Charge density wave transition of pristine and organic-intercalated 1T-VSe2 studied by infrared spectroscopy

Vanadium diselenide (VSe2), a rare layered metallic material with a three-dimensional charge density wave (CDW), has garnered interest due to its remarkable tunability. Recently, organic tetrabutyl ammonium (TBA) ions have been intercalated into bulk VSe2, resulting in a novel metal-insulator transition with a new twodimensional in-plane 3×3 periodicity lattice modulation. In this study, we employ infrared spectroscopy and first-principles calculations to investigate the electronic structure of both pristine VSe2 and TBA+-intercalated VSe2. Our findings reveal a gradual development of a CDW energy gap in pristine VSe2 during the CDW transition, whereas TBA+-intercalated VSe2 undergoes an abrupt and intricate electronic band reconstruction at the phase transition. The study directly distinguishes between traditional CDW order, characterized by a change in band structure at low energy with the formation of an energy gap, and a first-order phase transition with abrupt band reconstruction over broad energies, as seen in (TBA+)xVSe2. Infrared spectroscopy provides a straightforward method to distinguish between these two scenarios. These findings enhance our understanding of structural phase transitions driven by Fermi surface nesting or alternative mechanisms.