1,434 research outputs found
Critical current anisotropy in Nd-1111 single crystals and the infuence of neutron irradiation
We report on angle-resolved magnetization measurements on
NdFeAsOF (Nd-1111) single crystals. The field dependence of
the critical current density, , is non-monotonous in these crystals at all
orientations and temperatures due to the fishtail effect, which strongly
influences the angular dependence of . The currents decrease as the field
is tilted from the crystallographic c-axis at low fields, but increase at high
fields. A peak occurs in the angular dependence of at intermediate
fields. The critical currents are significantly enhanced after irradiation with
fast neutrons and the fishtail disappears. The different current anisotropies
at low and high fields, however, persist. We discuss the data in the framework
of the anisotropic scaling approach and propose a transition from dominant
pinning by large defects of low density at low fields to pinning by small
defects of high density at high fields in the pristine crystal. Strong pinning
dominates at all fields after the irradiation, and the angular dependence of
can be described by anisotropic scaling only after an appropriate
extension to this pinning regime
Pressure-induced lattice instabilities and superconductivity in YBa2Cu4O8 and optimally doped YBa2Cu3O7-{\delta}
Combined synchrotron angle-dispersive powder diffraction and micro-Raman
spectroscopy are used to investigate the pressure-induced lattice instabilities
that are accompanied by T anomalies in YBaCuO, in comparison with the optimally doped YBaCuO and the non-superconducting PrBaCuO. In the first two superconducting systems there is a clear
anomaly in the evolution of the lattice parameters and an increase of lattice
disorder with pressure, that starts at as well as
irreversibility that induces a hysteresis. On the contrary, in the Pr-compound
the lattice parameters follow very well the expected equation of state (EOS) up
to 7 GPa. In complete agreement with the structural data, the micro-Raman data
of the superconducting compounds show that the energy and width of the A phonons show anomalies at the same pressure range where the lattice
parameters deviate from the EOS and the average Cu2-O bond length
exhibits a strong contraction and correlate with the non-linear pressure
dependence of T. This is not the case for the non superconducting Pr
sample, clearly indicating a connection with the charge carriers. It appears
that the cuprates close to optimal doping are at the edge of lattice
instability.Comment: 6 pages, 7 figure
- …