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Applied Physics Letters

Quantitative measurement of the microwave distribution in superconducting tunnel junctions

Richard A. Gerber, D. Quenter, T. Doderer, C. A. Kruelle, R. P. Huebener, F. Müller, J. Niemeyer, R. Pöpel, Thomas Weimann

Published Mar 20, 1995• 6 citations
Publisher

Abstract

Low temperature scanning electron microscopy offers the possibility to measure the microwave distribution inside Josephson tunnel junctions in a quantitative way. The junctions are coupled to an external microwave source. Our results confirm the established model for the propagation of microwaves inside the junction stripline. Our method yields microwave specific parameters of the junction such as wavelength, microwave attenuation, reflection coefficients, and phase shifts.

DOI
10.1063/1.113644

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Low temperature scanning electron microscopy offers the possibility to measure the microwave distribution inside Josephson tunnel junctions in a quantitative way. The junctions are coupled to an external microwave source. Our results confirm the established model for the propagation of microwaves inside the junction stripline. Our method yields microwave specific parameters of the junction such as wavelength, microwave attenuation, reflection coefficients, and phase shifts.
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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 certain way of coupling to the thin-film Josephson tunneling junction has been studied. The coupling structure comprises a superconducting stripline which is terminated to the edge of the Nb-oxide-Pb junction. A mismatch of 23 dB has been obtained so far between the signal source and the junction at X-band compared to the earlier estimates of 40 dB between conventional size outer circuits and the junction. Prospects for improving the coupling are good. Methods of estimating the voltage and power which are coupled to the junction are given.
We measured the spatial distribution of microwave fields and dissipative surface currents in various passive and active superconducting thin-film devices during their operation at a temperature of about 4 K. We used low-temperature scanning electron microscopy which offers a spatial resolution of about 1 μm and we present sample response models for interpreting the imaging results. Such investigations may have important consequences for the improvement of the design and preparation of superconducting thin-film microwave devices.
We perform both theoretical and experimental investigations on the phase escape of a current-biased Josephson tunnel junction under microwave irradiation. The switching current distributions exhibit abundant nonlinear behaviors depending on the power and frequency of the applied microwave. We present a model to describe the behavior of the primary peak in the switching current distribution, which is confirmed by our experimental results. The obtained features can be used to characterize the damping parameter of Josephson junctions.
The microwave response of the dc quasiparticle tunneling current in Josephson tunnel junctions, where the Josephson current is suppressed by an external magnetic field, has been studied quantitatively in order to clarify its characteristics as a probe for the measurement of the junction capacitance. Extensive experiments for both small and long junctions are carried out for distinguishing between microwave behaviors of lumped and distributed constant junctions. It is shown that the observed voltage dependence of the dc quasiparticle tunneling current modified by an applied rf field is in good agreement with a theoretical result which takes into account the influence of the microwave circuit connected to the junction. The comparison between theory and experiment gives the magnitude of the internal rf field in the junction. Together with the applied rf field, this internal rf field leads to the junction rf impedance which is dominated by the junction capacitance in our experimental condition. In the case of lumped junctions, this experimental rf impedance is in reasonable agreement with the theoretical one with the junction capacitance estimated from the Fiske step of the distributed junction fabricated on the same substrate; the obtained ratio of the experimental impedance to the theoretical one is approximately 0.6–1.7. In the case of distributed junctions, however, experimental values of their characteristic impedances are approximately 0.2–0.3 of theoretical values calculated by assuming the one-dimensional junction model and taking account of the standing-wave effect in the junction.