35 research outputs found
A Hartree-Fock Study of Persistent Currents in Disordered Rings
For a system of spinless fermions in a disordered mesoscopic ring,
interactions can give rise to an enhancement of the persistent current by
orders of magnitude. The increase in the current is associated with a charge
reorganization of the ground state. The interaction strength for which this
reorganization takes place is sample-dependent and the log-averages over the
ensemble are not representative. In this paper we demonstrate that the
Hartree-Fock method closely reproduces results obtained by exact
diagonalization. For spinless fermions subject to a short-range Coulomb
repulsion U we show that due to charge reorganization the derivative of the
persistent current is a discontinuous function of U. Having established that
the Hartree-Fock method works well in one dimension, we present corresponding
results for persistent currents in two coupled chains.Comment: 4 pages, 6 figures, Submitted to Phys. Rev.
The Effect of Surface Roughness on the Universal Thermal Conductance
We explain the reduction of the thermal conductance below the predicted
universal value observed by Schwab et al. in terms of the scattering of thermal
phonons off surface roughness using a scalar model for the elastic waves. Our
analysis shows that the thermal conductance depends on two roughness
parameters: the roughness amplitude and the correlation length .
At sufficiently low temperatures the conductance decrease from the universal
value quadratically with temperature at a rate proportional to .
Values of equal to 0.22 and equal to about 0.75 of the width of
the conduction pathway give a good fit to the data.Comment: 10 pages, 5 figures. Ref. added, typo correcte
Temperature and Field Dependence of the Mobility in Liquid-Crystalline Conjugated Polymer Films
The transport properties of organic light-emitting diodes in which the
emissive layer is composed of conjugated polymers in the liquid-crystalline
phase have been investigated. We have performed simulations of the current
transient response to an illumination pulse via the Monte Carlo approach, and
from the transit times we have extracted the mobility of the charge carriers as
a function of both the electric field and the temperature. The transport
properties of such films are different from their disordered counterparts, with
charge carrier mobilities exhibiting only a weak dependence on both the
electric field and temperature. We show that for spatially ordered polymer
films, this weak dependence arises for thermal energy being comparable to the
energetic disorder, due to the combined effect of the electrostatic and thermal
energies. The inclusion of spatial disorder, on the other hand, does not alter
the qualitative behaviour of the mobility, but results in decreasing its
absolute value.Comment: 9 pages, 8 figures, submitted to Phys. Rev.
Ballistic thermal conductance limited by phonon roughness scattering: A comparison of power-law and Gaussian roughness
In this work, we have investigated the influence of power-law roughness on the ballistic thermal conductance KTH for a nanosized beam adiabatically connected between two heat reservoirs. The sideways wall beam roughness is assumed to be power-law type, which is described by the roughness amplitude w, the in-plane roughness correlation length ξ and the roughness exponent 0≤H≤1. Distinct differences occur in between power-law and Gaussian wall roughness. For power-law roughness with low roughness exponents H (<0.5), the influence of phonon scattering can be rather destructive leading to significant deviations from the universal conductance value for flat beam walls. On the other hand for large roughness exponents (H>0.5) the conductance drop is significantly smaller than that of Gaussian roughness assuming similar roughness ratios w/ξ.
