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ABSTRACT: Magnetic excitations in Ba(Fe_{0.94}Co_{0.06})_{2}As_{2} are studied by polarized inelastic neutron scattering above and below the superconducting transition. In the superconducting state, we find clear evidence for two resonancelike excitations. At a higher energy of about 8 meV, there is an isotropic resonance mode with weak dispersion along the c direction. In addition, we find a lower excitation at 4 meV that appears only in the c-polarized channel and whose intensity strongly varies with the l component of the scattering vector. These resonance excitations behave remarkably similar to the gap modes in the antiferromagnetic phase of the parent compound BaFe_{2}As_{2}.
Physical Review Letters 03/2013; 110(13):137001. · 7.37 Impact Factor
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ABSTRACT: Compounds with intermediate-size transition metals such as Fe or Mn are close
to the transition between charge-transfer systems and Mott-Hubbard systems. We
study the optical conductivity \sigma(\omega) of insulating layered LaSrFeO_4
in the energy range 0.5 - 5.5 eV from 15 K to 250 K by the use of spectroscopic
ellipsometry in combination with transmittance measurements. A multipeak
structure is observed in both \sigma^a(\omega) and \sigma^c(\omega). The
layered structure gives rise to a pronounced anisotropy, thereby offering a
means to disentangle Mott-Hubbard and charge-transfer absorption bands. We find
strong evidence that the lowest dipole-allowed excitation in LaSrFeO_4 is of
Mott-Hubbard type. This rather unexpected result can be attributed to Fe 3d - O
2p hybridization and in particular to the layered structure with the associated
splitting of the e_g level. In general, Mott-Hubbard absorption bands may show
a strong dependence on temperature. This is not the case in LaSrFeO_4, in
agreement with the fact that spin-spin and orbital-orbital correlations between
nearest neighbors do not vary strongly below room temperature in this compound
with a high-spin 3d^5 configuration and a Neel temperature of T_N = 366 K.
03/2013;
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ABSTRACT: We present elastic and inelastic neutron scattering data on LaSrFeO$_4$. We
confirm the known magnetic structure with the magnetic moments lying in the
tetragonal basal plane, but contrarily to previous reports our macroscopic and
neutron diffraction data do not reveal any additional magnetic phase transition
connected to a spin reorientation or to a redistribution of two irreducible
presentations. Our inelastic neutron scattering data reveals the magnon
dispersion along the main-symmetry directions [0 $\xi$ 0] and [$\xi$ -$\xi$ 0].
The dispersion can be explained within linear spin-wave theory yielding an
antiferromagnetic nearest-neighbour interaction parameter $J_{1}=7.4(1)$ meV
and a next-nearest neighbour interaction parameter $J_{2}=0.4(1)$ meV. The
dispersion is gapped with the out-of-plane anisotropy gap found at
$\Delta_{out}=5.26(2)$ meV, while evidence is present that the in-plane
anisotropy gap lies at lower energies, where it cannot be determined due to
limited instrument resolution.
01/2013;
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ABSTRACT: Magnetic excitations in Ba(Fe0.94Co0.06)2As2 are studied by polarized
inelastic neutron scattering (INS) above and below the superconducting
transition. In the superconducting state we find clear evidence for two
resonance-like excitations. At a higher energy of about 8 meV there is an
isotropic resonance mode with weak dispersion along the c-direction. In
addition we find a lower excitation at 4 meV that appears only in the
c-polarized channel and whose intensity strongly varies with the L-component of
the scattering vector. These resonance excitations behave remarkably similar to
the gap modes in the antiferromagnetic phase of the parent compound BaFe2As2.
10/2012;
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J. Knolle,
V. B. Zabolotnyy,
I. Eremin,
S. V. Borisenko, N. Qureshi,
M. Braden,
D. V. Evtushinsky,
T. K. Kim,
A. A. Kordyuk,
S. Sykora,
Ch. Hess,
I. V. Morozov,
S. Wurmehl,
R. Moessner,
B. Büchner
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ABSTRACT: Using the angle-resolved photoemission spectroscopy 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 (π,π) point of the BZ, and the large two-dimensional hole pocket, centered around the Γ 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 and the sign-preserving gap, depending on the assumptions made for the fermionic damping.
Phys. Rev. B. 10/2012; 86(17).
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N Qureshi,
P Steffens,
Y Drees,
A C Komarek,
D Lamago,
Y Sidis,
L Harnagea,
H-J Grafe,
S Wurmehl,
B Büchner,
M Braden
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ABSTRACT: Magnetic correlations in superconducting LiFeAs were studied by elastic and by inelastic neutron-scattering experiments. There is no indication for static magnetic ordering, but inelastic correlations appear at the incommensurate wave vector (0.5±δ,0.5-/+δ,0) with δ~0.07 slightly shifted from the commensurate ordering observed in other FeAs-based compounds. The incommensurate magnetic excitations respond to the opening of the superconducting gap by a transfer of spectral weight.
Physical Review Letters 03/2012; 108(11):117001. · 7.37 Impact Factor
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ABSTRACT: The anisotropy of the magnetic excitations in BaFe2As2 was studied by polarized inelastic neutron scattering, which allows one to separate the components of the magnetic response. Despite the in-plane orientation of the static ordered moment, we find the in-plane polarized magnons to exhibit a larger gap than the out-of-plane polarized ones, indicating very strong single-ion anisotropy within the layers. It costs more energy to rotate a spin within the orthorhombic a-b plane than to rotate it perpendicular to the FeAs layers.
Phys. Rev. B. 01/2012; 86(6).
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N. Qureshi,
P. Steffens,
Y. Drees,
A. C. Komarek,
D. Lamago,
Y. Sidis,
L. Harnagea,
H. -J. Grafe,
S. Wurmehl,
B. Büchner,
M. Braden
[show abstract]
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ABSTRACT: Magnetic correlations in superconducting LiFeAs were studied by elastic and
by inelastic neutron scattering experiments. There is no indication for static
magnetic ordering but inelastic correlations appear at the incommensurate wave
vector (0:5+-d 0:-+d 0) with d=0.07 slightly shifted from the commensurate
ordering observed in other FeAs-based compounds. The incommensurate magnetic
excitations respond to the opening of the superconducting gap by a transfer of
spectral weight.
08/2011;
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ABSTRACT: 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.
02/2010;
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ABSTRACT: High-resolution and high-flux neutron as well as x-ray powder-diffraction experiments were performed on the oxypnictide series LaO1−xFxFeAs 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) μB 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 magnetoelastic coupling in this material.
Phys. Rev. B. 82(18).
