Electronic structure of strain- and thickness- tuned epitaxial α-Sn_1-xGe_x thin films as studied by ARPES and spin-ARPES

α-Sn is a zero-gap semiconductor with inverted bands. Epitaxial tensile strain is suggested to produce a 3D topological insulator (TI) phase, while epitaxial compressive strain is suggested to produce a 3D Dirac semimetal (DSM) phase [1,2]. When this DSM phase is confined in an ultrathin film, it has been seen to transform to either a 3D TI phase [3] or a 2D TI phase [4].

Using angle-resolved photoemission spectroscopy (ARPES), we first explore the electronic structure of ultrathin compressive strained α-Sn/InSb(001) grown by molecular beam epitaxy. We find evidence of the confinement-induced 3D TI phase in compressive strained α-Sn. With this behavior benchmarked, we then alloy the α-Sn films with Ge to decrease the bulk lattice constant and switch from compressive to tensile strained α-Sn_1-xGe_x on InSb(001). The tensile strain induces a phase transition unexpectedly away from the 3D TI phase to a likely topologically trivial phase. We supplement these measurements with spin-ARPES and show the presence of multiple spin-polarized surface states in addition to the previously measured topological surface state. Our results pave the way for a better understanding of the effect of strain and confinement on α-Sn’s band structure.

[1] D. Zhang, et al., Phys. Rev. B 97, 195139 (2018).

[2] S. Küfner, et al., Phys. Rev. B 90, 125312 (2014).

[3] Y. Ohtsubo, et al., Phys. Rev. Lett 111, 216401 (2013).

[4] L. D. Anh, et al., Adv. Mater. 33, 2104645 (2021).