Microwave quasiparticle conductivity of LaFePO single crystals

We measured the temperature dependence of the microwave quasiparticle conductivity σ1(T) in single crystals of LaFePO (Tc≅7.5K) to study the presence/absence of the coherence peak expected in s-wave superconductors. We use sensitive cavity resonators at several frequencies (15,28,60GHz). We observe a broad enhancement of σ1(T) below Tc which is almost independent of frequency. We find that such a frequency-independent enhancement is incompatible with the conventional coherence peak, and rather it can be explained by the rapid change of quasiparticle dynamics below Tc due to strong suppression of electron-electron inelastic scattering.