Determination of the phase diagram of the electron-doped superconductorBa(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 $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$. These data delineate an $x\text{−}T$ 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 $x\sim 0.025$ and appears to coexist with the broken-symmetry state for an appreciable range of doping up to $x\sim 0.06$. 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 $x\sim 0.18$ and above. The form of this $x\text{−}T$ 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.