towards q = 1/2. These observations are interpreted to indicate a progressive reduction of the amplitude of the oscillating Josephson supercurrent in proximity effect bridges from I_c towards I_c /2 as the critical current and/or voltage are increased.
The first measurements on the Dayem bridge showed that the behaviour in an applied microwave field was quite different from that of the Josephson tunnel junction. Measurements on bridges one magnitude smaller later showed this difference to result from the bridge size. A very simple equivalent circuit was furthermore shown to characterize these small bridges. Analog computer calculations on this circuit will be compared to experimental results. At high temperatures a rounding of steps and supercurrent presumably due to noise is observed. A comparison with the theory of Ambegaokar and Halperin will be presented. At low temperatures a hysteresis develops. This may result from the bridge capacitance which also may give rise to subharmonic steps. However, the magnitude (1 pF) inferred from the hysteresis is too small to account for the subharmonic steps actually observed. In contrast to other types of junctions the bridge is mechanically very robust. We have therefore been interested in the applicability of the bridge. An upper limit to the sensitivity to 10 GHz radiation has been found in terms of NEP to 9 × 10-14 W/√Hz. Furthermore, we have demonstrated that mixing via the Josephson effect is possible. The power dependence of the sum and difference steps is in fair agreement with analog computer calculations on the equivalent circuit. It has been debated whether the bridge radiates at all. By the observation of a cavity induced step we have now established that bridges do radiate.