Description
In this talk I will present the theory of interplay between
different macroscopic orderings universally found in iron
based superconductors. In most of iron pnictides nematic
order is a byproduct of stripe magnetism. In FeSe, the
nematic transition is not followed by magnetism at ambient
conditions.
I will present a unifying description of both of these two
different scenarios. Next I'll present the parquet
Renormalization Group (RG) study of the interplay between
different ordering tendencies. The RG flow of susceptibilities
favors the nematic instability even though the magnetism is
the only interaction channel with attraction at the bare level.
In most of iron pnictides the Fermi energy is large and RG is
not capable of tipping the scale in favour of structural
transition. Hence, the leading instability is magnetic or
superconducting. In FeSe the small Fermi energy allows for
RG flow to run long enough to promote the nematic
instability, and at the same time to suppress magnetism.
