Epitaxial growth and electronic properties of mixed valence YbAl3 thin films

We report the growth of thin films of the mixed valence compound YbAl${}_3$ on MgO using molecular-beam epitaxy. Employing an aluminum buffer layer, epitaxial (001) films can be grown with sub-nm surface roughness. Using x-ray diffraction, in situ low-energy electron diffraction and aberration-corrected scanning transmission electron microscopy we establish that the films are ordered in the bulk as well as at the surface. Our films show a coherence temperature of 37 K, comparable to that reported for bulk single crystals. Photoelectron spectroscopy reveals contributions from both ${\mathit{\text{f}}}^{13}$ and ${\mathit{\text{f}}}^{12}$ final states establishing that YbAl${}_3$ is a mixed valence compound and shows the presence of a Kondo Resonance peak near the Fermi-level.