Study On Structure And Electronic States Of Bismuth-Lead Alloy,Bismuth-Copper System And The Dislocation On Bi(111)Films

The Bismuth and its related materials,such as topological insulator Bi2Te3 and Bi2Se3,topological superconductor Pb3Bi,Dirac semimetal Na3Bi,have been the hot topics and widely studied because of their intriguing properties.Bismuth,as the core element in these materials,has very large atomic mass,resulting in strong spin-orbit coupling and leading to spin splitting in the surface states of bismuth films.Because of its large Fermi wavelength(?40nm)and long mean free path of carriers(several micrometers),bismuth film is a good candidate to study the quantum transport and quantum size effect.Due to the small effective masses of carriers and the large Fermi wavelength,very large magnetoresistance effects were observed in single crystal bismuth.In this thesis,we will focus on the structural and electronic properties of Bi-Pb alloy films,self-assembled submonolayer Bi on Cu(111)surface,and the dislocation defects in single crystal Bi films,characterizing by high resolution scanning tunneling microscopy(STM)and angular resolved photoelectron spectroscopy(ARPES).In chapter one,we briefly introduce the background and some concepts,including the superconductivity of alloys with electrons-per-atom ratio,Dirac nodal line semimetal,band folding,electron beam supercollimation,and the topological one-dimensional states in the dislocation.STM and ARPES,the main techniques used in the ample characterizations are also introduced.In chapter two,we focus on the superconductivity of Bi-Pb alloy.We prepared two types of Bi-Pb alloy films with different Bi-Pb atom ratio by co-deposition of Bi and Pb,followed by post-annealing treatments at a low temperature.In the large scale images of the sample surfaces,we observed alloy phases separated from the Bi phase,shown as different domains with relatively flat surfaces,in which the atomic structures were characterized.According to the atomic structures,the derived compositions of the alloys are PbBi3 and Pb7Bi3,respectively.It is also found that the formation of the different alloy phases is mainly dependent on the symmetry of the substrates,and the effect of Pb/Bi ratios is negligible.From STS spectra,it can be seen that the alloy phases are superconductor,different from the normal metal phase of Bi.Using variable temperature measurements,we get TC=6.12K for PbBi3 and 7.77K for Pb7Bi3,both are strong coupling superconductor.Our experimental results are in satisfactory agreement with the empirical relation between superconductivity and the number of valence electrons per atom.In chapter three,we investigate the superconducting proximity effect.When a magnetic field is applied to the Bi-Pb alloy films,vortices with pinning effect are observed,indicating that the Bi-Pb alloy films are type II superconductor with a measured upper-critical field(HC2)of 1.68 T.By fitting the zero-bias conductance(ZBC)distribution on a vortex,we obtained the BCS coherence length.The proximity effect is studied at the superconductor-normal metal heterojunction.We demonstrate that the steps on the surface act as native Josephson barriers to disrupt the proximity effect.The proximity effect is obviously suppressed by the applied magnetic fields.In chapter four,we investigate the electronic properties of Bi/Cu system.Single layer Bi[2012]films were grown on Cu(111)surface.It is experimentally demonstrated that the self-assembled Bi superlattice with a nonsymmorphic symmetry on Cu(111)can effectively introduce the surface potential to cause the formation of nodal lines by the folding of Cu sp band.The nodal lines at a boundary of the superlattice Brillouin zone possess the gapless features,which can be ascribed to Dirac nodal lines,where the underlying physics has been revealed by the numerical calculations and the analytic solution,and also supported by the DFT calculations.Our results show the alternative approach to create Dirac nodal line by enforcing an extrinsic symmetry through the introduction of external periodic potentials on the normal metal of Cu.In chapter five,we study the small angle grain boundaries in Bi(111)film.The single crystal Bi(111)films were grown on Si(111)-(7x7)substrate.In the films,some small angle grain boundaries were observed.The boundaries consist of a series of(1,0)dislocations,and the core structure is identified to be pentagon-heptagon pairs.It is observed that very strong defect states distribute along the grain boundaries from the STS spectra.Our DFT calculations show that the defect states could be ascribed to the van Hove singularity(VHS)states due to the relatively ordered chains of(1,0)dislocations along the boundaries.