2,502 research outputs found
Coherent Particle Transfer in an On-Demand Single-Electron Source
Coherent electron transfer from a localized state trapped in a quantum dot
into a ballistic conductor, taking place in on-demand electron sources, in
general may result in excitation of particle-hole pairs. We consider a simple
model for these effects, involving a resonance level with time-dependent
energy, and derive Floquet scattering matrix describing inelastic transitions
of particles in the Fermi sea. We find that, as the resonance level is driven
through the Fermi level, particle transfer may take place completely without
particle-hole excitations for certain driving protocols. In particular, such
noiseless transfer occurs when the level moves with constant rapidity, its
energy changing linearly with time. A detection scheme for studying the
coherence of particle transfer is proposed.Comment: 5 pages, 3 figures. Updated introduction, Fig. 1, and reference
Global surface-ocean pCO2 and sea–air CO2 flux variability from an observation-driven ocean mixed-layer scheme
A temporally and spatially resolved estimate of the global surface-ocean CO<sub>2</sub> partial pressure field and the sea–air CO<sub>2</sub> flux is presented, obtained by fitting a simple data-driven diagnostic model of ocean mixed-layer biogeochemistry to surface-ocean CO<sub>2</sub> partial pressure data from the SOCAT v1.5 database. Results include seasonal, interannual, and short-term (daily) variations. In most regions, estimated seasonality is well constrained from the data, and compares well to the widely used monthly climatology by Takahashi et al. (2009). Comparison to independent data tentatively supports the slightly higher seasonal variations in our estimates in some areas. We also fitted the diagnostic model to atmospheric CO<sub>2</sub> data. The results of this are less robust, but in those areas where atmospheric signals are not strongly influenced by land flux variability, their seasonality is nevertheless consistent with the results based on surface-ocean data. From a comparison with an independent seasonal climatology of surface-ocean nutrient concentration, the diagnostic model is shown to capture relevant surface-ocean biogeochemical processes reasonably well. Estimated interannual variations will be presented and discussed in a companion paper
Non-equilibrium quantum condensation in an incoherently pumped dissipative system
We study spontaneous quantum coherence in an out of equilibrium system,
coupled to multiple baths describing pumping and decay. For a range of
parameters describing coupling to, and occupation of the baths, a stable
steady-state condensed solution exists. The presence of pumping and decay
significantly modifies the spectra of phase fluctuations, leading to
correlation functions that differ both from an isolated condensate and from a
laser.Comment: 5 pages, 2 eps figure
Stochastic oscillations in models of epidemics on a network of cities
We carry out an analytic investigation of stochastic oscillations in a
susceptible-infected-recovered model of disease spread on a network of
cities. In the model a fraction of individuals from city commute
to city , where they may infect, or be infected by, others. Starting from a
continuous time Markov description of the model the deterministic equations,
which are valid in the limit when the population of each city is infinite, are
recovered. The stochastic fluctuations about the fixed point of these equations
are derived by use of the van Kampen system-size expansion. The fixed point
structure of the deterministic equations is remarkably simple: a unique
non-trivial fixed point always exists and has the feature that the fraction of
susceptible, infected and recovered individuals is the same for each city
irrespective of its size. We find that the stochastic fluctuations have an
analogously simple dynamics: all oscillations have a single frequency, equal to
that found in the one city case. We interpret this phenomenon in terms of the
properties of the spectrum of the matrix of the linear approximation of the
deterministic equations at the fixed point.Comment: 13 pages, 7 figure
Phase lag in epidemics on a network of cities
We study the synchronisation and phase-lag of fluctuations in the number of
infected individuals in a network of cities between which individuals commute.
The frequency and amplitude of these oscillations is known to be very well
captured by the van Kampen system-size expansion, and we use this approximation
to compute the complex coherence function that describes their correlation. We
find that, if the infection rate differs from city to city and the coupling
between them is not too strong, these oscillations are synchronised with a well
defined phase lag between cities. The analytic description of the effect is
shown to be in good agreement with the results of stochastic simulations for
realistic population sizes.Comment: 10 pages, 6 figure
Monte Carlo simulation of the transmission of measles: Beyond the mass action principle
We present a Monte Carlo simulation of the transmission of measles within a
population sample during its growing and equilibrium states by introducing two
different vaccination schedules of one and two doses. We study the effects of
the contact rate per unit time as well as the initial conditions on the
persistence of the disease. We found a weak effect of the initial conditions
while the disease persists when lies in the range 1/L-10/L ( being
the latent period). Further comparison with existing data, prediction of future
epidemics and other estimations of the vaccination efficiency are provided.
