2,900 research outputs found
Impurity states in antiferromagnetic Iron Arsenides
We explore theoretically impurity states in the antiferromagnetic
spin-density wave state of the iron arsenide. Two types of impurity models are
employed: one has only the intraband scattering while the other has both the
intraband and interband scattering with the equal strength. Interestingly, the
impurity bound state is revealed around the impurity site in the energy gap for
both models. However, the impurity state is doubly degenerate with respect to
spin for the first case; while the single impurity state is observed in either
the spin-up or spin-down channel for the second one. The impurity-induced
variations of the local density of states are also examined.Comment: 4 pages, 2 figure
Effect of anisotropy on the ground-state magnetic ordering of the spin-one quantum -- model on the square lattice
We study the zero-temperature phase diagram of the
-- Heisenberg model for spin-1 particles on an
infinite square lattice interacting via nearest-neighbour () and
next-nearest-neighbour () bonds. Both bonds have the same -type
anisotropy in spin space. The effects on the quasiclassical N\'{e}el-ordered
and collinear stripe-ordered states of varying the anisotropy parameter
is investigated using the coupled cluster method carried out to high
orders. By contrast with the spin-1/2 case studied previously, we predict no
intermediate disordered phase between the N\'{e}el and collinear stripe phases,
for any value of the frustration , for either the -aligned () or -planar-aligned () states. The quantum phase
transition is determined to be first-order for all values of and
. The position of the phase boundary is determined
accurately. It is observed to deviate most from its classical position (for all values of ) at the Heisenberg isotropic point
(), where . By contrast, at the XY
isotropic point (), we find . In the
Ising limit () as expected.Comment: 20 pages, 5 figure
Doping evolution of itinerant magnetic excitations in Fe-based oxypnictides
Employing the four-band tight-binding model we study theoretically the doping
dependence of the spin response in the normal state of novel Fe-based pnictide
superconductors. We show that the commensurate spin density wave (SDW)
transition that arises due to interband scattering between the hole
-pockets and the electron -pockets disappears already at the
doping concentration reflecting the evolution of the Fermi
surfaces. Correspondingly, with further increase of the doping the
antiferromagnetic fluctuations are suppressed for and the
Im becomes nearly temperature independent. At the
same time, we observe that the uniform susceptibility deviates from the
Pauli-like behavior and is increasing with increasing temperature reflecting
the activation processes for the -Fermi surfaces up to temperatures of
about T=800K. With increase of the doping the absolute value of the uniform
susceptibility lowers and its temperature dependence changes. In particular, it
is a constant at low temperatures and then decreases with increasing
temperature. We discuss our results in a context of recent experimental data.Comment: 5 pages, accepted in EP
Effect of the tetrahedral distortion on the electronic properties of iron-pnictides
We study the dependence of the electronic structure of iron pnictides on the
angle formed by the arsenic-iron bonds. Within a Slater-Koster tight binding
model which captures the correct symmetry properties of the bands, we show that
the density of states and the band structure are sensitive to the distortion of
the tetrahedral environment of the iron atoms. This sensitivity is extremely
strong in a two-orbital (d_xz, d_yz) model due to the formation of a flat band
around the Fermi level. Inclusion of the d_xy orbital destroys the flat band
while keeping a considerable angle dependence in the band structure.Comment: 5 pages, including 5 figures. Fig. 5 replaced. Minor changes in the
tex
Raman spectra in iron-based quaternary CeOFFeAs and LaOFFeAs
Raman spectra have been measured on iron-based quaternary
CeOFFeAs and LaOFFeAs with varying fluorine doping at
room temperatures. A group analysis has been made to clarify the optical modes.
Based on the first principle calculations, the observed phonon modes can be
assigned accordingly. In LaOFFeAs, the E and A modes
related to the vibrations of La, are suppressed with increasing F doping.
However F doping only has a small effect on the E and A modes of Fe
and As. The Raman modes of La and As are absent in rare-earth substituted
CeOFFeAs, and the E mode of oxygen, corresponding to the
in-plane vibration of oxygen, moves to around 450 cm and shows a very
sharp peak. Electronic scattering background is low and electron-phonon
coupling is not evident for the observed phonon modes. Three features are found
above 500 cm, which may be associated with multi-phonon process.
Nevertheless it is also possible that they are related to magnetic fluctuations
or interband transitions of d orbitals considering their energies.Comment: 12 pages, 3 figures, 2 table
Pressure-induced magnetic transition and volume collapse in FeAs superconductors: An orbital-selective Mott scenario
Motivated by pressure experiments on FeAs-122 superconductors, we propose a
scenario based on local-moment physics to explain the simultaneous
disappearance of magnetism, reduction of the unit cell volume, and decrease in
resistivity. In this scenario, the low-pressure magnetic phase derives from Fe
moments, which become screened in the paramagnetic high-pressure phase. The
quantum phase transition can be described as an orbital-selective Mott
transition, which is rendered first order by coupling to the lattice, in
analogy to a Kondo volume collapse. Spin-fluctuation driven superconductivity
competes with antiferromagnetism and may be stabilized at low temperatures in
the high-pressure phase. The ideas are illustrated by a suitable mean-field
analysis of an Anderson lattice model.Comment: 9 pages, 3 figs; (v2) robustness of OS Mott transition vs. fragility
of superconductivity discussed, final version to be publishe
Observation of electron-antineutrino disappearance at Daya Bay
The Daya Bay Reactor Neutrino Experiment has measured a non-zero value for
the neutrino mixing angle with a significance of 5.2 standard
deviations. Antineutrinos from six 2.9 GW reactors were detected in
six antineutrino detectors deployed in two near (flux-weighted baseline 470 m
and 576 m) and one far (1648 m) underground experimental halls. With a 43,000
ton-GW_{\rm th}-day livetime exposure in 55 days, 10416 (80376) electron
antineutrino candidates were detected at the far hall (near halls). The ratio
of the observed to expected number of antineutrinos at the far hall is
. A rate-only analysis
finds in a
three-neutrino framework.Comment: 5 figures. Version to appear in Phys. Rev. Let
Nuclear magnetic relaxation and superfluid density in Fe-pnictide superconductors: An anisotropic \pm s-wave scenario
We discuss the nuclear magnetic relaxation rate and the superfluid density
with the use of the effective five-band model by Kuroki et al. [Phys. Rev.
Lett. 101, 087004 (2008)] in Fe-based superconductors. We show that a
fully-gapped anisotropic \pm s-wave superconductivity consistently explains
experimental observations. In our phenomenological model, the gaps are assumed
to be anisotropic on the electron-like \beta Fermi surfaces around the M point,
where the maximum of the anisotropic gap is about four times larger than the
minimum.Comment: 10 pages, 8 figures; Submitted versio
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