Jaydeep D. Joshi, Heather M. Hill, Sugata Chowdhury, Christos D. Malliakas, Francesca M. Tavazza, Utpal Chatterjee, Angela R. Hight Walker, Patrick M. Vora
2H-TaS2 undergoes a charge density wave (CDW) transition at T_CDW ~ 75 K, however key questions regarding the onset of CDW order remain under debate. In this study, we explore the CDW transition through a combination of temperature and excitation-dependent Raman spectroscopy, angle resolved photoemission spectroscopy (ARPES), and density functional theory (DFT). Below T_CDW we identify two CDW amplitude modes that redshift and broaden with increasing temperature and one zone-folded mode that disappears above T_CDW. Above T_CDW, we observe a strong two- phonon mode that softens substantially upon cooling, which suggests the presence of substantial lattice distortions at temperatures as high as 250 K. This correlates with the ARPES observation of the persistence of CDW energy gap above T_CDW and finite-temperature DFT calculations of the phonon band structure that indicate an instability occurring well above the CDW transition temperature. DFT also provides the atomic displacements of the CDW amplitude modes and reproduces their temperature-dependence. From these observations we suggest that short range CDW order exists well-above T_CDW which poses new questions regarding the interplay between electronic structure and vibrational modes in layered CDW materials.