210 research outputs found
MgB2 tunnel junctions and SQUIDs
Recent advances in the realization and understanding of MgB2 tunnel junctions and SQUIDs are surveyed. High quality MgB2 junctions with suitable tunnel barriers have been realized based on both oriented and epitaxial thin MgB2 films. Multiband transport properties, such as the existence of two energy gaps, phonon spectra and anisotropy have been investigated with these junctions. We review the suitability of different barrier materials and recent advances in obtaining reproducible all-MgB2 Josephson junctions for superconducting electronic circuitry. The development of epitaxial thin films has also led to high-quality multiband MgB2 SQUIDs and magnetometers that operate at high temperatures. The multiband nature of MgB2 provides new phenomena such as the Leggett mode. Manipulating the different phases of the condensates could lead to novel MgB2 devices with phase degrees of freedom.\ud
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Influence of temperature gradients on tunnel junction thermometry below 1 K: cooling and electron-phonon coupling
We have studied thermal gradients in thin Cu and AlMn wires, both
experimentally and theoretically. In the experiments, the wires were Joule
heated non-uniformly at sub-Kelvin temperatures, and the resulting temperature
gradients were measured using normal metal-insulator-superconducting tunnel
junctions. The data clearly shows that even in reasonably well conducting thin
wires with a short (m) non-heated portion, significant temperature
differences can form. In most cases, the measurements agree well with a model
which includes electron-phonon interaction and electronic thermal conductivity
by the Wiedemann-Franz law.Comment: J. Low Temp. Phys. in pres
Method for Measuring the Momentum-Dependent Relative Phase of the Superconducting Gap of High-Temperature Superconductors
The phase variation of the superconducting gap over the (normal) Fermi
surface of the high-temperature superconductors remains a significant
unresolved question. Is the phase of the gap constant, does it change sign, or
is it perhaps complex? A detailed answer to this question would provide
important constraints on various pairing mechanisms. Here we propose a new
method for measuring the relative gap PHASE on the Fermi surface which is
direct, is angle-resolved, and probes the bulk. The required experiments
involve measuring phonon linewidths in the normal and superconducting state,
with resolution available in current facilities. We primarily address the
La_1.85Sr_.15CuO_4 material, but also propose a more detailed study of a
specific phonon in Bi_2Sr_2CaCu_2O_8.Comment: 13 pages (revtex) + 5 figures (postscript-included), NSF-ITP-93-2
Size-effects in the Density of States in NS and SNS junctions
The quasiparticle local density of states (LDOS) is studied in clean NS and
SNS junctions with increasing transverse size, from quasi-one-dimensional to
three-dimensional. It is shown that finite transverse dimensions are related to
pronounced effects in the LDOS, such as fast oscillations superimposed on the
quasiparticle interference oscillations (for NS) and additional peaks in the
bound state spectrum in the subgap region (for SNS). Also, the validity of the
Andreev approximation is discussed. It turns out to be an acceptable
approximation in all situations tested.Comment: 9 pages, RevTex, 5 figures, accepted in Phys. Rev.
Inherent thermometry in a hybrid superconducting tunnel junction
We discuss inherent thermometry in a Superconductor - Normal metal -
Superconductor tunnel junction. In this configuration, the energy selectivity
of single-particle tunneling can provide a significant electron cooling,
depending on the bias voltage. The usual approach for measuring the electron
temperature consists in using an additional pair of superconducting tunnel
junctions as probes. In this paper, we discuss our experiment performed on a
different design with no such thermometer. The quasi-equilibrium in the central
metallic island is discussed in terms of a kinetic equation including injection
and relaxation terms. We determine the electron temperature by comparing the
micro-cooler experimental current-voltage characteristic with isothermal
theoretical predictions. The limits of validity of this approach, due to the
junctions asymmetry, the Andreev reflection or the presence of sub-gap states
are discussed
Mesoscopic proximity effect in double barrier Superconductor/Normal Metal junctions
We report transport measurements down to T=60mK of SININ and SNIN structures
in the diffusive limit. We fabricated Al-AlOx/Cu/AlOx/Cu (SININ) and
Al/Cu/AlOx/Cu (SNIN) vertical junctions. For the first time, a zero bias
anomaly was observed in a metallic SININ structure. We attribute this peak of
conductance to coherent multi-reflections of electrons between the two tunnel
barriers. This conductance maximum is quantitatively fitted by the relevant
theory of mesoscopic SININ structures. When the barrier at the SN interface is
removed (SNIN structure), we observe a peak of conductance at finite voltage
accompagnied by an excess of sub-gap conductance.Comment: 4 pages, 4 figures, editorially approved for publication in Phys.
