166 research outputs found
Josephson junctions with centered step and local variation of critical current density
Superconductor-insulator-ferromagnet-superconductor (SIFS) Josephson tunnel
junctions based on Nb\Al2O3\Ni\Cu\Nb stacks with a thickness step in the
metallic NiCu interlayer were fabricated. The step height of a few 0.1 nm was
defined by optical lithography and controlled etching of both Nb and NiCu
layers. Experimentally determined junction parameters by current-voltage
characteristics and Fraunhofer pattern indicate a uniform NiCu thickness and
similar interface transparencies for etched and non-etched parts. The critical
current diffraction pattern was calculated and measured for stepped junctions
having the same ground phase difference but different critical current
densities in both halves. The measured data show a good agreement with
simulations.Comment: slight modification
Fabrication of high quality ferromagnetic Josephson junctions
We present ferromagnetic Nb/Al2O3/Ni60Cu40/Nb Josephson junctions (SIFS) with
an ultrathin Al2O3 tunnel barrier. The junction fabrication was optimized
regarding junction insulation and homogeneity of current transport. Using
ion-beam-etching and anodic oxidation we defined and insulated the junction
mesas. The additional 2 nm thin Cu layer below the ferromagnetic NiCu (SINFS)
lowered interface roughness and ensured very homogeneous current transport. A
high yield of junctional devices with jc spreads less than 2% was obtained.Comment: 5 pages, 6 figures; VORTEX IV conference contribution; Submitted to
Physica
Magnetic anisotropy in ferromagnetic Josephson junctions
Magnetotransport measurements were done on \Nb/\Al_2\O_3/\Cu/\Ni/\Nb
superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions.
Depending on ferromagnetic \Ni interlayer thickness and geometry the standard
(1d) magnetic field dependence of critical current deviates from the text-book
model for Josephson junctions. The results are qualitatively explained by a
short Josephson junction model based on anisotropy and 2d remanent
magnetization.Comment: to be published in AP
Optimization of -Layer Systems for Josephson Junctions from a Microstructure Point of View
-layer systems are frequently used for Josephson junction-based
superconducting devices. Although much work has been devoted to the
optimization of the superconducting properties of these devices, systematic
studies on influence of deposition conditions combined with structural analyses
on the nanoscale are rare up to now. We have focused on the optimization of the
structural properties of -layer systems deposited on Si(111)
substrates with a particular focus on the thickness homogeneity of the
-tunnel barrier. A standard high-vacuum electron-beam deposition system
was used and the effect of substrate pretreatment, different Al-deposition
temperatures and Al-deposition rates was studied. Transmission electron
microscopy was applied to analyze the structural properties of the
-layer systems to determine the thickness homogeneity of the
layer, grain size distribution in the Al layers, Al-grain boundary
types and the morphology of the interface. We show that the
structural properties of the lower Al layer are decisive for the structural
quality of the whole -layer system. Optimum conditions yield an
epitaxial Al(111) layer on a Si(111) substrate with an Al-layer thickness
variation of only 1.6 nm over more than 10 and large lateral grain
sizes up to 1 . Thickness fluctuations of the -tunnel barrier are
minimized on such an Al layer which is essential for the homogeneity of the
tunnel current. Systematic variation of the Al-deposition rate and deposition
temperature allows to develop an understanding of the growth mechanisms
Observation of Josephson coupling through an interlayer of antiferromagnetically ordered chromium
The supercurrent transport in metallic Josephson tunnel junctions with an
additional interlayer made up by chromium, being an itinerant antiferromagnet,
was studied. Uniform Josephson coupling was observed as a function of the
magnetic field. The supercurrent shows a weak dependence on the interlayer
thickness for thin chromium layers and decays exponentially for thicker films.
The diffusion constant and the coherence length in the antiferromagnet were
estimated. The antiferromagnetic state of the barrier was indirectly verified
using reference samples. Our results are compared to macroscopic and
microscopic models.Comment: Phys. Rev. B (2009), in prin
Memory cell based on a Josephson junction
The Josephson junction has a doubly degenerate ground state with
the Josephson phases . We demonstrate the use of such a
Josephson junction as a memory cell (classical bit), where writing is done by
applying a magnetic field and reading by applying a bias current. In the
"store" state, the junction does not require any bias or magnetic field, but
just needs to stay cooled for permanent storage of the logical bit.
Straightforward integration with Rapid Single Flux Quantum logic is possible.Comment: to be published in AP
Correlating the nanostructure of Al-oxide with deposition conditions and dielectric contributions of two-level systems in perspective of superconducting quantum circuits
This work is concerned with Al/Al-oxide(AlO)/Al-layer systems which are
important for Josephson-junction-based superconducting devices such as quantum
bits. The device performance is limited by noise, which has been to a large
degree assigned to the presence and properties of two-level tunneling systems
in the amorphous AlO tunnel barrier. The study is focused on the
correlation of the fabrication conditions, nanostructural and nanochemical
properties and the occurrence of two-level tunneling systems with particular
emphasis on the AlO-layer. Electron-beam evaporation with two different
processes and sputter deposition were used for structure fabrication, and the
effect of illumination by ultraviolet light during Al-oxide formation is
elucidated. Characterization was performed by analytical transmission electron
microscopy and low-temperature dielectric measurements. We show that the
fabrication conditions have a strong impact on the nanostructural and
nanochemical properties of the layer systems and the properties of two-level
tunneling systems. Based on the understanding of the observed structural
characteristics, routes are derived towards the fabrication of
Al/AlO/Al-layers systems with improved properties.Comment: 28 pages, 4 figure
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
Critical current diffraction pattern of SIFS Josephson junctions with step-like F-layer
We present the latest generation of
superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions
with a step-like thickness of the ferromagnetic (F) layer. The F-layer
thicknesses and in both halves were varied to obtain different
combinations of positive and negative critical current densities and
. The measured dependences of the critical current on applied magnetic
field can be well described by a model which takes into account different
critical current densities (obtained from reference junctions) and different
net magnetization of the multidomain ferromagnetic layer in both halves.Comment: 7 pages, 3 figure
Phase retrapping in aφJosephson junction: onset of the butterfly effect
We investigate experimentally the retrapping of the phase in a
φ
Josephson junction upon return of the junction to the zero-voltage state. Since the Josephson energy profile
U
0
(
ψ
)
in
φ
JJ is a
2
Ï€
periodic double-well potential with minima at
ψ
=
±
φ
mod
2
Ï€
, the question is at which of the two minima
−
φ
or
+
φ
the phase will be trapped upon return from a finite voltage state during quasistatic decrease of the bias current (tilt of the potential). By measuring the relative population of two peaks in escape histograms, we determine the probability of phase trapping in the
±
φ
wells for different temperatures. Our experimental results agree qualitatively with theoretical predictions. In particular, we observe an onset of the butterfly effect with an oscillating probability of trapping. Unexpectedly, this probability saturates at a value different from 50% at low temperatures
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