Angle-resolved Photoemission Spectroscopy Study On Type Ⅱ Weyl Semimetal MoTe2

Weyl semimetal is a topological novel state of matter,hosting the condensed matter physics counterpart of relativistic Weyl fermions originally introduced in high energy physics with nontrivial topology of electronic structure.Thanks to the diverse structure symmetries and rich interactions in solid state systems,a new type of Weyl fermions that break Lorentz invariance are predicted to emerge as topologically protected touching points between electron and hole pockets in a group of transition metal dichalcogenides（TMDCs）,and have no counterpart in high energy physics.Although fruitful results have been achieved in TMDCs in the past decades such as valleytronics,bandgap tunable semiconductor,charge density wave（CDW）and superconductivity.The realizations of topological novel states in TMDCs still remain absent.Here we performed systematically study on orthogonal T_d phase Mo Te₂,including synthesis of the 1T’-Mo Te₂ crystal,inversion symmetry breaking of the crystal upon cooling as well as its topological nontrivial state of matter.We summarize main results as follows:（1）Large scale high quality Mo Te₂ single crystal with 1T’phase is obtained by chemical vapor transport（CVT）.Our transport measurement revealed a structural phase transition occurred at 250K.And the inversion symmetry breaking below 250K is observed by tracking the Raman inactive phonon vibration mode evolution upon temperature variation.Such inversion symmetry breaking in low temperature guaranteed the existance of type II Weyl semimetal state in T_d-Mo Te₂.（2）We revealed the electronic structure by using Angle-resolved photoelectron spectroscopy with taking advantage of the matrix element effect and bulk/surface-sensitive light sources.Electron pocket and hole pocket formed from bulk bands are clearly revealed by fourth-harmonic generation laser source（6.33 e V）,and the signature of Fermi arcs is observed by using surface sensitive linear polarized synchrotron readiation with suppression of bulk states.Combining complimentary idenpendent experimental evidence provided by surface-sensitive probe STM and first-principal calculation,we further confirm the existance of topological Fermi arcs on the surface of T_d-Mo Te₂.Our systematical studies provide direct experimental evidence for the inversion symmetry breaking in T_d-Mo Te₂ and successfully establish it as a type II Weyl semimetal.The study not only shed lights on TMDCs with nontrivial toplogy,but opened new opportunity to futher topological order manipulation in layered structural materials as well.