Spectroscopic investigation of CDW phase transitions 2H-NbSe₂, 2H-TaS₂ and 1T-TiSe₂

Layered transition-metal dichalcogenides (TMDs) are well-known for their rich phase diagrams, which encompass diverse quantum states including metals, semiconductors, Mott insulators, superconductors, and charge density waves (CDWs). For instance, 2H-NbSe₂, 2H-TaS₂and 1T-TiSe₂are canonical TMD CDW systems—2H-NbSe₂ and 2H-TaS₂host incommensurate CDW order, while 1T-TiSe₂ hosts a commensurate one. There is a coexistence/competition of CDW and superconductivity 2H-NbSe₂and 2H-TaS₂. This is, however, not the case for pristine 1T-TiSe₂. Nevertheless, subtle interplay of superconductivity and CDW order appears in each of these materials strain is applied to foreign atoms are introduced in these compounds. Quite generally, the phase diagrams of 2H-NbSe₂, 2H-TaS₂and 1T-TiSe₂ at the presence of strain or chemical intercalation reminisce those of High temperature superconductors and heavy fermion compounds. A critical step towards characterizing these phase diagrams is to first unveil the mechanism of the CDW transition in the parent compounds, which remain an intriguing puzzle despite decades of research. It is almost universally accepted that conventional Fermi surface nesting theory doesn’t apply to the CDW instability of these systems. In the context of alternative models for CDW orders in these systems, we will present our experimental data which combine Angle Resolved Photoemission Spectroscopy, Scanning Tunneling Microscopy , X-ray diffraction and transport measurements. We will also explore the distinctive impacts of disorder and change in carrier concentration of commensurate and incommensurate CDW systems.