Angle Resolved Photoemission Spectroscopy Study On A New Quantum Material WTe₂ And Graphite Intercalation Superconductors

The study of quantum topological materials has seen rapid progress from the discovery of topological insulators,to three-dimensional Dirac semimetals,and to threedimensional Weyl semimetals.These materials have attracted much attention because they represent new states of matter with unique electronic structures,spin textures,and associated novel physical properties.Also,around 50 years ago,superconductivity in graphite intercalation compounds（GICs）was firstly reported,and the corresponding superconductivity mechanism has stimulated great interests in GICs superconductors.This thesis is mainly divided into three parts: the angle-resolved photoemission spectroscopy（ARPES）study on the electronic structures of WTe₂,the crystal growth of a few GICs（CaC₆,SrC₆ and BaC₆）,and the ARPES study on the electronic structures of these GICs.The main results of the thesis are as following:1.The brief introduction of the background of the quantum topological materials has been given,especially for the Weyl semimetal.The background of the discovery and progress of the superconductors has been introduced,and the physical properties of GIC superconductors have been summarized.2.My thesis work includes the maintenance,improvement and utilization of the time-of-flight ARPES（ARTo F）system in our lab.The brief introduction of the basic theory of the ARPES and the configuration of the ARPES system have been given.Also,I provided a general introduction of the laser-based ARPES system,ARTo F system,spin resolved ARPES system and the He3 ARPES system developed in our lab.3.It is known that WTe₂ attracts considerable attention owing to its extremely large magnetoresistance（MR）which has been largely ascribed to the nature of a perfect compensation semimetal.By utilizing ARTo F system with high energy and momentum resolutions which can detect two dimensional momentum space simultaneously,we have revealed a full picture on the electronic structure of WTe₂.The temperature-induced chemical potential change was observed, which causes an ratio variation of the electron and hole concentration.Our results show that WTe₂ is unlikely to be a perfectly compensated system at low temperature which are not consistent with the perfect electron-hole compensation picture.Also,we found a flat band which is located at a few me V below the Fermi level.The decreasing of the excited carrier around Fermi level provided by the flat band with increasing temperature is consistent with the temperature evolution of the extremely large magnetoresistance,which means the flat band may play an important role in the extraordinary transport properties.Anyway,a further work will be required to clarify the mechanism involved.4.WTe₂ has ignited another surge of excitement because it is theoretically predicted to be the first material candidate that may realize a type-II Weyl state.As we know,in type-II Weyl semimetals,the Lorentz invariance is violated.This results in a highly tilted Weyl cone,where the Weyl points emerge at the contact points of the electron and hole pockets.By utilizing our ARTo F system,a clear surface state has been identified and its connection with the bulk electronic states in the momentum and energy space shows good agreement with band structure calculations.Our results provide electronic signatures that are consistent with type-II Weyl states in WTe₂.They lay a foundation for further investigations on the topological nature of WTe₂ and the exploration of unique phenomena as well as the peculiar physical properties in type-II Weyl semimetals.5.We have grown high quality single crystal samples CaC₆,BaC₆ and SrC₆ by utilizing alloy displacement method and vapor transport method.The characterization of the samples has been done by X-ray Diffraction（XRD）,specific heat measurement and magnetization measurement.Our measurements show that CaC₆ and SrC₆ exhibit superconducting transition at 11.6K and 1.65 K with very sharp transition,respectively.Limited by accessed experimental conditions,we did not measure the superconducting transition of BaC₆（the transition temperature～30m K）.The high quality single crystals we have obtained enable the ARPES experiment studies on these interesting GIC superconductors.6.These GICs（CaC₆,SrC₆ and BaC₆）were systematically measured by our labbased ARPES and the synchrotron radiation-based ARPES.The corresponding electronic structure calculations were also performed.From our results,for all these three compounds,we found that the π* bands show two kinks: one is located at ～150-200 me V,with electron coupling with Cxy phonon mode;and another is located at ～40-90 me V,with electron coupling with Cz phonon mode.Also,we found that the carrier concentration of CaC₆ and BaC₆ in K region is different,and the electron-phonon coupling is very anisotropic.In the Brilloin zone center at Γ point,we identified the folding bands and the interlayer state bands.So we could speculate that the interlayer band and π* band might both play key roles for the superconductivity in GICs.