4,341 research outputs found
Voltage probe model of spin decay in a chaotic quantum dot, with applications to spin-flip noise and entanglement production
The voltage probe model is a model of incoherent scattering in quantum
transport. Here we use this model to study the effect of spin-flip scattering
on electrical conduction through a quantum dot with chaotic dynamics. The spin
decay rate gamma is quantified by the correlation of spin-up and spin-down
current fluctuations (spin-flip noise). The resulting decoherence reduces the
ability of the quantum dot to produce spin-entangled electron-hole pairs. For
gamma greater than a critical value gamma_c, the entanglement production rate
vanishes identically. The statistical distribution P(gamma_c) of the critical
decay rate in an ensemble of chaotic quantum dots is calculated using the
methods of random-matrix theory. For small gamma_c this distribution is
proportional to gamma_c^(-1+beta/2), depending on the presence (beta=1) or
absence (beta=2) of time-reversal symmetry. To make contact with experimental
observables, we derive a one-to-one relationship between the entanglement
production rate and the spin-resolved shot noise, under the assumption that the
density matrix is isotropic in the spin degrees of freedom. Unlike the Bell
inequality, this relationship holds for both pure and mixed states. In the
tunneling regime, the electron-hole pairs are entangled if and only if the
correlator of parallel spin currents is at least twice larger than the
correlator of antiparallel spin currents.Comment: version 3: corrected a factor of two in Eq. (3.16), affecting the
final result
Counting statistics of coherent population trapping in quantum dots
Destructive interference of single-electron tunneling between three quantum
dots can trap an electron in a coherent superposition of charge on two of the
dots. Coupling to external charges causes decoherence of this superposition,
and in the presence of a large bias voltage each decoherence event transfers a
certain number of electrons through the device. We calculate the counting
statistics of the transferred charges, finding a crossover from sub-Poissonian
to super-Poissonian statistics with increasing ratio of tunnel and decoherence
rates.Comment: 4 pages, 2 figure
Koszul Theorem for S-Lie coalgebras
For a symmetry braid S-Lie coalgebras, as a dual object to algebras
introduced by Gurevich, are considered. For an Young antisymmetrizer an
S-exterior algebra is introduced. From this differential point of view S-Lie
coalgebras are investigated. The dual Koszul theorem in this case is proved.Comment: 8 pages, AMSLaTe
On the 3.35 ppm singlet resonance in proton NMR spectra of brain tissue: Scyllo-inositol or methanol contamination?
Tidal controls on trace gas dynamics in a seagrass meadow of the Ria Formosa lagoon (southern Portugal)
Coastal zones are important source regions for a variety of trace gases, including halocarbons and sulfur-bearing species. While salt marshes, macroalgae and phyto-plankton communities have been intensively studied, little is known about trace gas fluxes in seagrass meadows. Here we report results of a newly developed dynamic flux chamber system that can be deployed in intertidal areas over full tidal cycles allowing for highly time-resolved measurements. The fluxes of CO2, methane (CH4) and a range of volatile organic compounds (VOCs) showed a complex dynamic mediated by tide and light. In contrast to most previous studies, our data indicate significantly enhanced fluxes during tidal immersion relative to periods of air exposure. Short emission peaks occurred with onset of the feeder current at the sampling site. We suggest an overall strong effect of advective transport processes to explain the elevated fluxes during tidal immersion. Many emission estimates from tidally influenced coastal areas still rely on measurements carried out during low tide only. Hence, our results may have significant implications for budgeting trace gases in coastal areas. This dynamic flux chamber system provides intensive time series data of community respiration (at night) and net community production (during the day) of shallow coastal systems.German Federal Ministry of Education and Research (BMBF) [03F0611E, 03F0662E]; EU FP7 ASSEMBLE research infrastructure initiative
Enzyme kinetics for a two-step enzymic reaction with comparable initial enzyme-substrate ratios
We extend the validity of the quasi-steady state assumption for a model double intermediate enzyme-substrate reaction to include the case where the ratio of initial enzyme to substrate concentration is not necessarily small. Simple analytical solutions are obtained when the reaction rates and the initial substrate concentration satisfy a certain condition. These analytical solutions compare favourably with numerical solutions of the full system of differential equations describing the reaction. Experimental methods are suggested which might permit the application of the quasi-steady state assumption to reactions where it may not have been obviously applicable before
Michaelis-Menten dynamics in protein subnetworks
To understand the behaviour of complex systems it is often necessary to use
models that describe the dynamics of subnetworks. It has previously been
established using projection methods that such subnetwork dynamics generically
involves memory of the past, and that the memory functions can be calculated
explicitly for biochemical reaction networks made up of unary and binary
reactions. However, many established network models involve also
Michaelis-Menten kinetics, to describe e.g. enzymatic reactions. We show that
the projection approach to subnetwork dynamics can be extended to such
networks, thus significantly broadening its range of applicability. To derive
the extension we construct a larger network that represents enzymes and enzyme
complexes explicitly, obtain the projected equations, and finally take the
limit of fast enzyme reactions that gives back Michaelis-Menten kinetics. The
crucial point is that this limit can be taken in closed form. The outcome is a
simple procedure that allows one to obtain a description of subnetwork
dynamics, including memory functions, starting directly from any given network
of unary, binary and Michaelis-Menten reactions. Numerical tests show that this
closed form enzyme elimination gives a much more accurate description of the
subnetwork dynamics than the simpler method that represents enzymes explicitly,
and is also more efficient computationally
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