The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, for example in high-transition-temperature cuprates, heavy fermions and organic superconductors. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, for example charge-carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently, high-temperature superconductivity has been discovered in iron pnictides, providing a new class of unconventional superconductors. Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state and superconductivity is induced on charge-carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of X-ray scattering, muon spin relaxation and Mössbauer spectroscopy on the series LaO(1-x)F(x)FeAs. We find a discontinuous first-order-like change of the Néel temperature, the superconducting transition temperature and the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in iron pnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state.

A magnetic functionalization of microcrystalline MOF particles was realized using magnetic iron oxide particles. Such magnetic MOFs can be separated using a static magnetic field after use in catalytic processes and heated by an external alternating magnetic field to trigger desorption of encaged drug molecules.

Superconductivity in doped BaFe2As2 is controlled by the charge of the (FeAs)δ− layers. Adjustment from electron to hole doping in Ba1−xKxFe1.86Co0.14As2 tailors the system from superconductivity to static magnetic order and back to superconductivity. When the charges compensate each other, the magnetic phase similar to BaFe2As2 is recovered. Structural parameters play minor roles in the superconductivity but are important for the highest possible critical temperatures.

Supraleitung in dotiertem BaFe2As2 wird von der Ladung der (FeAs)δ−‐Schichten gesteuert. Durch Abstimmen der Elektronen/Lochdotierung in der Titelverbindung gelangt man von Supraleitung über statische magnetische Ordnung zurück zu Supraleitung. Kompensieren sich die Dotierungen, wird die magnetische Phase ähnlich der von BaFe2As2 wiederhergestellt. Strukturparameter sind weniger wichtig für die Supraleitung, wohl aber für höchstmögliche kritische Temperaturen Tc.