2,037 research outputs found
d-wave pairing symmetry in cuprate superconductors
Phase-sensitive tests of pairing symmetry have provided strong evidence for
predominantly d-wave pairing symmetry in both hole- and electron-doped high-Tc
cuprate superconductors. Temperature dependent measurements in YBCO indicate
that the d-wave pairing dominates, with little if any imaginary component, at
all temperatures from 0.5K through Tc. In this article we review some of this
evidence and discuss the implications of the universal d-wave pairing symmetry
in the cuprates.Comment: 4 pages, M2S 2000 conference proceeding
Meissner response of superconductors with inhomogeneous penetration depths
We discuss the Meissner response to a known field source of superconductors
having inhomogeneities in their penetration depth. We simplify the general
problem by assuming that the perturbations of the fields by the penetration
depth inhomogeneities are small. We present expressions for inhomogeneities in
several geometries, but concentrate for comparison with experiment on planar
defects, perpendicular to the sample surfaces, with superfluid densities
different from the rest of the samples. These calculations are relevant for
magnetic microscopies, such as Scanning Superconducting Quantum Interference
Device (SQUID) and Magnetic Force Microscope, which image the local diamagnetic
susceptibility of a sample.Comment: 9 pages, 4 figure
The integrated motivational–volitional model of suicidal behaviour
Suicide is a major public health concern accounting for 800 000 deaths globally each year. Although there have been many advances in understanding suicide risk in recent decades, our ability to predict suicide is no better now than it was 50 years ago. There are many potential explanations for this lack of progress, but the absence, until recently, of comprehensive theoretical models that predict the emergence of suicidal ideation distinct from the transition between suicidal ideation and suicide attempts/suicide is key to this lack of progress. The current article presents the integrated motivational–volitional (IMV) model of suicidal behaviour, one such theoretical model. We propose that defeat and entrapment drive the emergence of suicidal ideation and that a group of factors, entitled volitional moderators (VMs), govern the transition from suicidal ideation to suicidal behaviour. According to the IMV model, VMs include access to the means of suicide, exposure to suicidal behaviour, capability for suicide (fearlessness about death and increased physical pain tolerance), planning, impulsivity, mental imagery and past suicidal behaviour. In this article, we describe the theoretical origins of the IMV model, the key premises underpinning the model, empirical tests of the model and future research directions
Determining the vortex tilt relative to a superconductor surface
It is of interest to determine the exit angle of a vortex from a
superconducting surface, since this affects the intervortex interactions and
their consequences. Two ways to determine this angle are to image the vortex
magnetic fields above the surface, or the vortex core shape at the surface. In
this work we evaluate the field h(x, y, z) above a flat superconducting surface
x, y and the currents J(x,y) at that surface for a straight vortex tilted
relative to the normal to the surface, for both the isotropic and anisotropic
cases. In principle, these results can be used to determine the vortex exit
tilt angle from analyses of magnetic field imaging or density of states data.Comment: 5 pages, 4 figure
Probing the order parameter symmetry in the cuprate high temperature superconductors by SQUID microscopy
The orbital component of the order parameter in the cuprate high-Tc cuprate
superconductors is now well established, in large part because of phase
sensitive tests. Although it would be desirable to use such tests on other
unconventional superconductors, there are a number of favorable factors
associated with the properties of the cuprates, and a number of technical
advances, that were required for these tests to be successful. In this review I
will describe the development of phase sensitive pairing symmetry tests using
SQUID microscopy, underlining the factors favoring these experiments in the
cuprates and the technical advances that had to be made.Comment: 12 pages, 7 figure, invited review to be published in Comptes Rendus
de l'Academie des Sciences (Comptes Rendus Physique
Performance of ceramic superconductors in magnetic bearings
Magnetic bearings are large-scale applications of magnet technology, quite similar in certain ways to synchronous machinery. They require substantial flux density over relatively large volumes of space. Large flux density is required to have satisfactory force density. Satisfactory dynamic response requires that magnetic circuit permeances not be too large, implying large air gaps. Superconductors, which offer large magnetomotive forces and high flux density in low permeance circuits, appear to be desirable in these situations. Flux densities substantially in excess of those possible with iron can be produced, and no ferromagnetic material is required. Thus the inductance of active coils can be made low, indicating good dynamic response of the bearing system. The principal difficulty in using superconductors is, of course, the deep cryogenic temperatures at which they must operate. Because of the difficulties in working with liquid helium, the possibility of superconductors which can be operated in liquid nitrogen is thought to extend the number and range of applications of superconductivity. Critical temperatures of about 98 degrees Kelvin were demonstrated in a class of materials which are, in fact, ceramics. Quite a bit of public attention was attracted to these new materials. There is a difficulty with the ceramic superconducting materials which were developed to date. Current densities sufficient for use in large-scale applications have not been demonstrated. In order to be useful, superconductors must be capable of carrying substantial currents in the presence of large magnetic fields. The possible use of ceramic superconductors in magnetic bearings is investigated and discussed and requirements that must be achieved by superconductors operating at liquid nitrogen temperatures to make their use comparable with niobium-titanium superconductors operating at liquid helium temperatures are identified
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