5 research outputs found
Correlation between superfluid density and Tc of underdoped YBa2Cu3O6+x near the superconductor-insulator transition
We report measurements of the ab-plane superfluid density Ns (magnetic
penetration depth, \lambda) of severely underdoped films of YBa2Cu3O6+x, with
Tc's from 6 to 50 K. Tc is not proportional to Ns(0); instead, we find Tc ~
Ns^{1/2.3 +/- 0.4}. At the lowest dopings, Tc is as much as 5 times larger than
the upper limit set by the KTB transition temperature of individual CuO2
bilayers.Comment: 4 pages, 2 figures, Submitted to Phys. Rev. Let
Transition from columnar to point pinning in coated conductors: critical currents, that are independent of magnetic field direction
We identify a phase transition in the vortex system of a high-temperature
superconductor with nano-columnar stacks of precipitates as strong vortex
pinning centers. Above a particular, temperature-dependent field the
vortex response is no longer determined by the nano-columns, and is instead
determined by point-like pinning. This phase transition leads to the change in
the measured critical current density as a function of angle between the
applied magnetic field and the nano-columns. Below the unbinding, there is a
strong maximum in when field is aligned parallel to the columns. Above
the unbinding, there is a minimum for this orientation.Comment: Formatted for AP
Tuning flux-pinning in epitaxial NdBa2Cu3O7-x films via engineered, hybrid nanoscale defect structures
Epitaxial NdBa2Cu3O7-x films with a hybrid nanoscale defect structure
comprised of BaZrO3 nanodot arrays aligned along the c-axis in one half of the
film thickness and aligned perpendicular to the c-axis in the other half
thickness of the film were fabricated. Transmission electron microscopy images
confirm the orientation of the nanoscale defect structures. The angular
dependence of critical current density, Jc, at 77 K, 1 T, shows significantly
reduced angular variation of Jc. This study nicely demonstrates how pinning
characteristics can be tuned by tuning the nanoscale defect structures within
the films.Comment: 13 pages, 3figures, submitted to Applied Physics Expres
Aligned crystallite powder of NdFeAsOF: magnetic hysteresis and penetration depth
We report the basal-plane critical current and superfluid density of
magnetically aligned NdFeAsOF powder. This sample has
individual crystallite grains permanently oriented with their c axis along the
external field. Magnetic irreversibilities at high field suggest strong flux
pinning of basal-plane critical currents, with monotonic field dependence and
no evidence of the "fishtail" effect. The small particles provide a sensitive
indicator of \textit{dc} flux penetration, and allow analysis of the
temperature dependence of plane London penetration depth
, which is quadratic at low . This feature may not
necessarily be due to the nodes in the gap, but may be rather a sign of a
strong pair-breaking. A quantitative determination of the absolute magnitude of
is hindered by the need for accurate knowledge of the
particle size distribution.Comment: 6 pages, 6 figure