Description
The interplay of itinerant magnetism, electronic nematic
order and unconventional superconductivity in Fe based
superconductors and other intermetallic systems with
complex Fermi surfaces is a fascinating topic in contemporary
correlated electron physics.
In my talk I will discuss Mössbauer spectroscopy and muon
spin relaxation experiments on several series of hole doped
122 pnictides like $Ba_{1?x}K_xFe_2As_2$$^{1,2}$) and
$Ca_{1?x}Na_xFe_2As_2$. The parent compound
$CaFe_2As_2$ of the latter series shows a spin density wave
order below TN = 165 K 3). Our microscopic study proves
that with increasing Na-substitution level, the magnetic order
parameter as well as the magneto-structural phase transition
is suppressed. For $x = 0.50$ we find a microscopic
coexistence of magnetic and superconducting phases
accompanied by a reduction of the magnetic order parameter
below the superconducting transition temperature TC.
A systematic comparison with other 122 pnictides reveals a
linear correlation between the magnetic order parameter
reduction and the ratio of the transition temperatures, TC/TN,
which can be understood in the framework of a Landau-
theory$^3$). In the optimally doped specimen with $T_c
\approx 34 K$, the temperature dependence of the
penetration depth and superfluid density were obtained,
which proves the presence of two superconducting s-wave
gaps.
1) E Wiesenmayer - Phys. Rev. Lett. 107, 237001 (2011)
2) T Goltz et al - Phys. Rev. B 89, 144511 (2014)
3) P. Materne et al. - Phys. Rev. B 92, 134511 (2015)
