THz electrodynamics of mixed-valent YbAl

We present our results from time-domain THz spectroscopy measurements of thin films of mixed-valent YbAl$$_3$$ and its structural analogue LuAl$$_3$$. Combined with Fourier transform infrared (FTIR) spectroscopy, the extended Drude formalism is utilized to study the quasiparticle scattering rate and effective masses in YbAl$$_3$$. We find that LuAl$$_3$$ demonstrates conventional Drude transport whereas at low temperatures YbAl$$_3$$ 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$$_3$$ the extended Drude analysis demonstrates consistency with FL behavior below the FL coherence temperature $$T^* < 40$$ K with the scattering rate following $$T^2$$ proportionality and a moderate mass enhancement. Despite not observing a clear $$\omega ^2$$ 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 $$T^*$$ in YbAl$$_3$$, indicative of incoherent hybridization effects persisting to high temperatures