3,708 research outputs found
Third edge for a graphene nanoribbon: A tight-binding model calculation
The electronic and transport properties of an extended linear defect embedded
in a zigzag nanoribbon of realistic width are studied, within a tight binding
model approach. Our results suggest that such defect profoundly modify the
properties of the nanoribbon, introducing new conductance quantization values
and modifying the conductance quantization thresholds. The linear defect along
the nanoribbon behaves as an effective third edge of the system, which shows a
metallic behavior, giving rise to new conduction pathways that could be used in
nanoscale circuitry as a quantum wire.Comment: 6 pages, 6 figures. Two new figures and a few references adde
Photovoltaic Performance of Ultrasmall PbSe Quantum Dots
We investigated the effect of PbSe quantum dot size on the performance of Schottky solar cells made in an ITO/PEDOT/PbSe/aluminum structure, varying the PbSe nanoparticle diameter from 1 to 3 nm. In this highly confined regime, we find that the larger particle bandgap can lead to higher open-circuit voltages (~0.6 V), and thus an increase in overall efficiency compared to previously reported devices of this structure. To carry out this study, we modified existing synthesis methods to obtain ultrasmall PbSe nanocrystals with diameters as small as 1 nm, where the nanocrystal size is controlled by adjusting the growth temperature. As expected, we find that photocurrent decreases with size due to reduced absorption and increased recombination, but we also find that the open-circuit voltage begins to decrease for particles with diameters smaller than 2 nm, most likely due to reduced collection efficiency. Owing to this effect, we find peak performance for devices made with PbSe dots with a first exciton energy of ~1.6 eV (2.3 nm diameter), with a typical efficiency of 3.5%, and a champion device efficiency of 4.57%. Comparing the external quantum efficiency of our devices to an optical model reveals that the photocurrent is also strongly affected by the coherent interference in the thin film due to Fabry-PĆ©rot cavity modes within the PbSe layer. Our results demonstrate that even in this simple device architecture, fine-tuning of the nanoparticle size can lead to substantial improvements in efficiency
Austerity, political control and supplier selection in English local government: implications for autonomy in multi-level systems
Analysis of 60,000 contracts awarded by English councils between 2015-19 reveals that austerity constraints are a key predictor of councils outsourcing services to for-profit suppliers, regardless of their political control. Conservative Party-controlled councils are also more likely to contract with for-profit suppliers, although we found no link between Labour-controlled councils and not-for-profit suppliers, nor evidence that political or budgetary factors influence whether councils contract with providers based in their own region. We argue that centrally imposed funding cuts, and a belief that for-profit suppliers represent a cheaper option, could be overriding Labour Party councilsā ideological preference for not-for-profit providers
Bulk and shear relaxation in glasses and highly viscous liquids
The ratio between the couplings of a relaxational process to compression and
shear, respectively, is calculated in the Eshelby picture of structural
rearrangements within a surrounding elastic matrix, assuming a constant density
of stable structures in distortion space. The result is compared to
experimental data for the low-temperature tunneling states in glasses and to
Prigogine-Defay data at the glass transition from the literature.Comment: 6 pages, 2 figures, 53 references; version after understanding the
Prigogine-Defay ratio at the glass transition in the accompanying paper
arXiv:1203.3555 [cond-mat.dis-nn
A nearly closed ballistic billiard with random boundary transmission
A variety of mesoscopic systems can be represented as a billiard with a
random coupling to the exterior at the boundary. Examples include quantum dots
with multiple leads, quantum corrals with different kinds of atoms forming the
boundary, and optical cavities with random surface refractive index. The
specific example we study is a circular (integrable) billiard with no internal
impurities weakly coupled to the exterior by a large number of leads with one
channel open in each lead. We construct a supersymmetric nonlinear
-model by averaging over the random coupling strengths between bound
states and channels. The resulting theory can be used to evaluate the
statistical properties of any physically measurable quantity in a billiard. As
an illustration, we present results for the local density of states.Comment: 5 pages, 1 figur
Direct conversion of rheological compliance measurements into storage and loss moduli
We remove the need for Laplace/inverse-Laplace transformations of
experimental data, by presenting a direct and straightforward mathematical
procedure for obtaining frequency-dependent storage and loss moduli
( and respectively), from time-dependent experimental
measurements. The procedure is applicable to ordinary rheological creep
(stress-step) measurements, as well as all microrheological techniques, whether
they access a Brownian mean-square displacement, or a forced compliance. Data
can be substituted directly into our simple formula, thus eliminating
traditional fitting and smoothing procedures that disguise relevant
experimental noise.Comment: 4 page
Pengaruh Bentuk dan Dimensi Tapak Pondasi terhadap Daya Dukung Pondasi Dangkal Akibat Beban Aksial pada Tanah Pasir
The load distribution of shallow foundation spread out along the width of the foundation. Ultimate bearing capacity is defined as the maximum load that can be supported by the footing. Some form of shallow foundation were square, rectangular and circular shapes were they made to analyze the influence of the foundation over the same area of the shallow foundation bearing capacity due to axial loads on sandy soil. Variations foundation area of 100 cm2, 150 cm2 and 200 cm2. In the same area, the square shape can withstand greater load than in rectangular and circular shapes. Addition area will make 50-100% in weight-bearing capacity increased from 102,016 to 157,661% for squares, 135,751 to 228,497% for the rectangle and circle 187,413 to 341.259%. There are differences in the results of observation and empirical formula. Shape factors of squares and circles according to Mayerhof, Vesic and Hansen are the same. So, Terzaghi`s formula was similiar phenomenom with this research
Tunneling through nanosystems: Combining broadening with many-particle states
We suggest a new approach for transport through finite systems based on the
Liouville equation. By working in a basis of many-particle states for the
finite system, Coulomb interactions are taken fully into account and correlated
transitions by up to two different contact states are included. This latter
extends standard rate equation models by including level-broadening effects.
The main result of the paper is a general expression for the elements of the
density matrix of the finite size system, which can be applied whenever the
eigenstates and the couplings to the leads are known. The approach works for
arbitrary bias and for temperatures above the Kondo temperature. We apply the
approach to standard models and good agreement with other methods in their
respective regime of validity is found.Comment: 9 pages, 5 figures included to tex
One-Bead Microrheology with Rotating Particles
We lay the theoretical basis for one-bead microrheology with rotating
particles, i.e, a method where colloids are used to probe the mechanical
properties of viscoelastic media. Based on a two-fluid model, we calculate the
compliance and discuss it for two cases. We first assume that the elastic and
fluid component exhibit both stick boundary conditions at the particle surface.
Then, the compliance fulfills a generalized Stokes law with a complex shear
modulus whose validity is only limited by inertial effects, in contrast to
translational motion. Secondly, we find that the validity of the Stokes regime
is reduced when the elastic network is not coupled to the particleComment: 7 pages, 5 figures, submitted to Europhys. Let
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