Superconductivity, magnetism, and spin fluctuations in Fe-based superconductors

Since the discovery of a high temperature superconducting transition in
ferropnictides approximately two years ago, the highly magnetic
character of these compounds and the close relationship between
superconductivity and magnetism has been widely recognized and intensely
studied. Initially, debate about the nature of the magnetism was split
into two camps: localized moments (as in cuprates) and pure itineracy (a
spin-peierls type transition). But closer investigation shows that magnetism in pnictides
and in the related chalcogenides is between these two extremes,
consisting of Hund’s rule (or Stoner) derived moments on the Fe atoms.
Using density functional theory (DFT) calculations, it is shown that the
ordering mechanism is not Fermi surface driven and is also unlikely to
be of superexchange origin. From a computational
perspective, it can be explained how the magnetic and structural transitions are related and
doping and pressure dependent quantities can be compared to
experiment. Many quantities are well reproduced and
explainable using DFT, though remaining questions need to be answered
before magnetism, superconductivity and their relationship can be
considered as understood. Spin fluctuations are widely understood to be the
driving force behind the superconductivity with magnetic order as a
competing, and therefore detrimental, phase. In this context,
spin fluctuation scheme, known as the nematic phase, could explain why the structural transition
appears either in conjunction with or before the magnetic transition.
Spin fluctuations are also likely related to the suppression of LRO order with pressure and doping.

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