Observation of surface state enhancement and Fermi surface reconstruction in epitaxial ultrathin TaAs (001) films via ARPES

Using molecular-beam epitaxy, we have synthesized epitaxial thin films of the Weyl semimetal TaAs (001) on GaAs (001) substrates. Here, we report a comprehensive electronic structure for film thicknesses ranging from 200 nm and 1 nm. These are, to the best of our knowledge, the thinnest films reported to date, and thus provide access to the parameter space that was previously inaccessible. TaAs film thicknesses ranged from 200 nm to 1 nm. We reproduce the known electronic structure of single-crystal TaAs in epitaxially grown samples of thickness roughly greater than 10 nm. Further decreasing film thickness below about 10 nm shows a drastic change in the Fermi surface which, supported by first-principles calculations, we attribute to surface and quantum well states in the TaAs film. Slab and bulk DFT calculations indicate that we have accessed both As- and Ta-terminated surfaces, which are simultaneously accessible due to the ultrathin film growth. The appearance of Γ-centered circular pockets, altered connectivity between X and Y, and several new band features along high-symmetry paths indicate that the drastic change in Fermi surface topology below 10 nm film thickness constitutes a Fermi surface reconstruction driven by surface state hybridization and dimensional confinement.