We report the detection of unusual superconductivity up to 49 K in single crystalline CaFe 2 As 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 2 As 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 superconductivity with a T c up to 30 K. The unusual 49 K phase detected will be discussed.

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The magnetic properties of LiFeAs, as single crystalline and polycrystalline samples, were investigated. The lower critical field deduced from the vortex penetration of two single crystals appears to be almost isotropic with a temperature dependence closer to that of two-gap superconductors. The parameters extracted from the reversible magnetizations of sintered polycrystalline samples are in good agreement with those from the single-crystal data.

The zero-field specific heat of LiFeAs was measured on several single crystals selected from a bulk sample. A sharp ⌬C p / T c anomaly Ϸ20 mJ/ mole K 2 was observed. The value appears to be between those of SmFeAs͑O 0.9 F 0.1 ͒ and ͑Ba 0.6 K 0.4 ͒Fe 2 As 2 but bears no clear correlation with their Sommerfeld coefficients. The electronic specific heat below T c further reveals a two-gap structure with the narrower one only on the order of 0.7 meV. While the results are in rough agreement with the H c1 ͑T͒ previously reported on both LiFeAs and ͑Ba 0.6 K 0.4 ͒Fe 2 As 2 , they are different from the published specific-heat data of a ͑Ba 0.6 K 0.4 ͒Fe 2 As 2 single crystal.

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