Spin-orbit splitting in the valence bands of 1T-TaS2 and 1T-TaSe2

We perform angle-resolved photoemission spectroscopy on 1T-TaS2 and 1T-TaSe2 using synchrotron radiation. We observe a characteristic splitting of the chalcogen p-derived valence bands along high symmetry directions. Density functional theory calculation and group theory strongly suggest that this splitting is due to spin-orbit interaction along one direction, and to symmetry along the other direction. We note that, according to the Kramers degeneracy, the spin-orbit interaction leaves every state doubly degenerate. Furthermore, this study allows us to identify a mixing between bands with Ta 5d and Se 4p character, possibly relevant for the different temperature behaviours of the two compounds.