Abstract
In the heavily hole-doped iron-based superconductors (, 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 in clean crystals of down to low temperature K. In , the superfluid stiffness at low temperatures can be accounted for by the strongly band-dependent multiple gaps reported recently. Although the 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.
- Received 9 October 2015
- Revised 12 June 2016
DOI:https://doi.org/10.1103/PhysRevB.94.024508
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