Scanning Tunneling Spectroscopy of Graphene and Magnetic Nanostructures
This dissertation is divided into two parts, both of which describe measurements where a scanning tunneling microscope (STM) is operated to perform measurements of novel physical systems. In the first section I describe spin-polarized STM (SP-STM) measurements on individual adatoms on a ferromagnetic surface. The aim of the first section is to illustrate how a SP-STM can be used to probe magnetic phenomena at the atomic scale. After describing the explicit details of the SP-STM apparatus, I explain how to use SP-STM to study the magnetic coupling of different 3d adatoms (Fe, Cr and Cu) to a ferromagnetic surface. It is shown that the SP-STM is capable of distinguishing which species couple ferromagnetically vs. anti-ferromagnetically to the surface. The second section describes STM measurements performed on a graphene surface equipped with a back-gate electrode that can be used to vary the charge density of the graphene in situ. These measurements represent the first time an STM has been capable of measuring a gate-tunable surface, and this section describes the extra experimental and theoretical considerations that are required to set up such an experiment and accurately interpret the data. It is shown that electrons tunneling into graphene include a strong, phononmediated inelastic signal, and that an STM tip can be an invasive probe of graphene. After showing how to account for these effects, STM experiments are presented that examine (1) how impurities effect the large scale electronic structure of graphene, (2) how the quasiparticle lifetime in graphene depends on charge density, and (3) how Co adatoms can be reversibly ionized on graphene through the use of a back-gate electrode.