1,827 research outputs found
Absence of correlation between built-in electric dipole moment and quantum Stark effect in InAs/GaAs self-assembled quantum dots
We report significant deviations from the usual quadratic dependence of the
ground state interband transition energy on applied electric fields in
InAs/GaAs self-assembled quantum dots. In particular, we show that conventional
second-order perturbation theory fails to correctly describe the Stark shift
for electric field below kV/cm in high dots. Eight-band calculations demonstrate this effect is predominantly due to
the three-dimensional strain field distribution which for various dot shapes
and stoichiometric compositions drastically affects the hole ground state. Our
conclusions are supported by two independent experiments.Comment: 4 pages, 4 figure
Anomalous quantum confined Stark effects in stacked InAs/GaAs self-assembled quantum dots
Vertically stacked and coupled InAs/GaAs self-assembled quantum dots (SADs)
are predicted to exhibit a strong non-parabolic dependence of the interband
transition energy on the electric field, which is not encountered in single SAD
structures nor in other types of quantum structures. Our study based on an
eight-band strain-dependent Hamiltonian indicates that
this anomalous quantum confined Stark effect is caused by the three-dimensional
strain field distribution which influences drastically the hole states in the
stacked SAD structures.Comment: 4 pages, 4 figure
Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15: II. Kinematical Analysis and Dynamical Modeling
We analyze HST/STIS spectra (see Paper I) of the central region of the dense
globular cluster M15. We infer the velocities of 64 individual stars,
two-thirds of which have their velocity measured for the first time. This
triples the number of stars with measured velocities in the central 1 arcsec of
M15 and doubles the number in the central 2 arcsec. Combined with existing
ground-based data we obtain the radial profiles of the projected kinematical
quantities. The RMS velocity sigma_RMS rises to 14 km/s in the central few
arcsec, somewhat higher than the values of 10-12 km/s inferred previously from
ground-based data. To interpret the results we construct dynamical models based
on the Jeans equation, which imply that M15 must have a central concentration
of non-luminous material. If this is due to a single black hole, then its mass
is M_BH = (3.9 +/- 2.2) x 10^3 solar masses. This is consistent with the
relation between M_BH and sigma_RMS that has been established for galaxies.
Also, the existence of intermediate-mass black holes in globular clusters is
consistent with several scenarios for globular cluster evolution proposed in
the literature. Therefore, these results may have important implications for
our understanding of the evolution of globular clusters, the growth of black
holes, the connection between globular cluster and galaxy formation, and the
nature of the recently discovered `ultra-luminous' X-ray sources in nearby
galaxies. Instead of a single black hole, M15 could have a central
concentration of dark remnants (e.g., neutron stars) due to mass segregation.
However, the best-fitting Fokker-Planck models that have previously been
constructed for M15 do not predict a central mass concentration that is
sufficient to explain the observed kinematics.[ABRIDGED]Comment: 43 pages, LaTeX, with 14 PostScript figures. Astronomical Journal, in
press (Dec 2002). Please note that the results reported here are modified by
the Addendum available at astro-ph/0210158 (Astronomical Journal, in press,
Jan 2003). This second version submitted to astro-ph is identical to first,
with the exception of the preceeding remar
Electronic structure of strained InP/GaInP quantum dots
We calculate the electronic structure of nm scale InP islands embedded in
. The calculations are done in the envelope approximation
and include the effects of strain, piezoelectric polarization, and mixing among
6 valence bands. The electrons are confined within the entire island, while the
holes are confined to strain induced pockets. One pocket forms a ring at the
bottom of the island near the substrate interface, while the other is above the
island in the GaInP. The two sets of hole states are decoupled. Polarization
dependent dipole matrix elements are calculated for both types of hole states.Comment: Typographical error corrected in strain Hamiltonia
Theoretical interpretation of the experimental electronic structure of lens shaped, self-assembled InAs/GaAs quantum dots
We adopt an atomistic pseudopotential description of the electronic structure
of self-assembled, lens shaped InAs quantum dots within the ``linear
combination of bulk bands'' method. We present a detailed comparison with
experiment, including quantites such as the single particle electron and hole
energy level spacings, the excitonic band gap, the electron-electron, hole-hole
and electron hole Coulomb energies and the optical polarization anisotropy. We
find a generally good agreement, which is improved even further for a dot
composition where some Ga has diffused into the dots.Comment: 16 pages, 5 figures. Submitted to Physical Review
Tight-binding study of the influence of the strain on the electronic properties of InAs/GaAs quantum dots
We present an atomistic investigation of the influence of strain on the
