5,884 research outputs found
A Qualitative Examination of Graduate Advising Relationships:The Advisee Perspective
Sixteen 3rd-year counseling psychology doctoral students were interviewed about their relationships with their graduate advisors. Of those students, 10 were satisfied and 6 were unsatisfied with their advising relationships. Satisfied and unsatisfied students differed on several aspects of the advising relationship, including (a) the ability to choose their advisors, (b) the frequency of meetings with their advisors, (c) the benefits and costs associated with their advising relationships, and (d) how conflict was dealt with in the advising relationship. Furthermore, all of the satisfied students reported that their advising relationships became more positive over time, whereas many of the unsatisfied students reported that their advising relationships got worse (e.g., became more distant) over time
CMB Likelihood Functions for Beginners and Experts
Although the broad outlines of the appropriate pipeline for cosmological
likelihood analysis with CMB data has been known for several years, only
recently have we had to contend with the full, large-scale, computationally
challenging problem involving both highly-correlated noise and extremely large
datasets (). In this talk we concentrate on the beginning and end of
this process. First, we discuss estimating the noise covariance from the data
itself in a rigorous and unbiased way; this is essentially an iterated
minimum-variance mapmaking approach. We also discuss the unbiased determination
of cosmological parameters from estimates of the power spectrum or experimental
bandpowers.Comment: Long-delayed submission. In AIP Conference Proceedings "3K Cosmology"
held in Rome, Oct 5-10, 1998, edited by Luciano Maiani, Francesco Melchiorri
and Nicola Vittorio, 343-347, New York, American Institute of Physics 199
EUV and X-ray spectroheliograph study
The results of a program directed toward the definition of an EUV and X-ray spectroheliograph which has significant performance and operational improvements over the OSO-7 instrument are documented. The program investigated methods of implementing selected changes and incorporated the results of the study into a set of drawings which defines the new instrument. The EUV detector performance degradation observed during the OSO-7 mission was investigated and the most probable cause of the degradation identified
Cosmic Microwave Background Anisotropy Window Functions Revisited
The primary results of most observations of cosmic microwave background (CMB)
anisotropy are estimates of the angular power spectrum averaged through some
broad band, called band-powers. These estimates are in turn what are used to
produce constraints on cosmological parameters due to all CMB observations.
Essential to this estimation of cosmological parameters is the calculation of
the expected band-power for a given experiment, given a theoretical power
spectrum. Here we derive the "band power" window function which should be used
for this calculation, and point out that it is not equivalent to the window
function used to calculate the variance. This important distinction has been
absent from much of the literature: the variance window function is often used
as the band-power window function. We discuss the validity of this assumed
equivalence, the role of window functions for experiments that constrain the
power in {\it multiple} bands, and summarize a prescription for reporting
experimental results. The analysis methods detailed here are applied in a
companion paper to three years of data from the Medium Scale Anisotropy
Measurement.Comment: 5 pages, 1 included .eps figure, PRD in press---final published
versio
Theory of excitons in cubic III-V semiconductor GaAs, InAs and GaN quantum dots: fine structure and spin relaxation
Exciton fine structures in cubic III-V semiconductor GaAs, InAs and GaN
quantum dots are investigated systematically and the exciton spin relaxation in
GaN quantum dots is calculated by first setting up the effective exciton
Hamiltonian. The electron-hole exchange interaction Hamiltonian, which consists
of the long- and short-range parts, is derived within the effective-mass
approximation by taking into account the conduction, heavy- and light-hole
bands, and especially the split-off band. The scheme applied in this work
allows the description of excitons in both the strong and weak confinement
regimes. The importance of treating the direct electron-hole Coulomb
interaction unperturbatively is demonstrated. We show in our calculation that
the light-hole and split-off bands are negligible when considering the exciton
fine structure, even for GaN quantum dots, and the short-range exchange
interaction is irrelevant when considering the optically active doublet
splitting. We point out that the long-range exchange interaction, which is
neglected in many previous works, contributes to the energy splitting between
the bright and dark states, together with the short-range exchange interaction.
