Determination of the phase diagram of the electron-doped superconductor Ba(Fe1-xCox)2As2

Systematic measurements of the resistivity, heat capacity, susceptibility, and Hall coefficient are presented for single-crystal samples of the electron-doped superconductor Ba(Fe_1-xCo_x)₂As₂ . These data delineate an x-T phase diagram in which the single magnetic/structural phase transition that is observed for undoped BaFe₂As₂ at 134 K appears to split into two distinct phase transitions, both of which are rapidly suppressed with increasing Co concentration. Superconductivity emerges for Co concentrations above xtilde 0.025 and appears to coexist with the broken-symmetry state for an appreciable range of doping up to xtilde 0.06 . The optimal superconducting transition temperature appears to coincide with the Co concentration at which the magnetic/structural phase transitions are totally suppressed, at least within the resolution provided by the finite-step size between crystals prepared with different doping levels. Superconductivity is observed for a further range of Co concentrations before being completely suppressed for xtilde 0.18 and above. The form of this x-T phase diagram is suggestive of an association between superconductivity and a quantum critical point arising from suppression of the magnetic and/or structural phase transitions.