Superconductivity in Sm-doped CaFe₂As₂ single crystals

In this article, the Sm-doping single crystals Ca_1-xSm_xFe₂As₂ (x=0～0.2) were prepared by the CaAs flux method, and followed by a rapid quenching treatment after the high temperature growth. The samples were characterized by structural, resistive, and magnetic measurements. The successful Sm-substitution was revealed by the reduction of the lattice parameter c, due to the smaller ionic radius of Sm³⁺ than Ca²⁺. Superconductivity was observed in all samples with onset T_c varying from 27 K to 44 K upon Sm-doping. The coexistence of a collapsed phase transition and the superconducting transition was found for the lower Sm-doping samples. Zero resistivity and substantial superconducting volume fraction only happen in higher Sm-doping crystals with the nominal x >0.10. The doping dependences of the c-axis length and onset T_c were summarized. The high-T_c observed in these quenched crystals may be attributed to simultaneous tuning of electron carriers doping and strain effect caused by lattice reduction of Sm-substitution.