Microscopic, structural, transport and thermodynamic measurements of single
crystalline Ba(Fe1-xTMx)2As2 (TM = Ni and Cu) series, as well as two mixed TM =
Cu / Co series, are reported. 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)2As2 (x =< 0.072),
superconductivity, with Tc up to 19 K, is stabilized for 0.024 =< x =< 0.072.
In the Ba(Fe1-xCux)2As2 (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)2As2 series,
superconductivity, with Tc values up to 12 K (x ~ 0.022 series) and 20 K (x ~
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 reenforces the conclusion that
suppression of the structural / magnetic phase transition temperature enhances
Tc on the underdoped side, but for the overdoped side Tcmax 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.Comment: 42 pages, 27 figures, 5 table
