2,849 research outputs found
Time-dependent transport in interacting and non-interacting mesoscopic systems
We consider a mesoscopic region coupled to two leads under the influence of
external time-dependent voltages. The time dependence is coupled to source and
drain contacts, the gates controlling the tunnel- barrier heights, or to the
gates which define the mesoscopic region. We derive, with the Keldysh
nonequilibrium Green function technique, a formal expression for the fully
nonlinear, time-dependent current through the system. The analysis admits
arbitrary interactions in the mesoscopic region, but the leads are treated as
noninteracting. For proportionate coupling to the leads, the time-averaged
current is simply the integral between the chemical potentials of the
time-averaged density of states, weighted by the coupling to the leads, in
close analogy to the time-independent result of Meir and Wingreen (PRL {\bf
68}, 2512 (1992)). Analytical and numerical results for the exactly solvable
non-interacting resonant-tunneling system are presented.Comment: 42 pages, 13 figures (available either as ps-files, or as FAX, upon
request), RevTex 3.
Covariant localizations in the torus and the phase observables
We describe all the localization observables of a quantum particle in a
one-dimensional box in terms of sequences of unit vectors in a Hilbert space.
An alternative representation in terms of positive semidefinite complex
matrices is furnished and the commutative localizations are singled out. As a
consequence, we also get a vector sequence characterization of the covariant
phase observables.Comment: 16 pages, no figure, Latex2
Quantum theory of shuttling instability in a movable quantum dot array
We study the shuttling instability in an array of three quantum dots the
central one of which is movable. We extend the results by Armour and MacKinnon
on this problem to a broader parameter regime. The results obtained by an
efficient numerical method are interpreted directly using the Wigner
distributions. We emphasize that the instability should be viewed as a
crossover phenomenon rather than a clear-cut transition.Comment: 4 pages, 2 figures, presented at HCIS-13, Modena, July 200
Acoustic phonon limited mobility in two-dimensional semiconductors: Deformation potential and piezoelectric scattering in monolayer MoS2 from first principles
We theoretically study the acoustic phonon limited mobility in n-doped
two-dimensional MoS2 for temperatures T < 100 K and high carrier densities
using the Boltzmann equation and first-principles calculations of the acoustic
electron-phonon (el-ph) interaction. In combination with a continuum elastic
model, analytic expressions and the coupling strengths for the deformation
potential and piezoelectric interactions are established. We furthermore show
that the deformation potential interaction has contributions from both normal
and umklapp processes and that the latter contribution is only weakly affected
by carrier screening. Consequently, the calculated mobilities show a transition
from a high-temperature \mu T^{-1} behavior to a stronger \mu T^{-4} behavior
in the low-temperature Bloch-Gruneisen regime characteristic of unscreened
deformation potential scattering. Intrinsic mobilities in excess of 10^5 cm^2
V^{-1} s^{-1} are predicted at T 10^{11}
cm^{-2}). At 100 K, the mobility does not exceed ~7 x 10^3 cm2 V^{-1} s^{-1}.
Our findings provide new and important understanding of the acoustic el-ph
interaction and its screening by free carriers, and is of high relevance for
the understanding of acoustic phonon limited mobilities in general.Comment: Substantially revised version. 17 pages, 11 figure
Policy lessons from a simple open - economy model
The authors show how two-sector models can be used to derive policy lessons about adjustment in developing economies. In the past two decades, changes in the external environment and in economic policies have been the key factors in the performance of developing economies. By and large the shocks have involved the external sector: terms-of-trade shocks or cutbacks in foreign capital. The policy responses most commonly proposed have targeted the external sector: depreciating the real exchange rate or reducing distortionary taxes to make the economy more competitive. The authors provide a starting point for analyzing the relation between external shocks and policy responses. Starting from a small, one-country, two-sector, three-good (1-2-3) model, the authors outline how the effects of a foreign capital inflow and terms-of-trade shock can be analyzed. They derive the assumptions underlying the conventional policy recommendation of real exchange rate depreciation in response to adverse shocks. The implications of such trade and fiscal policy instruments as export subsidies, import tariffs, and domestic indirect taxes can also be studied in this framework. The authors show that the standard advice to depreciate the real exchange rate in the wake of an adverse terms-of-trade shock rests on the condition that the income effect of the external shock dominates its substitution effect. But, depending on the characteristics of the economy (for example, the trade elasticities), policy results may run counter to received wisdom. For example, when the substitution effect ofan adverse external shock dominates, real depreciation is inappropriate. An infusion of foreign capital does not necessarily benefit the nontradable sector, as the results of"Dutch disease"models suggest (for example, in the extreme case of nearly infinite substitution elasticity between imports and domestic goods). When import tariffs are significant sources of public revenue, potential revenue losses from tariff cuts must be offset by other revenue sources to maintain the external current account balance. The paper shows a simple way to calculate the necessary tax adjustment. A major advantage of small models is their simplicity. The example in this paper can be solved analytically - either graphically or algebraically. It also can be solved numerically, using such widely available PC-based spreadsheet programs as Excel. The numerical implementation involves only modest data requirements. The data that governments normally release on national income, fiscal, and balance of payments accounts are sufficient.Environmental Economics&Policies,Economic Theory&Research,Economic Stabilization,TF054105-DONOR FUNDED OPERATION ADMINISTRATION FEE INCOME AND EXPENSE ACCOUNT,Markets and Market Access
Master-equation analysis of accelerating networks
In many real-world networks, the rates of node and link addition are time
dependent. This observation motivates the definition of accelerating networks.
