Optical verification of H-point exciton in bulk transition metal dichalcogenides

During last decades, transition metal dichalcogenide (TMDC) compounds has been studied extensively. TMDCs provide an excellent platform to investigate many-body physics of exciton, and exciton binding energy increased significantly by reducing thickness. Mostly hydrogen models are used to obtain band gap and exciton binding energy from absoption spectra. On the other hand, recent reports on bulk TMDC claimed an peak previously assigned as n=2 state of A exciton at K-point is not true, but is an independent n=1 exciton arising at H-point [J. Kopaczek et al., Jour. Appl. Phys. 119, 235705 (2016)]. Following the argument, previous method to obtain binding energy of bulk TMDC may be incorrect. To verify the existence of H-point, we performed an ellipsometry measurements and two-dimensional correlation spectroscopy on TMDC compounds. We analyzed thermal sequential order of exciton peaks, which may be used to distinguish exciton states that arise from K-point and H-point. In this work, we will discuss on analysis and possible impact in future studies.