Angle-resolved Photoemission Spectroscopy Studies Of PtSe₂ Class Transition Metal Dichalcogenides

Transition metal dichalcogenides(TMDCs)with the chemical formula of MX2(M-transition metal,X-S,Se,Te)have attracted extensive research interests in recent years due to the rich physics and tunable photoelectrical properties.Here we focus on new members of TMDCs formed by group VIII transition metal Pt,i.e.PtSe2 and PtTe2.We have performed a systematic study on the growth of bulk single crystals and thin films,electronic and spin structure as well as the topological property of the band structure.Our main achievements are summarized as follows:(1)First discovery of type-II Dirac fermions,and its experimental realization in bulk single crystals of PtSe2 and PtTe2.High quality PtSe2 and PtTe2 single crystals were grown by chemical vapor transport and self-flux method.Our observation of a pair of strongly tilted 3D Dirac cones in the momentum space along the kz direction revealed by angle-resolved photoemission spectroscopy provides direct experimental evidence for the existence of type-II Dirac fermions in PtSe2 and PtTe2.The experimental realization of type-II Dirac fermions is particularly important for investigating topological phase transition since type-II Dirac semimetal can be tuned to type-II Weyl semimetal or topological insulator via magnetic doping or crystal distortions.(2)We reveal the transition from bulk semimetal to semiconducting thin films in PtSe2,and the band gap is tunable in atomically thin PtSe2 films.High quality single crystalline PtSe2 films with varying thickness were grown on bilayer graphene substrates by molecular beam epitaxy.Although bulk PtSe2 single crystals exhibit metallic behavior,atomically thin PtSe2 films show obvious semiconductor behavior with smaller band gap when increasing the film thickness,with the largest band gap of 1.2 eV for monolayer PtSe2 film.XPS measurements show the good air stability of atomically thin RSe2 films.The Raman experiments show the enhanced interlayer interaction in thicker PtSe2 films.The good air stability and tunable band gap make PtSe2 a promising candidate for future electronic devices.(3)We report the spin-layer locking effect induced by local Rashba effect in PtSe2.Unconventional R-2 Rashba effect is revealed in non-magnetic centrosymmetric PtSe2 films and single crystals by spin-and angle-resolved photoemission spectroscopy.In constrast to Rashba effect,the opposite spins are degenerate in energy but seperated in real space.This novel spin polarization is beneficial for finding new spin systems in non-magnetic centrosymmetric materials,and for developing electric field tunable spintronics.Our studies reveal a series of novel topological states,tunable band gap and rich spin textures in PtSe2 and PtTe2,which has motivated more researchers to further investigate these interesting members of TMDCs.