3,661 research outputs found
Effect of stoichiometry on oxygen incorporation in MgB2 thin films
The amount of oxygen incorporated into MgB2 thin films upon exposure to
atmospheric gasses is found to depend strongly on the material's stoichiometry.
Rutherford backscattering spectroscopy was used to monitor changes in oxygen
incorporation resulting from exposure to: (a) ambient atmosphere, (b) humid
atmospheres, (c) anneals in air and (d) anneals in oxygen. The study
investigated thin-film samples with compositions that were systematically
varied from Mg0.9B2 to Mg1.1B2. A significant surface oxygen contamination was
observed in all of these films. The oxygen content in the bulk of the film, on
the other hand, increased significantly only in Mg rich films and in films
exposed to humid atmospheres.Comment: 10 pages, 6 figures, 1 tabl
Inversion of magnetoresistance in magnetic tunnel junctions : effect of pinhole nanocontacts
Inverse magnetoresistance has been observed in magnetic tunnel junctions with
pinhole nanocontacts over a broad temperature range. The tunnel
magnetoresistance undergoes a change of sign at higher bias and temperature.
This phenomenon is attributed to the competition between the spin conserved
ballistic transport through the pinhole contact where the transmission
probability is close to unity and spin polarized tunneling across the
insulating spacer with weak transmittivity.Comment: Replaced with revised version and new figure, 6 figures, RevTex
The Reactivity of MgB2 with Common Substrate and Electronic Materials
The reactivity of MgB2 with powdered forms of common substrate and electronic
materials is reported. Reaction temperatures between 600 C and 800 C,
encompassing the range commonly employed in thin-film fabrication, were
studied. The materials tested for reactivity were ZrO2, yttria stabilized
zirconia (YSZ), MgO, Al2O3, SiO2, SrTiO3, TiN, TaN, AlN, Si, and SiC. At 600 C,
MgB2 reacted only with SiO2 and Si. At 800 C, however, reactions were observed
for MgB2 with Al2O3, SiO2, Si, SiC, and SrTiO3. The Tc of MgB2 decreased in the
reactions with SiC and Al2O3.Comment: 5 figure
MgB2 tunnel junctions with native or thermal oxide barriers
MgB2 tunnel junctions (MgB2/barrier/MgB2) were fabricated using a native
oxide grown on the bottom MgB2 film as the tunnel barrier. Such barriers
therefore survive the deposition of the second electrode at 300oC, even over
junction areas of ~1 mm2. Studies of such junctions, and those of the type
MgB2/native or thermal oxide/metal (Pb, Au, or Ag) show that tunnel barriers
grown on MgB2 exhibit a wide range of barrier heights and widths.Comment: 9 pages, 3 figure
The chemical ecology of herbivory on willows
Phenolic secondary compounds and trichomes are instrumental in the regulation of herbivory on Salicaceae. The roles of phenolics in willows as toxins or deterrents, as phagostimulants or ovipository signals, and as precursors in insect chemical defence are briefly reviewed. The interactions between salicaceous plants, herbivores and their predators are discussed in the context of theories on the evolution of interactions among three trophic level
The re-emission spectrum of digital hardware subjected to EMI
The emission spectrum of digital hardware under the influence of external electromagnetic interference is shown to contain information about the interaction of the incident energy with the digital circuits in the system. The generation mechanism of the re-emission spectrum is reviewed, describing how nonlinear effects may be a precursor to the failure of the equipment under test. Measurements on a simple circuit are used to demonstrate how the characteristics of the re-emission spectrum may be correlated with changes to the digital waveform within the circuit. The technique is also applied to a piece of complex digital hardware where Similar, though more subtle, effects can be measured. It is shown that the re-emission spectrum can be used to detect the interaction of the interference with the digital devices at a level well below that which is able to cause static failures in the circuits. The utility of the technique as a diagnostic tool for immunity testing of digital hardware, by identifying which subsystems are being affected by external interference, is also demonstrated
Limitations in cooling electrons by normal metal - superconductor tunnel junctions
We demonstrate both theoretically and experimentally two limiting factors in
cooling electrons using biased tunnel junctions to extract heat from a normal
metal into a superconductor. Firstly, when the injection rate of electrons
exceeds the internal relaxation rate in the metal to be cooled, the electrons
do no more obey the Fermi-Dirac distribution, and the concept of temperature
cannot be applied as such. Secondly, at low bath temperatures, states within
the gap induce anomalous heating and yield a theoretical limit of the
achievable minimum temperature.Comment: 4 pages, 4 figures, added Ref. [6] + minor correction
Experimental confirmation of the low B isotope coefficient in MgB2
Recent investigations have shown that the first proposed explanations of the
disagreement between experimental and theoretical value of isotope coefficient
in MgB2 need to be reconsidered. Considering that in samples with residual
resistivity of few mu-Ohm cm critical temperature variations produced by
disorder effects can be comparable with variations due to the isotopic effect,
we adopt a procedure in evaluating the B isotope coefficient which take account
of these effects, obtaining a value which is in agreement with previous results
and then confirming that there is something still unclear in the physics of
MgB2.Comment: 8 pages, 3 figures Title has been changed A statement has been added
in page 7 of the pdf file "Finally we would..." Reference 21 has been added
Figure 1 anf Figure 2 have been change
Spontaneous Fluxon Production in Annular Josephson Tunnel Junctions in the Presence of a Magnetic Field
We report on the spontaneous production of fluxons in the presence of a
symmetry-breaking magnetic field for annular Josephson tunnel junctions during
a thermal quench. The dependence on field intensity of the probability
to trap a single defect during the N-S phase transition drastically
depends on the sample circumferences. We show that the data can be understood
in the framework of the Kibble-Zurek picture of spontaneous defect formation
controlled by causal bounds.Comment: Submitted to Phys. Rev. B with 5 figures on Nov. 15, 200
Dynamic Spin-Polarized Resonant Tunneling in Magnetic Tunnel Junctions
Precisely engineered tunnel junctions exhibit a long sought effect that
occurs when the energy of the electron is comparable to the potential energy of
the tunneling barrier. The resistance of metal-insulator-metal tunnel junctions
oscillates with an applied voltage when electrons that tunnel directly into the
barrier's conduction band interfere upon reflection at the classical turning
points: the insulator-metal interface, and the dynamic point where the incident
electron energy equals the potential barrier inside the insulator. A model of
tunneling between free electron bands using the exact solution of the
Schroedinger equation for a trapezoidal tunnel barrier qualitatively agrees
with experiment.Comment: 4pgs, 3 fig
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