T. Doderer

Experiments with solitons in annular Josephson junctions

Abstract It is known that the long annular Josephson junction allows to avoid the influence of boundaries and soliton(fluxon)- soliton collisions on the dynamics of soliton chains. With new experimental methods and junction design a fully controlable way of introducing individual solitons into such a system is realized. For the homogeneous junction crossover at high velocities of the fluxon chain is seen and explained in terms of the fluxon bunching. In the periodically modulated junction a new resonant mode called a supersoliton is observed.

Multi-fluxon effects in long Josephson junctions

Experiments with fluxons moving in long Josephson junctions of different geometries are reviewed. A controllable way of introducing fluxons one by one into annular Josephson junctions has been realized. This allows a comparison with existing fluxon chain perturbation theory. Several new millimeter-wave stimulated regimes found in linear junctions are discussed. Large asymmetric zero-crossing steps of constant voltage have been observed in the I-V curves. These steps may have a potential application in Josephson voltage standards. Large narrowband radiation emitted from the flux-flow oscillator in the 62-77-GHz frequency range has been detected.<<ETX>>

Low-temperature scanning electron microscopy studies of superconducting thin films and Josephson junctions

Abstract Scanning techniques represent powerful methods for the characterization of condensed matter. Extending Scanning Electron Microscopy (SEM) to the range of low temperatures an interesting tool for studying low-temperature properties of solids with high-spatial and temporal resolution is obtained. By Low-Temperature Scanning Electron Microscopy (LTSEM) important new information on low-temperature phenomena in superconductors, semiconductors, and insulators is obtained by two-dimensional imaging. Here, we summarize the basic principles of LTSEM and show its application to the study of superconducting films and Josephson junctions.

Trapping of Several Solitons in Annular Josephson Junctions

We have observed for the first time a chain of sine-Gordon solitons (flux quanta) trapped in an annular Josephson junction. The low-temperature electron microscopy technique has been used in order to trap in a controlled way one by one up to several solitons in the junction. We attribute the deviations of the soliton chain velocity from the single-soliton velocity with the effects of the interaction between solitons and their bunching at high velocity.

Frequency dependence of the emission from 2D array Josephson oscillators

Coherent emission from 2-D arrays of Josephson junctions, coupled to a detector junction through a DC blocking stripline capacitor, was detected over a frequency range from 50 to 210 GHz. A power of 0.26 mu W, which is larger than the 0.1 mu W expected from the resistively shunted-junction model, was detected in a range from 140 to 150 GHz. Frequencies where no emission was detected correspond to standing waves in the capacitor when multiples of the half-wavelength match the capacitor length. Low-temperature scanning electron microscopy confirmed the presence of standing waves at these frequencies, but also revealed standing waves at other frequencies, indicating an impedance mismatch and a possible extension of the standing waves into the array.<<ETX>>

Fluxon pinning in annular Josephson junctions by an external magnetic field

The behavior of long annular Josephson junctions with various numbers n of trapped fluxons is studied in the presence of a uniform external magnetic field parallel to the plane of the junction’s tunnel barrier. We report on measurements of the magnetic dependencies of the critical current and current–voltage characteristics with different n. Experimental results are explained by the model assuming that a barrier-parallel magnetic field produces potential wells for fluxons and antifluxons at the opposite locations. We also present numerical simulations using the perturbed sine-Gordon equation with an additional magnetic-field-induced term. Good agreement is found between the measured critical current versus magnetic-field dependencies and the numerical simulations.

Detailed investigation of two-dimensional Josephson junction array circuits

We have investigated two-dimensional arrays of overdamped Josephson junctions that have been fabricated in Nb/Al/sub 2/O/sub 3//Nb-technology, together with microwave coupling structures. Different arrays have shown to emit coherent microwave radiation by using microwave-coupled on-chip detectors. Low-temperature scanning electron microscopy is used to study the coupling of the microwave radiation to an on-chip load. From the appearance of Shapiro steps in the current vs. voltage curves of detector junctions for different degrees of disorder in the array-junctions, margins of the spread in the critical currents of the Josephson junctions for coherent oscillation can be drawn.

Multi-fluxon zero-field modes in long Josephson tunnel junctions

The higher order zero field steps of long inline and overlap Josephson junctions have been investigated experimentally by means of low temperature scanning electron microscopy. The results indicate that several stable states are possible when more than one fluxon is present in the junction, corresponding to a different spacing among the fluxons during their propagation. These dynamic states should differ by the spectrum of the emitted radiation. Numerical simulations show that the fluxon interaction with plasma oscillations appears to be responsible for the observed behavior.

Fluxon pinning through interaction with the superconducting wiring of long annular Josephson junctions

The statics and dynamics of magnetic-flux quanta (fluxons) in long annular Josephson tunnel junctions have been investigated. Pinning by interaction of the fluxon field outside the junction with the superconducting wiring has been observed in spatially resolving measurements using low-temperature scanning electron microscopy. We were able to influence the characteristics of this field by carefully modifying the beam-induced fluxon trapping procedure. In this way we were able to select the pinning site acting on the fluxon.

Phase locking of Josephson junctions in two-dimensional arrays

We studied the phase-locking behavior of Josephson junctions in two-dimensional arrays under microwave irradiation or coupled to a resonant cavity. We are able to identify the phase-locked junctions by means of spatially resolving measurements. We focus on the degree of coherence in the arrays, changing from incoherent co coherent oscillations of the junctions when the microwave power is increased. We interpret our results in terms of a second order, phase transition to the phase-locked state.