Investigations On Topological Phase Transition In Na₃Bi Films And Correlation Effects Of Single Electron Scale In Pb Superconductor

Topological insulator and topological superconductor are two topological materials which have attracted great attention.The dissipation of topological edge states in the topological insulator can reduce energy consumption of devices and the phase transition of topological materials through external regulation will make the topological materials devices have more extensive applications in the future.In general,we need to construct p wave superconductors to realize topological superconductor.Theoretical and experimental results show that topological YSR bands generated by the coupling of YSR states can produce Majorana fermions at both ends of magnetic atomic chains or at the boundary of magnetic islands on superconducting substrates,which is undoubtedly a good platform for studying the properties of Majorana fermions.If magnetic atoms or molecules interact with superconductors to produce multiple YSR states,topological superconducting system will become more complex and interesting.Therefore,exploring the origin of multiple YSR states and regulating them will enrich the properties of topological superconductivity.When the topological superconductors are small enough,the coulomb blockade effect will interact with the superconductivity.In combination with the high critical magnetic field in the small superconductors,we can explore the fundamental properties of Majorana zero-energy mode including braiding,transportation and supersymmetry.In this paper,we combined molecular beam epitaxy(MBE)and scanning tunneling microscopy(STM)techniques to study the transformation of topological Dirac semi-metal Na₃Bi into topological insulator with decreasing film thickness,the YSR states in magnetic molecule Tb₂Pc₃ on Pb(111)and the relationship between superconductivity and coulomb blockade in small Pb islands.The main research results of this paper are following:(1)Na₃Bi films with various thickness have been grown on the graphene surface by molecular beam epitaxy.We found?3×?3 reconstruction on the surface of Na₃Bi film grown by MBE,and combined with DFT calculation,it was shown that the reconstruction was caused by the trimerization of surface Na atoms,and had no effect on the electronic properties of the material.By gradually decreasing the thickness,we measured the size of the65 Me V and 72 Me V energy gaps on the four and three layers of Na₃Bi films,indicating that Na₃Bi experencing a phase transion from semimetal to insulator.We found topological edge states at the boundary of three and four layers Na₃Bi films,indicating that two-dimensional Na₃Bi is a topological insulator.In combination with first-principles calculations,we find that the dimentional crossover point of Na₃Bi is seven monolayers,and all layers below are topological insulators,and all layers above are Dirac semimetal.(2)Pb(111)thin films are first grown on STO by molecular beam epitaxy,and then Tb Pc₂ molecules are grown on Pb(111)thin films.It is found that a small amount of Tb₂Pc₃molecules are generated during the growth of Tb Pc₂ molecules and will be inserted into the molecular film formed by Tb Pc₂ self-assembly.Because of the charge transfer between the molecule and Pb film,the isolated electrons in Tb Pc₂ pair up and lose the local magnetic moment,so the Kondo effect and YSR state can not be measured.The charge transfer enables Tb₂Pc₃ to obtain the local magnetic moment and then to measure the Kondo resonance and YSR states.We find that the strength of the Kondo peak and the molecular orbitals are perpendicular to each other with a two-fold symmetry respectively within a four-fold symmetry molecule.We found that there are two pairs of YSR states on the molecule,and the YSR states show the same distribution as the intensity of Kondo peak inside the molecule,indicating that there is a relationship between them.By changing the distance between tip and sample,we adjust the coupling strength between the molecule and the substrate,and then we can tune the YSR states to undergo a quantum phase transition,which means the magnetic ground state has changed.By combining DFT+U and NRG methods,we find that the double symmetry of the Kondo peak strength within the molecule and the molecular orbitals come from the splitting of the molecular orbitals near the Fermi surface,and one of the two molecular orbitals was semi-occupied and the other was almost empty.The combination of these two orbitals produces two pairs of YSR states.(3)Using molecular beam epitaxy,lead islands of various sizes have been grown on STO substrates.The coupling between superconductivity and Coulomb blockade effect was observed in lead islands of different sizes.When the tip is far away from the sample,the superconducting gap is the same as that of the bulk material on the lead island with the size of more than 200nm.When the tip approached the sample,the intensity of the superconducting coherence peaks gradually weakened and finally disappeared.At the same time,a new pair of superconducting coherence peaks appeared outside the superconducting energy gap.By applying 1T magnetic field,we find that the Coulomb blockade effect gradually increases with the decreased distance between tip and sample,which indicates that the enhancement of the Coulomb blocking effect will weaken the original superconducting coherence peak and produce superconducting energy gap superimposed with the Coulomb blocking effect.Coulomb staircase will appear on the 10nm island,and a small peak appears at 2?next to each Coulomb step peak,and that peak disappear as the magnetic field and temperature increased,indicating its relationship with superconducting.By changing the position of the tip relative to the sample,the number of charges in the island will change odd-even,and this change will make the coulomb stair peak near the Fermi surface disappear or appear.This indicates that the tunneling probability of electrons in the superconductor is modulated by the parity of the superconductor.