Observation of an Incommensurate Charge Density Wave in Monolayer TiSe2/CuSe/Cu(111) Heterostructure

TiSe2 is a layered material exhibiting a commensurate (2 x 2 x 2) charge density wave (CDW) with a transition temperature of similar to 200 K. Recently, incommensurate CDW in bulk TiSe2 draws great interest due to its close relationship with the emergence of superconductivity. Here, we report an incommensurate superstructure in monolayer TiSe2/CuSe/Cu(111) heterostructure. Characterizations by low-energy electron diffraction and scanning tunneling microscopy show that the main wave vector of the superstructure is similar to 0.41a* or similar to 0.59a* (here a* is in-plane reciprocal lattice constant of TiSe2). After ruling out the possibility of moire superlattices, according to the correlation of the wave vectors of the superstructure and the large indirect band gap below the Fermi level, we propose that the incommensurate superstructure is associated with an incommensurate charge density wave (I-CDW). It is noteworthy that the I-CDW is robust with a transition temperature over 600 K, much higher than that of commensurate CDW in pristine TiSe2. Based on our data and analysis, we present that interface effect may play a key role in the formation of the I-CDW state.