5,647 research outputs found
Low Power Superconducting Microwave Applications and Microwave Microscopy
We briefly review some non-accelerator high-frequency applications of
superconductors. These include the use of high-Tc superconductors in front-end
band-pass filters in cellular telephone base stations, the High Temperature
Superconductor Space Experiment, and high-speed digital electronics. We also
present an overview of our work on a novel form of near-field scanning
microscopy at microwave frequencies. This form of microscopy can be used to
investigate the microwave properties of metals and dielectrics on length scales
as small as 1 mm. With this microscope we have demonstrated quantitative
imaging of sheet resistance and topography at microwave frequencies. An
examination of the local microwave response of the surface of a heat-treated
bulk Nb sample is also presented.Comment: 11 pages, including 6 figures. Presented at the Eight Workshop on RF
Superconductivity. To appear in Particle Accelerator
Unusual effects of anisotropy on the specific heat of ceramic and single crystal MgB2
The two-gap structure in the superconducting state of MgB_2 gives rise to
unusual thermodynamic properties which depart markedly from the isotropic
single-band BCS model, both in their temperature- and field dependence. We
report and discuss measurements of the specific heat up to 16 T on ceramic, and
up to 14 T on single crystal samples, which demonstrate these effects in the
bulk. The behavior in zero field is described in terms of two characteristic
temperatures, a crossover temperature Tc_pi ~ 13 K, and a critical temperature
Tc = Tc_sigma ~ 38 K, whereas the mixed-state specific heat requires three
characteristic fields, an isotropic crossover field Hc2_pi ~ 0.35 T, and an
anisotropic upper critical field with extreme values Hc2_sigma_c ~ 3.5 T and
Hc2_sigma_ab ~ 19 T, where the indexes \pi and \sigma refer to the 3D and 2D
sheets of the Fermi surface. Irradiation-induced interband scattering tends to
move the gaps toward a common value, and increases the upper critical field up
to ~ 28 T when Tc = 30 K.Comment: 31 pages, 9 figures. Accepted in the Physica C special issue on MgB
SRF Cavity Fabrication and Materials
The technological and metallurgical requirements of material for highgradient
superconducting cavities are described. High-purity niobium, as the preferred
metal for the fabrication of superconducting accelerating cavities, should meet
exact specifications. The content of interstitial impurities such as oxygen,
nitrogen, and carbon must be below 10{\mu}g/g. The hydrogen content should be
kept below 2{\mu}g/g to prevent degradation of the Q-value under certain
cool-down conditions. The material should be free of flaws (foreign material
inclusions or cracks and laminations) that can initiate a thermal breakdown.
Defects may be detected by quality control methods such as eddy current
scanning and identified by a number of special methods. Conventional and
alternative cavity fabrication methods are reviewed. Conventionally, niobium
cavities are fabricated from sheet niobium by the formation of half-cells by
deep drawing, followed by trim machining and Electron-Beam Welding (EBW). The
welding of half-cells is a delicate procedure, requiring intermediate cleaning
steps and a careful choice of weld parameters to achieve full penetration of
the joints. The equator welds are particularly critical. A challenge for a
welded construction is the tight mechanical and electrical tolerances. These
can be maintained by a combination of mechanical and radio-frequency
measurements on halfcells and by careful tracking of weld shrinkage. The
established procedure is suitable for large series production. The main aspects
of quality assurance management are mentioned. Another cavity fabrication
approach is slicing discs from the ingot and producing cavities by deep drawing
and EBW. Accelerating gradients at the level of 35-45 MV.m-1 can be achieved by
applying Electropolishing (EP) treatment....Comment: 37 pages, contribution to the CAS-CERN Accelerator School:
Superconductivity for Accelerators, Erice, Italy, 24 April - 4 May 2013,
edited by R. Baile
Recent Advances in High-Temperature Superconductivity
Recent experimental and theoretical developments in high-temperature
superconductivity are reviewed, and the empirically asymmetric behavior between
hole-doped and electron-doped cuprates is contrasted. A number of phenomena
previously considered as essential for the formation of cuprate
superconductivity, such as the pairing symmetry, pseudogap phenomenon, gapped
incommensurate spin fluctuations and charged stripes, are found to be
non-universal, and are likely the consequence of competing orders. It is
suggested that the only ubiquitous properties among all cuprates are the strong
electronic correlation and antiferromagnetic spin interaction in the CuO2
planes.Comment: 24 pages, 17 figures, 166 references. Review article, to appear in
the Bulletin of Associations of Asia Pacific Physical Societies (AAPPS), Vol.
