157 research outputs found
The Impact of Micro-Finance on the Performance of Small-Scale Enterprises: A Comparison of Sinapi Aba Trust and Maata-N-Tudu Associations in Wa Municipality, Ghana
Literature on the impact of micro-finance institutions (MFIs) on small and medium scale enterprises (SMEs) is still fraught with inconsistent results, with some concluding on positive impacts while others reporting negative impacts. This paper therefore sets out to assess the impact that the study-MFIs are making on their SMEs-client in the Wa Municipality. The withand- without approach, coupled with a quasi-longitudinal approach was employed for the study. Data were gathered from the study-MFIs’ documents and questionnaire administration. Simple percentage change was used as a measure of growth of mean sales revenue. Analysis of variance (ANOVA), Spearman’s correlation coefficient (r) and coefficient of determination were also used in the analyses, with various hypotheses tests. It is concluded that the study-MFIs make positive impacts on the mean sales revenues of their client – SMEs, with great differences between those of SAT and MTA clients on the one hand and those of the control group on the other. A slight difference that exists between the two study-MFIs was accounted for by differences in their management approaches. A strong positive correlation exists between average sales revenue on one hand and micro loans, level of education and training on the other.Keywords: Small-scale Enterprises, Micro-Financing, Micro-Credit, Sales Revenue, Impac
One-dimensional potential for image-potential states on graphene
In the framework of dielectric theory the static non-local self-energy of an
electron near an ultra-thin polarizable layer has been calculated and applied
to study binding energies of image-states near free-standing graphene. The
corresponding series of eigenvalues and eigenfunctions have been obtained by
solving numerically the one-dimensional Schr{\"o}dinger equation.
Image-potential-state wave functions accumulate most of their probability
outside the slab. We find that a Random Phase Approximation (RPA) for the
non-local dielectric function yields a superior description for the potential
inside the slab, but a simple Fermi-Thomas theory can be used to get a
reasonable quasi-analytical approximation to the full RPA result that can be
computed very economically. Binding energies of the image-potential states
follow a pattern close to the Rydberg series for a perfect metal with the
addition of intermediate states due to the added symmetry of the potential. The
formalism only requires a minimal set of free parameters; the slab width and
the electronic density. The theoretical calculations are compared to
experimental results for work function and image-potential states obtained by
two-photon photoemission.Comment: 24 pages; 10 figures. arXiv admin note: text overlap with
arXiv:1301.448
Quantum Coherence of Image-Potential States
The quantum dynamics of the two-dimensional image-potential states in front
of the Cu(100) surface is measured by scanning tunneling microscopy (STM) and
spectroscopy (STS). The dispersion relation and the momentum resolved
phase-relaxation time of the first image-potential state are determined from
the quantum interference patterns in the local density of states (LDOS) at step
edges. It is demonstrated that the tip-induced Stark shift does not affect the
motion of the electrons parallel to the surface.Comment: Submitted to Phys. Rev. Lett., 4 pages, 4 figures; corrected typos,
minor change
Scanning tunneling microscopy and kinetic Monte Carlo investigation of Cesium superlattices on Ag(111)
Cesium adsorption structures on Ag(111) were characterized in a
low-temperature scanning tunneling microscopy experiment. At low coverages,
atomic resolution of individual Cs atoms is occasionally suppressed in regions
of an otherwise hexagonally ordered adsorbate film on terraces. Close to step
edges Cs atoms appear as elongated protrusions along the step edge direction.
At higher coverages, Cs superstructures with atomically resolved hexagonal
lattices are observed. Kinetic Monte Carlo simulations model the observed
adsorbate structures on a qualitative level.Comment: 8 pages, 7 figure
Self-energy and lifetime of Shockley and image states on Cu(100) and Cu(111): Beyond the GW approximation of many-body theory
We report many-body calculations of the self-energy and lifetime of Shockley
and image states on the (100) and (111) surfaces of Cu that go beyond the
approximation of many-body theory. The self-energy is computed in the framework
of the GW\Gamma approximation by including short-range exchange-correlation
(XC) effects both in the screened interaction W (beyond the random-phase
approximation) and in the expansion of the self-energy in terms of W (beyond
the GW approximation). Exchange-correlation effects are described within
time-dependent density-functional theory from the knowledge of an adiabatic
nonlocal XC kernel that goes beyond the local-density approximation.Comment: 8 pages, 5 figures, to appear in Phys. Rev.
