1,027 research outputs found
Paramagnetic Phase of a Heavy-Fermion Compound, CeFePO, Probed by 57Fe M\"{o}ssbauer Spectroscopy
57Fe M\"{o}ssbauer spectroscopy was applied to an iron-based layered compound
CeFePO. At temperatures from 9.4 to 293 K, no magnetic splitting was observed
in the M\"ossbauer spectra of CeFePO indicating a paramagnetic phase of the Fe
magnetic sublattice. All the spectra were fitted with a small quadrupole
splitting, and the Debye temperature of CeFePO was found to be \sim448 K. The
isomer shift at room temperature, 0.32 mm/s, was almost equal to those of
LnFeAsO (Ln = La, Ce, Sm). Comparing s-electron density using the isomer shifts
and unit cell volumes, it was found that the Fe of CeFePO has a similar valence
state to other layered iron-based quaternary oxypnictides except LaFePO
Superconducting Gap and Pseudogap in Iron-Based Layered Superconductor La(OF)FeAs
We report high-resolution photoemission spectroscopy of newly-discovered
iron-based layered superconductor La(OF)FeAs (Tc = 24 K). We
found that the superconducting gap shows a marked deviation from the isotropic
s-wave symmetry. The estimated gap size at 5 K is 3.6 meV in the s- or axial
p-wave case, while it is 4.1 meV in the polar p- or d-wave case. We also found
a pseudogap of 15-20 meV above Tc, which is gradually filled-in with increasing
temperature and closes at temperature far above Tc similarly to copper-oxide
high-temperature superconductors.Comment: 4 pages, 3 figures, J. Phys. Soc. Jpn. Vol. 77, No. 6 (2008), in
pres
Electronic states and pairing symmetry in the two-dimensional 16 band d-p model for iron-based superconductor
The electronic states of the FeAs plane in iron-based superconductors are
investigated on the basis of the two-dimensional 16-band d-p model, where the
tight-binding parameters are determined so as to fit the band structure
obtained by the density functional calculation for LaFeAsO. The model includes
the Coulomb interaction on a Fe site: the intra- and inter-orbital direct terms
U and U', the exchange coupling J and the pair-transfer J'. Within the random
phase approximation (RPA), we discuss the pairing symmetry of possible
superconducting states including s-wave and d-wave pairing on the U'-J plane.Comment: 2 pages, 4 figures; Proceedings of the Int. Symposium on
Fe-Oxipnictide Superconductors (Tokyo, 28-29th June 2008
Systematic Study on Fluorine-doping Dependence of Superconducting and Normal State Properties in LaFePO1-xFx
We have investigated the fluorine-doping dependence of lattice constants,
transports and specific heat for polycrystalline LaFePO1-xFx. F doping slightly
and monotonically decreases the in-plane lattice parameter. In the normal
state, electrical resistivity at low temperature is proportional to the square
of temperature and the electronic specific heat coefficient has large value,
indicating the existence of moderate electron-electron correlation in this
system. Hall coefficient has large magnitude, and shows large temperature
dependence, indicating the low carrier density and multiple carriers in this
system. Temperature dependence of the upper critical field suggests that the
system is a two gap superconductor. The F-doping dependence of these properties
in this system are very weak, while in the FeAs system (LaFeAsO), the F doping
induces the large changes in electronic properties. This difference is probably
due to the different F-doping dependence of the lattice in these two systems.
It has been revealed that a pure effect of electron doping on electronic
properties is very weak in this Fe pnictide compound.Comment: 8 pages, 5 figures, accepted for publication in J. Phys. Soc. Jp
The double life of electrons in magnetic iron pnictides, as revealed by NMR
We present a phenomenological, two-fluid approach to understanding the
magnetic excitations in Fe pnictides, in which a paramagnetic fluid with
gapless, incoherent particle-hole excitations coexists with an
antiferromagnetic fluid with gapped, coherent spin wave excitations. We show
that this two-fluid phenomenology provides an excellent quantitative
description of NMR data for magnetic "122" pnictides, and argue that it finds a
natural justification in LSDA and spin density wave calculations. We further
use this phenomenology to estimate the maximum renormalisation of the ordered
moment that can follow from low-energy spin fluctuations in Fe pnictides. We
find that this is too small to account for the discrepancy between ab intio
calculations and neutron scattering measurements.Comment: Accepted for publication in Europhys. Lett. 6 pages, 4 figure
Effect of Structural Parameters on Superconductivity in Fluorine-Free LnFeAsO1-y (Ln=La,Nd)
The crystal structure of LnFeAsO (Ln = La, Nd) has been studied by
the powder neutron diffraction technique. The superconducting phase diagram of
NdFeAsO is established as a function of oxygen content which is
determined by Rietveld refinement. The small As-Fe bond length suggests that As
and Fe atoms are connected covalently. FeAs-tetrahedrons transform toward a
regular shape with increasing oxygen deficiency. Superconducting transition
temperatures seem to attain maximum values for regular FeAs-tetrahedrons
Fe-based superconductors: unity or diversity?
Does the high temperature superconductivity observed in the newly discovered
iron-pnictide materials represent another example of the same essential physics
responsible for superconductivity in the cuprates, or does it embody a new
mechanism?Comment: Some minor errors in the figure and in the reference in the published
version are corrected. 2 pages, 2 figure
A possible ground state and its electronic structure of a mother material (LaOFeAs) of new superconductors
The electronic and magnetic properties of the mother material LaOFeAs of new
superconductors have been carefully studied using first-principles electronic
structure calculations based on the generalized gradient approximation in the
density functional theory. The present calculation predicts that the ground
state of LaOFeAs is antiferromagnetic with a stripe type magnetic moment
alignment leading to orthorhombic symmetry of the crystal. In this particular
magnetic state, the density of states at the Fermi level is very small. On the
other hand, LaOFeP has turned out to be paramagnetic and a good metal.
Implications of the results regarding the experimental observations will also
be presented.Comment: This letter will be published in J. Phys. Soc. Jpn. {\bf 77} (2008)
No.
Magnetism and Superconductivity in the Two-Dimensional 16 Band d-p Model for Iron-Based Superconductors
The electronic states of the Fe2As2 plane in iron-based superconductors are
investigated on the basis of the two-dimensional 16-band d-p model which
includes the Coulomb interaction on a Fe site: the intra- and inter-orbital
direct terms U and U', the Hund's coupling J and the pair-transfer J'. Using
the random phase approximation (RPA), we obtain the magnetic phase diagram
including the stripe and the incommensurate order on the U'-J plane. We also
solve the superconducting gap equation within the RPA and find that, for large
J, the most favorable pairing symmetry is extended s-wave whose order parameter
changes its sign between the hole pockets and the electron pockets, while it is
dxy-wave for small J.Comment: 4 pages, 5 figure
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