11,192 research outputs found
Theoretical study of the charge transport through C60-based single-molecule junctions
We present a theoretical study of the conductance and thermopower of
single-molecule junctions based on C60 and C60-terminated molecules. We first
analyze the transport properties of gold-C60-gold junctions and show that these
junctions can be highly conductive (with conductances above 0.1G0, where G0 is
the quantum of conductance). Moreover, we find that the thermopower in these
junctions is negative due to the fact that the LUMO dominates the charge
transport, and its magnitude can reach several tens of micro-V/K, depending on
the contact geometry. On the other hand, we study the suitability of C60 as an
anchoring group in single-molecule junctions. For this purpose, we analyze the
transport through several dumbbell derivatives using C60 as anchors, and we
compare the results with those obtained with thiol and amine groups. Our
results show that the conductance of C60-terminated molecules is rather
sensitive to the binding geometry. Moreover, the conductance of the molecules
is typically reduced by the presence of the C60 anchors, which in turn makes
the junctions more sensitive to the functionalization of the molecular core
with appropriate side groups.Comment: 9 pages, 7 figure
Bloch states in light transport through a perforated metal
Light transport in a metal with two-dimensional hole arrays is considered.
Analytical expression for a transmission coefficient in periodic, isolated and
disordered cases are derived, assuming the existence of waveguide modes
transverse tunneling in two-dimensional plane perpendicular to traveling
direction of light. The one dimensional case of periodic holes, due to its
simplicity, is investigated in detail. In the dilute metal regime, when metal
fraction is small, our numerical study of the transmission coefficient of
central diffracted wave indicates the existence of a minimum which is
completely independent of an incident wavelength. Further increasing of metal
fraction leads to the unusual monotonic increasing of central diffracted wave
transmission. The role of the surface plasmons is discussed.Comment: 14pages, 2figure
Simple universal models capture all classical spin physics
Spin models are used in many studies of complex systems---be it condensed
matter physics, neural networks, or economics---as they exhibit rich
macroscopic behaviour despite their microscopic simplicity.
Here we prove that all the physics of every classical spin model is
reproduced in the low-energy sector of certain `universal models'.
This means that (i) the low energy spectrum of the universal model reproduces
the entire spectrum of the original model to any desired precision, (ii) the
corresponding spin configurations of the original model are also reproduced in
the universal model, (iii) the partition function is approximated to any
desired precision, and (iv) the overhead in terms of number of spins and
interactions is at most polynomial.
This holds for classical models with discrete or continuous degrees of
freedom.
We prove necessary and sufficient conditions for a spin model to be
universal, and show that one of the simplest and most widely studied spin
models, the 2D Ising model with fields, is universal.Comment: v1: 4 pages with 2 figures (main text) + 4 pages with 3 figures
(supplementary info). v2: 12 pages with 3 figures (main text) + 35 pages with
6 figures (supplementary info) (all single column). v2 contains new results
and major revisions (results for spin models with continuous degrees of
freedom, explicit constructions, examples...). Close to published version.
v3: minor typo correcte
Transactions Costs and the Viability of Rural Financial Intermediaries
In its attempt to examine the transaction cost of banks, this study develops a method of estimating transaction cost for each bank activity. It also explains the differences and the composition cost among commercial private development and rural banks. Results in this paper is hoped to improve the efficient functioning of the formal financial system.financial sector, rural sector, transaction cost, financial intermediaries
Transaction Cost and the Viability of Rural Financial Intermediaries
Recognition of a continuous improvement in the formal financial system is necessary for economic growth. Over the years, the government has been infusing cheap funds into the rural sector. Unfortunately, it overlooks a number of serious issues regarding bank’s transaction costs. This article develops a method of estimating transaction costs for each bank activity. It also explains the differences and the composition of transaction costs among commercial banks, rural banks and private development banks.financial system, banking system, commercial banks, transaction cost, deposit liabilities
Field enhancement in subnanometer metallic gaps
Motivated by recent experiments [Ward et al., Nature Nanotech. 5, 732
(2010)], we present here a theoretical analysis of the optical response of
sharp gold electrodes separated by a subnanometer gap. In particular, we have
used classical finite difference time domain simulations to investigate the
electric field distribution in these nanojunctions upon illumination. Our
results show a strong confinement of the field within the gap region, resulting
in a large enhancement compared to the incident field. Enhancement factors
exceeding 1000 are found for interelectrode distances on the order of a few
angstroms, which are fully compatible with the experimental findings. Such huge
enhancements originate from the coupling of the incident light to the
evanescent field of hybrid plasmons involving charge density oscillations in
both electrodes.Comment: 4 pages, 3 figures, to appear in Physical Review
Role of electronic structure in photoassisted transport through atomic-sized contacts
We study theoretically quantum transport through laser-irradiated metallic
atomic-sized contacts. The radiation field is treated classically, assuming its
effect to be the generation of an ac voltage over the contact. We derive an
expression for the dc current and compute the linear conductance in one-atom
thick contacts as a function of the ac frequency, concentrating on the role
played by electronic structure. In particular, we present results for three
materials (Al, Pt, and Au) with very different electronic structures. It is
shown that, depending on the frequency and the metal, the radiation can either
enhance or diminish the conductance. This can be intuitively understood in
terms of the energy dependence of the transmission of the contacts in the
absence of radiation.Comment: 7 pages, 7 figures; four new figures adde
Nonequilibrium Green's function theory for nonadiabatic effects in quantum electron transport
We develop nonequilibribrium Green's function based transport theory, which
includes effects of nonadiabatic nuclear motion in the calculation of the
electric current in molecular junctions. Our approach is based on the
separation of slow and fast timescales in the equations of motion for the
Green's functions by means of the Wigner representation. Time derivatives with
respect to central time serves as a small parameter in the perturbative
expansion enabling the computation of nonadiabatic corrections to molecular
Green's functions. Consequently, we produce series of analytic expressions for
non-adiabatic electronic Green's functions (up to the second order in the
central time derivatives); which depend not solely on instantaneous molecular
geometry but likewise on nuclear velocities and accelerations. Extended formula
for electric current is derived which accounts for the non-adiabatic
corrections. This theory is concisely illustrated by the calculations on a
model molecular junction
Multibreather and vortex breather stability in Klein--Gordon lattices: Equivalence between two different approaches
In this work, we revisit the question of stability of multibreather
configurations, i.e., discrete breathers with multiple excited sites at the
anti-continuum limit of uncoupled oscillators. We present two methods that
yield quantitative predictions about the Floquet multipliers of the linear
stability analysis around such exponentially localized in space, time-periodic
orbits, based on the Aubry band method and the MacKay effective Hamiltonian
method and prove that their conclusions are equivalent. Subsequently, we
showcase the usefulness of the methods by a series of case examples including
one-dimensional multi-breathers, and two-dimensional vortex breathers in the
case of a lattice of linearly coupled oscillators with the Morse potential and
in that of the discrete model
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