Signatures of the charge density wave collective mode in the infrared optical response of VSe2

We present a detailed study of the bulk electronic structure of high-quality VSe2 single crystals using optical spectroscopy. Upon entering the charge density wave phase below the critical temperature of 112 K, the optical conductivity of VSe2 undergoes a significant rearrangement. A Drude response present above the critical temperature is suppressed while a new interband transition appears around 0.07 eV. From our analysis, we estimate that part of the spectral weight of the Drude response is transferred to a collective mode of the CDW phase. The remaining normal-state charge dynamics appears to become strongly damped by interactions with the lattice as evidenced by a mass enhancement factor m∗/m ≈ 3. In addition to the changes taking place in the electronic structure, we observe the emergence of infrared active phonons below the critical temperature associated with the 4a × 4a lattice reconstruction.