2,096 research outputs found
An imaging vector magnetograph for the next solar maximum
Researchers describe the conceptual design of a new imaging vector magnetograph currently being constructed at the University of Hawaii. The instrument combines a modest solar telescope with a rotating quarter-wave plate, an acousto-optical tunable prefilter as a blocker for a servo-controlled Fabry-Perot etalon, CCD cameras, and on-line digital image processing. Its high spatial resolution (1/2 arcsec pixel size) over a large field of view (5 by 5 arcmin) will be sufficient to significantly measure, for the first time, the magnetic energy dissipated in major solar flares. Its millisecond tunability and wide spectral range (5000 to 7000 A) enable nearly simultaneous vector magnetic field measurements in the gas-pressure-dominated photosphere and magnetically-dominated chromosphere, as well as effective co-alignment with Solar-A's X ray images. Researchers expect to have the instrument in operation at Mees Solar Observatory (Haleakala) in early 1991. They have chosen to use tunable filters as wavelength-selection elements in order to emphasize the spatial relationships between magnetic field elements, and to permit construction of a compact, efficient instrument. This means that spectral information must be obtained from sequences of images, which can cause line profile distortions due to effects of atmospheric seeing
Anisotropy Reversal of the Upper Critical Field at Low Temperatures and Spin-Locked Superconductivity in K2Cr3As3
We report the first measurements of the anisotropic upper critical field
for KCrAs single crystals up to 60 T and K. Our results show that the upper critical field parallel to the Cr
chains, , exhibits a paramagnetically-limited behavior,
whereas the shape of the curve (perpendicular to the Cr
chains) has no evidence of paramagnetic effects. As a result, the curves
and cross at K, so that
the anisotropy parameter
increases from near to at 0.6 K. This behavior of is inconsistent with triplet
superconductivity but suggests a form of singlet superconductivity with the
electron spins locked onto the direction of Cr chains
Field Dependence of the Superconducting Basal Plane Anisotropy of TmNi2B2C
The superconductor TmNi2B2C possesses a significant four-fold basal plane
anisotropy, leading to a square Vortex Lattice (VL) at intermediate fields.
However, unlike other members of the borocarbide superconductors, the
anisotropy in TmNi2B2C appears to decrease with increasing field, evident by a
reentrance of the square VL phase. We have used Small Angle Neutron Scattering
measurements of the VL to study the field dependence of the anisotropy. Our
results provide a direct, quantitative measurement of the decreasing
anisotropy. We attribute this reduction of the basal plane anisotropy to the
strong Pauli paramagnetic effects observed in TmNi2B2C and the resulting
expansion of vortex cores near Hc2.Comment: 8 pages, 6 figures, 1 tabl
Systematic effects of carbon doping on the superconducting properties of Mg(BC)
The upper critical field, , of Mg(BC) has been
measured in order to probe the maximum magnetic field range for
superconductivity that can be attained by C doping. Carbon doped boron
filaments are prepared by CVD techniques, and then these fibers are then
exposed to Mg vapor to form the superconducting compound. The transition
temperatures are depressed about C and rises at about C. This means that 3.5% C will depress from to and
raise from to . Higher fields are probably
attainable in the region of 5% C to 7% C. These rises in are
accompanied by a rise in resistivity at from about
to about . Given that the samples are polycrystalline wire
segments, the experimentally determined curves represent the upper
manifold associated with
Magnetic field induced orientation of superconducting MgB crystallites determined by X-ray diffraction
X-ray diffraction studies of fine polycrystalline samples of MgB in the
superconducting state reveal that crystals orient with their \emph{c}-axis in a
plane normal to the direction of the applied magnetic field. The MgB
samples were thoroughly ground to obtain average grain size 5 - 10 m in
order to increase the population of free single crystal grains in the powder.
By monitoring Bragg reflections in a plane normal to an applied magnetic field
we find that the powder is textured with significantly stronger (\emph{0,0,l})
reflections in comparison to (\emph{h,k,0}), which remain essentially
unchanged. The orientation of the crystals with the \emph{ab}-plane parallel to
the magnetic field at all temperatures below demonstrates that the sign
of the torque under magnetic field does not alter, in disagreement with current
theoretical predictions
Magnetic excitations in underdoped Ba(Fe1-xCox)2As2 with x=0.047
The magnetic excitations in the paramagnetic-tetragonal phase of underdoped
Ba(Fe0.953Co0.047)2As2, as measured by inelastic neutron scattering, can be
well described by a phenomenological model with purely diffusive spin dynamics.
At low energies, the spectrum around the magnetic ordering vector Q_AFM
consists of a single peak with elliptical shape in momentum space. At high
energies, this inelastic peak is split into two peaks across the direction
perpendicular to Q_AFM. We use our fittings to argue that such a splitting is
not due to incommensurability or propagating spin-wave excitations, but is
rather a consequence of the anisotropies in the Landau damping and in the
magnetic correlation length, both of which are allowed by the tetragonal
symmetry of the system. We also measure the magnetic spectrum deep inside the
magnetically-ordered phase, and find that it is remarkably similar to the
spectrum of the paramagnetic phase, revealing the strongly overdamped character
of the magnetic excitations.Comment: 12 pages, 7 figure
Effect of field dependent core size on reversible magnetization of high- superconductors
The field dependence of the vortex core size is incorporated in the
London model, in order to describe reversible magnetization for a
number of materials with large Ginzburg-Landau parameter . The
dependence is directly related to deviations in from linear
behavior prescribed by the standard London model. A simple method to extract
from the magnetization data is proposed. For most materials examined,
so obtained decreases with increasing field and is in qualitative
agreement both with behavior extracted from SR and small angle neutron
scattering data and with that predicted theoretically
Unpaired Electrons in the Heavy-Fermion Superconductor CeCoIn_{5}
Thermal conductivity and specific heat were measured in the superconducting
state of the heavy fermion material Ce_{1-x}La_{x}CoIn_{5}. With increasing
impurity concentration x, the suppression of T_{c} is accompanied by the
increase in the residual electronic specific heat expected of a d-wave
superconductor, but it occurs in parallel with a decrease in residual
electronic thermal conductivity. This contrasting behavior reveals the presence
of uncondensed electrons coexisting with nodal quasiparticles. An extreme
multiband scenario is proposed, with a d-wave superconducting gap on the
heavy-electron sheets of the Fermi surface and a negligible gap on the light,
three-dimensional pockets.Comment: 4 pages, 3 figure
Superconductivity in Dense Wires
becomes superconducting just below 40 K. Whereas porous
polycrystalline samples of can be synthesized from boron powders, in
this letter we demonstrate that dense wires of can be prepared by
exposing boron filaments to vapor. The resulting wires have a diameter of
160 , are better than 80% dense and manifest the full shielding in the superconducting state. Temperature-dependent
resistivity measurements indicate that is a highly conducting metal in
the normal state with = 0.38 -. Using this value, an
electronic mean free path, can be estimated, indicating
that wires are well within the clean limit. , , and
data indicate that manifests comparable or better superconducting
properties in dense wire form than it manifests as a sintered pellet.Comment: Figures' layout fixe
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