Semimetal-to-semiconductor transition and charge-density-wave suppression in 1T-TiSe2-xSx single crystals

The transition-metal dichalcogenide 1T-TiSe2 is a quasi-two-dimensional layered material with a phase transition towards a commensurate charge-density wave (CDW) at a critical temperature T-c approximate to 200 K. The relationship between the origin of the CDW instability and the semimetallic or semiconducting character of the normal state, i.e., with the nonreconstructed Fermi-surface topology, remains elusive. By combining angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations, we investigate 1T-TiSe2-xSx single crystals. Using STM, we first show that the long-range phase-coherent CDW state is stable against S substitutions with concentrations, at least, up to x = 0.34. The ARPES measurements then reveal a slow but continuous decrease in the overlap between the electron and the hole (e-h) bands of the semimetallic normal state well reproduced by DFT and related to slight reductions of both the CDW order parameter and T-c. Our DFT calculations further predict a semimetal-to-semiconductor transition of the normal state at a higher critical S concentration of x(c) = 0.9 +/- 0.1 that coincides with a suppressed CDW state in TiSeS as measured with STM. Finally, we rationalize the x dependence of the e-h band overlap in terms of isovalent substitution-induced competing chemical pressure and charge localization effects. Our study highlights the key role of the e-h band overlap for the CDW instability.