Novel Topological State And Band Structure Engineering In Metal Chalcogenide

In recent years,with the discovery of topological quantum states,the combination of topological states and two-dimensional materials has provided new opportunities for investigating the novel properties.Past researches have focused on the rich structure of two-dimensional material family members,as well as traditional problems such as CDW and superconductivity.In this paper,we focus on the topological properties of transition metal chalcogenides 1T’phase MoTe2 and 1T phase PtTe2/Pt Se2,as well as the band structure and modulation of monochalcogenides SnSe and CuTe.The main research results are as follows:(1)We provide first direct evidence for the realization of type-II 3D Dirac fermions in 1T phase single crystal PtTe2.We prepared the large-area,high-quality PtTe2 single crystal samples which are necessary for the experiment,and systematically studied the band structure of the PtTe2 single crystal by angular resolved photoelectron spectroscopy(ARPES).The anisotropic 3D Dirac cone that are highly tilted along the kz direction provides direct evidence of type-II Dirac fermions.We further found the type-II Dirac fermion in its isostructural Pt Se2.(2)The symmetry breaking and phase transition in the type-II Weyl semi-metal MoTe2 was confirmed by Raman spectroscopy for the first time after the previous ARPES work.We revealed the inversion symmetry breaking in low temperature phase MoTe2,which indicates that the MoTe2 is an ideal system to achieve temperature-induced topological phase transition.(3)We reveal the electronic origin of the excellent thermoelectric properties upon hole doping by performing a systematic ARPES study of semiconducting SnSe single crystals.We also find the electronic structure of SnSe is widely tunable via surface doping of K on SnSe.(4)We study the electronic structure and provide experimental evidence for the CDW phase in CuTe containing quasi-1DTe chains.ARPES experiments clearly show that the two quasi-1D like parallel bands from theTe px orbitals are gapped in CDW transition,and the gap is collapsed by increasing temperature or electron doping.Our experimental results together with first principles calculations suggest that electron-phonon coupling and electron-electron scattering by Fermi surface nesting both contribute to the formation of the CDW phase of CuTe.In summary,we reveal a series of novel topological properties in transition metal chalcogenides,and systematically study the band structure and its modulation in monochalcogenides.This would attract a broad interest in investigating the fascinating properties of such materials.