Possible Multiple Gap Superconductivity with Line Nodes in Heavily Hole-Doped Superconductor KFe2AS2 Studied by 75As Nuclear Quadrupole Resonance and Specific Heat

We report the As-75 nuclear quadrupole resonance (NQR) and specific heat measurements of the heavily hole-doped superconductor KFe2As2 (superconducting transition temperature Tc similar or equal to 3.5 K). The spin-lattice relaxation rate 1/T-1 in the superconducting state exhibits a gradual temperature dependence with no coherence peak below T-c. The quasiparticle specific heat C-QP/T shows a small jump, which is about 30% of the electronic specific heat coefficient just below T-c. The C-QP/T suggests the existence of low-energy quasiparticle excitation at the lowest measurement temperature T = 0.4 K similar or equal to T-c/10. The T dependences of 1/T-1 and C-QP/T can be explained by a multiple nodal superconducting gap scenario rather than by a multiple fully gapped s(+/-)-wave scenario determined using simple gap analysis.