THz electrodynamics of mixed-valent YbAl$$_3$$ and LuAl$$_3$$ thin films

We present our results from time-domain THz spectroscopy measurements of thin films of mixed-valent YbAl and its structural analogue LuAl. Combined with Fourier transform infrared (FTIR) spectroscopy, the extended Drude formalism is utilized to study the quasiparticle scattering rate and effective masses in YbAl. We find that LuAl demonstrates conventional Drude transport whereas at low temperatures YbAl demonstrates a renormalized Drude peak and a mid-infrared (MIR) peak in the conductivity, indicative of the formation of a mass-enhanced Fermi liquid (FL). In YbAl the extended Drude analysis demonstrates consistency with FL behavior below the FL coherence temperature K with the scattering rate following proportionality and a moderate mass enhancement. Despite not observing a clear Fermi liquid-like frequency dependence the evidence is consistent with a moderate mass Fermi liquid, albeit one with a smaller mass than observed in single crystals. The extended Drude analysis also demonstrates a slow crossover between the FL state and the normal state above the in YbAl, indicative of incoherent hybridization effects persisting to high temperatures.