Finally, we compare our approach to the models using the mass action
principle in the first and another epidemic region and found the incidence
independent of the number of susceptibles after the epidemic peak while it
strongly fluctuates in its growing region. This method can be easily applied to
other human, animals and vegetable diseases and includes more complicated
parameters.Comment: 15 pages, 4 figures, 1 table, Submitted to Phys.Rev.
Extracting the time-dependent transmission rate from infection data via solution of an inverse ODE problem
The transmission rate of many acute infectious diseases varies significantly in time, but the underlying mechanisms are usually uncertain. They may include seasonal changes in the environment, contact rate, immune system response, etc. The transmission rate has been thought difficult to measure directly. We present a new algorithm to compute the time-dependent transmission rate directly from prevalence data, which makes no assumptions about the number of susceptible or vital rates. The algorithm follows our complete and explicit solution of a mathematical inverse problem for SIR-type transmission models. We prove that almost any infection profile can be perfectly fitted by an SIR model with variable transmission rate. This clearly shows a serious danger of overfitting such transmission models. We illustrate the algorithm with historic UK measles data and our observations support the common belief that measles transmission was predominantly driven by school contacts
Spin-glasses in optical cavity
Recent advances in nanofabrication and optical control have garnered
tremendous interest in multi-qubit-cavity systems. Here we analyze a spin-glass
version of such a nanostructure, solving analytically for the phase diagrams in
both the matter and radiation subsystems in the replica symmetric regime.
Interestingly, the resulting phase transitions turn out to be tunable simply by
varying the matter-radiation coupling strength
The effects of CO2, climate and land-use on terrestrial carbon balance, 1920-1992: An analysis with four process-based ecosystem models
The concurrent effects of increasing atmospheric CO2 concentration, climate variability, and cropland establishment and abandonment on terrestrial carbon storage between 1920 and 1992 were assessed using a standard simulation protocol with four process-based terrestrial biosphere models. Over the long-term(1920–1992), the simulations yielded a time history of terrestrial uptake that is consistent (within the uncertainty) with a long-term analysis based on ice core and atmospheric CO2 data. Up to 1958, three of four analyses indicated a net release of carbon from terrestrial ecosystems to the atmosphere caused by cropland establishment. After 1958, all analyses indicate a net uptake of carbon by terrestrial ecosystems, primarily because of the physiological effects of rapidly rising atmospheric CO2. During the 1980s the simulations indicate that terrestrial ecosystems stored between 0.3 and 1.5 Pg C yr−1, which is within the uncertainty of analysis based on CO2 and O2 budgets. Three of the four models indicated (in accordance with O2 evidence) that the tropics were approximately neutral while a net sink existed in ecosystems north of the tropics. Although all of the models agree that the long-term effect of climate on carbon storage has been small relative to the effects of increasing atmospheric CO2 and land use, the models disagree as to whether climate variability and change in the twentieth century has promoted carbon storage or release. Simulated interannual variability from 1958 generally reproduced the El Niño/Southern Oscillation (ENSO)-scale variability in the atmospheric CO2 increase, but there were substantial differences in the magnitude of interannual variability simulated by the models. The analysis of the ability of the models to simulate the changing amplitude of the seasonal cycle of atmospheric CO2 suggested that the observed trend may be a consequence of CO2 effects, climate variability, land use changes, or a combination of these effects. The next steps for improving the process-based simulation of historical terrestrial carbon include (1) the transfer of insight gained from stand-level process studies to improve the sensitivity of simulated carbon storage responses to changes in CO2 and climate, (2) improvements in the data sets used to drive the models so that they incorporate the timing, extent, and types of major disturbances, (3) the enhancement of the models so that they consider major crop types and management schemes, (4) development of data sets that identify the spatial extent of major crop types and management schemes through time, and (5) the consideration of the effects of anthropogenic nitrogen deposition. The evaluation of the performance of the models in the context of a more complete consideration of the factors influencing historical terrestrial carbon dynamics is important for reducing uncertainties in representing the role of terrestrial ecosystems in future projections of the Earth system
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