5,644 research outputs found
Dynamics of Electrons in Graded Semiconductors
I present a theory of electron dynamics in semiconductors with slowly varying
composition. I show that the frequency-dependent conductivity, required for the
description of transport and optical properties, can be obtained from a
knowledge of the band structures and momentum matrix elements of homogeneous
semiconductor alloys. New sum rules for the electronic oscillator strengths,
which apply within a given energy band or between any two bands, are derived,
and a general expression for the width of the intraband absorption peak is
given. Finally, the low-frequency dynamics is discussed, and a correspondence
with the semiclassical motion is established.Comment: 4 pages, Revte
Sources Of Student Engagement In Introductory Physics For Life Sciences
We explore the sources of student engagement with curricular content in an Introductory Physics for Life Science (IPLS) course at Swarthmore College. Do IPLS students find some life-science contexts more interesting than others, and, if so, what are the sources of these differences? We draw on three sources of student data to answer this question: (1) quantitative survey data illustrating how interested students were in particular contexts from the curriculum, (2) qualitative survey data in which students describe the source of their interest in these particular contexts, and (3) interview data in which students reflect on the contexts that were and were not of interest to them. We find that examples that make interdisciplinary connections with students’ other coursework in biology and chemistry, and examples that make connections to what students perceive to be the “real world,” are particularly effective at fostering interest. More generally, students describe being deeply engaged with contexts that foster a sense of coherence or have personal meaning to them. We identify various “engagement pathways” by which different life-science students engage with IPLS content, and suggest that a curriculum needs to be flexible enough to facilitate these different pathways
Adaptive Density Estimation on the Circle by Nearly-Tight Frames
This work is concerned with the study of asymptotic properties of
nonparametric density estimates in the framework of circular data. The
estimation procedure here applied is based on wavelet thresholding methods: the
wavelets used are the so-called Mexican needlets, which describe a nearly-tight
frame on the circle. We study the asymptotic behaviour of the -risk
function for these estimates, in particular its adaptivity, proving that its
rate of convergence is nearly optimal.Comment: 30 pages, 3 figure
Discovery of a Large-scale Wall in the Direction of Abell 22
We report on the discovery of a large-scale wall in the direction of Abell
22. Using photometric and spectroscopic data from the Las Campanas Observatory
and Anglo-Australian Telescope Rich Cluster Survey, Abell 22 is found to
exhibit a highly unusual and striking redshift distribution. We show that Abell
22 exhibits a foreground wall-like structure by examining the galaxy
distributions in both redshift space and on the colour-magnitude plane. A
search for other galaxies and clusters in the nearby region using the 2dF
Galaxy Redshift Survey database suggests that the wall-like structure is a
significant large-scale, non-virialized filament which runs between two other
Abell clusters either side of Abell 22. The filament stretches over at least
>40 Mpc in length and 10 Mpc in width at the redshift of Abell 22.Comment: 6 pages, 4 figures, accepted for publication in MNRAS letter
An excess of damped Lyman alpha galaxies near QSOs
We present a sample of 33 damped Lyman alpha systems (DLAs) discovered in the
Sloan Digital Sky Survey (SDSS) whose absorption redshifts (z_abs) are within
6000 km/s of the QSO's systemic redshift (z_sys). Our sample is based on 731
2.5 < z_sys < 4.5 non-broad-absorption-line (non-BAL) QSOs from Data Release 3
(DR3) of the SDSS. We estimate that our search is ~100 % complete for absorbers
with N(HI) >= 2e20 cm^-2. The derived number density of DLAs per unit redshift,
n(z), within v < 6000 km/s is higher (3.5 sigma significance) by almost a
factor of 2 than that of intervening absorbers observed in the SDSS DR3, i.e.
there is evidence for an overdensity of galaxies near the QSOs. This provides a
physical motivation for excluding DLAs at small velocity separations in surveys
of intervening 'field' DLAs. In addition, we find that the overdensity of
proximate DLAs is independent of the radio-loudness of the QSO, consistent with
the environments of radio-loud and radio-quiet QSOs being similar.Comment: Accepted for publication in MNRAS (13 pages, 6 figures
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