UBC Theses and Dissertations
Investigations in Flatland : scanning tunnelling microscopy measurements of noble metal surface states and magnetic atoms on magnesium oxide Macdonald, Andrew James
At the atomic scale, surfaces exhibit a rich variety of physical phenomena that can be probed using a scanning tunnelling microscope (STM). The STM measures the quantum tunnelling of electrons between a metallic tip and conducting sample and can be used to characterize the nanoscale surface. This thesis presents STM measurements taken at low-temperature in ultra-high vacuum, which are used to characterize two different nanoscale environments: the two-dimensional surface states of Ag(111) and Cu(111) and the magnetic moments of iron and cobalt atoms deposited on a thin-film of magnesium oxide. Fourier-transform scanning tunnelling spectroscopy (FT-STS) analysis of quasiparticle interference, created by impurity scattering on the surfaces of the noble metals Ag(111) and Cu(111), is used to compare the most common modes of acquiring FT-STS data and shows, through both experiment and simulations, that artifact features can arise that depend on how the STM tip height is stabilized throughout the course of the measurement. Such artifact features are similar to those arising from physical processes in the sample and are susceptible to misinterpretation in the analysis of FT-STS in a wide range of important materials. A prescription for characterizing and avoiding these artifacts is proposed, which details how to check for artifacts using measurement acquisition modes that do not depend on tip height as a function of lateral position and careful selection of the tunnelling energy. In a separate set of experiments a spin resonance technique is coupled to an STM to probe the spin states of individual iron atoms on a magnesium oxide bilayer. The magnetic interaction between the iron atoms and surrounding spin centres shows an inverse-cubic distance dependence at distances greater than one nanometre. This distance-dependence demonstrates that the spins are coupled via a magnetic dipole-dipole interaction. By characterizing this interaction and combining it with atomic manipulation techniques a new form of nanoscale magnetometry is invented. This nanoscale magnetometer can be combined with trilateration to probe the spin structure of individual atoms and nanoscale structures. The information gained characterizing these new forms of magnetic sensing sets the stage for the study of complex magnetic systems like molecular magnets.
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