116 research outputs found
Elliptic Annular Josephson Tunnel Junctions in an external magnetic field: The dynamics
We analyze the dynamics of a magnetic flux quantum (current vortex) trapped
in a current-biased long planar elliptic annular Josephson tunnel junction. The
system is modeled by a perturbed sine-Gordon equation that determines the
spatial and temporal behavior of the phase difference across the tunnel barrier
separating the two superconducting electrodes. In the absence of an external
magnetic field the fluxon dynamics in an elliptic annulus does not differ from
that of a circular annulus where the stationary fluxon speed merely is
determined by the system losses. The interaction between the vortex magnetic
moment and a spatially homogeneous in-plane magnetic field gives rise to a
tunable periodic non-sinusoidal potential which is strongly dependent on the
annulus aspect ratio. We study the escape of the vortex from a well in the
tilted potential when the bias current exceeds the depinning current. The
smallest depinning current as well as the lowest sensitivity of the annulus to
the external field is achieved when the eccentricity is equal to -1. The
presented extensive numerical results are in good agreement with the findings
of the perturbative approach. We also probe the rectifying properties of an
asymmetric potential implemented with an egg-shaped annulus formed by two
semi-elliptic arcs.Comment: 17 pages, 9 figure
Josephson Tunnel Junctions in a Magnetic Field Gradient
We measured the magnetic field dependence of the critical current of high
quality Nb-based planar Josephson tunnel junctions in the presence of a
controllable non-uniform field distribution. We found skewed and slowly
changing magnetic diffraction patterns quite dissimilar from the
Fraunhofer-like ones typical of a homogeneous field. Our findings can be well
interpreted in terms of recent theoretical predictions [R. Monaco, J. Appl.
Phys. vol.108, 033906 (2010)] for a uniform magnetic field gradient leading to
Fresnel-like magnetic diffraction patterns. We also show that Fiske resonances
can be suppressed by an asymmetric magnetic field profile.Comment: 8 pages, 4 figure
Flux Flow Effects in Annular Josephson Tunnel Junctions
We investigate Josephson flux-flow in annular Josephson tunnel junctions
(AJTJs) under the application of magnetic fields generating finite-voltage
steps in their current-voltage characteristics. Experimental data are presented
for confocal AJTJs which are the natural generalization of the well studied
circular AJTJs for which flux flow effects have never been reported. Displaced
linear slopes, Fiske step staircases and Eck steps were sequentially recorded
at with high-quality Nb/Al-AlOx/Nb confocal AJTJs when increasing the
strength of a uniform magnetic field applied in the plane of the junction.
Their amplitude was found to strongly depend not only on the strength, but also
on the orientation, of the external field. Extensive numerical simulations
based on a phenomenological sine-Gordon model developed for confocal AJTJs were
carried out to disclose the basic flux-flow mechanism responsible for the
appearance of magnetically induced steps and to elucidate the role of several
critical parameters, namely, the field orientation, the system loss and the
annulus eccentricity. It was found that in a topologically closed system, such
as the AJTJ, where the number of trapped fluxons is conserved and new fluxons
can be created only in the form of fluxon-antifluxon pairs, the existence of a
steady viscous flow of Josephson vortices only relies on the capability of the
fluxons and antifluxons to be generated and to annihilate each other inside the
junction. This also implies that flux-flow effects are not observable in
circular AJTJs.Comment: 26 pages, 8 figure
Spontaneous Fluxon Formation in Annular Josephson Tunnel Junctions
It has been argued by Zurek and Kibble that the likelihood of producing
defects in a continuous phase transition depends in a characteristic way on the
quench rate. In this paper we discuss our experiment for measuring the
Zurek-Kibble scaling exponent sigma for the production of fluxons in annular
symmetric Josephson Tunnel Junctions. The predicted exponent is sigma = 0.25,
and we find sigma = 0.27 +/- 0.05. Further, there is agreement with the ZK
prediction for the overall normalisation
Field Cooled Annular Josephson Tunnel Junctions
We investigate the physics of planar annular Josephson tunnel junctions
quenched through their transition temperature in the presence of an external
magnetic field. Experiments carried out with long Nb/Al-AlOx/Nb annular
junctions showed that the magnetic flux trapped in the high-quality
doubly-connected superconducting electrodes forming the junction generates a
persistent current whose associated magnetic field affects the both the static
and dynamics properties of the junctions. More specifically, the field trapped
in the hole of one electrode combined with a d.c. bias current induces a
viscous flow of dense trains of Josephson vortices which manifests itself
through the sequential appearance of displaced linear slopes, Fiske step
staircases and Eck steps in the junction's current-voltage characteristic.
Furthermore, a field shift is observed in the first lobe of the magnetic
diffraction pattern. The effects of the persistent current can be mitigated or
even canceled by an external magnetic field perpendicular to the junction
plane. The radial field associated with the persistent current can be
accurately modeled with the classical phenomenological sine-Gordon model for
extended one-dimensional Josephson junctions. Extensive numerical simulations
were carried out to disclose the basic flux-flow mechanism responsible for the
appearance of the magnetically induced steps and to elucidate the role of
geometrical parameters. It was found that the imprint of the field cooling is
enhanced in confocal annular junctions which are the natural generalization of
the well studied circular annular junctions.Comment: 26 pages, 10 figures. Supercond. Sci. Technol (2020
Josephson Vortex Qubit based on a Confocal Annular Josephson Junction
We report theoretical and experimental work on the development of a Josephson
vortex qubit based on a confocal annular Josephson tunnel junction (CAJTJ). The
key ingredient of this geometrical configuration is a periodically variable
width that generates a spatial vortex potential with bistable states. This
intrinsic vortex potential can be tuned by an externally applied magnetic field
and tilted by a bias current. The two-state system is accurately modeled by a
one-dimensional sine-Gordon like equation by means of which one can numerically
calculate both the magnetic field needed to set the vortex in a given state as
well as the vortex depinning currents. Experimental data taken at 4.2K on
high-quality Nb/Al-AlOx/Nb CAJTJs with an individual trapped fluxon advocate
the presence of a robust and finely tunable double-well potential for which
reliable manipulation of the vortex state has been classically demonstrated.
The vortex is prepared in a given potential by means of an externally applied
magnetic field, while the state readout is accomplished by measuring the
vortex-depinning current in a small magnetic field. Our proof of principle
experiment convincingly demonstrates that the proposed vortex qubit based on
CAJTJs is robust and workable.Comment: 20 pages, 11 figure
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