26 research outputs found
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
Normal State Spin Dynamics of Five-band Model for Iron-pnictides
Normal state spin dynamics of the recently discovered iron-pnictide
superconductors is discussed by calculating spin structure factor S(q, omega)
in an itinerant five-band model within RPA approximation. Due to the
characteristic Fermi surface structure of iron-pnictide, column like response
is found at (pi, 0) in extended Brillouin zone in the undoped case, which is
consistent with the recent neutron scattering experiment. This indicates that
the localized spin model is not necessary to explain the spin dynamics of this
system. Furthermore, we show that the temperature dependence of inelastic
neutron scattering intensity can be well reproduced in the itinerant model. We
also study NMR 1/T_1T in the same footing calculation and show that the
itinerant model can capture the magnetic property of iron-pnictide
superconductors.Comment: 4 page
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
Structural and superconducting properties in LaFeAs1-xSbxO1-yFy
We report the antimony (Sb) doping effect in a prototype system of iron-based
supercon-ductors LaFeAsO1-yFy (y=0, 0.1, 0.15). X-ray powder diffraction
indicates that the lattice pa-rameters increase with Sb content within the
doping limit. Rietveld structural refinements show that, with the partial
substitution of Sb for As, while the thickness of the Fe2As2 layers increases
significantly, that of the La2O2 layers shrinks simultaneously. So a negative
chemical pressure is indeed "applied" to the superconducting-active Fe2As2
layers, in con-trast to the effect of positive chemical pressure by the
phosphorus doping. Electrical resis-tance and magnetic susceptibility
measurements indicate that, while the Sb doping hardly influences the SDW
anomaly in LaFeAsO, it recovers SDW order for the optimally-doped sample of
y=0.1. In the meantime, the superconducting transition temperature can be
raised up to 30 K in LaFeAs1-xSbxO1-yFy with x=0.1 and y=0.15. The Sb doping
effects are discussed in term of both J1-J2 model and Fermi Surface (FS)
nesting scenario.Comment: 7 pages, 4 figures, 1 table. to be published in Science in China
Series
Superconductivity above 50K in LnFeAsO1-y (Ln= Nd, Sm, Gd, Tb and Dy) Synthesized by High-pressure Technique
We have succeeded in synthesizing single-phase polycrystalline samples of
oxygen-deficient oxypnictide superconductors, LnFeAsO1-y (Ln: lanthanide
elements) with Ln=La, Ce, Pr, Nd, Sm, Gd, Tb and Dy using high-pressure
synthesis technique. It is found out that the synthesis pressure is the most
important parameter for synthesizing single-phase samples, in particular for
the heavier Ln?s, such as Tb and Dy. The lattice parameters systematically
decrease with the atomic number of Ln, reflecting the shrinkage of Ln ionic
radius. For the lighter Ln?s (La, Ce, Pr, Nd), Tc increases monotonously with
decreasing the lattice parameters from 26K for Ln=La to 54K for Ln=Nd, then
stays at the constant value around 53K for the heavier counterpart (Nd, Sm, Gd,
Tb and Dy). The results suggest the intimate relationship between the crystal
structural parameters and the superconductivity on the one hand, as well as the
possible existence of the inherent maximum Tc on the other, which is located
around 50 K in the LnFeAsO based materials.Comment: 14 pages, 10 figures,Accepted for publication in Journal of the
Physical Society of Japan Vol.78No.
Heavily electron-doped electronic structure and isotropic superconducting gap in AxFe2Se2 (A=K,Cs)
The low energy band structure and Fermi surface of the newly discovered
superconductor, AxFe2Se2 (A=K,Cs), have been studied by angle-resolved
photoemission spectroscopy. Compared with iron pnictide superconductors,
AxFe2Se2 (A=K,Cs) is the most heavily electron-doped with Tc~30 K. Only
electron pockets are observed with an almost isotropic superconducting gap of
~10.3 meV, while there is no hole Fermi surface near the zone center, which
indicates the inter-pocket hopping or Fermi surface nesting is not a necessary
ingredient for the unconventional superconductivity in iron-based
superconductors. Thus, the sign changed s pairing symmetry, a leading
candidate proposed for iron-based superconductors, becomes conceptually
irrelevant in describing the superconducting state here. A more conventional
s-wave pairing is a better description.Comment: 4 pages, 4 figures, published online in Nature Materials 201
First-principles Electronic Structure of Superconductor CaAlOFeP: Comparison with LaFePO and CaAlOFeAs
We investigate the electronic structures of iron-based superconductors having
perovskite-like blocking layers, %CaAlOFe(AsP)
from first principles. CaAlOFeP and
CaAlOFeAs from first principles.
