277 research outputs found
Crystal and magnetic structure of the oxypnictide superconductor LaO(1-x)FxFeAs: evidence for magnetoelastic coupling
High-resolution and high-flux neutron as well as X-ray powder-diffraction
experiments were performed on the oxypnictide series LaO(1-x)FxFeAs with
0<x<0.15 in order to study the crystal and magnetic structure. The magnetic
symmetry of the undoped compound corresponds to those reported for ReOFeAs
(with Re a rare earth) and for AFe2As2 (A=Ba, Sr) materials. We find an ordered
magnetic moment of 0.63(1)muB at 2 K in LaOFeAs, which is significantly larger
than the values previously reported for this compound. A sizable ordered
magnetic moment is observed up to a F-doping of 4.5% whereas there is no
magnetic order for a sample with a F concentration of x=0.06. In the undoped
sample, several interatomic distances and FeAs4 tetrahedra angles exhibit
pronounced anomalies connected with the broad structural transition and with
the onset of magnetism supporting the idea of strong magneto-elastic coupling
in this material.Comment: 8 pages, 7 figures, regular articl
Magnetodielectric and magnetoelastic coupling in TbFe3(BO3)4
We have studied the magnetodielectric and magnetoelastic coupling in
TbFe3(BO3)4 single crystals by means of capacitance, magnetostriction and Raman
spectroscopy measurements. The data reveal strong magnetic field effects on the
dielectric constant and on the macroscopic sample length which are associated
to long range magnetic ordering and a field-driven metamagnetic transition. We
discuss the coupling of the dielectric, structural, and magnetic order
parameters and attribute the origin of the magnetodielectric coupling to phonon
mode shifts according to the Lyddane-Sachs-Teller (LST) relation.Comment: Accepted for publication in Physical Review
Highly Dispersive Spin Excitations in the Chain Cuprate Li2CuO2
We present an inelastic neutron scattering investigation of Li2CuO2 detecting
the long sought quasi-1D magnetic excitations with a large dispersion along the
CuO2-chains studied up to 25 meV. The total dispersion is governed by a
surprisingly large ferromagnetic (FM) nearest-neighbor exchange integral
J1=-228 K. An anomalous quartic dispersion near the zone center and a
pronounced minimum near (0,0.11,0.5) r.l.u. (corresponding to a spiral
excitation with a pitch angle about 41 degree point to the vicinity of a 3D
FM-spiral critical point. The leading exchange couplings are obtained applying
standard linear spin-wave theory. The 2nd neighbor inter-chain interaction
suppresses a spiral state and drives the FM in-chain ordering below the Ne'el
temperature. The obtained exchange parameters are in agreement with the results
for a realistic five-band extended Hubbard Cu 3d O 2p model and L(S)DA+U
predictions.Comment: 6 pages, 4 figures, submitted to Europhys. Let
Incommensurate magnetic fluctuations and Fermi surface topology in LiFeAs
Using the angle-resolved photoemission spectroscopy (ARPES) data accumulated
over the whole Brillouin zone (BZ) in LiFeAs we analyze the itinerant component
of the dynamic spin susceptibility in this system in the normal and
superconducting state. We identify the origin of the incommensurate magnetic
inelastic neutron scattering (INS) intensity as scattering between the electron
pockets, centered around the point of the BZ and the large
two-dimensional hole pocket, centered around the -point of the BZ. As
the magnitude of the superconducting gap within the large hole pocket is
relatively small and angle dependent, we interpret the INS data in the
superconducting state as a renormalization of the particle-hole continuum
rather than a true spin exciton. Our comparison indicates that the INS data can
be reasonably well described by both the sign changing symmetry of the
superconducting gap between electron and hole pockets as well as sign
preserving gap, depending on the assumptions made for the fermionic damping.Comment: 7 pages, 5 figure
High-Field Pauli-Limiting Behavior and Strongly Enhanced Upper Critical Magnetic Fields near the Transition Temperature of an Arsenic-Deficient LaO_0.9F_0.1FeAs_(1-\delta) Superconductor
We report upper critical field B_c2(T) data for disordered
(arsenic-deficient) LaO_0.9F_0.1FeAs_(1-delta) in a wide temperature and
magnetic field range up to 47 T. Because of the large linear slope of Bc2 about
-5.4 T/K to -6.6 T/K near Tc = 28.5 K the T-dependence of the in-plane Bc2(T)
shows a flattening near 23 K above 30 T which points to Pauli-limited behavior
with Bc2(0) about 63-68 T. Our results are discussed in terms of disorder
effects within conventional and unconventional superconducting pairings.Comment: Change of the title as suggested by the Editors, one author added,
typos corrected, references updated, final published versio
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