Evolution of quasiparticle excitations with enhanced electron correlations in superconducting AFe2As2 (A= K, Rb, and Cs)

In the heavily hole-doped iron-based superconductors AFe2As2 (A=K, Rb, and Cs), the electron effective mass increases rapidly with alkali-ion radius. To study the superconducting gap structure in this series, we measure the in-plane London penetration depth λab(T) in clean crystals of AFe2As2 down to low temperature T∼0.1 K. In KFe2As2, the superfluid stiffness ρsab(T)=λab2(0)/λab2(T) at low temperatures can be accounted for by the strongly band-dependent multiple gaps reported recently. Although the λab(T) in all systems exhibits similar nonexponential temperature dependence indicating nodes or small minima in the gap, we find that the quasiparticle excitations at low temperatures show a systematic suppression with increasing alkali-ion radius. A possible origin of such evolution of low-energy excitations is discussed in terms of the momentum-dependent effect of enhanced quasiparticle mass near a quantum critical point.