Coexistence of Supercondcutivity and Magnetism in LaFeAs(O0.94F0.06) Probed by Muon Spin Relaxation

The presence of macroscopic phase separation into superconducting and spin-glass-like magnetic phases in LaFeAs(O1-xFx) is demonstrated by muon spin rotation measurement in a sample near the phase boundary (x = 0.06). Both magnetism and superconductivity develop simultaneously below a common critical temperature, T-m similar or equal to T-c similar or equal to 18 K. This remarkable accordance strongly suggests that the electronic correlations leading to these two competing ground states share a common origin.