700 research outputs found
Carbon–water flux coupling under progressive drought
Water-use efficiency, defined as the ratio of carbon assimilation over evapotranspiration (ET), is a key metric to assess ecosystem functioning in response to environmental conditions. It remains unclear which factors control this ratio during periods of extended water-limitation, and current semi-empirical water-use efficiency models fail to reproduce observed ET dynamics for these periods. Here, we use dry-down events occurring at eddy-covariance flux tower sites in the FLUXNET database as natural experiments to assess if and how decreasing soil-water availability modifies water-use efficiency on ecosystem scale. We demonstrate that an attenuating soil-water availability factor in junction with a previously discovered additive radiation term is necessary to accurately predict ET flux magnitudes and dry-down lengths of these water-limited periods. In an analysis of the attenuation, 20–33% of the observed decline in ET was due to the previously unconsidered soil-water availability effect. We conclude by noting the rates of ET decline differ significantly between FLUXNET sites with tall and short vegetation types and discuss the dependency of this rate on the variability of seasonal dryness
Thermopower of Kondo Effect in Single Quantum Dot Systems with Orbital at Finite Temperatures
We investigate the thermopower due to the orbital Kondo effect in a single
quantum dot system by means of the noncrossing approximation. It is elucidated
how the asymmetry of tunneling resonance due to the orbital Kondo effect
affects the thermopower under gate-voltage and magnetic-field control.Comment: 4 pages, 4 figures, proceeding of Second International Symposium on
Nanometer-Scale Quantum Physic
Electromechanics of charge shuttling in dissipative nanostructures
We investigate the current-voltage (IV) characteristics of a model
single-electron transistor where mechanical motion, subject to strong
dissipation, of a small metallic grain is possible. The system is studied both
by using Monte Carlo simulations and by using an analytical approach. We show
that electromechanical coupling results in a highly nonlinear IV-curve. For
voltages above the Coulomb blockade threshold, two distinct regimes of charge
transfer occur: At low voltages the system behave as a static asymmetric double
junction and tunneling is the dominating charge transfer mechanism. At higher
voltages an abrupt transition to a new shuttle regime appears, where the grain
performs an oscillatory motion back and forth between the leads. In this regime
the current is mainly mediated by charges that are carried on the grain as it
moves from one lead to the other.Comment: 8 pages, 10 figures, final version to be published in PR
Low frequency current noise of the single-electron shuttle
Coupling between electronic and mechanical degrees of freedom in a single
electron shuttle system can cause a mechanical instability leading to shuttle
transport of electrons between external leads. We predict that the resulting
low frequency current noise can be enhanced due to amplitude fluctuations of
the shuttle oscillations. Moreover, at the onset of mechanical instability a
pronounced peak in the low frequency noise is expected.Comment: 14 pages, 3 figures, 1 tabl
Status and Strategy for Moisture Metrology in European Metrology Institutes
Measurement of moisture in materials presents many challenges, due to diverse measuring principles, sample interactions with atmosphere, and variation in what is measured (either water content alone or moisture including other liquids). Calibrations are variously referenced to published standard methods, primary calibration facilities, or certified reference materials, but each of these addresses limited substances and ranges of measurement. Overall, metrology infrastructure is not as fully developed or coherent for this field as it is for many other areas of measurement. In order to understand the metrology needs and to support developments, several European national metrology institutes (NMIs) have undertaken some collaborative activities. These have included a “cooperation in research” project for sharing of information, a survey of moisture capabilities at NMIs, the formulation of a strategy for moisture metrology at the NMI level, and a funded research project to develop improved metrology for the moisture field. This paper summarizes the information gathered, giving an overview of the status of moisture metrology at NMIs, and it reports a proposed strategy to improve the current situation
Impact of van der Waals forces on the classical shuttle instability
The effects of including the van der Waals interaction in the modelling of
the single electron shuttle have been investigated numerically. It is
demonstrated that the relative strength of the vdW-forces and the elastic
restoring forces determine the characteristics of the shuttle instability. In
the case of weak elastic forces and low voltages the grain is trapped close to
one lead, and this trapping can be overcome by Coulomb forces by applying a
bias voltage larger than a threshold voltage . This allows for
grain motion leading to an increase in current by several orders of magnitude
above the transition voltage . Associated with the process is also
hysteresis in the I-V characteristics.Comment: minor revisions, updated references, Article published in Phys. Rev.
B 69, 035309 (2004
Steering of a Bosonic Mode with a Double Quantum Dot
We investigate the transport and coherence properties of a double quantum dot
coupled to a single damped boson mode. Our numerically results reveal how the
properties of the boson distribution can be steered by altering parameters of
the electronic system such as the energy difference between the dots.
Quadrature amplitude variances and the Wigner function are employed to
illustrate how the state of the boson mode can be controlled by a stationary
electron current through the dots.Comment: 10 pages, 6 figures, to appear in Phys. Rev.
Microscopic mechanisms of dephasing due to electron-electron interactions
We develop a non-perturbative numerical method to study tunneling of a single
electron through an Aharonov-Bohm ring where several strongly interacting
electrons are bound. Inelastic processes and spin-flip scattering are taken
into account. The method is applied to study microscopic mechanisms of
dephasing in a non-trivial model. We show that electron-electron interactions
described by the Hubbard Hamiltonian lead to strong dephasing: the transmission
probability at flux is high even at small interaction strength. In
addition to inelastic scattering, we identify two energy conserving mechanisms
of dephasing: symmetry-changing and spin-flip scattering. The many-electron
state on the ring determines which of these mechanisms will be at play:
transmitted current can occur either in elastic or inelastic channels, with or
without changing the spin of the scattering electron.Comment: 11 pages, 16 figures Submitted to Phys. Rev.
Temperature dependence of polaronic transport through single molecules and quantum dots
Motivated by recent experiments on electric transport through single
molecules and quantum dots, we investigate a model for transport that allows
for significant coupling between the electrons and a boson mode isolated on the
molecule or dot. We focus our attention on the temperature dependent properties
of the transport. In the Holstein picture for polaronic transport in molecular
crystals the temperature dependence of the conductivity exhibits a crossover
from coherent (band) to incoherent (hopping) transport. Here, the temperature
dependence of the differential conductance on resonance does not show such a
crossover, but is mostly determined by the lifetime of the resonant level on
the molecule or dot.Comment: 8 pages, 7 figure
Conductance oscillations in strongly correlated fractional quantum Hall line junctions
We present a detailed theory of transport through line junctions formed by
counterpropagating single-branch fractional-quantum-Hall edge channels having
different filling factors. Intriguing transport properties are exhibited when
strong Coulomb interactions between electrons from the two edges are present.
Such strongly correlated line junctions can be classified according to the
value of an effective line-junction filling factor n that is the inverse of an
even integer. Interactions turn out to affect transport most importantly for
n=1/2 and n=1/4. A particularly interesting case is n=1/4 corresponding to,
e.g., a junction of edge channels having filling factor 1 and 1/5,
respectively. We predict its differential tunneling conductance to oscillate as
a function of voltage. This behavior directly reflects the existence of novel
Majorana-fermion quasiparticle excitations in this type of line junction.
Experimental accessibility of such systems in current cleaved-edge overgrown
samples enables direct testing of our theoretical predictions.Comment: 2 figures, 10 pages, RevTex4, v2: added second figure for clarit
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