UC Berkeley

This work presents first-principles theoretical studies on two topics of condensed matter physics. The first topic is the adsorption of metal adatoms on graphene. Graphene, a two-dimensional material made of carbon atoms arranged in a honeycomb lattice, has many outstanding properties that can be enhanced or tailored by adsorbing adatoms on its surface. The second topic involves the coupling of spins or electrons to phonons in a solid. The interaction between different degrees of freedom of a material complicates the study of its properties but also leads to fascinating phenomena, such as superconductivity, and potential device applications. This dissertation is organized into six chapters:

· In Chapter One, we give an overview of this work and review the first-principles theory and methods used in our studies.

· Chapter Two focuses on structural, energetic, and electronic properties for a variety of adatom species adsorbed on the graphene surface. We classify different species as having mostly ionic or covalent character of bonding to graphene. For ionically bonded adatoms, charge transfer between the adatom and graphene is signficant. We find general trends relating the surface dipole moment, work function, and atomic ionization potential of the adatom species.

· In Chapter Three, we study the electronic structure of adatoms on graphene when a gate voltage is applied to control the number of electrons in the system. Lithium on graphene, a prototype system, and cobalt on graphene, an experimentally relevant case, are studied. We find that localized states on the adatom can be charged or discharged by the application of gate voltage, and we study the changes in potential and charge density of the system as electrons are added or removed.

· In Chapter Four, we extend the work in Chapter Three to consider the possibility of transforming the electronic structure of one species of adatom on graphene into that of another by applying a gate voltage. We find that within our model, such transformations are possible for certain adatom species.

· In Chapter Five, the zone-center phonons for the frustrated antiferromagnetic compound ZnCr2O4 are calculated. We find that the transition from nonmagnetic to antiferromagnetic ordering causes a splitting of certain degnerate phonon frequencies, in agreement with experimental results.

· In Chapter Six, the pressure dependence of electron-phonon coupling and the superconducting transition temperature (Tc ) in elemental arsenic is studied. We find that an experimentally observed peak in Tc as a function of pressure is related to a structural transition and can be explained mainly by changes in electronic structure and phonon frequencies with pressure.