N. Thyssen

The role of surface roughness in the fabrication of stacked Nb/Al–AlOx/Nb tunnel junctions

The surface roughness of sputtered Nb films was determined with high precision using x‐ray specular reflectivity measurements in the 10 keV range. The roughness of Nb films increased from 0.9 nm for a 70‐nm‐thick film to 1.8 nm for a 210‐nm‐thick film. The roughness of the Nb surface strongly influences the tunnel barrier formation and the electrical properties of that barrier. For stacked tunnel junctions each thermal Al oxidation has to be adjusted as a function of the underlying Nb film thickness. Alternatively, one can use an Al or Al/AlOx underlayer in order to obtain stacks with small spread in critical currents.

Narrow long Josephson junctions

Long Josephson junctions of width down to less than 0.3 /spl mu/m are fabricated using electron beam lithography. The junctions are made in niobium-aluminum-oxide trilayer technology using cross-linked PMMA for insulation. We measured the fluxon penetration field, the magnetic field period of the critical current modulation, and the Fiske step voltages of the junctions. A strong dependence of these quantities on the junction width is observed. Assuming a general-type relation between the spatial derivative of the phase and the spatial variation of the magnetic field along the plane of the junction, we derive a scaling relation between the measured quantities depending on the junction width. The derived relation is consistent with the experimental data.

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.

Soliton trapping in a periodic potential: experiment

Abstract The dynamics of a single soliton in a ring-shaped Josephson junction is studied experimentally. An externally applied magnetic field H forms a sinusoidal potential relief for the soliton in the ring. The effects of soliton pinning and trapping by the potential are observed and measured as a function of the potential height and losses in the junction. An analogy between the soliton dynamics and motion of a particle in a wash-board potential is clearly demonstrated. The experimentally found trapping threshold for small H fairly well agrees with the theoretical prediction using the dissipation-based power balance. With increasing H , the observed large discrepancy between theory and experiment is attributed to additional soliton energy losses due to radiation.

Flux flow and resonant modes in multi-junction Josephson stacks

Magnetic field dependent current-voltage characteristics of stackled Nb/(Al−AlOx/Nb)n long Josephson junctions are investigated experimentally. The thickness of their common superconducting electrodes provides the magnetic coupling between the junctions. For stacks of n=7 Josephson junctions the current-voltage characteristics display collective flux-flow behaviour of Josephson vortices. In the interior layers Josephson vortices move simultaneously under the influence of the bias current. The flux-flow behaviour is modulated by a complicated structure of cavity-like resonances which show broad range of characteristic frequencies. The measurements can be qualitatively explained by the Kleiner model for the resonances in stacks. Mutual locking of junctions in the stack is indicated by pronounced cavity resonances with large voltage spacing.

Experimental investigation of Cherenkov flux-flow oscillators

This paper is devoted to the experimental investigation of a new class of superconducting microwave oscillators based on Cherenkov radiation from Josephson vortices. Samples consisting of a long Josephson junction embedded in a dispersive transmission line with space-periodical inhomogeneities were fabricated and tested. We report observation of new resonances on the I-V curve of the system and present the first radiation measurements in the 80-120 GHz frequency range.

Experimental study of fluxon dynamics in a potential well

Dynamics of a single fluxon in an annular Josephson junction is studied in the presence of an externally applied magnetic field H. Due to the interaction of the fluxon with the radial field component, the fluxon moves in a potential well which height is proportional to H. Several spectacular features of fluxon dynamics analogous to the motion of a particle in a washboard potential are clearly observed experimentally. Periodic revolutions of the fluxon in the junction lead to a potential induced emission of plasma waves. These waves are indicated by a resonance at a certain fluxon velocity which depends on the junction length. Good agreement between experiment, theoretical model and numerical simulations is found.

Experimental study of flux flow and resonant modes in multi-junction Josephson stacks

Magnetic field dependence of current-voltage characteristics of 7-layer stacked Nb/Al-AlO/sub x//Nb long Josephson junctions are investigated experimentally. The magnetic coupling between the junctions is provided by their common superconducting electrodes of the thickness smaller than the London penetration depth. The current-voltage characteristics clearly display collective flux-flow behaviour of Josephson vortices (fluxons) which simultaneously move in up to 5 layers of the stack under the influence of the bias current. The flux-flow behaviour is modulated by a complicated structure of cavity-like resonances which show a broad range of characteristic frequencies. Our observations can be qualitatively explained as the interplay between the Fiske modes and the two-dimensional resonances in these stacks. For the intermediate magnetic field range, we find pronounced resonant modes with large voltage spacing which indicates mutually coherent operating junctions.

Trapped fluxons in annular Josephson junctions in the external magnetic field

The behavior of a long annular Josephson junction is studied in presence of an externally applied magnetic field. We report on measurements of the critical current dependencies on magnetic field with various numbers of trapped fluxons. The magnetic field produces opposite-located potential wells for fluxon and antifluxon. The images obtained by low temperature scanning electron microscopy prove that fluxon and antifluxon are pinned by these field-induced wells. Good agreement is found between experiment and numerical simulations.