Transport Properties of Highly Aligned Polymer Light-Emitting-Diodes
We investigate hole transport in polymer light-emitting-diodes in which the
emissive layer is made of liquid-crystalline polymer chains aligned
perpendicular to the direction of transport. Calculations of the current as a
function of time via a random-walk model show excellent qualitative agreement
with experiments conducted on electroluminescent polyfluorene demonstrating
non-dispersive hole transport. The current exhibits a constant plateau as the
charge carriers move with a time-independent drift velocity, followed by a long
tail when they reach the collecting electrode. Variation of the parameters
within the model allows the investigation of the transition from non-dispersive
to dispersive transport in highly aligned polymers. It turns out that large
inter-chain hopping is required for non-dispersive hole transport and that
structural disorder obstructs the propagation of holes through the polymer
film.Comment: 4 pages, 5 figure
Mesoscopic Electron and Phonon Transport through a Curved Wire
There is great interest in the development of novel nanomachines that use
charge, spin, or energy transport, to enable new sensors with unprecedented
measurement capabilities. Electrical and thermal transport in these mesoscopic
systems typically involves wave propagation through a nanoscale geometry such
as a quantum wire. In this paper we present a general theoretical technique to
describe wave propagation through a curved wire of uniform cross-section and
lying in a plane, but of otherwise arbitrary shape. The method consists of (i)
introducing a local orthogonal coordinate system, the arclength and two locally
perpendicular coordinate axes, dictated by the shape of the wire; (ii)
rewriting the wave equation of interest in this system; (iii) identifying an
effective scattering potential caused by the local curvature; and (iv), solving
the associated Lippmann-Schwinger equation for the scattering matrix. We carry
out this procedure in detail for the scalar Helmholtz equation with both
hard-wall and stress-free boundary conditions, appropriate for the mesoscopic
transport of electrons and (scalar) phonons. A novel aspect of the phonon case
is that the reflection probability always vanishes in the long-wavelength
limit, allowing a simple perturbative (Born approximation) treatment at low
energies. Our results show that, in contrast to charge transport, curvature
only barely suppresses thermal transport, even for sharply bent wires, at least
within the two-dimensional scalar phonon model considered. Applications to
experiments are also discussed.Comment: 9 pages, 11 figures, RevTe
Low-temperature heat transfer in nanowires
The new regime of low-temperature heat transfer in suspended nanowires is
predicted. It takes place when (i) only ``acoustic'' phonon modes of the wire
are thermally populated and (ii) phonons are subject to the effective elastic
scattering. Qualitatively, the main peculiarities of heat transfer originate
due to appearance of the flexural modes with high density of states in the wire
phonon spectrum. They give rise to the temperature dependence of the
wire thermal conductance. The experimental situations where the new regime is
likely to be detected are discussed.Comment: RevTex file, 1 PS figur
Electron transport in the dye sensitized nanocrystalline cell
Dye sensitised nanocrystalline solar cells (Gr\"{a}tzel cells) have achieved
solar-to-electrical energy conversion efficiencies of 12% in diffuse daylight.
The cell is based on a thin film of dye-sensitised nanocrystalline TiO
interpenetrated by a redox electrolyte. The high surface area of the TiO
and the spectral characteristics of the dye allow the device to harvest 46% of
the solar energy flux. One of the puzzling features of dye-sensitised
nano-crystalline solar cells is the slow electron transport in the titanium
dioxide phase. The available experimental evidence as well as theoretical
considerations suggest that the driving force for electron collection at the
substrate contact arises primarily from the concentration gradient, ie the
contribution of drift is negligible. The transport of electrons has been
characterised by small amplitude pulse or intensity modulated illumination.
Here, we show how the transport of electrons in the Gr\"{a}tzel cell can be
described quantitatively using trap distributions obtained from a novel charge
extraction method with a one-dimensional model based on solving the continuity
equation for the electron density. For the first time in such a model, a back
reaction with the I ions in the electrolyte that is second order in the
electron density has been included.Comment: 6 pages, 5 figures, invited talk at the workshop 'Nanostructures in
Photovoltaics' to appear in Physica
A Hartree-Fock study of charge redistribution in a two-dimensional mesoscopic structure
Abstract In this letter, we investigate the ground state of two-dimensional disordered cylinders which contain spinless, interacting electrons using the HartreeFock approximation. Calculations of the deviation of the polarization from uniformity reveal a tendency of the charge to rearrange towards the ends of the system. The presence of disorder results in fluctuations of the deviation around its mean value, which are more pronounced when the disorder strength is of the order of the interaction between the electrons. The existence of persistent currents in normal mesoscopic rings threaded by a magnetic flux In the present letter, we study the ground state of two-dimensional cylinders which contain spinless, interacting electrons via Coulomb interactions, by solving self-consistently the Hartree-Fock equation. The validity of the method has already been established The system under investigation is a two-dimensional cylinder, formed by a tight-binding lattice with M sites in the longitudinal direction (x) and L sites in the transverse direction (y). We take periodic boundary conditions along the longitudinal direction and free boundar
Phonon-mediated thermal conductance of mesoscopic wires with rough edges
We present an analysis of acoustic phonon propagation through long,
free-standing, insulating wires with rough surfaces. Due to a crossover from
ballistic propagation of the lowest-frequency phonon mode at to a diffusive (or even localized) behavior upon the increase of
phonon frequency, followed by re-entrance into the quasi-ballistic regime, the
heat conductance of a wire acquires an intermediate tendency to saturate within
the temperature range .Comment: 4 pages, 3 figures included; minor changes and corrections, figures 1
and 2 replaced by better versions; to appear in PRB Brief Report