Investigation of charge density wave transitions in a proximally coupled single-layer TiTe₂/TiSe₂ system

TiSe₂, a prototypical charge density wave (CDW) system with a 1T structure in the bulk, undergoes a (2×2×2) CDW transition at 205 K. By comparison, a single layer of TiSe₂ prepared on a bilayer-graphene-terminated SiC(0001) undergoes a (2×2) CDW transition at 232 K. The bonding of the single layer to the substrate is expected to be of the van der Waals type. Furthermore, the structures are incommensurate, which makes the interfacial bonding even weaker. A reasonable premise is that the single layer is nearly freestanding with minimal influence from the substrate on its (2×2) CDW. TiTe₂, by contrast, shows no CDWs in the bulk or in films grown on the same bilayer-graphene-terminated SiC; the only exception is a (2×2) CDW transition at 92 K in the single layer. The unique behavior of the single layer is an anomaly. To explore the physics further, we have grown a composite system made of a single layer of TiTe₂ on a single layer of TiSe₂ on a bilayer-graphene-terminated SiC. The crystal structures of TiSe₂, TiTe₂, and graphene are all incommensurate. Questions abound: Is there a CDW transition in this composite system? Or, are there actually two transitions? Results of band structure mapping by angle-resolved photoemission spectroscopy will be presented to address these issues.