ABSTRACT
A comprehensive understanding of the relationship between mag-
netism and superconductivity in the Fe-based superconductors,
discovered in 2008 by Hideo Hosono and collaborators [Kamihara
et al. (2008)], ultimately requires an analysis of the magnetic ground states in these compounds and their evolution with
doping. In particular, the origin of magnetism in the FeSC parent
compounds is hotly debated especially because it is believed that
the same magnetic interactions that drive the magnetic ordering
also produce the Cooper-pairing [Hirschfeld et al. (2011)]. The
phase diagram of ferropnictides (FPs) is similar to high-Tc cuprates
and contains an antiferromagnetic (AF) phase in close proximity
to the superconducting (SC) one. Most iron-based superconductors
exhibit an AF state at low carrier concentrationswhich is suppressed
with doping, pressure, or disorder allowing for the emergence of
superconductivity. This shows strong similarities to the generic
cuprate phase diagram and is evidence for the interplay of
magnetism and superconductivity in the Fe-based materials. There
are two important distinctions, however. First, parent compounds
of iron-based superconductors are antiferromagnetic metals, and second, the superconducting pairing symmetry in most of the
materials is, most likely, an extended s-wave, with or without nodes [Hirschfeld et al. (2011)]. The electronic structure of parent FPs in the normal state has been measured by angle-resolved
photoemission (ARPES) [Liu et al. (2008); Terashima et al. (2009); Zabolotnyy et al. (2009); Yang et al. (2009); Lu et al. (2008); Ding et al. (2011)] and by magneto-oscillations [Coldea et al. (2008); Carrington (2011)]. Both agree largely with ab-initio band structure calculations [Singh and Du (2008); Boeri et al. (2008)]. It consists of two quasi-two-dimensional near-circular hole pockets of unequal
size, centered around the -point (0,0), and two quasi-2D elliptic
electron pockets centered around (0,π) and (π,0) points in the
unfolded Brillouin zone (BZ) which includes only Fe atoms. For
tetragonal symmetry, the two electron pockets transform into each
other under rotation by 90◦. In the folded BZ, which is used for experimental measurements because of two nonequivalent As
positions with respect to an Fe plane, both electron pockets are
centered around (π, π). The dispersions near electron pockets and
near hole pockets are reasonably close to each other apart from the
sign change, i.e., there is a substantial degree of nesting between hole
and electron bands. One has to mention that nesting of electron and
hole bands is not always present in iron-based superconductors and
we comment on these systems at the end of this chapter.
