The education of Walter Kohn and the creation of density functional theory

The theoretical solid-state physicist Walter Kohn was awarded one-half of the 1998 Nobel Prize in Chemistry for his mid-1960's creation of an approach to the many-particle problem in quantum mechanics called density functional theory (DFT). In its exact form, DFT establishes that the total charge density of any system of electrons and nuclei provides all the information needed for a complete description of that system. This was a breakthrough for the study of atoms, molecules, gases, liquids, and solids. Before DFT, it was thought that only the vastly more complicated many-electron wave function was needed for a complete description of such systems. Today, fifty years after its introduction, DFT (in one of its approximate forms) is the method of choice used by most scientists to calculate the physical properties of materials of all kinds. In this paper, I present a biographical essay of Kohn's educational experiences and professional career up to and including the creation of DFT

Scepticism about Scepticism

Skeptical arguments are intuitively gripping. Or at least they seem to be. They readily capture the imagination and curiosity of beginners in philosophy. The arguments are easy to state but seemingly impossible to answer. Furthermore there is a powerful pessimistic induction. Those who think they have a reply inevitably haven’t appreciated the force of skeptical arguments. So, at least, I believed for many years, along with most of my fellow philosophers. In this paper I reconsider epistemological skepticism within a framework in which the dependence of epistemic properties on non-epistemic properties plays a central role. I argue that a notable consequence of foregrounding dependence is that skeptical arguments no longer have even a prima facie grip on us. At very least, parity is established between skepticism and its opposite. The presumption in favor of skepticism is obliterated. At most, the main types of skeptical argument are refuted. It sounds unlikely, I know, given the history of failures to refute skepticism, and the number of papers and books that begin with similar bravado yet end up failing with panache. Nevertheless, let’s see

Phase Field Modelling of Submonolayer Epitaxial Growth

We report simulations of submonolayer epitaxial growth using a continuum phase field model. The island density and the island size distribution both show scaling behavior. When the capillary length is small, the island size distribution is consistent with irreversible aggregation kinetics. As the capillary length increases, the island size distribution reflects the effects of reversible aggregation. These results are in quantitative agreement with other simulation methods and with experiments. However, the scaling of the island total density does not agree with known results. The reasons are traced to the mechanisms of island nucleation and aggregation in the phase field model.Comment: 6 pages, 5 figure

Regimes of Precursor-Mediated Epitaxial Growth

A discussion of epitaxial growth is presented for those situations (OMVPE, CBE, ALE, MOMBE, GSMBE, etc.) when the kinetics of surface processes associated with molecular precursors may be rate limiting. Emphasis is placed on the identification of various {\it characteristic length scales} associated with the surface processes. Study of the relative magnitudes of these lengths permits one to identify regimes of qualitatively different growth kinetics as a function of temperature and deposition flux. The approach is illustrated with a simple model which takes account of deposition, diffusion, desorption, dissociation, and step incorporation of a single precursor species, as well as the usual processes of atomic diffusion and step incorporation. Experimental implications are discussed in some detail.Comment: 10 pages, 2 figure

Model and Simulations of the Epitaxial Growth of Graphene on Non-Planar 6H-SiC Surfaces

We study step flow growth of epitaxial graphene on 6H-SiC using a one dimensional kinetic Monte Carlo model. The model parameters are effective energy barriers for the nucleation and propagation of graphene at the SiC steps. When the model is applied to graphene growth on vicinal surfaces, a strip width distribution is used to characterize the surface morphology. Additional kinetic processes are included to study graphene growth on SiC nano-facets. Our main result is that the original nano-facet is fractured into several nano-facets during graphene growth. This phenomenon is characterized by the angle at which the fractured nano-facet is oriented with respect to the basal plane. The distribution of this angle across the surface is found to be related to the strip width distribution for vicinal surfaces. As the terrace propagation barrier decreases, the fracture angle distribution changes continously from two-sided Gaussian to one-sided power-law. Using this distribution, it will be possible to extract energy barriers from experiments and interpret the growth morphology quantitatively.Comment: 6 pages, 7 figure

Non-collinear spin transfer in Co/Cu/Co multilayers

This paper has two parts. The first part uses a single point of view to discuss the reflection and averaging mechanisms of spin-transfer between current-carrying electrons and the ferromagnetic layers of magnetic/non-magnetic heterostructures. The second part incorporates both effects into a matrix Boltzmann equation and reports numerical results for current polarization, spin accumulation, magnetoresistance, and spin-transfer torques for Co/Cu/Co multilayers. When possible, the results are compared quantitatively with relevant experiments.Comment: The following article has been submitted to J. Appl. Phys. After it is published, it will be found at http://ojps.aip.org/japo

Strong Electron Confinement By Stacking-fault Induced Fractional Steps on Ag(111) Surfaces

The electron reflection amplitude $R$ at stacking-fault (SF) induced fractional steps is determined for Ag(111) surface states using a low temperature scanning tunneling microscope. Unexpectedly, $R$ remains as high as $0.6 \sim 0.8$ as energy increases from 0 to 0.5 eV, which is in clear contrast to its rapidly decreasing behavior for monatomic (MA) steps [L. B{\"u}rgi et al., Phys. Rev. Lett. \textbf{81}, 5370 (1998)]. Tight-binding calculations based on {\em ab-initio} derived band structures confirm the experimental finding. Furthermore, the phase shifts at descending SF steps are found to be systematically larger than counterparts for ascending steps by $\approx 0.4 \pi$. These results indicate that the subsurface SF plane significantly contributes to the reflection of surface states