Rev. B Rapid Com
Kondo effect induced by a magnetic field
We study peculiarities of transport through a Coulomb blockade system tuned
to the vicinity of the spin transition in its ground state. Such transitions
can be induced in practice by application of a magnetic field. Tunneling of
electrons between the dot and leads mixes the states belonging to the ground
state manifold of the dot. Remarkably, both the orbital and spin degrees of
freedom of the electrons are engaged in the mixing at the singlet-triplet
transition point. We present a model which provides an adequate theoretical
description of recent experiments with semiconductor quantum dots and carbon
nanotubes
Using isotopic dilution to assess chemical extraction of labile Ni, Cu, Zn, Cd and Pb in soils
Chemical extractants used to measure labile soil metal must ideally select for and solubilise the labile fraction, with minimal solubilisation of non-labile metal. We assessed four extractants (0.43 M HNO3, 0.43 M CH3COOH, 0.05 M Na2H2EDTA and 1 M CaCl2) against these requirements. For soils contaminated by contrasting sources, we compared isotopically exchangeable Ni, Cu, Zn, Cd and Pb (EValue, mg kg-1), with the concentrations of metal solubilised by the chemical extractants (MExt, mg kg-1). Crucially, we also determined isotopically exchangeable metal in the soil–extractant systems (EExt, mg kg-1). Thus ‘EExt - EValue’ quantifies the concentration of mobilised non-labile metal, while ‘EExt - MExt’ represents adsorbed labile metal in the presence of the extractant. Extraction with CaCl2 consistently underestimated EValue for Ni, Cu, Zn and Pb, while providing a reasonable estimate of EValue for Cd. In contrast, extraction with HNO3 both consistently mobilised non-labile metal and overestimated the EValue. Extraction with CH3COOH appeared to provide a good estimate of EValue for Cd; however, this was the net outcome of incomplete solubilisation of labile metal, and concurrent mobilisation of non-labile metal by the extractant (MExt EValue). The Na2H2EDTA extractant mobilised some non-labile metal in three of the four soils, but consistently solubilised the entire labile fraction for all soil-metal combinations (MExt ≈ EExt). Comparison of EValue, MExt and EExt provides a rigorous means of assessing the underlying action of soil chemical extraction methods and could be used to refine long-standing soil extraction methodologies
Modulation of radial blood flow during Braille character discrimination task
Purpose: Human hands are excellent in performing sensory and motor function. We have hypothesized that blood flow of the hand is dynamically regulated by sympathetic outflow during concentrated finger perception. To identify this hypothesis, we measured radial blood flow (RBF), radial vascular conductance (RVC), heart rate (HR), and arterial blood pressure (AP) during Braille reading performed under the blind condition in nine healthy subjects. The subjects were instructed to read a flat plate with raised letters (Braille reading) for 30 s by the forefinger, and to touch a blank plate as control for the Braille discrimination procedure. Results: HR and AP slightly increased during Braille reading but remained unchanged during the touching of the blank plate. RBF and RVC were reduced during the Braille character discrimination task (decreased by -46% and -49%, respectively). Furthermore, the changes in RBF and RVC were much greater during the Braille character discrimination task than during the touching of the blank plate (decreased by -20% and -20%, respectively). Conclusions: These results have suggested that the distribution of blood flow to the hand is modulated via sympathetic nerve activity during concentrated finger perception
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