Doctoral Dissertations
Date of Award
8-2020
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Major
Physics
Major Professor
Hanno H. Weitering
Committee Members
Steven S. Johnston, Cristian D. Batista, Gong Gu
Abstract
The analysis of the electronic structure and morphology of 1/3 monolayers (ML) of Sn or Pb on Si(111) and Ge(111) has played an important role in understanding the role of electronic correlations in two dimensions. Specifically, the two-dimensional lattice of partially filled dangling bonds of these so-called α-phases has been an important testbed for studying structural phase transitions and correlated electronic phenomena ever since the discovery of a surface charge density wave in the Pb/Ge(111) system more than two decades ago. With the exception of the novel Sn/Si(111) system, all $\alpha$-phases undergo a charge ordering transition at low temperature. The uniqueness of this surface has further been observed in modulation hole doping experiments, where we have shown that the Sn/Si(111) system shows clear signs of doped Mott physics. This provides a rare insight into cuprate-like physics in a sp3 bonded non-oxide system and suggests the possibility of realizing unconventional superconductivity on a conventional semiconductor platform. Indeed, recent measurements on this system reveals that the hole-doped Sn layers on a degenerately doped p-type Si(111) wafer are superconducting with a critical temperature of 4.7 ± 0.3 K. The triangular lattice symmetry and strong Mott correlations of the Sn-derived dangling bond surface state strongly suggest that the superconductivity may be unconventional and topologically non-trivial. This fascinating result motivates attempts to electron-dope this system using low-dose potassium deposition. Here, we find that potassium induces a charge ordering transition in the Sn sublattice. Temperature dependent studies on this surface reveal the complex interplay between the site location and diffusion of the K adatoms, and the charge ordering transition in the Sn sublattice. As the temperature increases from absolute zero, the K atoms start fluctuating about their equilibrium positions and eventually hop back and forth between neighboring lattice sites. During this process, long-range order is maintained. At higher temperature the K adatoms begin to diffuse over much longer distance, ultimately leading to a complete melting of the K sublattice. K diffusion induces charge fluctuations between the dangling bonds of the Sn sublattice, and ultimately leads to melting of the charge order of the Sn lattice through a displacive and subsequently an order-disorder transition. In combination with theoretical work, this provides a rare atomically resolved insight into a multistep 2D melting transition.
Further studies are performed on the Pb/Si(111) system, as the true electronic ground state and mechanism behind the charge ordering transition has been controversial due to the still limited number of studies of this surface. A unique growth method is developed to grow larger domains on this surface, which greatly reduces the effects of defects and domain boundaries on the structure and electronic structure of the interface. This allows a comprehensive study of both the electronic and structural properties through quasiparticle interference imaging. To explore the role of spin-orbit coupling for heavy adatoms such as Pb, we introduced magnetic scattering centers (Mn) so as to break time-reversal symmetry in the QPI patterns. Our results provide clear evidence of Rashba-type spin-orbital coupling, although more detailed interpretation awaits further theoretical studies.
Recommended Citation
Smith, Tyler S., "Exploring Structural and Electronic Properties of Triangular Adatom Layers on the Silicon Surface Through Adsorbate Doping. " PhD diss., University of Tennessee, 2020.
https://trace.tennessee.edu/utk_graddiss/6760