The landscape of topological quantum materials has expanded greatly with the discovery of topological Dirac states in both the bulk and surface of certain semimetals. This talk provides an overview of the synthesis by molecular beam epitaxy (MBE) of topological semimetal thin films (Cd₃As₂[1], ZrTe₂ [2], TaAs [3], NbAs [4] and their characterization using x-ray diffraction, angle resolved photoemission spectroscopy, and quantum transport. The potential application of these films for spintronics is studied by measuring spin to charge interconversion after interfacing them with conventional metallic ferromagnets (permalloy) or two dimensional ferromagnets (CrTe₂).

This work was supported by SMART/nCORE, a Semiconductor Research Corporation program, sponsored by NIST, the Institute for Quantum Matter (DOE EFRC grant DE-SC0019331) and the Penn State Two-Dimensional Crystal Consortium-Materials Innovation Platform (2DCC-MIP) under NSF DMR 2039351.

1. W. Yanez et al., Phys. Rev. Applied 16, 054031 (2021).

2. Y. Ou et al., Nat. Commun. 13, 2972 (2022).

3. R. Xiao et al., Phys. Rev. B 106, L201101 (2022).

4. W. Yanez et al., Phys. Rev. Applied 18, 054004 (2022).

Understanding how functional phenomena can be modified in epitaxial thin films is crucial for designing and manipulating properties. In this talk I will discuss several interesting examples where novel routes to control magnetic properties arose from understanding why defects form and ultimately how to control their formation. I will discuss the large electronic and magnetic response that is induced in the layered magnetic topological insulator MnBi₂Te₄ by controlling the propagation of surface oxidation as well as native defects imparted during synthesis. I will also discuss how ferromagnetism can be externally turned on with a high level of continuous control through the application of low energy helium implantation in the ultra-high conductivity, non-magnetic layered oxide PdCoO₂. These two examples highlight how a detailed understanding of synthesis by molecular beam epitaxy is critical to understanding and designing properties which is critical to driving new applications.