NMR/NQR and Specific Heat Studies of Iron Pnictide Superconductor KFe2As2

We report the 75 As nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR) and specific heat measurements of the heavily hole-doped superconductor KFe 2 As 2 (superconducting transition temperature T c ≃3.5 K) using high quality single crystals. NMR Knight shifts in superconducting state clearly indicate that spin–singlet superconductivity realizes in this material. It is noted that the electronic specific heat coefficient of the material is 93.0 mJ/K 2 mol-fu and that the value is quite heavy for d electron system.