263 research outputs found
Field and Temperature Dependence of the Superfluid Density in LaO_{1-x}F_xFeAs Superconductors: A Muon Spin Relaxation Study
We present zero field and transverse field \muSR experiments on the recently
discovered electron doped Fe-based superconductor LaO_{1-x}F_xFeAs. The zero
field experiments on underdoped (x=0.075) and optimally doped (x=0.1) samples
rule out any static magnetic order above 1.6 K in these superconducting
samples. From transverse field experiments in the vortex phase we deduce the
temperature and field dependence of the superfluid density. Whereas the
temperature dependence is consistent with a weak coupling BCS s-wave or a dirty
d-wave gap function scenario, the field dependence strongly evidences
unconventional superconductivity. We obtain the in-plane penetration depth of
\lambda_{ab} (0) = 254(2)nm for LaO_{0.9}F_{0.1}FeAs and \lambda_{ab} (0) =
364(8)nm for LaO_{0.925}F_{0.075}FeAs. Further evidence for unconventional
superconductivity is provided by the ratio of T_c versus the superfluid
density, which is close to the Uemura line of hole doped high-T_c cuprates.Comment: 5 pages, 5 figure
Optical Study of LaO_0.9F_0.1FeAs: Evidence for a Weakly Coupled Superconducting State
We have studied the reflectance of the recently discovered superconductor
LaO_0.9F0.FeAs in a wide energy range from the far infrared to the visible
regime. We report on the observation of infrared active phonons, the plasma
edge (PE) and possible interband transitions. On the basis of this data and the
reported in-plane penetration depth lambda_L(0) about 254 nm [H. Luetkens et
al., Phys. Rev. Lett. v. 101, 0970009 (2008)] a disorder sensitive relatively
small value of the total electron electron-boson coupling constant
lambda_tot=lambda_e-ph+lambda_e-sp ~ 0.6 +- 0.35 can be estimated adopting an
effective single-band picture.Comment: Changed title, updated references, final published versio
Electronic properties of LaOFFeAs in the normal state probed by NMR/NQR
We report 139La, 57Fe and 75As nuclear magnetic resonance (NMR) and nuclear
quadrupole resonance (NQR) measurements on powders of the new LaO1-xFxFeAs
superconductor for x = 0 and x = 0.1 at temperatures up to 480 K, and compare
our measured NQR spectra with local density approximation (LDA) calculations.
For all three nuclei in the x = 0.1 material, it is found that the local Knight
shift increases monotonically with an increase in temperature, and scales with
the macroscopic susceptibility, suggesting a single magnetic degree of freedom.
Surprisingly, the spin lattice relaxation rates for all nuclei also scale with
one another, despite the fact that the form factors for each site sample
different regions of q-space. This result suggests a lack of any q-space
structure in the dynamical spin susceptibility that might be expected in the
presence of antiferromagnetic correlations. Rather, our results are more
compatible with simple quasi-particle scattering. Furthermore, we find that the
increase in the electric field gradient at the As cannot be accounted for by
LDA calculations, suggesting that structural changes, in particular the
position of the As in the unit cell, dominate the NQR response.Comment: 17 pages, 6 figure
Universal linear-temperature dependence of static magnetic susceptibility in iron-pnictides
A universal linear-temperature dependence of the uniform magnetic
susceptibility has been observed in the nonmagnetic normal state of
iron-pnictides. This non-Pauli and non-Curie-Weiss-like paramagnetic behavior
cannot be understood within a pure itinerant picture. We argue that it results
from the existence of a wide antiferromagnetic fluctuation window in which the
local spin-density-wave correlations exist but the global directional order has
not been established yet.Comment: 4 pages, 2 figure
Rare earth magnetism in CeFeAsO: A single crystal study
Single crystals of CeFeAsO, large enough to study the anisotropy of the
magnetic properties, were grown by an optimized Sn-flux technique. The high
quality of our single crystals is apparent from the highest residual
resistivity ratio, RRR = 12, reported among undoped RFeAsO compounds (R=rare
earth) as well as sharp anomalies in resistivity, specific heat, C(T), and
thermal expansion at the different phase transitions. The magnetic
susceptibility chi(T) presents a large easy-plane anisotropy consistent with
the lowest crystal electric field doublet having a dominant Gamma_6 character.
Curie-Weiss like susceptibilities for magnetic field parallel and perpendicular
to the crystallographic c-axis do not reveal an influence of a staggered field
on the Ce site induced by magnetic ordering of the Fe. Furthermore, the
standard signatures for antiferromagnetic order of Ce at T_N = 3.7 K observed
in chi(T) and C(T) are incompatible with a Zeeman splitting Delta = 10 K of the
CEF ground state doublet at low temperature due to the Fe-magnetic order as
previously proposed. Our results can be reconciled with the earlier observation
by assuming a comparatively stronger effect of the Ce-Ce exchange leading to a
reduction of this Zeeman splitting below 15 K.Comment: 15 pages, 6 figures, added section on magn. susceptibilit
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