20,554 research outputs found
Nonmagnetic Impurity Resonances as a Signature of Sign-Reversal Pairing in the FeAs-based Superconductors
The low energy band structure of the FeAs based superconductors is fitted by
a tight binding model with two Fe ions per unit cell and two degenerate
orbitals per Fe ion. Based on this, superconductivity with extended s-wave
pairing symmetry of the form is examined. The local
density of states near an impurity is also investigated by using T-matrix
approach. For the nonmagnetic scattering potential, we found that there exist
two major resonances inside the gap. The height of the resonance peaks depends
on the strength of the impurity potential. These in-gap resonances are
originated in the Andreev's bound states due to the quasiparticle scattering
between the hole Fermi surfaces around point with positive order
parameter and the electron Fermi surfaces around point with negative order
parameter.Comment: 5 pages, 5 figure
Strong Correlations and Magnetic Frustration in the High Tc Iron Pnictides
We consider the iron pnictides in terms of a proximity to a Mott insulator.
The superexchange interactions contain competing nearest-neighbor and
next-nearest-neighbor components. In the undoped parent compound, these
frustrated interactions lead to a two-sublattice collinear antiferromagnet
(each sublattice forming a Neel ordering), with a reduced magnitude for the
ordered moment. Electron or hole doping, together with the frustration effect,
suppresses the magnetic ordering and allows a superconducting state. The
exchange interactions favor a d-wave superconducting order parameter; in the
notation appropriate for the Fe square lattice, its orbital symmetry is
. A number of existing and future experiments are discussed in light of
the theoretical considerations.Comment: (v2) 4+ pages, 4 figures, discussions on several points expanded;
references added. To appear in Phys. Rev. Let
Possibility of Unconventional Pairing Due to Coulomb Interaction in Fe-Based Pnictide Superconductors: Perturbative Analysis of Multi-Band Hubbard Models
Possibility of unconventional pairing due to Coulomb interaction in
iron-pnictide superconductors is studied by applying a perturbative approach to
realistic 2- and 5-band Hubbard models. The linearized Eliashberg equation is
solved by expanding the effective pairing interaction perturbatively up to
third order in the on-site Coulomb integrals. The numerical results for the
5-band model suggest that the eigenvalues of the Eliashberg equation are
sufficiently large to explain the actual high Tc for realistic values of
Coulomb interaction and the most probable pairing state is spin-singlet s-wave
without any nodes just on the Fermi surfaces, although the superconducting
order parameter changes its sign between the small Fermi pockets. On the other
hand the 2-band model is quite insufficient to explain the actual high Tc.Comment: 2 pages, 3 figures. Proceedings of the Intl. Symposium on
Fe-Oxypnictide Superconductors (Tokyo, 28-29th June 2008
Thorium-doping induced superconductivity up to 56 K in Gd1-xThxFeAsO
Following the discovery of superconductivity in an iron-based arsenide
LaO1-xFxFeAs with a superconducting transition temperature (Tc) of 26 K[1], Tc
was pushed up surprisingly to above 40 K by either applying pressure[2] or
replacing La with Sm[3], Ce[4], Nd[5] and Pr[6]. The maximum Tc has climbed to
55 K, observed in SmO1-xFxFeAs[7, 8] and SmFeAsO1-x[9]. The value of Tc was
found to increase with decreasing lattice parameters in LnFeAsO1-xFx (Ln stands
for the lanthanide elements) at an apparently optimal doping level. However,
the F- doping in GdFeAsO is particularly difficult[10,11] due to the lattice
mismatch between the Gd2O2 layers and Fe2As2 layers. Here we report observation
of superconductivity with Tc as high as 56 K by the Th4+ substitution for Gd3+
in GdFeAsO. The incorporation of relatively large Th4+ ions relaxes the lattice
mismatch, hence induces the high temperature superconductivity.Comment: 4 pages, 3 figure
Magnetic Excitations in the High Tc Iron Pnictides
We calculate the expected finite frequency neutron scattering intensity based
on the two-sublattice collinear antiferromagnet found by recent neutron
scattering experiments as well as by theoretical analysis on the iron
oxypnictide LaOFeAs. We consider two types of superexchange couplings between
Fe atoms: nearest-neighbor coupling J1 and next-nearest-neighbor coupling J2.
We show how to distinguish experimentally between ferromagnetic and
antiferromagnetic J1. Whereas magnetic excitations in the cuprates display a
so-called resonance peak at (pi,pi) (corresponding to a saddlepoint in the
magnetic spectrum) which is at a wavevector that is at least close to nesting
Fermi-surface-like structures, no such corresponding excitations exist in the
iron pnictides. Rather, we find saddlepoints near (pi,pi/2) and (0,pi/2)(and
symmetry related points). Unlike in the cuprates, none of these vectors are
close to nesting the Fermi surfaces.Comment: 4 pages, 5 figure
Theory of the Magnetic Moment in Iron Pnictides
We show that the combined effects of spin-orbit, monoclinic distortion, and
p-d hybridization in tetrahedrally coordinated Fe in LaOFeAs invalidates the
naive Hund's rule filling of the Fe d-levels. The two highest occupied levels
have one electron each but as a result of the p-d hybridization have very
different on-site repulsions. As a result, electrons in the upper level are
more itinerant while those in the lower level are more localized. It is the
xy-projection of the spin in the lower level that orders antiferromagnetically
as the z-components of the spins in the two levels is shown to be vanishingly
small in the ground state. The resulting magnetic moment is highly anisotropic
with an in-plane value of per Fe and a z-projection of
, both of which are in agreement with experiment. As a consequence,
we arrive the minimal model that describes the electronic properties of
LaOFeAs.Comment: Published Versio
Wilson Fermions on a Randomly Triangulated Manifold
A general method of constructing the Dirac operator for a randomly
triangulated manifold is proposed. The fermion field and the spin connection
live, respectively, on the nodes and on the links of the corresponding dual
graph. The construction is carried out explicitly in 2-d, on an arbitrary
orientable manifold without boundary. It can be easily converted into a
computer code. The equivalence, on a sphere, of Majorana fermions and Ising
spins in 2-d is rederived. The method can, in principle, be extended to higher
dimensions.Comment: 18 pages, latex, 6 eps figures, fig2 corrected, Comment added in the
conclusion sectio
Superconductivity in heavily boron-doped silicon carbide
The discoveries of superconductivity in heavily boron-doped diamond (C:B) in
2004 and silicon (Si:B) in 2006 renew the interest in the superconducting state
of semiconductors. Charge-carrier doping of wide-gap semiconductors leads to a
metallic phase from which upon further doping superconductivity can emerge.
Recently, we discovered superconductivity in a closely related system:
heavily-boron doped silicon carbide (SiC:B). The sample used for that study
consists of cubic and hexagonal SiC phase fractions and hence this lead to the
question which of them participates in the superconductivity. Here we focus on
a sample which mainly consists of hexagonal SiC without any indication for the
cubic modification by means of x-ray diffraction, resistivity, and ac
susceptibility.Comment: 9 pages, 5 figure
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