1,319 research outputs found
Subgap conductivity in SIN-junctions of high barrier transparency
We investigate the current-voltage characteristics of high-transparency
superconductor-insulator-normal metal (SIN) junctions with the specific tunnel
resistance below 30 kOhm per square micron. The junctions were fabricated from
different superconducting and normal conducting materials, including Nb, Al,
AuPd and Cu. The subgap leakage currents were found to be appreciably larger
than those given by the standard tunnelling model. We explain our results using
the model of two-electron tunnelling in the coherent diffusive transport
regime. We demonstrate that even in the high-transparency SIN-junctions, a
noticeable reduction of the subgap current can be achieved by splitting a
junction into several submicron sub-junctions. These structures can be used as
nonlinear low-noise shunts in Rapid-Single-Flux-Quantum (RSFQ) circuitry for
controlling Josephson qubits.Comment: 6 pages, 5 figures, 1 tabl
Reduced leakage current in Josephson tunnel junctions with codeposited barriers
Josephson junctions were fabricated using two different methods of barrier
formation. The trilayers employed were Nb/Al-AlOx/Nb on sapphire, where the
first two layers were epitaxial. The oxide barrier was formed either by
exposing the Al surface to O2 or by codepositing Al in an O2 background. The
codeposition process yielded junctions that showed the theoretically predicted
subgap current and no measurable shunt conductance. In contrast, devices with
barriers formed by thermal oxidation showed a small shunt conductance in
addition to the predicted subgap current.Comment: 3 pages, 4 figure
A 275–425-GHz Tunerless Waveguide Receiver Based on AlN-Barrier SIS Technology
We report on a 275–425-GHz tunerless waveguide receiver with a 3.5–8-GHz IF. As the mixing element, we employ a high-current-density Nb–AlN–Nb superconducting–insulating– superconducting (SIS) tunnel junction. Thanks to the combined use of AlN-barrier SIS technology and a broad bandwidth waveguide to thin-film microstrip transition, we are able to achieve an unprecedented 43% instantaneous bandwidth, limited by the receiver's corrugated feedhorn.
The measured double-sideband (DSB) receiver noise temperature, uncorrected for optics loss, ranges from 55 K at 275 GHz, 48 K at 345 GHz, to 72 K at 425 GHz. In this frequency range, the mixer has a DSB conversion loss of 2.3 1 dB. The intrinsic mixer noise is found to vary between 17–19 K, of which 9 K is attributed to shot noise associated with leakage current below the gap. To improve reliability, the IF circuit and bias injection are entirely planar by design. The instrument was successfully installed at the Caltech Submillimeter Observatory (CSO), Mauna Kea, HI, in October 2006
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
Parity effect in Al and Nb single electron transistors in a tunable environment
Two different types of Cooper pair transistors, with Al and Nb islands, have
been investigated in a tunable electromagnetic environment. The device with an
Al island demonstrates gate charge modulation with 2e-periodicity in a wide
range of environmental impedances at bath temperatures below 340 mK. Contrary
to the results of the Al sample, we were not able to detect 2e-periodicity
under any conditions on similar samples with Nb island. We attribute this to
the material properties of Nb.Comment: 3 pages, 3 figure
Micrometre-scale refrigerators
A superconductor with a gap in the density of states or a quantum dot with
discrete energy levels is a central building block in realizing an electronic
on-chip cooler. They can work as energy filters, allowing only hot
quasiparticles to tunnel out from the electrode to be cooled. This principle
has been employed experimentally since the early 1990s in investigations and
demonstrations of micrometre-scale coolers at sub-kelvin temperatures. In this
paper, we review the basic experimental conditions in realizing the coolers and
the main practical issues that are known to limit their performance. We give an
update of experiments performed on cryogenic micrometre-scale coolers in the
past five years
Micrometre-scale refrigerators
A superconductor with a gap in the density of states or a quantum dot with
discrete energy levels is a central building block in realizing an electronic
on-chip cooler. They can work as energy filters, allowing only hot
quasiparticles to tunnel out from the electrode to be cooled. This principle
has been employed experimentally since the early 1990s in investigations and
demonstrations of micrometre-scale coolers at sub-kelvin temperatures. In this
paper, we review the basic experimental conditions in realizing the coolers and
the main practical issues that are known to limit their performance. We give an
update of experiments performed on cryogenic micrometre-scale coolers in the
past five years
The Detection of Defects in a Niobium Tri-layer Process
Niobium (Nb) LTS processes are emerging as the technology for future ultra high-speed systems especially in the digital domain. As the number of Josephson Junctions (JJ) per chip has recently increased to around 90000, the quality of the process has to be assured so as to realize these complex circuits. Until now, very little or no information is available in the literature on how to achieve this. In this paper we present an approach and results of a study conducted on an RSFQ process. Measurements and SEM inspection were carried out on sample chips and a list of possible defects has been identified and described in detail. We have also developed test-structures for detection of the top-ranking defects, which will be used for yield analysis and the determination of the probability distribution of faults in the process. A test chip has been designed, based on the results of this study, and certain types of defects were introduced in the design to study the behavior of faulty junctions and interconnections
Fabrication of planarized small area Nb/Al,AlO<sub>x</sub>/Al/Nb Josephson tunnel junctions using reactive ion etching in SF<sub>6</sub>
High quality Nb/Al,AlOx/Al/Nb Josephson tunnel junctions have been made with the help of a fabrication process based on reactive ion etching of Nb in SF6. The Vm value of these junctions is typically 60–70 mV at 4.2 K. At 1.6 K, a Vm of 4.1 V has been measured, which is the highest value that has ever been reported for this type of junction. The area of the junctions ranges from 1 to 25 μm2. By burying the Nb/Al,AlOx/Al/Nb trilayer in the substrate, a planarized junction configuration has been obtained. Reactive ion etching of Nb in SF6 plasmas has been studied in detail. Anisotropic etch profiles can be obtained because of the formation of a resistant layer during etching, which prevents etching of Nb under the photoresist. The etching process has been monitored with a spectrometer. The fluorine emission at 703.7 nm is shown to be suitable for end point detection
- …