| Bismuth possesses unique electronic properties both in bulk and surfaces, and has attracted renewed interests. Some intriguing phenomena, like electron fractionalization, valley polarization, and phase transitions of Dirac electrons, were reported in bulk bismuth recently. However, their physical origins have become highly controversial issues due to the lack of a complete understanding of how the Landau levels (LLs) of bismuth surfaces interplay with the bulk under magnetic fields. The electronic properties of bulk bismuth were measured by detecting the LL signals intersecting the Fermi energy (EF) under magnetic fields in many previous experiments. It has been found that the quantum limit of bulk bismuth can be easily reached by applying a magnetic field up to 9 T along the high-symmetry trigonal axis. At the quantum limit, the carriers should be confined to the lowest LL. In other words, no more LLs could appear above this limit in the single-particle picture. The observation of unexpected signals above 9 T thus led to the suggestion of electron fractionalization in bulk bismuth. However, it was argued that these signals could be from the surface states or the presence of twin boundaries in bulk. To pinpoint the exact physical origin of the observed unexpected phenomenon, the surface-state properties of bismuth under magnetic fields have to be carefully examined.The Bi(lll) surface is particularly suitable for this purpose. Due to the rhombohedral symmetry in bismuth crystal, the normal of the (111) surface is parallel to the trigonal axis. The surface is terminated by Bi bilayer, forming a honeycomb-like lattice. Owing to the strong spin-orbit coupling (SOC) with the broken space-inversion symmetry, the electronic properties of the surfaces become distinctly different from the bulk ones. The Bi(111) surface possesses metallic and spin-split Rashba surface states with a vortical spin texture, where the filled states were experimentally characterized by angle-resolved photoemission spectroscopy (ARPES). The transport measurements of Bi(111) thin films showed that there are significant contributions to the conductance from the metallic surface states.In this work, the epitaxial Bi(111) ultrathin films on Si(111)-7×7 substructure were studied using scanning tunneling microscopy/spectroscopy (STM/STS) under a magnetic field up to 11 T. Joint with the first principles calculations, the Landau quantization and the scattering of the surfaces were characterized. Our results highlight the important role of the surface states and provide alternative pictures for the properties of bismuth.In chapter 1, we briefly introduce the principle of STM and its performance at the ultra-low temperature, strong magnetic field, and ultra-high vacuum conditions.In chapter 2, the properties of Bi bulk and film are introduced. We interpret the significance of Bi(111) ultrathin film in understanding the surface states. The method of epitaxial growth of Bi(111) ultrathin films on Si(111)-7×7 is introduced. We compare the electronic properties from different techniques and also compare with the first-principles calculations.In chapter 3, we characterize the Landau quantization of the Bi(111) surface states under magnetic fields measured using scan tunneling spectroscopy. In the STS spectra, the field-depended discrete peaks are associated to Landau level (LL). With an analysis analogous to the traditional measurements of Shubnikov-de Haas oscillations (SdHOs), the discrete peaks in the STS spectra can be well attributed to the Landau levels from the surface-state band structure that are well described by the DFT calculated results of Bi(111) surface.In chapter 4, the scattering of surface states are investigated through the adsorbed defects and steps by measuring the standing wave in STS maps. The standing waves are the interference of the quasi-particle waves in the material, known as quasi-particle interference (QPI). The Fourier transform (FT) patterns of standing waves are in good agreement with the simulated pattern using the surface-state band structure of Bi(111) by considering its chiral spin texture, which gives experimental evidence of the vortical spin-textured surface states of Bi (111).In chapter 5, the adsorption of Co atoms on the surface of Bi(111) film is studied. STM images joint with the DFT calculations confirm the interstitial adsorption structure of Co atoms. The dl/dV spectra taken at adsorbed Co atoms show a shoulder feature at the Fermi level. The shoulder feature is probable attributed to the Kondo resonance with a Fano shape, which become wider with increasing temperature. |
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