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 $text{Ba}{({text{Fe}}_{1ensuremath{-}x}{text{Co}}_{x})}_{2}{text{As}}_{2}$. These data delineate an $xtext{ensuremath{-}}T$ phase diagram in which the single magnetic/structural phase transition that is observed for undoped ${text{BaFe}}_{2}{text{As}}_{2}$ 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 $xensuremath{sim}0.025$ and appears to coexist with the broken-symmetry state for an appreciable range of doping up to $xensuremath{sim}0.06$. 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 $xensuremath{sim}0.18$ and above. The form of this $xtext{ensuremath{-}}T$ 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.