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Le Journal de Physique Colloques

A SIMPLE QUALITATIVE DETERMINATION OF JOSEPHSON TUNNEL JUNCTION PARAMETERS NEAR THE TRANSITION TEMPERATURE

N. F. Pedersen, J. Mygind, O. H. Soerensen, Bjørn Dueholm

Published Aug 1, 1978• 3 citations
Publisher

Abstract

The junction capacitance may be determined and the cosϕ-amplitude may be estimated from the switching characteristics seen in the IV curve near the critical temperature. The method relies on the empirical observation that a small area tunnel junction returns from finite to zero voltage when the Josephson frequency is about equal to the plasma frequency.

DOI
10.1051/jphyscol:19786544

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The junction capacitance may be determined and the cosϕ-amplitude may be estimated from the switching characteristics seen in the IV curve near the critical temperature. The method relies on the empirical observation that a small area tunnel junction returns from finite to zero voltage when the Josephson frequency is about equal to the plasma frequency.
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 current-voltage characteristics of small area tunnel junctions at temperatures below 1 K. The junctions were made in an edge geometry with a Nb base electrode and had areas less than .05 μm2and critical currents in the nA range. Although the measured I-V characteristics resemble those of ordinary hysteretic junctions, the supposed zero-voltage portion of the curve proved to have a finite slope and to deviate from zero voltage. For these junctions it is apparently possible for occasional 2π phase slips to occur without switching to the usual voltage state. This behavior can be explained either by macroscopic quantum tunneling or by a model in which the effective shunt conductance of the junction is frequency dependent.
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.
The capacitance of a Josephson tunnel junction is an important parameter, which determines its plasma frequency ωp and thus the energy level spacing in the quantum regime. When a microwave signal at the frequency of ω is applied to a current-biased junction in its zero-voltage state, the escape rate from this state is somehow enhanced. The enhancement is expected to go through a pronounced maximum when ωp=ω. Making use of such microwave generated resonant activation in the thermal regime, we have developed a time-resolved measurement scheme for determining the junction capacitance with an uncertainty of less than 2%.