There has been relatively little investigation of accelerating networks and
previous efforts at analyzing their degree distributions have employed
mean-field techniques. By contrast, we show that it is possible to apply a
master-equation approach to such network development. We provide full
time-dependent expressions for the evolution of the degree distributions for
the canonical situations of random and preferential attachment in networks
undergoing constant acceleration. These results are in excellent agreement with
results obtained from simulations. We note that a growing, non-equilibrium
network undergoing constant acceleration with random attachment is equivalent
to a classical random graph, bridging the gap between non-equilibrium and
classical equilibrium networks.Comment: 6 pages, 1 figure, 1 tabl
Plasmonic eigenmodes in individual and bow-tie graphene nanotriangles
Serving as a new two-dimensional plasmonic material, graphene has stimulated
an intensive study of its optical properties which benefit from the unique
electronic band structure of the underlying honeycomb lattice of carbon atoms.
In classical electrodynamics, nanostructured graphene is commonly modeled by
the computationally demanding problem of a three-dimensional conducting film of
atomic-scale thickness. Here, we propose an efficient alternative
two-dimensional electrostatic approach where all the calculation procedures are
restricted to the plane of the graphene sheet. To explore possible quantum
effects, we perform tight-binding calculations, adopting a random-phase
approximation. We investigate the multiple plasmon modes in triangles of
graphene, treating the optical response classically as well as quantum
mechanically in the case of both armchair and zigzag edge termination of the
underlying atomic lattice. Compared to the classical plasmonic spectrum which
is "blind" to the edge termination, we find that the quantum plasmon
frequencies exhibit blueshifts in the case of armchair edge termination, while
redshifts are found for zigzag edges. Furthermore, we find spectral features in
the zigzag case which are associated with electronic edge states not present
for armchair termination. Merging pairs of such triangles into dimers, the
plasmon hybridization leads to energy splitting in accordance with
plasmon-hybridization theory, with a lower energy for the antisymmetric modes
and a smaller splitting for modes with less confinement to the gap region. The
hybridization appears strongest in classical calculations while the splitting
is lower for armchair edges and even more reduced for zigzag edges. Our various
results illustrate a surprising phenomenon: Even 20 nm large graphene
structures clearly exhibit quantum plasmonic features due to atomic-scale
details in the edge termination.Comment: 27 pages including 7 figures. Supplementary information available
upon request to author
Electronic transport in Si nanowires: Role of bulk and surface disorder
We calculate the resistance and mean free path in long metallic and
semiconducting silicon nanowires (SiNWs) using two different numerical
approaches: A real space Kubo method and a recursive Green's function method.
We compare the two approaches and find that they are complementary: depending
on the situation a preferable method can be identified. Several numerical
results are presented to illustrate the relative merits of the two methods. Our
calculations of relaxed atomic structures and their conductance properties are
based on density functional theory without introducing adjustable parameters.
Two specific models of disorder are considered: Un-passivated, surface
reconstructed SiNWs are perturbed by random on-site (Anderson) disorder whereas
defects in hydrogen passivated wires are introduced by randomly removed H
atoms. The un-passivated wires are very sensitive to disorder in the surface
whereas bulk disorder has almost no influence. For the passivated wires, the
scattering by the hydrogen vacancies is strongly energy dependent and for
relatively long SiNWs (L>200 nm) the resistance changes from the Ohmic to the
localization regime within a 0.1 eV shift of the Fermi energy. This high
sensitivity might be used for sensor applications.Comment: 9 pages, 7 figures, submitted to Phys. Rev.
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