3,763 research outputs found
Quantum wires from coupled InAs/GaAs strained quantum dots
The electronic structure of an infinite 1D array of vertically coupled
InAs/GaAs strained quantum dots is calculated using an eight-band
strain-dependent k-dot-p Hamiltonian. The coupled dots form a unique quantum
wire structure in which the miniband widths and effective masses are controlled
by the distance between the islands, d. The miniband structure is calculated as
a function of d, and it is shown that for d>4 nm the miniband is narrower than
the optical phonon energy, while the gap between the first and second minibands
is greater than the optical phonon energy. This leads to decreased optical
phonon scattering, providing improved quantum wire behavior at high
temperatures. These miniband properties are also ideal for Bloch oscillation.Comment: 5 pages revtex, epsf, 8 postscript figure
Electrical manipulation of an electronic two-state system in Ge/Si quantum dots
We calculate that the electron states of strained self-assembled Ge/Si
quantum dots provide a convenient two-state system for electrical control. An
electronic state localized at the apex of the quantum dot is nearly degenerate
with a state localized at the base of the quantum dot. Small electric fields
shift the electronic ground state from apex-localized to base-localized, which
permits sensitive tuning of the electronic, optical and magnetic properties of
the dot. As one example, we describe how spin-spin coupling between two Ge/Si
dots can be controlled very sensitively by shifting the individual dot's
electronic ground state between apex and base
Arkansas Soybean Performance Tests 2011
Soybean variety and strain performance tests are conducted each year in Arkansas by the University of Arkansas System Division of Agriculture Arkansas Crop Variety Improvement Program. The tests provide information to companies developing varieties and/or marketing seed within the state, and aid the Arkansas Cooperative Extension Service in formulating variety recommendations for soybean producers
Arkansas Soybean Performance Tests 2012
Soybean variety and strain performance tests are conducted each year in Arkansas by the University of Arkansas System Division of Agriculture Arkansas Crop Variety Improvement Program. The tests provide information to companies developing varieties and/or marketing seed within the state, and aid the Arkansas Cooperative Extension Service in formulating variety recommendations for soybean producers
Growth and optical properties of self-assembled InGaAs Quantum Posts
We demonstrate a method to grow height controlled, dislocation-free InGaAs
quantum posts (QPs) on GaAs by molecular beam epitaxy (MBE) which is confirmed
by structural investigations. The optical properties are compared to realistic
8-band k.p calculations of the electronic structure which fully account for
strain and the structural properties of the QP. Using QPs embedded in n-i-p
junctions we find wide range tunability of the interband spectrum and giant
static dipole moments.Comment: Proccedings paper for MSS-13, 7 pages, 4 figure
g-Factors and diamagnetic coefficients of electrons, holes and excitons in InAs/InP quantum dots
The electron, hole, and exciton g-factors and diamagnetic coefficients have
been calculated using envelope-function theory for cylindrical InAs/InP quantum
dots in the presence of a magnetic field parallel to the dot symmetry axis. A
clear connection is established between the electron g-factor and the amplitude
of the those valence-state envelope functions which possess non-zero orbital
momentum associated with the envelope function. The dependence of the exciton
diamagnetic coefficients on the quantum dot height is found to correlate with
the energy dependence of the effective mass. Calculated exciton g-factor and
diamagnetic coefficients, constructed from the values associated with the
electron and hole constituents of the exciton, match experimental data well,
however including the Coulomb interaction between the electron and hole states
improves the agreement. Remote-band contributions to the valence-band
electronic structure, included perturbatively, reduce the agreement between
theory and experiment.Comment: 12 pages, 7 figure
Structure of the Draco Dwarf Spheroidal Galaxy
This article studies the structure of the Draco dwarf spheroidal galaxy with
an emphasis on the question of whether the spatial distribution of its stars
has been affected by the tidal interaction with the Milky Way, using R- and
V-band CCD photometry for eleven fields. The article reports coordinates for
the center, a position angle of the major axis, and the ellipticity. It also
reports the results of searches for asymmetries in the structure of Draco.
These results, and searches for a ``break'' in the radial profile and for the
presence of principal sequences of Draco in a color-magnitude diagram for
regions more than 50 arcmin from the center, yield no evidence that tidal
forces from the Milky Way have affected the structure of Draco.Comment: 25 pages, 11 figures, 3 tables. Accepted for publication in A
Eight-band calculations of strained InAs/GaAs quantum dots compared with one, four, and six-band approximations
The electronic structure of pyramidal shaped InAs/GaAs quantum dots is
calculated using an eight-band strain dependent Hamiltonian. The
influence of strain on band energies and the conduction-band effective mass are
examined. Single particle bound-state energies and exciton binding energies are
computed as functions of island size. The eight-band results are compared with
those for one, four and six bands, and with results from a one-band
approximation in which m(r) is determined by the local value of the strain. The
eight-band model predicts a lower ground state energy and a larger number of
excited states than the other approximations.Comment: 8 pages, 7 figures, revtex, eps
MoleculeâInduced Radical Formation (MIRF) ReactionsâA Reappraisal
Radical chain reactions are commonly initiated through the thermal or photochemical activation of purposeâbuilt initiators, through photochemical activation of substrates, or through wellâdesigned redox processes. Where radicals come from in the absence of these initiation strategies is much less obvious and are often assumed to derive from unknown impurities. In this situation, moleculeâinduced radical formation (MIRF) reactions should be considered as wellâdefined alternative initiation modes. In the most general definition of MIRF reactions, two closedâshell molecules react to give a radical pair or biradical. The exact nature of this transformation depends on the Ïâ or Ïâbonds involved in the MIRF process, and this Minireview specifically focuses on reactions that transform two Ïâbonds into two radicals and a closedâshell product molecule
Classroom assessment and education: challenging the assumptions of socialisation and instrumentality
The opportunity offered by the Umea Symposium to probe the intersection of quality and assessment immediately brings into focus a wider issue â that of the quality of education which assessment aspires to support. Prompted by recent research into formative assessment in Scottish primary school contexts, the paper explores how formative assessment has become associated with an overly benign understanding of learning which misrecognises the possibility of undesirable learning and does not seem to address the inherently political nature of education. Having illuminated the potential inequities of formative assessment practices, the paper then asks what role formative assessment might play to support an understanding of education that is not simply about the transmission of traditional social norms, but also aspires to illuminate their social construction and their political nature
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