749 research outputs found
Creating new rural development strategies : the role of nonprofits
Rural areas ; Rural development ; Nonprofit organizations
Why invest in rural America and how? : a critical public policy question for the 21st century
Significant portions of rural America are in trouble. For some parts of rural America, the slow slide to no longer being economically, socially, or politically viable is within sight. At the same time, without intending it, rural America appears headed for a land of the rich and the poor—a rural America of resorts and pockets of persistent poverty.> Yet current rural policies are designed for the past, not the future. In terms of public dollars committed, today’s rural policy focuses primarily on two areas—agriculture and manufacturing. Neither focus can meet the future needs of rural people and their communities.> In his paper presented at this year’s conference sponsored by the Center for the Study of Rural America, Exploring Policy Options for a New Rural America, Dr. Stauber discusses how a successful rural policy must be crafted with three key societal benefits in mind—the survival of the rural middle class, reducing concentrated rural poverty, and sustaining and improving the quality of the natural environment.Rural areas ; Rural development
Dynamical polarizability of graphene beyond the Dirac cone approximation
We compute the dynamical polarizability of graphene beyond the usual Dirac
cone approximation, integrating over the full Brillouin zone. We find
deviations at ( the hopping parameter) which amount to a
logarithmic singularity due to the van Hove singularity and derive an
approximate analytical expression. Also at low energies, we find deviations
from the results obtained from the Dirac cone approximation which manifest
themselves in a peak spitting at arbitrary direction of the incoming wave
vector \q. Consequences for the plasmon spectrum are discussed.Comment: 8 pages, 6 figure
Conductivity of suspended and non-suspended graphene at finite gate voltage
We compute the DC and the optical conductivity of graphene for finite values
of the chemical potential by taking into account the effect of disorder, due to
mid-gap states (unitary scatterers) and charged impurities, and the effect of
both optical and acoustic phonons. The disorder due to mid-gap states is
treated in the coherent potential approximation (CPA, a self-consistent
approach based on the Dyson equation), whereas that due to charged impurities
is also treated via the Dyson equation, with the self-energy computed using
second order perturbation theory. The effect of the phonons is also included
via the Dyson equation, with the self energy computed using first order
perturbation theory. The self-energy due to phonons is computed both using the
bare electronic Green's function and the full electronic Green's function,
although we show that the effect of disorder on the phonon-propagator is
negligible. Our results are in qualitative agreement with recent experiments.
Quantitative agreement could be obtained if one assumes water molelcules under
the graphene substrate. We also comment on the electron-hole asymmetry observed
in the DC conductivity of suspended graphene.Comment: 13 pages, 11 figure
Transport in a Clean Graphene Sheet at Finite Temperature and Frequency
We calculate the conductivity of a clean graphene sheet at finite
temperatures starting from the tight-binding model. We obtain a finite value
for the dc-conductivity at zero temperature. For finite temperature, the
spontaneous electron-hole creation, responsible for the finite conductivity at
zero temperature, is washed out and the dc-conductivity yields zero. Our
results are in agreement with calculations based on the field-theoretical model
for graphene.Comment: 8 pages, 1 figur
Phenomenological study of the electronic transport coefficients of graphene
Using a semi-classical approach and input from experiments on the
conductivity of graphene, we determine the electronic density dependence of the
electronic transport coefficients -- conductivity, thermal conductivity and
thermopower -- of doped graphene. Also the electronic density dependence of the
optical conductivity is obtained. Finally we show that the classical Hall
effect (low field) in graphene has the same form as for the independent
electron case, characterized by a parabolic dispersion, as long as the
relaxation time is proportional to the momentum.Comment: 4 pages, 1 figur
The optical conductivity of graphene in the visible region of the spectrum
We compute the optical conductivity of graphene beyond the usual Dirac cone
approximation, giving results that are valid in the visible region of the
conductivity spectrum. The effect of next nearest neighbor hoping is also
discussed. Using the full expression for the optical conductivity, the
transmission and reflection coefficients are given. We find that even in the
optical regime the corrections to the Dirac cone approximation are surprisingly
small (a few percent). Our results help in the interpretation of the
experimental results reported by Nair {\it et al.} [Science {\bf 320}, 1308
(2008)].Comment: 8 pages, 6 figure
Universal Dynamic Conductivity and Quantized Visible Opacity of Suspended Graphene
We show that the optical transparency of suspended graphene is defined by the
fine structure constant, alpha, the parameter that describes coupling between
light and relativistic electrons and is traditionally associated with quantum
electrodynamics rather than condensed matter physics. Despite being only one
atom thick, graphene is found to absorb a significant (pi times alpha=2.3%)
fraction of incident white light, which is a consequence of graphene's unique
electronic structure. This value translates into universal dynamic conductivity
G =e^2/4h_bar within a few percent accuracy
Efficient graphene-based photodetector with two cavities
We present an efficient graphene-based photodetector with two Fabri-P\'erot
cavities. It is shown that the absorption can reach almost 100% around a given
frequency, which is determined by the two-cavity lengths. It is also shown that
hysteresis in the absorbance is possible, with the transmittance amplitude of
the mirrors working as an external driving field. The role of non-linear
contributions to the optical susceptibility of graphene is discussed.Comment: 10 pages, 8 figures. published version: minor revisio
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