Unoccupied Topological States on Bismuth Chalcogenides
The unoccupied part of the band structure of topological insulators
BiTeSe () is studied by angle-resolved two-photon
photoemission and density functional theory. For all surfaces
linearly-dispersing surface states are found at the center of the surface
Brillouin zone at energies around 1.3 eV above the Fermi level. Theoretical
analysis shows that this feature appears in a spin-orbit-interaction induced
and inverted local energy gap. This inversion is insensitive to variation of
electronic and structural parameters in BiSe and BiTeSe. In
BiTe small structural variations can change the character of the local
energy gap depending on which an unoccupied Dirac state does or does not exist.
Circular dichroism measurements confirm the expected spin texture. From these
findings we assign the observed state to an unoccupied topological surface
state
Ultrafast Optical Excitation of a Persistent Surface-State Population in the Topological Insulator Bi2Se3
Using femtosecond time- and angle- resolved photoemission spectroscopy, we
investigated the nonequilibrium dynamics of the topological insulator Bi2Se3.
We studied p-type Bi2Se3, in which the metallic Dirac surface state and bulk
conduction bands are unoccupied. Optical excitation leads to a meta-stable
population at the bulk conduction band edge, which feeds a nonequilibrium
population of the surface state persisting for >10ps. This unusually long-lived
population of a metallic Dirac surface state with spin texture may present a
channel in which to drive transient spin-polarized currents
The role of surface plasmons in the decay of image-potential states on silver surfaces
The combined effect of single-particle and collective surface excitations in
the decay of image-potential states on Ag surfaces is investigated, and the
origin of the long-standing discrepancy between experimental measurements and
previous theoretical predictions for the lifetime of these states is
elucidated. Although surface-plasmon excitation had been expected to reduce the
image-state lifetime, we demonstrate that the subtle combination of the spatial
variation of s-d polarization in Ag and the characteristic non-locality of
many-electron interactions near the surface yields surprisingly long
image-state lifetimes, in agreement with experiment.Comment: 4 pages, 2 figures, to appear in Phys. Rev. Let
Adiabatic-Connection-Fluctuation-Dissipation approach to the long-range behavior of the exchange-correlation energy at metal surfaces: A numerical study for jellium slabs
A still open issue in many-body theory is the asymptotic behavior of the
exchange-correlation energy and potential in the vacuum region of a metal
surface. Here we report a numerical study of the position-dependent
exchange-correlation energy for jellium slabs, as obtained by combining the
formally exact adiabatic-connection-fluctuation-dissipation theorem with either
time-dependent density-functional theory or an inhomogeneous
Singwi-Tosi-Land-Sj\"olander approach. We find that the inclusion of
correlation allows to obtain well-converged semi-infinite-jellium results
(independent of the slab thickness) that exhibit an image-like asymptotic
behavior close to the classical image potential .Comment: 6 pages, 4 Figure
Electronic structure of superconducting graphite intercalate compounds: The role of the interlayer state
Although not an intrinsic superconductor, it has been long--known that, when
intercalated with certain dopants, graphite is capable of exhibiting
superconductivity. Of the family of graphite--based materials which are known
to superconduct, perhaps the most well--studied are the alkali metal--graphite
intercalation compounds (GIC) and, of these, the most easily fabricated is the
CK system which exhibits a transition temperature K. By increasing the alkali metal concentration (through high pressure
fabrication techniques), the transition temperature has been shown to increase
to as much as K in CNa. Lately, in an important recent
development, Weller \emph{et al.} have shown that, at ambient conditions, the
intercalated compounds \cyb and \cca exhibit superconductivity with transition
temperatures K and K respectively, in excess
of that presently reported for other graphite--based compounds. We explore the
architecture of the states near the Fermi level and identify characteristics of
the electronic band structure generic to GICs. As expected, we find that charge
transfer from the intercalant atoms to the graphene sheets results in the
occupation of the --bands. Yet, remarkably, in all those -- and only
those -- compounds that superconduct, we find that an interlayer state, which
is well separated from the carbon sheets, also becomes occupied. We show that
the energy of the interlayer band is controlled by a combination of its
occupancy and the separation between the carbon layers.Comment: 4 Figures. Please see accompanying experimental manuscript
"Superconductivity in the Intercalated Graphite Compounds C6Yb and C6Ca" by
Weller et a
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