12. Contact author: Nai-Chang Yeh (e-mail: [email protected]
Concentration Dependence of Superconductivity and Order-Disorder Transition in the Hexagonal Rubidium Tungsten Bronze RbxWO3. Interfacial and bulk properties
We revisited the problem of the stability of the superconducting state in
RbxWO3 and identified the main causes of the contradictory data previously
published. We have shown that the ordering of the Rb vacancies in the
nonstoichiometric compounds have a major detrimental effect on the
superconducting temperature Tc.The order-disorder transition is first order
only near x = 0.25, where it cannot be quenched effectively and Tc is reduced
below 1K. We found that the high Tc's which were sometimes deduced from
resistivity measurements, and attributed to compounds with .25 < x < .30, are
to be ascribed to interfacial superconductivity which generates spectacular
non-linear effects. We also clarified the effect of acid etching and set more
precisely the low-rubidium-content boundary of the hexagonal phase.This work
makes clear that Tc would increase continuously (from 2 K to 5.5 K) as we
approach this boundary (x = 0.20), if no ordering would take place - as its is
approximately the case in CsxWO3. This behaviour is reminiscent of the
tetragonal tungsten bronze NaxWO3 and asks the same question : what mechanism
is responsible for this large increase of Tc despite the considerable
associated reduction of the electron density of state ? By reviewing the other
available data on these bronzes we conclude that the theoretical models which
are able to answer this question are probably those where the instability of
the lattice plays a major role and, particularly, the model which call upon
local structural excitations (LSE), associated with the missing alkali atoms.Comment: To be published in Physical Review
Evidence for length-dependent wire expansion, filament dedensification and consequent degradation of critical current density in Ag-alloy sheathed Bi-2212 wires
It is well known that longer Bi-2212 conductors have significantly lower
critical current density (Jc) than shorter ones, and recently it has become
clear that a major cause of this reduction is internal gas pressure generated
during heat treatment, which expands the wire diameter and dedensifies the
Bi-2212 filaments. Here we report on the length-dependent expansion of 5 to 240
cm lengths of state-of-the-art, commercial Ag alloy-sheathed Bi-2212 wire after
full and some partial heat treatments. Detailed image analysis along the wire
length shows that the wire diameter increases with distance from the ends,
longer samples often showing evident damage and leaks provoked by the internal
gas pressure. Comparison of heat treatments carried out just below the melting
point and with the usual melt process makes it clear that melting is crucial to
developing high internal pressure. The decay of Jc away from the ends is
directly correlated to the local wire diameter increase, which decreases the
local Bi-2212 filament mass density and lowers Jc, often by well over 50%. It
is clear that control of the internal gas pressure is crucial to attaining the
full Jc of these very promising round wires and that the very variable
properties of Bi-2212 wires are due to the fact that this internal gas pressure
has so far not been well controlled
DEFECT PHENOMENA IN SUPERCONDUCTING OXIDES AND ANALOGOUS CERAMIC OXIDES
In this review we discuss defect phenomena in superconducting oxides. We survey those aspects of oxide superconductors which relate them most closely to conventional ceramic oxides, concentrating on processes and behaviour related to defects. We also identify areas of difference between two types of oxide.Theoretical modelling of conventional oxides has been extremely effective, and we emphasize that some of these modelling tools can be exploited for the superconducting oxides too. In particular, we stress those methods and ideas that provide a framework for understanding behaviour, those that provide a datasbase of good quantitative experiments and those that provide an established and tested approach to quantitative modelling as a guide to prediction, optimization and extrapolation.Much progress has been made in both theory and experiment, but some problems do remain and these have not been omitted from our discussions. There is potential to exploit past work on defects in oxides, so as to control defect processes and microstructure and hence to enhance performance
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