Abstract
Microscopic, structural, transport, and thermodynamic measurements of single crystalline ( and Cu) series, as well as two mixed series, are reported. In addition, high-magnetic field, anisotropic data were measured up to 33 T for the optimally Ni-doped sample. All the transport and thermodynamic measurements indicate that the structural and magnetic phase transitions at 134 K in pure are monotonically suppressed and increasingly separated in a similar manner by these dopants. In the , superconductivity, with up to 19 K, is stabilized for . In the 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 samples. In the series, superconductivity, with values up to 12 K ( series) and 20 K ( series), is stabilized. Quantitative analysis of the detailed temperature-dopant concentration and temperature-extra electrons 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 on the underdoped side, but for the overdoped side is determined by . 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 .
20 More- Received 14 June 2010
DOI:https://doi.org/10.1103/PhysRevB.82.024519
©2010 American Physical Society