2,182 research outputs found
Coulomb Drag in the Exciton Regime in Electron-Hole Bilayers
We report electrical transport measurements on GaAs/AlGaAs based
electron-hole bilayers. These systems are expected to make a transition from a
pair of weakly coupled two-dimensional systems to a strongly coupled exciton
system as the barrier between the layers is reduced. Once excitons form,
phenomena such as Bose-Einstein condensation of excitons could be observed. In
our devices, electrons and holes are confined in double quantum wells, and
carriers in the devices are induced with top and bottom gates leading to
variable density in each layer. Separate contact to each layer allows Coulomb
drag transport measurements where current is driven in one layer while voltage
is measured in the other. Coulomb drag is sensitive to interlayer coupling and
has been predicted to provide a strong signature of exciton condensation. Drag
measurement on EHBLs with a 30 nm barrier are consistent with drag between two
weakly coupled 2D Fermi systems where the drag decreases as the temperature is
reduced. When the barrier is reduced to 20 nm, we observe a consistent increase
in the drag resistance as the temperature is reduced. These results indicate
the onset of a much stronger coupling between the electrons and holes which
leads to exciton formation and possibly phenomena related to exciton
condensation.Comment: 12 pages, 3 figure
The Onset of Anisotropic Transport of Two-Dimensional Electrons in High Landau Levels: An Isotropic-to-Nematic Liquid Crystal Phase Transition?
The recently discovered anisotropy of the longitudinal resistance of
two-dimensional electrons near half filling of high Landau levels is found to
persist to much higher temperatures T when a large in-plane magnetic field B||
is applied. Under these conditions we find that the longitudinal resistivity
scales quasi-linearly with B||/T. These observations support the notion that
the onset of anisotropy at B||=0 does not reflect the spontaneous development
of charge density modulations but may instead signal an isotropic-to-nematic
liquid crystal phase transition.Comment: 5 pages, 4 figure
Final design proposal: Delta Group-Nood Rider 821(tm)
The Nood Rider 821 (trademark) twin-engine, prop passenger aircraft is described. It is argued that the aircraft is very economical to operate and maintain, offering competitive advantages in the air travel marketplace. The aircraft was designed to operate in 'Aeroworld', a fictional world where the passengers are ping pong balls and the distances between cites are on the order of thousands of feet
Fluorescent Ly-alpha emission from the high-redshift intergalactic medium
We combine a high-resolution hydro-simulation of the LambdaCDM cosmology with
two radiative transfer schemes (for continuum and line radiation) to predict
the properties, spectra and spatial distribution of fluorescent Ly-alpha
emission at z~3. We focus on line radiation produced by recombinations in the
dense intergalactic medium ionized by UV photons. In particular, we consider
both a uniform background and the case where gas clouds are illuminated by a
nearby quasar. We find that the emission from optically thick regions is
substantially less than predicted from the widely used static, plane-parallel
model. The effects induced by a realistic velocity field and by the complex
geometric structure of the emitting regions are discussed in detail. We make
predictions for the expected brightness and size distributions of the
fluorescent sources.Our results account for recent null detections and can be
used to plan new observational campaigns both in the field (to measure the
intensity of the diffuse UV background) and in the proximity of bright quasars
(to understand the origin of high colum-density absorbers).Comment: 15 pages, 12 figures, accepted for publication in Ap
Anisotropic States of Two-Dimensional Electron Systems in High Landau Levels: Effect of an In-Plane Magnetic Field
We report the observation of an acute sensitivity of the anisotropic
longitudinal resistivity of two-dimensional electron systems in half-filled
high Landau levels to the magnitude and orientation of an in-plane magnetic
field. In the third and higher Landau levels, at filling fractions nu=9/2,
11/2, etc., the in-plane field can lead to a striking interchange of the "hard"
and "easy" transport directions. In the second Landau level the normally
isotropic resistivity and the weak nu=5/2 quantized Hall state are destroyed by
a large in-plane field and the transport becomes highly anisotropic.Comment: 5 pages, 4 figures, minor errors correcte
Literacy practices of primary education children in Andalusia (Spain): a family-based perspective
Primary school children develop literacy practices in various domains and situations in everyday life.
This study focused on the analysis of literacy practices of children aged 8–12 years from the perspec-
tive of their families. 1,843 families participated in the non-experimental explanatory study. The
children in these families speak Spanish as a first language and are schooled in this language. The
instrument used was a self-report questionnaire about children’s home-literacy practices. The data
obtained were analysed using categorical principal components analysis (CATPCA) and analysis of
variance (ANOVA). The results show the complex relationship between literacy practices developed
by children in the domains of home and school and the limited development of a literacy-promoting
‘third space’. In conclusion, the families in our study had limited awareness of their role as literacy-
promoting agents and thought of literacy learning as restricted to formal or academic spaces
Finite Temperature Density Instability at High Landau Level Occupancy
We study here the onset of charge density wave instabilities in quantum Hall
systems at finite temperature for Landau level filling . Specific
emphasis is placed on the role of disorder as well as an in-plane magnetic
field. Beyond some critical value, disorder is observed to suppress the charge
density wave melting temperature to zero. In addition, we find that a
transition from perpendicular to parallel stripes (relative to the in-plane
magnetic field) exists when the electron gas thickness exceeds \AA.
The perpendicular alignment of the stripes is in agreement with the
experimental finding that the easy conduction direction is perpendicular to the
in-plane field.Comment: 4 pages, 2 eps figures. We show explicitly that a transition from
perpendicular to parallel stripes (relative to the in-plane magnetic field)
exists when the electron gas thickness exceeds \AA. The
perpendicular alignment of the stripes is in agreement with the experimental
finding that the easy conduction direction is perpendicular to the in-plane
fiel
- …