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

In the heavily hole-doped iron-based superconductors AFe(2)As(2) (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 lambda(ab) (T) in clean crystals of AFe(2)As(2) down to low temperature T similar to 0.1 K. In KFe2As2, the superfluid stiffness rho(ab)(s)(T) = lambda(2)(ab) (0)/lambda(2)(ab) (T) at low temperatures can be accounted for by the strongly band-dependent multiple gaps reported recently. Although the lambda(ab) (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.