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IEEE Transactions on Magnetics

Observations on small tunnel junctions: Anomalous critical-current temperature dependence, periodic structure, and microwave detection

James T. C. Yeh, D. N. Langenberg

Published Mar 1, 1975• 4 citations
Publisher

Abstract

Observations on very small Pb-Pb oxide-Pb tunnel junctions prepared by a new method are reported. The zero-voltage Josephson critical current is found to increase, then decrease with decreasing temperature. The current-voltage characteristics show very prominent periodic structure. The junctions have microwave (X-band) power sensitivities of 10-14W, more than two orders of magnitude better than conventional-size junctions.

DOI
10.1109/tmag.1975.1058726

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Journal of Applied Physics•Sep 1, 1980•5 citations
Observations on very small Pb-Pb oxide-Pb tunnel junctions prepared by a new method are reported. The zero-voltage Josephson critical current is found to increase, then decrease with decreasing temperature. The current-voltage characteristics show very prominent periodic structure. The junctions have microwave (X-band) power sensitivities of 10-14W, more than two orders of magnitude better than conventional-size junctions.
Unexpected partly dramatic temperature dependences were registered for the basic tunnel parameters used to characterize a Josephson junction. Measurements of the magnetic field variation of the microwave-power reflection, i.e., a mapping of the Josephson plasma resonance, enabled determinations of the rf small-voltage resistance RJ, the cosφ amplitude ζ, and the effective junction capacitance C in both Pb-O-Pb and PbTl-O-Pb junctions. The uncertainties in the parameter values are large, particularly in the low temperature values of ζ, for which we confirm earlier estimates of ζ, but as the superconducting transition temperature Tc is approached, ζ increases rapidly and goes through a positive maximum close to +1 before it decreases towards zero at Tc. Also, RJ displays a pronounced maximum in the same temperature interval; it decreases rapidly within a narrow range below the temperature of the maximum before it again increases slowly well below Tc. The location in temperature for the two maxima depends upon the barrier via the critical current density, but seems to be material independent. Well below Tc, the ratios of RJ to the normal-state tunnel resistance are also affected by the barrier transmission. These latter properties seem hard to explain within a model based only on the relaxation of the order parameters in the superconductors. A considerable decrease in C close to Tc can be explained by temperature-dependent surface losses in the junction electrodes. The Q-value associated with these could be determined as a function of temperature. In accordance with previous measurements, the Josephson current IJ is reduced at low temperature in small high-resistance tunnel junctions. Smaller deviations in IJ from theoretical estimates can be explained by strong coupling effects.
We have studied the power dependence of the microwave-induced dc voltages across "unbiased" Pb---Pb oxide---Pb Josephson tunnel junctions and found a systematic dependence. We have also found that the Josephson effect in the form of induced quantum voltages was observable in a large dc magnetic field as high as 0.5 kG, about one-half of the Hc2{H}_{c2}Hc2​ of the superconducting Pb film.
Preparation of high quality single crystal Nb tunnel junctions based on Nb/Nb2O5/Pb structures is reported. Vm values of 30±10 mV were obtained. The average specific capacitance of these junctions is 0.020±0.002 pF/μm2. From measurements of the oxidation rate, the conductance at high voltage bias, and observations of the tunneling behavior, we show that some characteristics previously associated with oxidized niobium barriers are not intrinsic and are significantly improved when the underlying niobium film is single crystal.
A previously published analysis of the magnetic field variation of the reflected microwave power from small unbiased Pb-O-Pb Josephson junctions found strong temperature dependences for the basic tunnel parameters. We reanalyze those measurements employing independent variables, compensating for spurious external reflections, and accounting for noise reduction of the dc critical current. The cosφ amplitude ζ is positive near Tc and becomes negative at lower temperatures. In contrast to the previous work ζ is less sharply peaked, the junction rf resistance varies smoothly, and the capacitance is temperature independent. Our results clearly disagree with recent theoretical work.