The electronic phase diagram of the LaO1−xFxFeAs superconductor

In non-conventional superconductors, it is usually found that superconductivity emerges in the vicinity of a critical point where antiferromagnetic order gradually disappears—corresponding to a second-order transition. Investigation of the newly discovered iron pnictide superconductors challenges this picture, showing an abrupt, first-order transition. 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 1 , heavy fermions 2 and organic superconductors 3 . 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 4 , 5 , 6 . Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state 7 , 8 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.