Terahertz Electrodynamics of Mixed-Valent YbAl$_3$ and LuAl$_3$ Thin Films

TL;DR

This study uses THz and FTIR spectroscopy to investigate the electrodynamics of YbAl$_3$ and LuAl$_3$ thin films, revealing heavy Fermi liquid behavior in YbAl$_3$ at low temperatures and contrasting transport properties.

Contribution

It provides the first detailed THz spectroscopic analysis of mixed-valent YbAl$_3$ thin films, demonstrating Fermi-liquid formation and its temperature-dependent evolution.

Findings

YbAl$_3$ shows a renormalized Drude peak and MIR peak indicating heavy Fermi liquid formation.

LuAl$_3$ exhibits conventional Drude transport with no heavy fermion features.

The Fermi liquid state in YbAl$_3$ persists below 40 K with moderate mass enhancement.

Abstract

We present THz measurements of thin films of mixed-valent YbAl$_3$ and its structural analogue LuAl$_3$. Combined with traditional Fourier transform infrared (FTIR) spectroscopy, the extended Drude formalism is utilized to study the low-frequency transport of these materials. We find that LuAl$_3$ demonstrates conventional Drude transport whereas at low temperatures YbAl$_3$ demonstrates a sharply renormalized Drude peak and a mid-infrared (MIR) peak in the conductivity, indicative of the formation of a heavy Fermi liquid. In YbAl$_3$ the extended Drude framework shows a consistency of the scattering rate with Fermi-liquid behavior below $T < 40$ K and a moderate mass enhancement. While a $\omega^2$ Fermi liquid-like frequency dependence is not clearly exhibited, the temperature dependence of the Drude scattering rate and effective mass is consistent with the formation of a low-temperature moderately heavy Fermi liquid, albeit one with a smaller mass than observed in single crystals. The extended Drude analysis also supports a slow crossover between the Fermi liquid state and the normal state in YbAl$_3$.