Strong dependence of the optically active doublet splitting on the anisotropy
of dot shape is reported. Large doublet splittings up to 600 eV, and even
up to several meV for small dot size with large anisotropy, is shown in GaN
quantum dots. The spin relaxation between the lowest two optically active
exciton states in GaN quantum dots is calculated, showing a strong dependence
on the dot anisotropy. Long exciton spin relaxation time is reported in GaN
quantum dots. These findings are in good agreement with the experimental
results.Comment: 22+ pages, 16 figures, several typos in the published paper are
corrected in re
Analysis of the exciton-exciton interaction in semiconductor quantum wells
The exciton-exciton interaction is investigated for quasi-two-dimensional
quantum structures. A bosonization scheme is applied including the full spin
structure. For generating the effective interaction potentials, the
Hartree-Fock and Heitler-London approaches are improved by a full two-exciton
calculation which includes the van der Waals effect. With these potentials the
biexciton formation in bilayer systems is investigated. For coupled quantum
wells the two-body scattering matrix is calculated and employed to give a
modified relation between exciton density and blue shift. Such a relation is of
central importance for gauging exciton densities in experiments which pave the
way toward Bose-Einstein condensation of excitons
Theory of exciton fine structure in semiconductor quantum dots: quantum dot anisotropy and lateral electric field
Theory of exciton fine structure in semiconductor quantum dots and its
dependence on quantum dot anisotropy and external lateral electric field is
presented. The effective exciton Hamiltonian including long range electron-hole
exchange interaction is derived within the k*p effective mass approximation
(EMA). The exchange matrix elements of the Hamiltonian are expressed explicitly
in terms of electron and hole envelope functions. The matrix element
responsible for the "bright" exciton splitting is identified and analyzed. An
excitonic fine structure for a model quantum dot with quasi- two-dimensional
anisotropic harmonic oscillator (2DLAHO) confining potential is analyzed as a
function of the shape anisotropy, size and applied lateral electric field
Constraining Large Scale Structure Theories with the Cosmic Background Radiation
We review the relevant 10+ parameters associated with inflation and matter
content; the relation between LSS and primary and secondary CMB anisotropy
probes; COBE constraints on energy injection; current anisotropy band-powers
which strongly support the gravitational instability theory and suggest the
universe could not have reionized too early. We use Bayesian analysis methods
to determine what current CMB and CMB+LSS data imply for inflation-based
Gaussian fluctuations in tilted CDM, hCDM and oCDM model
sequences with age 11-15 Gyr, consisting of mixtures of baryons, cold (and
possibly hot) dark matter, vacuum energy, and curvature energy in open
cosmologies. For example, we find the slope of the initial spectrum is within
about 5% of the (preferred) scale invariant form when just the CMB data is
used, and for CDM when LSS data is combined with CMB; with both, a
nonzero value of is strongly preferred ( for a 13
Gyr sequence, similar to the value from SNIa). The CDM sequence prefers
, but is overall much less likely than the flat
sequence with CMB+LSS. We also review the rosy forecasts
of angular power spectra and parameter estimates from future balloon and
satellite experiments when foreground and systematic effects are ignored.Comment: 20 pages, LaTeX, 5 figures, 2 tables, uses rspublic.sty To appear in
Philosophical Transactions of the Royal Society of London A, 1998.
"Discussion Meeting on Large Scale Structure in the Universe," Royal Society,
London, March 1998. Text and colour figures also available at
ftp://ftp.cita.utoronto.ca/bond/roysoc9
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Revitalizing a Mature Oil Play: Strategies for Finding and Producing Unrecovered Oil in Frio Fluvial-Deltaic Reservoirs of South Texas
Advanced reservoir characterization techniques are being applied to selected reservoirs in the Frio Fluvial-Deltaic Sandstone (Vicksburg Fault zone) oil play of South Texas to maximize the productivity of resources in this mature oil play. This mature play has already produced nearly 1 billion barrels (Bbbl) of oil, yet still contains about 1.6 Bbbl of unrecovered mobile oil and nearly the same amount of residual oil resources. More than half of the reservoirs in this depositionally complex play have already been abandoned, and large volumes of oil may remain unproduced unless advanced characterization techniques are applied to define untapped, incompletely drained, and new pool reservoirs as suitable targets for near-term recovery methods. Primary technical objectives of this project are to develop interwell-scale geological facies models of Frio fluvial-deltaic reservoirs and combine them with engineering assessments and geophysical evaluations in order to characterize the Frio fluvial-deltaic reservoir architecture and flow unit boundaries and to determine the controls that these characteristics exert on the location and volume of unrecovered mobile and residual oil. These results will lead directly to the identification of specific opportunities to exploit these heterogeneous reservoirs for incremental recovery by recompletion and strategic infill drilling.Bureau of Economic Geolog
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