412 research outputs found
Stripe antiferromagnetic correlations in LaFeAsO1-xFx probed by 75As NMR
The anisotropy of the nuclear spin-lattice relaxation rate of
As was investigated in the iron-based superconductor
LaFeAs(OF) ( and 0.14) as well as LaFeAsO. While
the temperature dependence of the normal-state in the superconducting
(SC) is different from that in the SC , their anisotropy
of , in
the normal state is almost the same ( 1.5). The observed anisotropy is
ascribable to the presence of the local stripe correlations with
or . In contrast, is isotropic and is approximately 1 in
the overdoped sample, where superconductivity is almost suppressed.
These results suggest that the presence of the local stripe correlations
originating from the nesting between hole and electron Fermi surfaces is linked
to high- superconductivity in iron pnictides.Comment: 4 pages, 3 figures, Accepted for publication in Phys. Rev.
Electronic Structure and Electron Correlation in LaFeAsO_{1-x}F_x and LaFePO_{1-x}F_x
Photoemission spectroscopy is used to investigate the electronic structure of
the newly discovered iron-based superconductors LaFeAsO_{1-x}F_x and
LaFePO_{1-x}F_x. Line shapes of the Fe 2p core-level spectra suggest an
itinerant character of Fe 3d electrons. The valence-band spectra are generally
consistent with band-structure calculations except for the shifts of Fe
3d-derived peaks toward the Fermi level. From spectra taken in the Fe 3p -> 3d
core-absorption region, we have obtained the experimental Fe 3d partial density
of states, and explained it in terms of a band-structure calculation with a
phenomenological self-energy correction, yielding a mass renormalization factor
of ~< 2.Comment: 4 pages, 5 figure
Itinerant Ferromagnetism in layered crystals LaCoOX (X = P, As)
The electronic and magnetic properties of cobalt-based layered oxypnictides,
LaCoOX (X = P, As), are investigated. LaCoOP and LaCoOAs show metallic type
conduction, and the Fermi edge is observed by hard x-ray photoelectron
spectroscopy. Ferromagnetic transitions occur at 43 K for LaCoOP and 66 K for
LaCoOAs. Above the transition temperatures, temperature dependence of the
magnetic susceptibility follows the Curie-Weiss law. X-ray magnetic circular
dichroism (XMCD) is observed at the Co L2,3-edge, but not at the other edges.
The calculated electronic structure shows a spin polarized ground state. These
results indicate that LaCoOX are itinerant ferromagnets and suggest that their
magnetic properties are governed by spin fluctuation.Comment: 16 pages, 9 figures, Physical Review B, in press. Received 17
February 2008. Accepted 29 May 200
Spin Dynamics in Iron-based Layered Superconductor (La_{0.87}Ca_{0.13})FePO Revealed by ^{31}P and ^{139}La NMR Studies
We report ^{31}P and ^{139}La NMR studies of (La_{0.87}Ca_{0.13})FePO, which
is a family member of the recently discovered superconductor
LaFeAs(O_{1-x}F_x). In the normal state, Knight shift and show that a
Fermi-liquid state with moderate ferromagnetic fluctuations emerges below 30K.
From 1/T_1T of ^{31}P and ^{139}La, quasi-two dimensional electronic
structure is suggested, in which the FeP layer is more conductive than the LaO
layer. In the superconducting (SC) state, although a clear Meissner signal was
observed, 1/T_1T increases below T_c, in contrast to a decrease of 1/T_1T due
to the opening of a SC gap, suggesting that novel low-energy spin dynamics
develop in the SC state.Comment: Phys. Rev. Lett. 101, 077006 (2008
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