  In rare-earth doped single crystalline CaFe2As2, the mysterious small volume
fraction which superconducts up to 49 K, much higher than the bulk Tc ~ 30s K,
has prompted a long search for a hidden variable that could enhance the Tc by
more than 30% in iron-based superconductors of the same structure. Here we
report a chemical, structural, and magnetic study of CaFe2As2 systematically
doped with La, Ce, Pr, and Nd. Coincident with the high Tc phase, we find
extreme magnetic anisotropy, accompanied by an unexpected doping-independent Tc
and equally unexpected superparamagnetic clusters associated with As vacancies.
These observations lead us to conjecture that the tantalizing Tc enhancement
may be associated with naturally occurring chemical interfaces and may thus
provide a new paradigm in the search for superconductors with higher Tc.
