10 research outputs found
Origin of Gap Anisotropy in Spin Fluctuation Models of the Fe-pnictides
We discuss the large gap anisotropy found for the A1g (s-wave) state in RPA
spin-fluctuation and functional renormalization group calculations and show how
the simple arguments leading to isotropic sign-switched s-wave states in these
systems need to be supplemented by a consideration of pair scattering within
Fermi surface sheets and between the individual electron sheets as well. In
addition, accounting for the orbital makeup of the states on the Fermi surface
is found to be crucial.Comment: 6 pages, 7 figure
d-wave pairing from spin fluctuations in the KxFe{2-y}Se2 superconductors
Angle-resolved photoemission spectroscopy measurements on the recently
discovered superconduc- tors in the KFe2Se2 family with critical temperatures
up to - 33K suggest that no Fermi pockets of hole character centered on the
{\Gamma} point of the Brillouin zone are present, in contrast to all other
known ferropnictide and ferrochalcogenide superconductors. Using a fluctuation
exchange approximation and a 5-orbital tight-binding description of the band
structure, we calculate the effective pairing interaction. We find that the
pairing state in this system is most likely to have d-wave symmetry due to pair
scattering between the remaining electron Fermi pockets at wave vector q -
({\pi}, {\pi}), but without any symmetry-imposed nodes for the given Fermi
surface. We propose experimental tests of this result, including the form of
the resonance spectrum probed by inelastic neutron scattering.Comment: 4 pages, 5 figures, submitted to Rapid Communication
Neutron Scattering Resonance and the Fe-pnictide Superconducting Gap
The existence of a neutron scattering resonance at a wavevector q* implies a
sign change of the gap between two Fermi surface regions separated by
wavevector q* . For the Fe pnictides, a resonance has been observed for a
wavevector q* which connects a hole Fermi surface around the point
with an electron Fermi surface around the X or Y points of the 1 Fe/unit cell
Brillouin zone. Here we study the neutron scattering resonance for a five
orbital model within an RPA-BCS approximation. Our results show that both
sign-switched and extended s-wave gaps are consistent with the present data for
q* near (, 0) and that scattering at other momentum transfers can be
useful in distinguishing between gap structures.Comment: 5 pages, 4 figure
Spin fluctuations and superconductivity in a 3D tight-binding model for BaFe2As2
Despite the wealth of experimental data on the Fe-pnictide compounds of the
KFe2As2-type, K = Ba, Ca, or Sr, the main theoretical work based on
multiorbital tight-binding models has been restricted so far to the study of
the related 1111 compounds. This can be ascribed to the more three dimensional
electronic structure found by ab initio calculations for the 122 materials,
making this system less amenable to model development. In addition, the more
complicated Brillouin zone (BZ) of the body-centered tetragonal symmetry does
not allow a straightforward unfolding of the electronic band structure into an
effective 1Fe/unit cell BZ. Here we present an effective 5-orbital
tight-binding fit of the full DFT band structure for BaFeAs including the kz
dispersions. We compare the 5-orbital spin fluctuation model to one previously
studied for LaOFeAs and calculate the RPA enhanced susceptibility. Using the
fluctuation exchange approximation to determine the leading pairing
instability, we then examine the differences between a strictly two dimensional
model calculation over a single kz cut of the BZ and a completely three
dimensional approach. We find pairing states quite similar to the 1111
materials, with generic quasi-isotropic pairing on the hole sheets and nodal
states on the electron sheets at kz = 0 which however are gapped as the system
is hole doped. On the other hand, a substantial kz dependence of the order
parameter remains, with most of the pairing strength deriving from processes
near kz = pi. These states exhibit a tendency for an enhanced anisotropy on the
hole sheets and a reduced anisotropy on the electron sheets near the top of the
BZ.Comment: 12 pages, 15 figure
Lifting of nodes by disorder in extended- state superconductors: application to ferropnictides
We show, using a simple model, how ordinary disorder can gap an extended-
() symmetry superconducting state with nodes. The concommitant
crossover of thermodynamic properties, particularly the -dependence of the
superfluid density, from pure power law behavior to an activated one is
exhibited. We discuss applications of this scenario to experiments on the
ferropnictide superconductors.Comment: 9 page
Sensitivity of the superconducting state and magnetic susceptibility to key aspects of electronic structure in ferropnictides
Experiments on the iron-pnictide superconductors appear to show some
materials where the ground state is fully gapped, and others where low-energy
excitations dominate, possibly indicative of gap nodes. Within the framework of
a 5-orbital spin fluctuation theory for these systems, we discuss how changes
in the doping, the electronic structure or interaction parameters can tune the
system from a fully gapped to nodal sign-changing gap with s-wave ()
symmetry (). In particular we focus on the role of the hole pocket at
the point of the unfolded Brillouin zone identified as crucial to
the pairing by Kuroki {\it et al.}, and show that its presence leads to
additional nesting of hole and electron pockets which stabilizes the isotropic
state. The pocket's contribution to the pairing can be tuned by doping,
surface effects, and by changes in interaction parameters, which we examine.
Analytic expressions for orbital pairing vertices calculated within the RPA
fluctuation exchange approximation allow us to draw connections between aspects
of electronic structure, interaction parameters, and the form of the
superconducting gap
Pairing in the iron arsenides: a functional RG treatment
We study the phase diagram of a microscopic model for the superconducting
iron arsenides by means of a functional renormalization group. Our treatment
establishes a connection between a strongly simplified two-patch model by
Chubukov et al. and a five-band- analysis by Wang et al.. For a wide parameter
range, the dominant pairing instability occurs in the extended s-wave channel.
The results clearly show the relevance of pair scattering between electron and
hole pockets. We also give arguments that the phase transition between the
antiferromagnetic phase for the undoped system and the superconducting phase
may be first order
Near-degeneracy of several pairing channels in multiorbital models for the Fe-pnictides
Weak-coupling approaches to the pairing problem in the iron pnictide
superconductors have predicted a wide variety of superconducting ground states.
We argue here that this is due both to the inadequacy of certain approximations
to the effective low-energy band structure, and to the natural near-degeneracy
of different pairing channels in superconductors with many distinct Fermi
surface sheets. In particular, we review attempts to construct two-orbital
effective band models, the argument for their fundamental inconsistency with
the symmetry of these materials, and the comparison of the dynamical
susceptibilities in two- and five-orbital models. We then present results for
the magnetic properties, pairing interactions, and pairing instabilities within
a five-orbital Random Phase Approximation model. We discuss the robustness of
these results for different dopings, interaction strengths, and variations in
band structure. Within the parameter space explored, an anisotropic,
sign-changing s-wave state and a d_x2-y2 state are nearly degenerate, due to
the near nesting of Fermi surface sheets.Comment: 17 pages, 23 figure
Magnetism and its microscopic origin in iron-based high-temperature superconductors
High-temperature superconductivity in the iron-based materials emerges from,
or sometimes coexists with, their metallic or insulating parent compound
states. This is surprising since these undoped states display dramatically
different antiferromagnetic (AF) spin arrangements and Nel
temperatures. Although there is general consensus that magnetic interactions
are important for superconductivity, much is still unknown concerning the
microscopic origin of the magnetic states. In this review, progress in this
area is summarized, focusing on recent experimental and theoretical results and
discussing their microscopic implications. It is concluded that the parent
compounds are in a state that is more complex than implied by a simple Fermi
surface nesting scenario, and a dual description including both itinerant and
localized degrees of freedom is needed to properly describe these fascinating
materials.Comment: 14 pages, 4 figures, Review article, accepted for publication in
Nature Physic