CaAlOFeP is found to have two hole-like Fermi surfaces
around , and one hole-like Fermi surface around M in the unfolded
Brillouin zone. This is in contrast with LaFePO, where no Fermi surface is
found around M. The relationship of their band structures and measured
transition temperatures of superconductivity is discussed. The number of Fermi
surfaces in CaAlOFeP is also different from that of
CaAlOFeAs, in which only one Fermi surface is formed around
. Analysis using maximally localized Wannier functions clarifies that
the differences between their band structures originate mainly from the
pnictogen height. We then analyze the alloying effect on the electronic
structure of CaAlOFeAsP. It is found that its electronic
structure is similar to that of CaAlOFeP and
CaAlOFeAs with the average crystal structure, though
CaAlOFeAsP contains the pnictogen height disorder. We calculate
the generalized susceptibility for CaAlOFe(AsP)
and clarify the factors determining its tendency.Comment: 5 figures, to be published in J. Phys. Soc. Jp
d- and f-orbital correlations in the REFeAsO compounds
We estimate theoretically the strength of the local Coulomb interaction for
the Fe 3d and Ce 4f shells in the REFeAsO compunds. In LaFeAsO and CeFeAsO we
obtain values of the local Coulomb interaction parameter U for both Fe and Ce
which are larger than those of elemental Fe and Ce metals. The Fe 3d bandwidth
of REFeAsO is found to increase slightly as one moves along the RE-series.
Using a combined local density approximation and dynamical mean-field theory
(LDA+DMFT) approach, we study the behaviour of the localized 4f states along
the rare-earth oxyarsenides REFeAsO series (RE=Ce,Pr,Nd). In CeFeAsO the
occupied Ce 4f band is located just below the Fe 3d band leading possibly to a
Kondo screening of the 4f local moment under applied pressure, while the
unscreened local moment behaviour is expected for the Pr and Nd compounds.Comment: 7 pages, 2 figures, 1 tabl
Local moment versus Kondo behavior of the 4f-electrons in rare-earth iron oxypnictides
We consider the role played by the 4f states in the rare-earth oxyarsenides
REOFeAs (RE=Ce,Pr,Nd) and the oxyphosphate CeOFeP, using a first-principles
technique that combines the local density approximation and dynamical
mean-field theory (LDA+DMFT). In the Pr and Nd compounds, the 4f states are
located well below and above the Fermi level E_F, and essentially do not
interact with the iron 3d bands located near E_F, resulting in local moment
behavior. In the Ce compounds, our results reveal a qualitatively different
picture, with the 3d-4f hybridization being sufficiently strong to give rise to
an observable Kondo screening of the local 4f moment. Our LDA+DMFT electronic
structure calculations allow us to estimate the Kondo temperature T_K for both
CeOFeP and CeOFeAs. For the phosphate, the order of magnitude of our estimate
is consistent with the experimental observation of T_K around 10 K. At ambient
pressure, T_K is found to be negligibly small for CeOFeAs. Under applied
hydrostatic pressure, we predict an exponential increase of T_K which reaches
values comparable to the superconducting T_c around 40 K at pressures above 10
GPa. We conjecture that the competition between the Kondo effect and
superconductivity may be at the origin of the monotonous decrease of T_c
observed in CeOFeAs under pressure. We argue that the quantitative aspects of
this competition are inconsistent with a weak-coupling BCS description of the
superconductivity in the oxyarsenides.Comment: 6 pages, 3 figure
Electronic Structure Calculation by First Principles for Strongly Correlated Electron Systems
Recent trends of ab initio studies and progress in methodologies for
electronic structure calculations of strongly correlated electron systems are
discussed. The interest for developing efficient methods is motivated by recent
discoveries and characterizations of strongly correlated electron materials and
by requirements for understanding mechanisms of intriguing phenomena beyond a
single-particle picture. A three-stage scheme is developed as renormalized
multi-scale solvers (RMS) utilizing the hierarchical electronic structure in
the energy space. It provides us with an ab initio downfolding of the global
band structure into low-energy effective models followed by low-energy solvers
for the models. The RMS method is illustrated with examples of several
materials. In particular, we overview cases such as dynamics of semiconductors,
transition metals and its compounds including iron-based superconductors and
perovskite oxides, as well as organic conductors of kappa-ET type.Comment: 44 pages including 38 figures, to appear in J. Phys. Soc. Jpn. as an
invited review pape