Temperature versus doping phase diagrams for Ba(Fe1-xTMx)2As2(TM=Ni,Cu,Cu/Co) single crystals

Microscopic, structural, transport, and thermodynamic measurements of single crystalline Ba(Fe1-xTMx)(2)As-2 (TM=Ni and Cu) series, as well as two mixed TM=Cu/Co series, are reported. In addition, high-magnetic field, anisotropic H-c2(T) data were measured up to 33 T for the optimally Ni-doped BaFe2As2 sample. All the transport and thermodynamic measurements indicate that the structural and magnetic phase transitions at 134 K in pure BaFe2As2 are monotonically suppressed and increasingly separated in a similar manner by these dopants. In the Ba(Fe1-xNix)(2)As-2 (x <= 0.072), superconductivity, with T-c up to 19 K, is stabilized for 0.024 <= x <= 0.072. In the Ba(Fe1-xCux)(2)As-2 (x <= 0.356) series, although the structural and magnetic transitions are suppressed, there is only a very limited region of superconductivity: a sharp drop of the resistivity to zero near 2.1 K is found only for the x=0.044 samples. In the Ba(Fe1-x-yCoxCuy)(2)As-2 series, superconductivity, with T-c values up to 12 K (x similar to 0.022 series) and 20 K (x similar to 0.047 series), is stabilized. Quantitative analysis of the detailed temperature-dopant concentration (T-x) and temperature-extra electrons (T-e) phase diagrams of these series shows that there exists a limited range of the number of extra electrons added, inside which the superconductivity can be stabilized if the structural and magnetic phase transitions are suppressed enough. Moreover, comparison with pressure-temperature phase diagram data, for samples spanning the whole doping range, further re-enforces the conclusion that suppression of the structural/magnetic phase transition temperature enhances T-c on the underdoped side, but for the overdoped side T-max(C) is determined by e. Therefore, by choosing the combination of dopants that are used, we can adjust the relative positions of the upper phase lines (structural and magnetic phase transitions) and the superconducting dome to control the occurrence and disappearance of the superconductivity in transition metal, electron-doped BaFe2As2.