2 research outputs found
Magnetic order in orbital models of the iron pnictides
We examine the appearance of the experimentally-observed stripe
spin-density-wave magnetic order in five different orbital models of the iron
pnictide parent compounds. A restricted mean-field ansatz is used to determine
the magnetic phase diagram of each model. Using the random phase approximation,
we then check this phase diagram by evaluating the static spin susceptibility
in the paramagnetic state close to the mean-field phase boundaries. The momenta
for which the susceptibility is peaked indicate in an unbiased way the actual
ordering vector of the nearby mean-field state. The dominant orbitally resolved
contributions to the spin susceptibility are also examined to determine the
origin of the magnetic instability. We find that the observed stripe magnetic
order is possible in four of the models, but it is extremely sensitive to the
degree of the nesting between the electron and hole Fermi pockets. In the more
realistic five-orbital models, this order competes with a strong-coupling
incommensurate state which appears to be controlled by details of the
electronic structure below the Fermi energy. We conclude by discussing the
implications of our work for the origin of the magnetic order in the pnictides.Comment: 19 pages, 19 figures; published version, typos corrected, references
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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