Scanning Tunneling Microscopy Imaging of Electronic Waves on the Surface of Molecular Beam Epitaxy Grown Ag₂Se

Recently, silver chalcogenide systems are being studied as possible candidates for topological insulators (TIs). While experimental works have been reported only on the electronic transport of anisotropic Dirac fermions in silver chalcogenides, no local scanning tunneling microscopy (STM) studies have been done on these materials. Here, we report the synthesis of epitaxial Ag₂Se nanostructures using molecular beam epitaxy (MBE) method. Based on atomic-resolved STM images, a new monoclinic structure is proposed for the MBE grown Ag₂Se, which was not seen in this system before. The electronic structures of the Ag₂Se have been investigated by means of scanning tunneling spectroscopy (STS). To test the unique feature of topological surface states (TSSs), we conducted a low-temperature STM/S study on the surface states of Ag₂Se films. On the selenium (Se)-terminated surfaces, evidence for TSSs are observed in the quasi-particle interference patterns. The existence of standing waves strongly supports the surface nature of topological states. Our results may help resolve the current controversy on the topological nature of Ag₂Se.