STM Study Of Majorana Bound State In Iron-based Superconductor

In 1937,Ettore Majorana,an Italian theoretical physicist,predict an elemen-tary particle which is equal to its anti-particle.Later,this kind of particle is named by Majorana Fermion.Neutrino could be Majorana fermion but without direc-tion evidence.Majorana Fermion is charge neutral and thought to be composed by particle and anti-particle.In recent years,Majorana Fermion is predicted to exist in condensed matter systems in an quasi particle form.This kind of quasi particle is usually bounded and can also be called Majorana anyon because it obeys non-Abelian statistics.These kind of anyons can be used as quantum qubit and are building blocks of next generation self-tolerant quantum computer.As is well known,quantum computer achieves faster computing speed and bigger computing task compared to traditional computer.But in traditional quantum computer,the qubit is very easy to be decoherenced by the environment and lead to errors in the computation.However,these problems are solved by the topological quantum computer based on Majorana anyon as the qubit is topological protected.There-fore,it is very meaningful to discovery Majorana Fermion in real materials for the construction of highly stable quantum computer.Searching for Majorana Fermion in condensed matter systems is still very chal-lenging.It was predicted that Majorana Fermion can be exist in a p wave super-conductor.However,there still rear evidence to show the existence of a p wave superconductor.In 2008,Liang Fu and Kane pointed out that Majorana Fermion can also be exist in a topological insulator-s wave superconductor heterostructure.Based on this idea,research group of Kowenhoven,Yazdani,Jinfeng Jia and Mar-cus declared the observation of Majorana Fermion in 2012,2014,2015 and 2016respectively.And in 2017,Kanglong Wang and Shoucheng Zhang showed the ev-idence of chiral Majorana mode in a quantum anomalous Hall system.However,all these systems above are made by complex heterostructure which have a very complex fabrication process and the observation of Majorana Fermion need a very low temperature,usually less than 1 K.Using scanning tunneling microscope,we observe a zero energy bound state in an iron-based superconductor FeTe_0.55Se_0.45with low temperature and high magnetic field.Spatial,temperature and magnet-ic field dependent experiment of this zero energy bound state show clear evidence that we are tunneling into a pristine Majorana bound state.Compared to previous result,Majorana bound state in the iron-based superconductor is more pure and no interface problem.More important is that braiding is much easier in this 2D system than 1D nanowire.Our work find a new direction in the exploration of the non-Abelian nature of Majorana Fermion.In this thesis,we also do the STM study of two topological structure in two dimensional-grain boundary in monolayer MoS₂and stacked zigzag graphene nano ribbon structure.On one hand,Grain boundaries are widely found in 2D materials due to the limitation of growing method and can affect transport property of thin films.Until now,no systematically research on the grain boundary on monolayer MoS₂.Previous papers reported that the band gap of monolayer MoS₂will shrink in some certain grain boundary.However,in our research,the probability to find a shrinking and boardening band gap in the grain boundary is almost equal after we make a statistic of more than ten different grain boundaries.DFT investigation provide an insight view that the band gap is mainly by the local strain effect on the grain boundary and is not that relevant to the grain boundary structure.This finding renewed our understanding of the origin of band gap on grain boundary and is useful for the device application.On the other hand,zigzag graphene nano ribbon attracted much attention these years because it was predicted to be an ideal platform for the quantum Hall effect.The electrons will favour ferromagnetic coupling on one zigzag edge and favour anti-ferromagnetic coupling with the other zigzag edge.The main difficulty is the fabrication of high quality nano ribbon and characterization of magnetic signal at the zigzag edge.As for now,there are two method to fabricate such a ribbon namely the top-down method and bottom-up method.The ribbon prepared by the first method is usually of poor quality with irregular shape and defects on the edge.The sample prepared by the second method is usually of good quality but the electronic signal of the edge electron is buried in the itinerate electron signal on the metal surface.Here,using STM tip manipulation technique,we succeed to fabricate a stacked graphene nano ribbon structure and detect a decoupled edge electronic signal.The on-top graphene nano ribbon is decoupled by the bottom graphene nano ribbon which serves as an intercalation layer between on-top graphene nano ribbon and the metal surface.This artificial structure provides an ideal platform for the investigation of magnetism of ribbon edge.