249 research outputs found
Distribution of the superconducting gap in an YNi2B2C film studied by point contact spectroscopy
The differential resistances of point contacts between a
normal metal and a c axis oriented YNi2B2C film ( = 15.2K) in the
superconducting (SC) state have been investigated. contains clear
"gap" features connected with processes of Andreev reflection at the boundary
between normal metal and superconductor that allow the determination of the SC
gap and its temperature and magnetic field dependence. A distribution
of from 1.5 meV to 2.4 meV is
revealed; however the critical temperature in all cases corresponded to
that of the film. The value 23.66 is close to the BCS
value of 3.52, and the temperature dependence is BCS-like,
irrespective of the actual value. It is supposed that the distribution
of can be attributed to a gap anisotropy or to a multiband nature of
the SC state in YNi2B2C, rather than to the presence of nodes in the gap.Comment: 6 two-column pages, 7 figs; V2: as published, Fig.4 is modifie
Depairing critical current achieved in superconducting thin films with through-thickness arrays of artificial pinning centers
Large area arrays of through-thickness nanoscale pores have been milled into
superconducting Nb thin films via a process utilizing anodized aluminum oxide
thin film templates. These pores act as artificial flux pinning centers,
increasing the superconducting critical current, Jc, of the Nb films. By
optimizing the process conditions including anodization time, pore size and
milling time, Jc values approaching and in some cases matching the
Ginzburg-Landau depairing current of 30 MA/cm^2 at 5 K have been achieved - a
Jc enhancement over as-deposited films of more than 50 times. In the field
dependence of Jc, a matching field corresponding to the areal pore density has
also been clearly observed. The effect of back-filling the pores with magnetic
material has then been investigated. While back-filling with Co has been
successfully achieved, the effect of the magnetic material on Jc has been found
to be largely detrimental compared to voids, although a distinct influence of
the magnetic material in producing a hysteretic Jc versus applied field
behavior has been observed. This behavior has been tested for compatibility
with currently proposed models of magnetic pinning and found to be most closely
explained by a model describing the magnetic attraction between the flux
vortices and the magnetic inclusions.Comment: 9 pages, 10 figure
Variability in the central equatorial Pacific, 1985–1989
We describe variability in the equatorial Pacific Ocean near 160°W during the 5‐year period 1985–1989, encompassing “normal”, El Niño, and La Niña conditions. This description is based on conductivity‐temperature‐depth and acoustic Doppler current profiler data acquired during five cruises between 21°N and 4°S and on dynamic‐height time series from an array based mainly on the Line Islands. At Jarvis Island, near the equator, the time series of dynamic height and near‐surface temperature go back to 1981 and show the 1986–1987 El Niño anomalies starting later in the year and having longer duration than those of the 1982–1983 El Niño. Dynamic‐height anomaly was less strong for the 1986–1987 event, but the near‐surface temperature anomaly was of similar magnitude for the two El Niños. The Jarvis near‐surface temperature drop from 1986–1987 El Niño maximum to 1988–1989 La Niña minimum was 8°C. Empirical orthogonal function analysis of the time series shows that interannual and interseasonal variability in dynamic height was dominated by a mode with meridional form similar to a first‐vertical‐mode Kelvin wave, while intraseasonal variability had a primary mode with a single peak at 6°N and a secondary mode with peak at 6°N and trough at 2°N. While the equatorial thermocline deepened to the east and shoaled to the west during the 1986–1987 El Niño, at 160°W it did not change depth during either this El Niño or the subsequent La Niña. Nevertheless, just before El Niño and just after La Niña, the thermocline was observed to be about 50 m deeper than at other times. The South Equatorial Current and North Equatorial Countercurrent had markedly reduced (increased) transports during this El Niño (La Niña). However, the Northern Tsuchiya Jet strengthened during El Niño and weakened during La Niña
Spin singlet small bipolarons in Nb-doped BaTiO3
The magnetic susceptibility and electrical resistivity of n-type
BaTi{1-x}Nb{x}O3 have been measured over a wide temperature range. It is found
that, for 0 < x < 0.2, dopant electrons form immobile spin singlet small
bipolarons with binding energy around 110 meV. For x = 0.2, a maximum in the
electrical resistivity around 15 K indicates a crossover from band to hopping
transport of the charge carriers, a phenomenon expected but rarely observed in
real polaronic systems.Comment: 5 pages, 4 figure
The interpretation of the field angle dependence of the critical current in defect-engineered superconductors
We apply the vortex path model of critical currents to a comprehensive
analysis of contemporary data on defect-engineered superconductors, showing
that it provides a consistent and detailed interpretation of the experimental
data for a diverse range of materials. We address the question of whether
electron mass anisotropy plays a role of any consequence in determining the
form of this data and conclude that it does not. By abandoning this false
interpretation of the data, we are able to make significant progress in
understanding the real origin of the observed behavior. In particular, we are
able to explain a number of common features in the data including shoulders at
intermediate angles, a uniform response over a wide angular range and the
greater discrimination between individual defect populations at higher fields.
