155 research outputs found
Aluminum Hard Mask Technique for the Fabrication of High-Quality Submicron Nb/Al-AlOx/Nb Josephson Junctions
We have developed a combined photolithography and electron-beam lithography
fabrication process for sub-\mum to \mum-size Nb/Al-AlOx/Nb Josephson
junctions. In order to define the junction size and protect its top electrode
during anodic oxidation, we developed and used the new concept of an aluminum
hard mask. Josephson junctions of sizes down to 0.5 \mum2 have been fabricated
and thoroughly characterized. We found that they have a very high quality,
which is witnessed by the IV curves with quality parameters Vm > 50 mV and Vgap
= 2.8 mV at 4.2 K, as well as IcRN products of 1.75-1.93 mV obtained at lower
temperatures. In order to test the usability of our fabrication process for
superconducting quantum bits, we have also designed, fabricated and
experimentally investigated phase qubits made of these junctions. We found a
relaxation time of T1 = 26 ns and a dephasing time of T2 = 21 ns
Disorder and transport in cuprates: weak localization and magnetic contributions
We report resistivity measurements in underdoped YBaCuOand overdoped TlBaCuO single crystals in which the
concentration of defects in the CuO planes is controlled by electron
irradiation. Low upturns of the resistivity are observed in both cases for
large defect content. In the Tl compound the decrease of conductivity scales as
expected from weak localization theory. On the contrary in
YBaCuO the much larger low T contribution to the
resistivity is proportional to the defect content and might then be associated
to a Kondo like spin flip scattering term. This would be consistent with the
results on the magnetic properties induced by spinless defects.Comment: latex rullier1.tex, 5 files, 4 pages [SPEC-S01/003], submitted to
Phys. Rev. Let
Electrical stress effect on Josephson tunneling through ultrathin AlOx barrier in Nb/Al/AlOx/Nb junctions
The effect of dc electrical stress and breakdown on Josephson and
quasiparticle tunneling in Nb/Al/AlOx/Nb junctions with ultrathin AlOx barriers
typical for applications in superconductor digital electronics has been
investigated. The junctions' conductance at room temperature and
current-voltage (I-V) characteristics at 4.2 K have been measured after the
consecutive stressing of the tunnel barrier at room temperature. Electrical
stress was applied using current ramps with increasing amplitude ranging from 0
to ~1000 Ic corresponding to voltages across the barrier up to 0.65 V where Ic
is the Josephson critical current. A very soft breakdown has been observed with
polarity-dependent breakdown current (voltage). A dramatic increase in subgap
conductance of the junctions, the appearance of subharmonic current steps, and
gradual increase in both the critical and the excess currents as well as a
decrease in the normal-state resistance have been observed. The observed
changes in superconducting tunneling suggest a model in which a progressively
increasing number of defects and associated additional conduction channels
(superconducting quantum point contacts (SQPCs)) are induced by electric field
in the tunnel barrier. By comparing the I-V characteristics of these conduction
channels with the nonstationary theory of current transport in SQPCs based on
multiple Andreev reflections by Averin and Bardas, the typical transparency D
of the induced SQPCs was estimated as D ~ 0.7. The number of induced SQPCs was
found to grow with voltage across the barrier as sinh(V/V_0) with V_0 = 0.045
V, in good agreement with the proposed model of defect formation by ion
electromigration. The observed polarity dependence of the breakdown current
(voltage) is also consistent with the model.Comment: 11 pages, 10 figure
Diffusion stop-layers for superconducting integrated circuits and qubits with Nb-based Josephson junctions
New technology for superconductor integrated circuits has been developed and
is presented. It employs diffusion stoplayers (DSLs) to protect Josephson
junctions (JJs) from interlayer migration of impurities, improve JJ critical
current (Ic) targeting and reproducibility, eliminate aging, and eliminate
pattern-dependent effects in Ic and tunneling characteristics of Nb/Al/AlOx/Nb
junctions in integrated circuits. The latter effects were recently found in
Nb-based JJs integrated into multilayered digital circuits. E.g., it was found
that Josephson critical current density (Jc) may depend on the JJ's
environment, on the type and size of metal layers making contact to niobium
base (BE) and counter electrodes (CE) of the junction, and also change with
time. Such Jc variations within a circuit reduce circuit performance and yield,
and restrict integration scale. This variability of JJs is explained as caused
by hydrogen contamination of Nb layers during wafer processing, which changes
the height and structural properties of AlOx tunnel barrier. Redistribution of
hydrogen impurities between JJ electrodes and other circuit layers by diffusion
along Nb wires and through contacts between layers causes long-term drift of
Jc. At least two DSLs are required to completely protect JJs from impurity
diffusion effects - right below the junction BE and right above the junction
CE. The simplest and the most technologically convenient DSLs we have found are
thin (from 3 nm to 10 nm) layers of Al. They were deposited in-situ under the
BE layer, thus forming an Al/Nb/Al/AlOx/Nb penta-layer, and under the first
wiring layer to junctions' CE, thus forming an Al/Nb wiring bi-layer. A
significant improvement of Jc uniformity on 150-mm wafer has also been obtained
along with large improvements in Jc targeting and run-to-run reproducibility.Comment: 7 pages, 9 figures; to be published in IEEE Transactions of Applied
Superconductivit
High-Throughput Method of Whole-Brain Sectioning, Using the Tape-Transfer Technique
Cryostat sectioning is a popular but labor-intensive method for preparing histological brain sections. We have developed a modification of the commercially available CryoJane tape collection method that significantly improves the ease of collection and the final quality of the tissue sections. The key modification involves an array of UVLEDs to achieve uniform polymerization of the glass slide and robust adhesion between the section and slide. This report presents system components and detailed procedural steps, and provides examples of end results; that is, 20mum mouse brain sections that have been successfully processed for routine Nissl, myelin staining, DAB histochemistry, and fluorescence. The method is also suitable for larger brains, such as rat and monkey
Combined potential and spin impurity scattering in cuprates
We present a theory of combined nonmagnetic and magnetic impurity scattering
in anisotropic superconductors accounting for the momentum-dependent impurity
potential. Applying the model to the d-wave superconducting state, we obtain a
quantitative agreement with the initial suppression of the critical temperature
due to Zn and Ni substitutions as well as electron irradiation defects in the
cuprates. We suggest, that the unequal pair-breaking effect of Zn and Ni may be
related to a different nature of the magnetic moments induced by these
impurities.Comment: 5 pages, 3 tables, RevTex, to be published in Phys. Rev.
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