Microscopic co-existence of superconductivity and magnetism in Ba1-xKxFe2As2

It is widely believed that, in contrast to its electron doped counterparts, the hole doped compound Ba1-xKxFe2As2 exhibits a mesoscopic phase separation of magnetism and superconductivity in the underdoped region of the phase diagram. Here, we report a combined high-resolution x-ray powder diffraction and volume sensitive muon spin rotation study of underdoped Ba1-xKxFe2As2 (0 \leq x \leq 0.25) showing that this paradigm is wrong. Instead we find a microscopic coexistence of the two forms of order. A competition of magnetism and superconductivity is evident from a significant reduction of the magnetic moment and a concomitant decrease of the magneto-elastically coupled orthorhombic lattice distortion below the superconducting phase transition.Comment: 4 pages, 4 figure

Microscopic Coexistence of Magnetism and Superconductivity in charge compensated Ba1-xKx(Fe1-yCoy)2As2

We present a detailed investigation of the electronic phase diagram of effectively charge compensated Ba1-xKx(Fe1-yCoy)2As2 with x/2 = y. Our experimental study by means of x-ray diffraction, M\"ossbauer 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 = 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 = y \< 0.13. At intermediate substitution levels in the range between 0.13 and 0.19, we find superconductivity with a maximum Tc of 15 K coexisting with static magnetic order on a microscopic length scale. For higher substitution levels x/2 = y \> 0.25 a tetragonal non-magnetic ground state is observed. Our DFT calculations yield a signifcant 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 non-magnetic impurities to a system with a constant charge carrier density. Our experimental study by means of x-ray diffraction, M\"ossbauer spectroscopy, muon spin relaxation and ac susceptibility measurements on polycrystalline samples is complemented by density functional electronic structure calculations.Comment: 16 pages, 14 figure

Structural and Magnetic Properties of the Insulating T' − RE2CuO4 Parent Compounds of Electron-Doped Superconductors

The rare earth cuprates RE2 CuO4 crystallizing in the T ′ -structure are the parent compounds for electron-doped superconductors. Various investigations illustrated that these systems turn superconducting upon electron-doping and/or fine-tuning of the oxygen reduction annealing conditions. These undoped parent compounds are characterized by a very strong antiferro- magnetic coupling between the Cu moment within the CuO2 planes and a very weak cou- pling between adjacent planes. The reason for this weak interlayer coupling lies in the body- centered tetragonal (BCT) structure of these compounds which cancels out the isotropic Heisenberg interaction between one and the next nearest layer. Therefore, the BCT cuprates can be regarded as quasi-2 dimensional (2D) magnetic systems. Even though the Mermin- Wagner theorem forbids the magnetic ordering of a 2D magnetic system, the BCT cuprates are known to order at temperatures around 280 K due to the weak interlayer coupling along the third spatial direction. The actual process how these quasi-2D systems approach the ul- timate 3D magnetic order and which kinds of spin structures are realized, are anyhow still a matter of current debate. Moreover, the rare-earth cuprates exhibit underlying interactions involving both the copper and the magnetic rare-earth subsystems that demonstrated inter- esting phenomena such as spin reorientation transitions. In this work, a systematic investigation of the 3D magnetic ordering process, the influence of a magnetic rare-earth ion and the effect of oxygen reduction on the magnetism of these undoped compounds, were focused upon. The rich magnetic behavior of the BCT cuprates T ′ -RE2 CuO4 (RE= La, La/Sm, and Pr) is investigated primarily using the muon spin rotation and relaxation (µSR) technique. Complementary experimental studies were also carried out with nuclear magnetic resonance (NMR) and neutron scattering techniques. The structural properties, primarily the oxygen content and site occupation, are determined by neutron scattering and synchrotron x-ray diffraction. All the studied T ′ -RE2 CuO4 (RE= La, La/Sm, and Pr) compounds revealed series of magnetic transitions as a function of temperature