We also correct several misconceptions including the idea that a peak in the
angular dependence of the critical current is a necessary signature of strong
correlated pinning, and conversely that the existence of such a peak implies
the existence of correlated pinning aligned to the particular direction. The
consistency of the vortex path model with the principle of maximum entropy is
introduced.Comment: 14 pages, 7 figure
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Book Review Roundtable: Alexander Cooley’s Great Games, Local Rules: The New Great Power Contest in Central Asia
In this book review roundtable, seven experts discuss Alexander Cooley’s Great Games, Local Rules: The New Great Power Contest in Central Asia, followed by a response essay from the author
p-wave superconductivity in iron-based superconductors
The possibility of p-wave pairing in superconductors has been proposed more than five decades ago, but has not yet been convincingly demonstrated. One difficulty is that some p-wave states are thermodynamically indistinguishable from s-wave, while others are very similar to d-wave states. Here we studied the self-field critical current of NdFeAs(O,F) thin films in order to extract absolute values of the London penetration depth, the superconducting energy gap, and the relative jump in specific heat at the superconducting transition temperature, and find that all the deduced physical parameters strongly indicate that NdFeAs(O,F) is a bulk p-wave superconductor. Further investigation revealed that single atomic layer FeSe also shows p-wave pairing. In an attempt to generalize these findings, we re-examined the whole inventory of superfluid density measurements in iron-based superconductors and show quite generally that single-band weak-coupling p-wave superconductivity is exhibited in iron-based superconductors. © 2019, The Author(s).Japan Science and Technology Corporation, JST: JPMJCR18J4Government Council on Grants, Russian FederationJapan Society for the Promotion of Science, JSPS: 16H04646АААА-А18-118020190104-3The authors thank Prof. Jeffery L. Tallon (Victoria University of Wellington, New Zealand) and Prof. Christian Bernhard (University of Fribourg, Switzerland) for helpful discussions, and also for reading and commenting on the manuscript. EFT is grateful for financial support provided by the state assignment of Minobrnauki of Russia (theme “Pressure” No. АААА-А18-118020190104-3) and by Act 211 of the Government of the Russian Federation, contract No. 02.A03.21.0006. KI and HI acknowledge support by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (B) Grant Number 16H04646, as well as JST CREST Grant Number JPMJCR18J4. The Article Processing Charge for this publication was provided by Ural Federal University, Russia
The onset of dissipation in high-temperature superconductors: magnetic hysteresis and field dependence
Recently, we showed that the self-field transport critical current, Ic(sf),
of a superconducting wire can be defined in a more fundamental way than the
conventional (and arbitrary) electric field criterion, Ec = 1 microV/cm. We
defined Ic(sf) as the threshold current, Ic,B, at which the perpendicular
component of the local magnetic flux density, measured at any point on the
surface of a high-temperature superconducting tape, abruptly crosses over from
a non-linear to a linear dependence with increasing transport current. This
effect results from the current distribution across the tape width
progressively transitioning from non-uniform to uniform. The completion of this
progressive transition was found to be singular. It coincides with the first
discernible onset of dissipation and immediately precedes the formation of a
measureable electric field. Here, we show that the same Ic,B definition of
critical currents applies in the presence of an external applied magnetic
field. In all experimental data presented here Ic,B is found to be
significantly (10-30%) lower than Ic,E determined by the common electric field
criterion of Ec = 1 microV/cm, and Ec to be up to 50 times lower at Ic,B than
at Ic,E.Comment: 14 pages, 10 figure
Effect of annealing high-dose heavy-ion irradiated high-temperature superconductor wires
Heavy-ion irradiation of high-temperature superconducting thin films has long been known to generate damage tracks of amorphized material that are of close-to-ideal dimension to effectively contribute to pinning of magnetic flux lines and thereby enhance the in-field critical current. At the same time, though, the presence of these tracks reduces the superconducting volume fraction available to transport current while the irradiation process itself generates oxygen depletion and disorder in the remaining superconducting material. We have irradiated commercially available superconducting coated conductors consisting of a thick film of (Y,Dy)Ba2Cu3O7 deposited on a buffered metal tape substrate in a continuous reel-to-reel process. Irradiation was by 185 MeV 197Au ions. A high fluence of 3 × 1011 ions/cm2 was chosen to emphasize the detrimental effects. The critical current was reduced following this irradiation, but annealing at relatively low temperatures of 200 °C and 400 °C substantially restore the critical current of the irradiated material. At high fields and high temperatures there is a net benefit of critical current compared to the untreated materialThe authors wish to thank M. Rupich of American Superconductor for supplying the coated conductor material for our irradiation
trials and for useful discussions. PK and MCR thank the Australian
Research Council for financial support
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