36 research outputs found
Non-ideal artificial phase discontinuity in long Josephson 0-kappa-junctions
We investigate the creation of an arbitrary -discontinuity of the
Josephson phase in a long Nb-AlO_x-Nb Josephson junction (LJJ) using a pair of
tiny current injectors, and study the formation of fractional vortices formed
at this discontinuity. The current I_inj, flowing from one injector to the
other, creates a phase discontinuity kappa ~ I_inj. The calibration of
injectors is discussed in detail. The small but finite size of injectors leads
to some deviations of the properties of such a 0-kappa-LJJ from the properties
of a LJJ with an ideal kappa-discontinuity. These experimentally observed
deviations in the dependence of the critical current on I_inj$ and magnetic
field can be well reproduced by numerical simulation assuming a finite injector
size. The physical origin of these deviations is discussed.Comment: Submitted to Phys. Rev. B (12 figures). v 2: refs updated, long eqs
fixed v 3: major changes, fractional vortex dynamics exclude
Experimental evidence of a {\phi} Josephson junction
We demonstrate experimentally the existence of Josephson junctions having a
doubly degenerate ground state with an average Josephson phase \psi=\pm{\phi}.
The value of {\phi} can be chosen by design in the interval 0<{\phi}<\pi. The
junctions used in our experiments are fabricated as 0-{\pi} Josephson junctions
of moderate normalized length with asymmetric 0 and {\pi} regions. We show that
(a) these {\phi} Josephson junctions have two critical currents, corresponding
to the escape of the phase {\psi} from -{\phi} and +{\phi} states; (b) the
phase {\psi} can be set to a particular state by tuning an external magnetic
field or (c) by using a proper bias current sweep sequence. The experimental
observations are in agreement with previous theoretical predictions
Quantum tunneling of semifluxons
We consider a system of two semifluxons of opposite polarity in a 0-pi-0 long
Josephson junction, which classically can be in one of two degenerate states:
up-down or down-up. When the distance between the 0-pi boundaries
(semifluxon's centers) is a bit larger than the crossover distance , the
system can switch from one state to the other due to thermal fluctuations or
quantum tunneling. We map this problem to the dynamics of a single particle in
a double well potential and estimate parameters for which quantum effects
emerge. We also determine the classical-to-quantum crossover temperature as
well as the tunneling rate (energy level splitting) between the states up-down
and down-up.Comment: submitted to PRB, comments/questions are welcom
Geometric pi Josephson junction in d-wave superconducting thin films
A novel way to realize a pi Josephson junction is proposed, based on a weak
link in an unconventional d-wave superconductor with appropriately chosen
boundary geometry. The critical current of such a junction is calculated from a
fully selfconsistent solution of microscopic Eilenberger theory of
superconductivity. The results clearly show, that a transition to a pi
Josephson junction occurs for both low temperatures and small sizes of the
geometry.Comment: 3 pages, 3 figure
Oscillatory eigenmodes and stability of one and two arbitrary fractional vortices in long Josephson 0-kappa-junctions
We investigate theoretically the eigenmodes and the stability of one and two
arbitrary fractional vortices pinned at one and two -phase
discontinuities in a long Josephson junction. In the particular case of a
single -discontinuity, a vortex is spontaneously created and pinned at
the boundary between the 0 and -regions. In this work we show that only
two of four possible vortices are stable. A single vortex has an oscillatory
eigenmode with a frequency within the plasma gap. We calculate this
eigenfrequency as a function of the fractional flux carried by a vortex.
For the case of two vortices, pinned at two -discontinuities situated
at some distance from each other, splitting of the eigenfrequencies occur.
We calculate this splitting numerically as a function of for different
possible ground states. We also discuss the presence of a critical distance
below which two antiferromagnetically ordered vortices form a strongly coupled
``vortex molecule'' that behaves as a single object and has only one eigenmode.Comment: submitted to Phys. Rev. B (
Theory of fractional vortex escape in a 0-kappa long Josephson junction
We consider a fractional Josephson vortex in an infinitely long 0-kappa
Josephson junction. A uniform bias current applied to the junction exerts a
Lorentz force acting on a vortex. When the bias current becomes equal to the
critical (or depinning) current, the Lorentz force tears away an integer fluxon
and the junction switches to the resistive state. In the presence of thermal
and quantum fluctuations this escape process takes place with finite
probability already at subcritical values of the bias current. We analyze the
escape of a fractional vortex by mapping the Josephson phase dynamics to the
dynamics of a single particle in a metastable potential and derive the
effective parameters of this potential. This allows us to predict the behavior
of the escape rate as a function of the topological charge of the vortex
Controllable plasma energy bands in a 1D crystal of fractional Josephson vortices
We consider a 1D chain of fractional vortices in a long Josephson junction
with alternating phase discontinuities. Since each vortex has its
own eigenfrequency, the inter-vortex coupling results in eigenmode splitting
and in the formation of an oscillatory energy band for plasma waves. The band
structure can be controlled at the design time by choosing the distance between
vortices or \emph{during experiment} by varying the topological charge of
vortices or the bias current. Thus one can construct an artificial vortex
crystal with controllable energy bands for plasmons.Comment: 4 pages, 2 Fig
0-pi Josephson tunnel junctions with ferromagnetic barrier
We fabricated high quality Nb/Al_2O_3/Ni_{0.6}Cu_{0.4}/Nb
superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions.
Using a ferromagnetic layer with a step-like thickness, we obtain a 0-pi
junction, with equal lengths and critical currents of 0 and pi parts. The
ground state of our 330 microns (1.3 lambda_J) long junction corresponds to a
spontaneous vortex of supercurrent pinned at the 0-pi step and carrying ~6.7%
of the magnetic flux quantum Phi_0. The dependence of the critical current on
the applied magnetic field shows a clear minimum in the vicinity of zero field.Comment: submitted to PR
Dynamics of semifluxons in Nb long Josephson 0-pi junctions
We propose, implement and test experimentally long Josephson 0-pi junctions
fabricated using conventional Nb-AlOx-Nb technology. We show that using a pair
of current injectors, one can create an arbitrary discontinuity of the
Josephson phase and in particular a pi-discontinuity, just like in
d-wave/s-wave or in d-wave/d-wave junctions, and study fractional Josephson
vortices which spontaneously appear. Moreover, using such junctions, we can
investigate the \emph{dynamics} of the fractional vortices -- a domain which is
not yet available for natural 0-pi-junctions due to their inherently high
damping. We observe half-integer zero-field steps which appear on the
current-voltage characteristics due to hopping of semifluxons.Comment: Fractional vortices in conventional superconductors ;-
Evidence of Spatially Inhomogeous Pairing on the Insulating Side of a Disorder-Tuned Superconductor-Insulator Transition
Measurements of transport properties of amorphous insulating indium oxide
thin films have been interpreted as evidence of the presence of superconducting
islands on the insulating side of a disorder-tuned superconductor-insulator
transition. Although the films are not granular, the behavior is similar to
that observed in granular films. The results support theoretical models in
which the destruction of superconductivity by disorder produces spatially
inhomogenous pairing with a spectral gap.Comment: Revised title and content/argument. Totals: 4 pages, 3 figure