Abstract
Systematic measurements of the resistivity, heat capacity, susceptibility, and Hall coefficient are presented for single-crystal samples of the electron-doped superconductor . These data delineate an phase diagram in which the single magnetic/structural phase transition that is observed for undoped 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 and appears to coexist with the broken-symmetry state for an appreciable range of doping up to . 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 and above. The form of this 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.
- Received 15 November 2008
DOI:https://doi.org/10.1103/PhysRevB.79.014506
©2009 American Physical Society