Unusual superconducting state at 49 K in electron-doped CaFe ₂As ₂at ambient pressure

We report the detection of unusual superconductivity up to 49 K in single crystalline CaFe ₂As ₂ via electron-doping by partial replacement of Ca by rare-earth. The superconducting transition observed suggests the possible existence of two phases: one starting at 49 K, which has a low critical field <4 Oe, and the other at 21 K, with a much higher critical field >5 T. Our observations are in strong contrast to previous reports of doping or pressurizing layered compounds AeFe ₂As ₂ (or Ae122), where Ae = Ca, Sr, or Ba. In Ae122, hole-doping has been previously observed to generate superconductivity with a transition temperature (T _c ) only up to 38 K and pressurization has been reported to produce super-conductivity with a T _c up to 30 K. The unusual 49 K phase detected will be discussed.