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

Abstract

Vanadium diselenide (VSe₂), a layered metallic material with a three-dimensional charge density wave (CDW), has received considerable attention due to the high tunability of its CDW phase. Recently, organic tetrabutyl ammonium (TBA) cations have been intercalated into bulk VSe₂, resulting in a novel metal-insulator transition with a new two-dimensional in-plane 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.