Possible Multiple Gap Superconductivity with Line Nodes in Heavily Hole-Doped Superconductor KFe₂As₂ Studied by
⁷⁵As Nuclear Quadrupole Resonance and Specific Heat

Abstract

We report the ⁷⁵As nuclear quadrupole resonance (NQR) and specific heat measurements of the heavily hole-doped superconductor KFe₂As₂ (superconducting transition temperature T_c\\simeq3.5 K). The spin-lattice relaxation rate 1⁄T₁ 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=0.4K\\simeqT_c⁄10. The T dependences of 1⁄T₁ 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.