electronic properties of quantum dots (QD's) within the empirical tight-binding (ETB) model with interactions up to 2nd nearest neighbors
and spin-orbit coupling. Results for the model system of capped pyramid-shaped
InAs QD's in GaAs, with supercells containing atoms are presented and
compared with previous empirical pseudopotential results. The good agreement
shows that ETB is a reliable alternative for an atomistic treatment. The strain
is incorporated through the atomistic valence force field model. The ETB
treatment allows for the effects of bond length and bond angle deviations from
the ideal InAs and GaAs zincblende structure to be selectively removed from the
electronic-structure calculation, giving quantitative information on the
importance of strain effects on the bound state energies and on the physical
origin of the spatial elongation of the wave functions. Effects of dot-dot
coupling have also been examined to determine the relative weight of both
strain field and wave function overlap.Comment: 22 pages, 7 figures, submitted to Phys. Rev. B (in press) In the
latest version, added Figs. 3 and 4, modified Fig. 5, Tables I and II,.and
added new reference
White Dwarfs in Globular Clusters: HST Observations of M4
Using WFPC2 on the Hubble Space Telescope, we have isolated a sample of 258
white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial
distances from the cluster center were observed and sizeable WD populations
were found in all three. The location of these WDs in the color-magnitude
diagram, their mean mass of 0.51()M, and their luminosity
function confirm basic tenets of stellar evolution theory and support the
results from current WD cooling theory. The WDs are used to extend the cluster
main-sequence mass function upward to stars that have already completed their
nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and
found to be at most a few percent of all the main-sequence stars. The most
ancient WDs found are about 9 Gyr old, a level which is set solely by the
photometric limits of our data. Even though this is less than the age of M4, we
discuss how these cooling WDs can eventually be used to check the turnoff ages
of globular clusters and hence constrain the age of the Universe.Comment: 46 pages, latex, no figures included, figures available at
ftp://ftp.astro.ubc.ca/pub/richer/wdfig.uu size 2.7Mb. To be published in the
Astrophysical Journa
Wavefunction and level statistics of random two dimensional gauge fields
Level and wavefunction statistics have been studied for two dimensional
clusters of the square lattice in the presence of random magnetic fluxes.
Fluxes traversing lattice plaquettes are distributed uniformly between - (1/2)
Phi_0 and (1/2) Phi_0 with Phi_0 the flux quantum. All considered statistics
start close to the corresponding Wigner-Dyson distribution for small system
sizes and monotonically move towards Poisson statistics as the cluster size
increases. Scaling is quite rapid for states close to the band edges but really
difficult to observe for states well within the band. Localization properties
are discussed considering two different scenarios. Experimental measurement of
one of the considered statistics --wavefunction statistics seems the most
promising one-- could discern between both possibilities. A real version of the
previous model, i.e., a system that is invariant under time reversal, has been
studied concurrently to get coincidences and differences with the Hermitian
model.Comment: 12 twocolumnn pages in revtex style, 17 postscript figures, to be
published in PRB, send comments to [email protected]
Density of states for the -flux state with bipartite real random hopping only: A weak disorder approach
Gade [R. Gade, Nucl. Phys. B \textbf{398}, 499 (1993)] has shown that the
local density of states for a particle hopping on a two-dimensional bipartite
lattice in the presence of weak disorder and in the absence of time-reversal
symmetry(chiral unitary universality class) is anomalous in the vicinity of the
band center whenever the disorder preserves the sublattice
symmetry. More precisely, using a nonlinear-sigma-model that encodes the
sublattice (chiral) symmetry and the absence of time-reversal symmetry she
argues that the disorder average local density of states diverges as
with some non-universal
positive constant and a universal exponent. Her analysis has been
extended to the case when time-reversal symmetry is present (chiral orthogonal
universality class) for which the same exponent was predicted.
Motrunich \textit{et al.} [O. Motrunich, K. Damle, and D. A. Huse, Phys. Rev. B
\textbf{65}, 064206 (2001)] have argued that the exponent does not
apply to the typical density of states in the chiral orthogonal universality
class. They predict that instead. We confirm the analysis of
Motrunich \textit{et al.} within a field theory for two flavors of Dirac
fermions subjected to two types of weak uncorrelated random potentials: a
purely imaginary vector potential and a complex valued mass potential. This
model is believed to belong to the chiral orthogonal universality class. Our
calculation relies in an essential way on the existence of infinitely many
local composite operators with negative anomalous scaling dimensions.Comment: 30 pages, final version published in PR
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