596 research outputs found
Analysis of surface waves generated on subwavelength-structured silver films
Using transmission electron microscopy (TEM) to analyse the physical-chemical
surface properties of subwavlength structured silver films and
finite-difference time-domain (FDTD) numerical simulations of the optical
response of these structures to plane-wave excitation, we report on the origin
and nature of the persistent surface waves generated by a single slit-groove
motif and recently measured by far-field optical interferometry. The surface
analysis shows that the silver films are free of detectable oxide or sulfide
contaminants, and the numerical simulations show very good agreement with the
results previously reported.Comment: 9 Figure
Efficiency in nanostructured thermionic and thermoelectric devices
Advances in solid-state device design now allow the spectrum of transmitted
electrons in thermionic and thermoelectric devices to be engineered in ways
that were not previously possible. Here we show that the shape of the electron
energy spectrum in these devices has a significant impact on their performance.
We distinguish between traditional thermionic devices where electron momentum
is filtered in the direction of transport only and a second type, in which the
electron filtering occurs according to total electron momentum. Such 'total
momentum filtered' kr thermionic devices could potentially be implemented in,
for example, quantum dot superlattices. It is shown that whilst total momentum
filtered thermionic devices may achieve efficiency equal to the Carnot value,
traditional thermionic devices are limited to efficiency below this. Our second
main result is that the electronic efficiency of a device is not only improved
by reducing the width of the transmission filter as has previously been shown,
but also strongly depends on whether the transmission probability rises sharply
from zero to full transmission. The benefit of increasing efficiency through a
sharply rising transmission probability is that it can be achieved without
sacrificing device power, in contrast to the use of a narrow transmission
filter which can greatly reduce power. We show that devices which have a
sharply-rising transmission probability significantly outperform those which do
not and it is shown such transmission probabilities may be achieved with
practical single and multibarrier devices. Finally, we comment on the
implications of the effect the shape of the electron energy spectrum on the
efficiency of thermoelectric devices.Comment: 11 pages, 15 figure
Measuring Temperature Gradients over Nanometer Length Scales
When a quantum dot is subjected to a thermal gradient, the temperature of
electrons entering the dot can be determined from the dot's thermocurrent if
the conductance spectrum and background temperature are known. We demonstrate
this technique by measuring the temperature difference across a 15 nm quantum
dot embedded in a nanowire. This technique can be used when the dot's energy
states are separated by many kT and will enable future quantitative
investigations of electron-phonon interaction, nonlinear thermoelectric
effects, and the effciency of thermoelectric energy conversion in quantum dots.Comment: 6 pages, 5 figure
Hard X-ray Emission Associated with White Dwarfs
We have used the WGACAT to search for hard X-ray sources associated with
white dwarfs (WDs) from the catalog of McCook & Sion (1999). We find 17 X-ray
sources coincident with WDs showing significant hard X-ray emission at energies
>0.5 keV. Twelve of these WDs are in known binary systems, in two of which the
accretion of the close companion's material onto the white dwarf produces the
hard X-ray emission, and in the other ten of which the late-type companions'
coronal activity emits hard X-rays. One WD is projected near an AGN which is
responsible for the hard X-ray emission. The remaining four WDs and two
additional white dwarfs with hard X-ray emission appear single. The lack of
near-IR excess from the apparently single WDs suggests that either X-ray
observations are more effective than near-IR photometry in diagnosing faint
companions or a different emission mechanism is needed. It is intriguing that
50% of the six apparently single WDs with hard X-ray emission are among the
hottest WDs. We have compared X-ray properties of 11 hot WDs with different
spectral types, and conclude that stellar pulsation and fast stellar winds are
not likely the origin of the hard X-ray emission, but a leakage of the
high-energy Wien tail of emission from deep in the stellar atmosphere remains a
tantalizing source of hard X-ray emission from hot DO and DQZO WDs. (This
abstract is an abridged version.)Comment: 35 pages, 8 figures, 4 tables, accepted for publication in AJ, April
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Merit, Tenure, and Bureaucratic Behavior: Evidence From a Conjoint Experiment in the Dominican Republic
Bureaucratic behavior in developing countries remains poorly understood. Why do some
public servants – yet not others – work hard to deliver public services, misuse state
resources, and/or participate in electoral mobilization? A classic answer comes from Weber:
bureaucratic structures shift behavior towards integrity, neutrality, and commitment to
public service. Our paper conducts the first survey experimental test of the effects of
bureaucratic structures. It does so through a conjoint experiment with public servants in the
Dominican Republic. Looking at merit examinations and job stability, we find that Weber
was right – but only partially. Recruitment by examination curbs corruption and political
services by bureaucrats, while enhancing work motivation. Job stability, by contrast, only
decreases political services: tenured bureaucrats are less likely to participate in electoral
mobilization. Examinations thus enhance the quality of bureaucracy (motivation and lower
corruption) and democracy (electoral competition); job stability only enhances the quality
of democracy
Effect of Thermoelectric Cooling in Nanoscale Junctions
We propose a thermoelectric cooling device based on an atomic-sized junction.
