  The competition of magnetic order and superconductivity is a key element in
the physics of all unconventional superconductors, e.g. in
high-transition-temperature cuprates 1, heavy fermions 2 and organic
superconductors3. 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, e.g. 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 superconductors4,5,6. Similar to other
unconventional superconductors the parent compounds of the pnictides exhibit a
magnetic ground state7,8 and superconductivity is induced upon charge carrier
doping. In this Letter the structural and electronic phase diagram is
investigated by means of x-ray scattering, MuSR and Moessbauer spectroscopy on
the series LaO1-xFxFeAs. We find a discontinuous first-order-like change of the
Neel temperature, the superconducting transition temperature and of the
respective order parameters. Our results strongly question the relevance of
quantum critical behaviour in ironpnictides and prove a strong coupling of the
structural orthorhombic distortion and the magnetic order both disappearing at
the phase boundary to the superconducting state.
