Angle-Resolved Photoemission Spectroscopy Studies On Topological Semimetals

Since the concept of topological insulator was built,the fever of exploring new topological materials spread fast in the field of condensed matter physics.In materials with topological properties,the transformation of quantum state in matter can happen without spontaneous symmetry breaking,but with the change of topological order.Topological insulator is one of the various quantum states with topological order.After the discovery of topological insulator,plenty of exotic topological quantum states were predicted theoretically or discovered experimentally,among of which investigations on topological semimetals made a great breakthrough.According to the type of band crossing in the bulk electronic structure of topological semimetals,we can clarify them as:Topological semimetals with(?)isolated crossing points,such as 4-fold degenerate Dirac fermion,2-fold degenerate Weyl fermion,three-component degenerate new fermions,new chiral fermions.(?)With one-dimensional crossing line,such as nodal ring fermion,nodal chain fermion.(?)With two-dimensional crossing surface.(?)With three-dimensional crossing sphere.Angle-resolved photoemission spectroscopy(ARPES)plays an important role in the discovery of new topological phase,mostly because of the advantage of observing band dispersions in momentum space directly.The subject of this dissertation is about ARPES studies on several topological semimetals(Ir1-xPtx)Te2,ZrSiS and TiB2.The main research results are listed below:1.In the superconductor of(Ir1-xPtx)Te2,we successfully observed type-? Dirac fermion and tuned the Dirac point to Fermi level.In Dirac semimetals with superconductivity,the cooper pairs could correlate with Dirac fermions and the resulted topological surface states,generating new quantum states,which requires the presence of Dirac fermions around Fermi level.via doping Pt into IrTe2,we managed to pull down the Dirac point to Fermi level in the sample with x = 0.1 and observed superconductivity with Te?2 K.By continuously changing photon energies to measure the band dispersions along k,direction,we demonstrated that the bulk Dirac fermions in(Ir1-xPtx)Te2 belong to type-?.2.Observation of nodal line structures in ZrSiS and TiB2 single crystals.Taking advantage of soft-X-ray ARPES,we have successfully probed the bulk electronic structures of ZrSiS,excluding the interference of surface states reported by other groups.We find that the three-dimensional Fermi surfaces are completely composed of Dirac bands at Fermi level.These Dirac bands also form nodal lines and nodal rings,connecting with each other.In TiB2,we discovered nodal rings as well and they intersect with each other resulting the nodal chain structure.The signs of topological Dirac surface state originating from nodal chain were also observed.3.First experimental discovery of nodal surface state in ZrSiS.Utilizing soft-X-ray ARPES,we can probe pure bulk states of ZrSiS,finding the existence of 4-fold degenerate bands in deep binding energy.Going through a careful check of symmetries,we conclude that there are nodal surfaces locating at the boundary of Brillouin zone,kx=±? and ky = ±?.Up to now,several materials probably containing nodal surface structure have been proposed theoretically.However,this is the first realization in experiment.The combination of mature theories and rich experimental techniques promotes the fast development of topological physics.So far,the catalogue of topological electronic materials has been established.A more thorough development in topological theory and potential applications of topological materials are desperately needed in the near future.