  NiTe2, a type-II Dirac semimetal with strongly tilted Dirac band, has been
explored extensively to understand its intriguing topological properties. Here,
using density-functional theory (DFT) calculations, we report that the strength
of spin-orbit coupling (SOC) in NiTe2 can be tuned by Se substitution. This
results in negative shifts of the bulk Dirac point (BDP) while preserving the
type-II Dirac band. Indeed, combined studies using scanning tunneling
spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES)
confirm that the BDP in the NiTe2-xSex alloy moves from +0.1 eV (NiTe2) to -0.3
eV (NiTeSe) depending on the Se concentrations, indicating the effective
tunability of type-II Dirac fermions. Our results demonstrate an approach to
tailor the type-II Dirac band in NiTe2 by controlling the SOC strength via
chalcogen substitution. This approach can be applicable to different types of
topological materials.
