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Abstract:
We present a detailed investigation of the electronic phase diagram of
effectively charge compensated Ba1-xKx(Fe1-yCoy)(2)As-2 with x/ 2
approximate to y. Our experimental study by means of x-ray diffraction,
Mossbauer spectroscopy, muon spin relaxation and ac-susceptibility
measurements on polycrystalline samples is complemented by density
functional electronic structure calculations. For low substitution
levels of x/ 2 approximate to y <= 0.13, the system displays an
orthorhombically distorted and antiferromagnetically ordered ground
state. The low-temperature structural and magnetic order parameters are
successively reduced with increasing substitution level. We observe a
linear relationship between the structural and the magnetic order
parameter as a function of temperature and substitution level for x/ 2
approximate to y <= 0.13. At intermediate substitution levels in the
range between 0.13 and 0.19, we find superconductivity with a maximum
T-c of 15 K coexisting with static magnetic order on a microscopic
length scale. For higher substitution levels x/ 2 approximate to y >=
0.25, a tetragonal nonmagnetic ground state is observed. Our DFT
calculations yield a significant reduction of the Fe 3d density of
states at the Fermi energy and a strong suppression of the ordered
magnetic moment in excellent agreement with experimental results. The
appearance of superconductivity within the antiferromagnetic state can
by explained by the introduction of disorder due to nonmagnetic
impurities to a system with a constant charge carrier density.
