Evidence of two superconducting phases in Ca1−xLaxFe2As2

Single crystals of Ca1−xLaxFe2As2 with x ranging from 0 to 0.25, have been grown and characterized by structural, transport, and magnetic measurements. Coexistence of two superconducting phases is observed, in which the phase with the lower superconducting transition temperature (Tc) has Tc ∼ 20 K and the higher Tc phase has Tc higher than 40 K. These data also delineate an x-T phase diagram in which the single magnetic/structural phase transition in undoped CaFe2As2 appears to split into two distinct phase transitions, both of which are suppressed with increasing La substitution. Superconductivity emerges when x is about 0.06 and coexists with the structural/magnetic transition until x is ∼ 0.13. With increasing concentration of La, the structural/magnetic transition is totally suppressed, and Tc reaches its maximum value of about 45 K for 0.15 ⩽ x ⩽ 0.19. A domelike superconducting region is not observed in the phase diagram, however, because no obvious over-doping region can be found. Two superconducting phases coexist in the x-T phase diagram of Ca1−xLaxFe2As2. The formation of the two separate phases and the origin of the high Tc in Ca1−xLaxFe2As2 have been studied and discussed in detail.