Scanning Tunneling Microscopy of Thin Films Grown by Molecular Beam Epitaxy to Study Topological Surface States in Silver Chalcogenides and Atomic and Electronic Properties of Intermetallic CaBi2

Silver chalcogenides have attracted significant interest as promising candidates for novel topological properties due to their highly anisotropic Dirac cones. We used our combined molecular beam epitaxy (MBE) and low temperature-scanning tunneling microscopy/spectroscopy (LT-STM/S) combined system to explore these binary topological insulators. Using LT-STM/S, we have characterized the atomic structure and electronic properties of Ag2Se thin films grown on SrTiO3 (STO)(001) substrates by MBE for the first time. Based on atomic-resolved STM images, a monoclinic structure is proposed for the MBE-grown Ag2Se films, which has not been clearly discussed in this system previously. Three different types of Ag2Se atomic terminations were observed on the surface stemming from different growth directions. STS analysis of these atomic terminations uncovered different features near the Fermi level, indicating constituent- and direction-dependent electronic properties. We also obtained epitaxial monoclinic Ag2Te(001) thin films grown on STO(001) substrates using MBE, which strongly supports the growth of new monoclinic Ag2Se(001) thin films on the STO(001) substrate. To test the unique features of topological surface states (TSSs), we conducted a LT-STM/S study on the surface states of Ag2Se thin films. On the selenium (Se)-terminated surfaces with different types of defect densities, evidence for TSSs was observed in the quasiparticle interference (QPI) patterns. By studying the voltage-dependent standing wave patterns, we determined the energy dispersion E(k), which confirms the Dirac cone structure of the topological states. To further confirm the topological nature of monoclinic Ag2Se, density functional theory (DFT) calculations were also performed and compared to the experimental results. The existence of standing waves strongly supports the topological nature of surface states. Our findings provide a convenient method to produce the monoclinic Ag2Se thin film and lead to a deeper understanding of the topological nature of this compound. Separately, we systematically characterized the atomic structure and electronic properties of epitaxial CaBi2(010) thin films grown on (STO)(001) substrates by MBE. Intermetallic bismuth-based compounds have attracted great interest as promising candidates for novel topological superconductivity. Our findings on epitaxial CaBi2(010) thin films provide insight for deeper understanding of the physical properties of this two-dimensional layered material compound.