125 research outputs found
Longitudinal spin excitations and magnetic anisotropy in antiferromagnetically ordered BaFe2As2
We report on a spin-polarized inelastic neutron scattering study of spin
waves in the antiferromagnetically ordered state of BaFe2As2. Three distinct
excitation components are identified, with spins fluctuating along the c-axis,
perpendicular to the ordering direction in the ab-plane, and parallel to the
ordering direction. While the first two "transverse" components can be
described by a linear spin-wave theory with magnetic anisotropy and inter-layer
coupling, the third "longitudinal" component is generically incompatible with
the local moment picture. It points towards a contribution of itinerant
electrons to the magnetism already in the parent compound of this family of
Fe-based superconductors.Comment: 4 pages, 4 figures, plus Supplemental Materia
Nematic crossover in BaFeAs under uniaxial stress
Raman scattering can detect spontaneous point-group symmetry breaking without
resorting to single-domain samples. Here we use this technique to study
, the parent compound of the "122" Fe-based
superconductors. We show that an applied compression along the Fe-Fe direction,
which is commonly used to produce untwinned orthorhombic samples, changes the
structural phase transition at temperature into a crossover
that spans a considerable temperature range above . Even in
crystals that are not subject to any applied force, a distribution of
substantial residual stress remains, which may explain phenomena that are
seemingly indicative of symmetry breaking above . Our results
are consistent with an onset of spontaneous nematicity only below
.Comment: 4 pages, 4 figure
Growth and post-annealing studies of La-Bi2201 single crystals
BiSrLaCuO (01.00) single crystals
with high-quality have been grown successfully using the travelling-solvent
floating-zone technique. The patterns of X-ray diffraction suggest high
crystalline quality of the samples. After post-annealing in flowing oxygen at
600 C, the crystals show sharp superconducting transitions revealed by AC
susceptibility. The hole concentration is deduced from superconducting
transition temperature (), which exhibits a linear relation with La doping
level . It ranges from the heavily overdoped regime ( 0.2) to the
extremely underdoped side ( 0.08) where the superconductivity is
absent. Comparing with the superconducting dome in
BiSrCuO system, the effects from out-of-plane
disorders show up in our samples. Besides the La doping level , the
superconductivity is also sensitive to the content of oxygen which could be
tuned by post-annealing method over the whole doping range. The post-annealing
effects on and for each La doping level are studied, which give some
insights on the different nature between overdoped and underdoped regime.Comment: 6 pages, 6 figure
Fishtail effect and the vortex phase diagram of single crystal Ba0.6K0.4Fe2As2
By measuring the magnetization hysteresis loops of superconducting
Ba0.6K0.4Fe2As2 single crystals, we obtained the high upper critical field and
large current carrying ability, which point to optimistic applications. The
fishtail (or second peak) effect is also found in the material, and the
position of the vortex pinning force shows a maximum at 1/3 of the reduced
field, being consistent with the picture of vortex pinning by small size normal
cores in the sample. Together with the resistive measurements, for the first
time the vortex phase diagram is obtained for superconductor Ba0.6K0.4Fe2As2.Comment: 4 pages, 5 figure
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