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Journal of Applied Physics

Basic Josephson tunnel parameters at microwave frequency—Strong temperature variations

S. Rudner, T. Claeson

Published Nov 1, 1979• 13 citations
Publisher

Abstract

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.

DOI
10.1063/1.325812

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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.

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N. F. Pedersen, J. Mygind, O. H. Soerensen +1 more

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Le Journal de Physique Colloques•Aug 1, 1978•3 citations
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.
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.
We report the electrical properties of all niobium nitride(NbN) Josephson tunnel junctions with magnesium oxide(MgO) films as barriers in the temperature range 4.2-15 K. NbN/MgO/NbN Josephson tunnel junctions have large gap voltages(Vg=4.8-5.4 mV), large products of the maximum critical currents and the normal tunneling resistances(IcRn=3.15-3.35 mV), and small subgap leakage currents(Vm=13-138 mV) at 4.2 K. The junction parameters decreased with increase in temperature, but reasonably large junction parameters(for example, Vg=4.9 mV, IcRn=2.67 mV, and Vm=22 mV) have been obtained even at 9 K. The magnetic penetration depth in NbN electrodes has been measured up to 15 K using two-junction interferometers which consist of NbN/MgO/NbN junctions, NbN ground planes, and NbN control lines.
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%.
High quality NbN-NbOX-Pb Josephson devices have been fabricated by r.f. diode sputtering. The critical temperatures of the NbN films were as high as 16.8 K. The junction geometry was defined by photolithography and the oxide barrier was obtained by thermal oxidation or by a plasma oxidation technique. Very low leakage currents have been obtained, corresponding to values of the parameter Vm = 80 mV (Vm = IO× 2 mV/I (2mV)). The junction microwave quality factor Q and other a.c. properties have been measured in the frequency range 20 - 300 GHz by looking at the devices self-resonant modes. Q values up to 120 at 4.2 K have been found in the low frequency range, indicating potential performances of these devices for microwave applications such as Josephson voltage standards. Theoretical analysis of the results shows that the main contribution to the losses is related to the NbN films surface impedance.