Using first-principles approaches, we investigate the working conditions and
the coefficient of performance (COP) of an atomic-scale electronic refrigerator
where the effects of phonon's thermal current and local heating are included.
It is observed that the functioning of the thermoelectric nano-refrigerator is
restricted to a narrow range of driving voltages. Compared with the bulk
thermoelectric system with the overwhelmingly irreversible Joule heating, the
4-Al atomic refrigerator has a higher efficiency than a bulk thermoelectric
refrigerator with the same due to suppressed local heating via the
quasi-ballistic electron transport and small driving voltages. Quantum nature
due to the size minimization offered by atomic-level control of properties
facilitates electron cooling beyond the expectation of the conventional
thermoelectric device theory.Comment: 8 figure
Особенности и закономерности изменения восстановлености углей башкирского яруса Западного Донбасса
В статье приведена детальная петрографическая характеристика углей башкирского яруса Западного Донбасса. Проведена классификация по восстановлености в соответствии с петрографическими типами. Установлены стратиграфические и площадные закономерности изменения степени восстановлености.У статті наведена детальна петрографічна характеристика вугілля башкирського ярусу Західного Донбасу. Проведена класифікація відновленості, згідно з петрографічними типами. Встановлені стратиграфічні та просторові закономірності зміни ступеню відновленості.The article gives detailed petrographic characteristics of coal of Bashkirian formation of Western Donbas. The classifications for recovery in accordance with petrographic types are given. The stratigraphic and areal patterns of change in the degree of recovery are established
Measurement of the scintillation time spectra and pulse-shape discrimination of low-energy beta and nuclear recoils in liquid argon with DEAP-1
The DEAP-1 low-background liquid argon detector was used to measure
scintillation pulse shapes of electron and nuclear recoil events and to
demonstrate the feasibility of pulse-shape discrimination (PSD) down to an
electron-equivalent energy of 20 keV.
In the surface dataset using a triple-coincidence tag we found the fraction
of beta events that are misidentified as nuclear recoils to be (90% C.L.) for energies between 43-86 keVee and for a nuclear recoil
acceptance of at least 90%, with 4% systematic uncertainty on the absolute
energy scale. The discrimination measurement on surface was limited by nuclear
recoils induced by cosmic-ray generated neutrons. This was improved by moving
the detector to the SNOLAB underground laboratory, where the reduced background
rate allowed the same measurement with only a double-coincidence tag.
The combined data set contains events. One of those, in the
underground data set, is in the nuclear-recoil region of interest. Taking into
account the expected background of 0.48 events coming from random pileup, the
resulting upper limit on the electronic recoil contamination is
(90% C.L.) between 44-89 keVee and for a nuclear recoil
acceptance of at least 90%, with 6% systematic uncertainty on the absolute
energy scale.
We developed a general mathematical framework to describe PSD parameter
distributions and used it to build an analytical model of the distributions
observed in DEAP-1. Using this model, we project a misidentification fraction
of approx. for an electron-equivalent energy threshold of 15 keV for
a detector with 8 PE/keVee light yield. This reduction enables a search for
spin-independent scattering of WIMPs from 1000 kg of liquid argon with a
WIMP-nucleon cross-section sensitivity of cm, assuming
negligible contribution from nuclear recoil backgrounds.Comment: Accepted for publication in Astroparticle Physic
Atomic nanolithography patterning of submicron features: writing an organic self-assembled monolayer with cold, bright Cs atom beams
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