1,070 research outputs found
Effects of Co substitution on thermodynamic and transport properties and anisotropic in Ba(FeCo)As single crystals
Single crystalline samples of Ba(FeCo)As with
have been grown and characterized via microscopic, thermodynamic and transport
measurements. With increasing Co substitution, the thermodynamic and transport
signatures of the structural (high temperature tetragonal to low temperature
orthorhombic) and magnetic (high temperature non magnetic to low temperature
antiferromagnetic) transitions are suppressed at a rate of roughly 15 K per
percent Co. In addition, for superconductivity is stabilized,
rising to a maximum of approximately 23 K for and
decreasing for higher values. The phase diagram for
Ba(FeCo)As indicates that either superconductivity can
exist in both low temperature crystallographic phases or that there is a
structural phase separation. Anisotropic, superconducting, upper critical field
data () show a significant and clear change in anisotropy between
samples that have higher temperature structural phase transitions and those
that do not. These data show that the superconductivity is sensitive to the
suppression of the higher temperature phase transition
Gary Wall, Plaintiff-Appellant v. James Wade, et al., Defendants-Appellees: Reply Brief of Appellant
Momentum dependence of the superconducting gap in NdFeAsO1-xFx single crystals measured by angle resolved photoemission spectroscopy
We use angle resolved photoemission spectroscopy (ARPES) to study the
momentum dependence of the superconducting gap in NdFeAsO1-xFx single crystals.
We find that the Gamma hole pocket is fully gapped below the superconducting
transition temperature. The value of the superconducting gap is 15 +- 1.5 meV
and its anisotropy around the hole pocket is smaller than 20% of this value.
This is consistent with an isotropic or anisotropic s-wave symmetry of the
order parameter or exotic d-wave symmetry with nodes located off the Fermi
surface sheets. This is a significant departure from the situation in the
cuprates, pointing to possibility that the superconductivity in the iron
arsenic based system arises from a different mechanism.Comment: 4 pages, 3 figure
Flux pinning in (1111) iron-pnictide superconducting crystals
Local magnetic measurements are used to quantitatively characterize
heterogeneity and flux line pinning in PrFeAsO_1-y and NdFeAs(O,F)
superconducting single crystals. In spite of spatial fluctuations of the
critical current density on the macroscopic scale, it is shown that the major
contribution comes from collective pinning of vortex lines by microscopic
defects by the mean-free path fluctuation mechanism. The defect density
extracted from experiment corresponds to the dopant atom density, which means
that dopant atoms play an important role both in vortex pinning and in
quasiparticle scattering. In the studied underdoped PrFeAsO_1-y and NdFeAs(O,F)
crystals, there is a background of strong pinning, which we attribute to
spatial variations of the dopant atom density on the scale of a few dozen to
one hundred nm. These variations do not go beyond 5% - we therefore do not find
any evidence for coexistence of the superconducting and the antiferromagnetic
phase. The critical current density in sub-T fields is characterized by the
presence of a peak effect, the location of which in the (B,T)-plane is
consistent with an order-disorder transition of the vortex lattice.Comment: 12 pages, submitted to Phys Rev.
Hydrostatic pressure study of pure and doped La1-xRxAgSb2 (R = Ce, Nd) charge-density-wave compounds
The intermetallic compound LaAgSb2 displays two charge-density-wave (CDW)
transitions, which were detected with measurements of electrical resistivity
(rho), magnetic susceptibility, and X-ray scattering; the upper transition
takes place at T1 approx. 210 K, and it is accompanied by a large anomaly in
rho(T), whereas the lower transition is marked by a much more subtle anomaly at
T2 approx. 185 K. We studied the effect of hydrostatic pressure (P) on the
formation of the upper CDW state in pure and doped La1-xRxAgSb2 (R = Ce, Nd)
compounds, by means of measurements of rho(T) for P < 23 kbar. We found that
the hydrostatic pressure, as well as the chemical pressure introduced by the
partial substitution of the smaller Ce and Nd ions for La, result in the
suppression of the CDW ground state, e.g. the reduction of the ordering
temperature T1. The values of dT1/dP are approx. 2-4 times higher for the
Ce-doped samples as compared to pure LaAgSb2, or even La0.75Nd0.25AgSb2
Nd-doped with a comparable T1 (P=0). This increased sensitivity to pressure may
be due to increasing Ce- hybridization under pressure. The magnetic ordering
temperature of the cerium-doped compounds is also reduced by pressure, and the
high pressure behavior of the Ce-doped samples is dominated by Kondo impurity
scattering.Comment: 22 pages, 11 figure
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