Experimental and theoretical investigations of chiral surfaces

  • Nathan Lewis

Student thesis: Phd


Spintronic devices have the potential to transform the electronics industry by increasing the efficiency and clock frequency of Si-based transistors. The suitability of using chiral materials, in particular Si, for generating spin-polarised electrons in spintronic devices is explored in this thesis by presenting spin-resolved photoemission studies on chiral Si and W surfaces. This is supported by theoretical research in which spin-polarisation effects arising from the surface chirality are revealed. Rashba bands are modelled for two-dimensional chiral and achiral lattices and these are used to determine the effects of chirality on the components of the spin polarisation. In contrast to the achiral case, the component parallel to the electron crystal momentum for a chiral lattice is found to have a maximum of 2.5% for a Bi lattice and is predicted to invert between the enantiomorphs; these effects are ascribed to the influence of the spin-orbit interaction. Parameters in the model are varied to shed light on their influence on this longitudinal spin component. The chiral Si(110) 16 x 2 reconstruction is studied using LEED, STM, spin- and angle-resolved photoemission. The mechanism that controls which enantiomorph forms upon reconstruction is determined from LEED and STM images. Polishing the sample surface with a particular misalignment angle is shown to influence the orientation of the vicinal surface observed at high temperatures, subsequently controlling the enantiomorph generated. The surface states observed in the band-dispersion maps of the chiral Si surface are attributed to different structural elements of the reconstruction. An upper limit of 1% is associated with the inverting longitudinal spin polarisation component of the Si(110) 16 x 2 reconstruction. The larger atomic number of W relative to Si increases the magnitude of the spin-orbit coupling and the associated magnitude of the longitudinal spin polarisation. The chiral W(321) and W(-3-2-1) surfaces are investigated using spin- and angle-resolved photoemission at the APE-LE beamline of the Elettra synchrotron. A Rashba-like state is observed in the band-dispersion maps of the W(-3-2-1) surface, and spin-resolved photoemission measurements of this state show all components of the spin-polarisation are non-zero. This is discussed in terms of the surface chirality and step structure.
Date of Award31 Dec 2019
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorWendy Flavell (Supervisor) & Elaine Seddon (Supervisor)

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