240 research outputs found
The Equilibrium Shape of Quantum Dots
The formation of dislocation-free three-dimensional islands during the
heteroepitaxial growth of lattice-mismatched materials has been observed
experimentally for several material systems. The equilibrium shape of the
islands is governed by the competition between the surface energy and the
elastic relaxation energy of the islands as compared to the uniform strained
film. As an exemplification we consider the experimentally intensively
investigated growth of InAs quantum dots on a GaAs(001) substrate, deriving the
equilibrium shape as a function of island volume. For this purpose InAs surface
energies have been calculated within density-functional theory, and a continuum
approach has been applied to compute the elastic relaxation energies.Comment: 10 pages, 4 figures. Submitted to Nuovo Cimento (November 27, 1996)
Surface-State Localization at Adatoms
Low-temperature scanning tunneling spectroscopy of magnetic and non-magnetic
metal atoms on Ag(111) and on Cu(111) surfaces reveals the existence of a
common electronic resonance at an energy below the binding energies of the
surface states. Using an extended Newns-Anderson model, we assign this
resonance to an adsorbate-induced bound state, split off from the bottom of the
surface-state band, and broadened by the interaction with bulk states. A
lineshape analysis of the bound state indicates that native adatoms decrease
the surface-state lifetime, while a cobalt adatom causes no significant change.Comment: 4 pages, 4 figure
What Elephant? The Challenge of Political: Pluralism in the Academy
Does displaying one’s political affiliation in the workplace present a moral dilemma for educators striving to be true pluralists? Two recent graduates of the Higher Education & Student Affairs Administration program at The University of Vermont pose challenging questions to practitioners regarding the intersection of pluralism and political affiliation, highlighting the oft-assumed understanding that to be pluralistic one must be liberal minded. Higher education faculty and administrators, who are overwhelmingly self-identified liberals, risk marginalizing conservative students and colleagues and thwarting opportunities for dialogue around the complexities of the political climate in the academy and beyond. This article examines the paradox of political pluralism and considers the role of advocacy in creating a safe environment for liberals and conservatives alike
Highly site-specific H2 adsorption on vicinal Si(001) surfaces
Experimental and theoretical results for the dissociative adsorption of H_2
on vicinal Si(001) surfaces are presented. Using optical second-harmonic
generation, sticking probabilities at the step sites are found to exceed those
on the terraces by up to six orders of magnitude. Density functional theory
calculations indicate the presence of direct adsorption pathways for
monohydride formation but with a dramatically lowered barrier for step
adsorption due to an efficient rehybridization of dangling orbitals.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Lett. (1998). Other
related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Evolution of unoccupied resonance during the synthesis of a silver dimer on Ag(111)
Silver dimers were fabricated on Ag(111) by single-atom manipulation using
the tip of a cryogenic scanning tunnelling microscope. An unoccupied electronic
resonance was observed to shift toward the Fermi level with decreasing
atom-atom distance as monitored by spatially resolved scanning tunnelling
spectroscopy. Density functional calculations were used to analyse the
experimental observations and revealed that the coupling between the adsorbed
atoms is predominantly direct rather than indirect via the Ag(111) substrate.Comment: 9 pages, 3 figure
The influence of surface stress on the equilibrium shape of strained quantum dots
The equilibrium shapes of InAs quantum dots (i.e., dislocation-free, strained
islands with sizes >= 10,000 atoms) grown on a GaAs (001) substrate are studied
using a hybrid approach which combines density functional theory (DFT)
calculations of microscopic parameters, surface energies, and surface stresses
with elasticity theory for the long-range strain fields and strain relaxations.
In particular we report DFT calculations of the surface stresses and analyze
the influence of the strain on the surface energies of the various facets of
the quantum dot. The surface stresses have been neglected in previous studies.
Furthermore, the influence of edge energies on the island shapes is briefly
discussed. From the knowledge of the equilibrium shape of these islands, we
address the question whether experimentally observed quantum dots correspond to
thermal equilibrium structures or if they are a result of the growth kinetics.Comment: 7 pages, 8 figures, submitted to Phys. Rev. B (February 2, 1998).
Other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
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