591 research outputs found
Quantum Particles Constrained on Cylindrical Surfaces with Non-constant Diameter
We present a theoretical formulation of the one-electron problem constrained
on the surface of a cylindrical tubule with varying diameter. Because of the
cylindrical symmetry, we may reduce the problem to a one-dimensional equation
for each angular momentum quantum number along the cylindrical axis. The
geometrical properties of the surface determine the electronic structures
through the geometry dependent term in the equation. Magnetic fields parallel
to the axis can readily be incorporated. Our formulation is applied to simple
examples such as the catenoid and the sinusoidal tubules. The existence of
bound states as well as the band structures, which are induced geometrically,
for these surfaces are shown. To show that the electronic structures can be
altered significantly by applying a magnetic field, Aharonov-Bohm effects in
these examples are demonstrated.Comment: 7 pages, 7 figures, submitted to J. Phys. Soc. Jp
Participation in contract farming and farm performance: Insights from cashew farmers in Ghana
The global demand for cashew nuts continues to increase steadily. However, many
African countries face difficulties in marketing and adding value to the product. Using
recent survey data of 391 cashew farmers in Ghana, this paper contributes to the grow-
ing evidence on the significance of contract farming (CF) in improving the welfare
of rural households in developing countries. Specifically, the paper analyzes the fac-
tors that influence cashew farmersā decisions to participate in CF, and the impact
of participation on farmersā performance. We employ a recently developed switch-
ing regression model with endogenous explanatory variables and endogenous switch-
ing to control for selection bias caused by observable and unobservable factors. The
empirical results show that participation in CF significantly increases labor produc-
tivity and price margins, as well as cashew yields, and net revenues. A disaggregated
analysis of the sample into farm size categories reveals that small-sized cashew farms
tend to benefit more through CF, compared to medium- and large-sized farms
Spontaneous DC Current Generation in a Resistively Shunted Semiconductor Superlattice Driven by a TeraHertz Field
We study a resistively shunted semiconductor superlattice subject to a
high-frequency electric field. Using a balance equation approach that
incorporates the influence of the electric circuit, we determine numerically a
range of amplitude and frequency of the ac field for which a dc bias and
current are generated spontaneously and show that this region is likely
accessible to current experiments. Our simulations reveal that the Bloch
frequency corresponding to the spontaneous dc bias is approximately an integer
multiple of the ac field frequency.Comment: 8 pages, Revtex, 3 Postscript figure
Electrically Driven Light Emission from Individual CdSe Nanowires
We report electroluminescence (EL) measurements carried out on three-terminal
devices incorporating individual n-type CdSe nanowires. Simultaneous optical
and electrical measurements reveal that EL occurs near the contact between the
nanowire and a positively biased electrode or drain. The surface potential
profile, obtained by using Kelvin probe microscopy, shows an abrupt potential
drop near the position of the EL spot, while the band profile obtained from
scanning photocurrent microscopy indicates the existence of an n-type Schottky
barrier at the interface. These observations indicate that light emission
occurs through a hole leakage or an inelastic scattering induced by the rapid
potential drop at the nanowire-electrode interface.Comment: 12 pages, 4 figure
How the world's collective attention is being paid to a pandemic: COVID-19 related n-gram time series for 24 languages on Twitter
In confronting the global spread of the coronavirus disease COVID-19 pandemic
we must have coordinated medical, operational, and political responses. In all
efforts, data is crucial. Fundamentally, and in the possible absence of a
vaccine for 12 to 18 months, we need universal, well-documented testing for
both the presence of the disease as well as confirmed recovery through
serological tests for antibodies, and we need to track major socioeconomic
indices. But we also need auxiliary data of all kinds, including data related
to how populations are talking about the unfolding pandemic through news and
stories. To in part help on the social media side, we curate a set of 2000
day-scale time series of - and -grams across 24 languages on Twitter that
are most `important' for April 2020 with respect to April 2019. We determine
importance through our allotaxonometric instrument, rank-turbulence divergence.
We make some basic observations about some of the time series, including a
comparison to numbers of confirmed deaths due to COVID-19 over time. We broadly
observe across all languages a peak for the language-specific word for `virus'
in January followed by a decline through February and a recent surge through
April. The world's collective attention dropped away while the virus spread out
from China. We host the time series on Gitlab, updating them on a daily basis
while relevant. Our main intent is for other researchers to use these time
series to enhance whatever analyses that may be of use during the pandemic as
well as for retrospective investigations.Comment: 12 pages, 6 figures, 3 table
Topological insulator quantum dot with tunable barriers
Thin (6-7 quintuple layer) topological insulator Bi2Se3 quantum dot devices
are demonstrated using ultrathin (2~4 quintuple layer) Bi2Se3 regions to
realize semiconducting barriers which may be tuned from Ohmic to tunneling
conduction via gate voltage. Transport spectroscopy shows Coulomb blockade with
large charging energy >5 meV, with additional features implying excited states
- ā¦