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

Abstract

We report the detection of unusual superconductivity up to 49 K in single crystalline CaFe₂As₂ 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 AeFe₂As₂ (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 superconductivity with a T_c up to 30 K. The unusual 49 K phase detected will be discussed.