Angle Resolved Photoemission Spectroscopy Study On Quantum Topological Materials ZrTe₅ And HfTe₅

The topological materials,including topological insulators,three-dimensional Dirac semimetals and three-dimensional Weyl semimetals,have attracted much attention recently for their peculiar physical properties,unique electronic structure and strong application potential.The quantum spin Hall effect,a phenomenon that the net spin current can be detected in 2D materials,has great application potential and significance in basic research.However,few systems have been found to have this phenomenon experimentally.It is predicted that single-layer ZrTe₅ and HfTe₅ are two-dimensional topological insulators?also named as quantum spin Hall insulators?with large bulk gap and there may be possibly a topological phase transition?based on the layer distance?in bulk ZrTe₅ and HfTe₅.What is more interesting is that,ZrTe₅ and HfTe₅ have host a long-standing puzzle on its anomalous transport properties since 1970 s manifested by its anomalous resistivity peak;its underlying origin remains elusive.All the above-mentioned aspects have triggered vast research on these materials.In this thesis,we use the Angle-resolved photoemission spectroscope?ARPES?to study the electronic structure of ZrTe₅ and HfTe₅,the conclusions are listed as following:1.The temperature dependent ARPES data indicated a clear electronic evidence of the temperature induced Lifshitz transition at their corresponding transport peak?T?in ZrTe₅ and HfTe₅.It provides a natural understanding on the underlying origin of the resistivity anomaly at T and its associated reversal of the charge carrier type.The overall electronic structure evolves from a p-type semimetal with a hole-like Fermi pocket at high temperature,to a semiconductor around T where its resistivity exhibits a peak,to an n-type semimetal with an electronlike Fermi pocket at low temperature.In addition,we also found that the overall band structure shifting down with decreasing temperature is not a rigid band shift.The direct gap at ? point rapidly decreases with the sample cooling down,this may be related to the samples' topological nature.2.Through the comparison between experiment and theory,we found in ZrTe₅ that evolution of the direct gap at ? point with decreasing temperature is consistent with the topological phase transition behavior from weak topological insulator to strong topological insulator.At the lowest temperature of 2K,the direct gap is still existing and no surface state is observed in the gap region.We detected an additional one-dimensional feature which is associated with the edge along c axis of streaked ZrTe₅ sample surface.In ZrTe₅,from the room temperature to2 K,with decreasing temperature,there is a trend of the topological transition,but the strong topological insulator has not been realized at the lowest temperature we measured?2 K?.Further enhancement of interlayer coupling,either by external high pressure or internal chemical pressure,may facilitate to the realization of such a transition.For HfTe₅,it has the similar behavior of direct gap size reduction with the sample cooling down when the temperature is higher than T.It indicated that the HfTe₅ is a weak topological insulator and has the trend to become a strong topological insulator when the temperature is lower than T.Different from ZrTe₅,when the temperature is lower than T,the band shifting and the gap size change with temperature become very small.Our observations suggest that HfTe₅ is a weak topological insulator that is located at the phase boundary between weak and strong topological insulators at very low temperature.For the lower temperature than that we measured,we cannot judge the topological nature of HfTe₅.3.Using S,P linear polarized and L,R circularly polarized laser to detect the electronic structure of ZrTe₅ and HfTe₅,we found that it shows some differences of spectral weight redistribution under different polarized laser.And this difference is not consistent with the spin orbital coupling in topological nontrivial surface state.Therefore,we think it is unlikely that ZrTe₅ and HfTe₅ are strong topological insulator.The difference of spectral weight redistribution may be related to the matrix element effect based on different polarization and bulk band orbital attributes,which needs further study.The surface potassium deposition on HfTe₅ shows that it is easy to form surface reconstruction.This is not good to study the direct gap and conduction band of HfTe₅.We should think about doping other elements in order to study the topological nature of HfTe₅.Graphene is another hot topic in condensed mater physics,superconductivity in graphene has invoked a lot of research because of its great application potential.Since2005,the report of the superconductivity in graphite intercalation compound Ca C6(T?8=11.5K?has led to a lot of controversy about the superconducting mechanism in the general graphite intercalation compounds.The understanding of such a mechanism can provide valuable information on the study of graphene superconductivity.Motivated by this problem,we synthesized a series of high quality graphite intercalation compounds?Ca C6,Ba C6,KC₈?,and employed Angle-resolved photoemission spectroscope?ARPES?to study their electronic structure.In the study of KC₈,the first discovered graphite intercalation compound superconductor,people focused on the *band of graphite in the past.For the first time,we studied the interlayer state of KC₈,and estimated the size of the interlayer state Fermi pocket.We also detected two kinks at the binding energy of around 50?70me V and 150?170me V in both *band and interlayer state band,which are believed to play a role on the emergence of superconductivity in KC₈.