Sub-Poissonian Shot Noise In A Diffusive Conductor

A review is given of the shot-noise properties of metallic, diffusive conductors. The shot noise is one third of the Poisson noise, due to the bimodal distribution of transmission eigenvalues. The same result can be obtained from a semiclassical calculation. Starting from Oseledec's theorem it is shown that the bimodal distribution is required by Ohm's law.Comment: 9 pages, LaTeX, including 2 figure

Doubled Shot Noise In Disordered Normal-Metal-Superconductor Junctions

The low-frequency shot-noise power of a normal-metal-superconductor junction is studied for arbitrary normal region. Through a scattering approach, a formula is derived which expresses the shot-noise power in terms of the transmission eigenvalues of the normal region. The noise power divided by the current is enhanced by a factor two with respect to its normal-state value, due to Cooper-pair transport in the superconductor. For a disordered normal region, it is still smaller than the Poisson noise, as a consequence of noiseless open scattering channels.Comment: 4 pages, RevTeX v3.0, including 1 figure, Submitted to Physical Review

Gap theory of rectification in ballistic three-terminal conductors

We introduce a model for rectification in three-terminal ballistic conductors, where the central connecting node is modeled as a chaotic cavity. For bias voltages comparable to the Fermi energy, a strong nonlinearity is created by the opening of a gap in the transport window. Both noninteracting cavity electrons at arbitrary temperature as well as the hot electron regime are considered. Charging effects are treated within the transmission formalism using a self-consistent analysis. The conductance of the third lead in a voltage probe configuration is varied to also model inelastic effects. We find that the basic transport features are insensitive to all of these changes, indicating that the nonlinearity is robust and well suited to applications such as current rectification in ballistic systems. Our findings are in broad agreement with several recent experiments.Comment: 8 pages, 6 figure

Transport of interacting electrons in arrays of quantum dots and diffusive wires

We develop a detailed theoretical investigation of the effect of Coulomb interaction on electron transport in arrays of chaotic quantum dots and diffusive metallic wires. Employing the real time path integral technique we formulate a new Langevin-type of approach which exploits a direct relation between shot noise and interaction effects in mesoscopic conductors. With the aid of this approach we establish a general expression for the Fano factor of 1D quantum dot arrays and derive a complete formula for the interaction correction to the current which embraces all perturbative results previously obtained for various quasi-0D and quasi-1D disordered conductors and extends these results to yet unexplored regimes.Comment: 12 pages, 2 figure

The evolution of the stellar populations in low surface brightness galaxies

We investigate the star formation history and chemical evolution of low surface brightness (LSB) disk galaxies by modelling their observed spectro-photometric and chemical properties using a galactic chemical and photometric evolution model incorporating a detailed metallicity depen dent set of stellar input data. For a large fraction of the LSB galaxies in our sample, observed properties are best explained by models incorporating an exponentially decreasing global star formation rate (SFR) ending at a present-day gas fraction (M_{gas}/(M_{gas}+M_{stars}) = 0.5 for a galaxy age of 14 Gyr. For some galaxies small amplitude star formation bursts are required to explain the contribution of the young (5-50 Myr old) stellar population to the galaxy integrated luminosity. This suggests that star formation has proceeded in a stochastic manner. The presence of an old stellar population in many late-type LSB galaxies suggests that LSB galaxies roughly follow the same evolutionary history as HSB galaxies, except at a much lower rate. In particular, our results imply that LSB galaxies do not form late, nor have a delayed onset of star formation, but simply evolve slowly.Comment: To be published in A&

On Star Formation and the Non-Existence of Dark Galaxies

We investigate whether a baryonic dark galaxy or `galaxy without stars' could persist indefinitely in the local universe, while remaining stable against star formation. To this end, a simple model has been constructed to determine the equilibrium distribution and composition of a gaseous protogalactic disk. Specifically, we determine the amount of gas that will transit to a Toomre unstable cold phase via the H2 cooling channel in the presence of a UV--X-ray cosmic background radiation field. All but one of the models are predicted to become unstable to star formation. Moreover, we find that all our model objects would be detectable via HI line emission, even in the case that star formation is potentially avoided. These results are consistent with the non-detection of isolated extragalactic HI clouds with no optical counterpart (galaxies without stars) by HIPASS. Additionally, where star formation is predicted to occur, we determine the minimum interstellar radiation field required to restore gravothermal stability, which we then relate to a minimum global star formation rate. This leads to the prediction of a previously undocumented relation between HI mass and star formation rate that is observed for a wide variety of dwarf galaxies in the HI mass range 10^8--10^10 M_sun. The existence of such a relation strongly supports the notion that the well observed population of dwarf galaxies represent the minimum rates of self-regulating star formation in the universe. (Barely abridged)Comment: 19 pages, 8 figures, TeX using emulateapj.cls, v2 accepted for publication in ApJ (16/8/5) with one figure deleted and a number of minor clarifying revision

Field efficacy of new compounds to replace copper for scab control in organic apple production

Efficacy of compounds was investigated in field experiments in Denmark and the Netherlands according to EPPO guidelines. Some Yucca extracts and potassium bicarbonate had an efficacy similar to sulphur on leaves. Addition of sulphur to Yucca 1 and to potassium bicarbonate increased the level of efficacy to that of copper. This confirms results from earlier years. Although a dose increase resulted in better efficacy, this was more prominent for Yucca1 than for potassium bicarbonate

Transport Theoretical Approach to the Nucleon Spectral Function in Nuclear Matter

The nucleon spectral function in infinite nuclear matter is calculated in a quantum transport theoretical approach. Exploiting the known relation between collision rates and correlation functions the spectral function is derived self-consistently. By re-inserting the spectral functions into the collision integrals the description of hard processes from the high-momentum components of wave functions and interactions is improved iteratively until convergence is achieved. The momentum and energy distributions and the nuclear matter occupation probabilities are in very good agreement with the results obtained from many-body theory.Comment: minor changes in the text, additional curves in fig.

Universality of Shot-Noise in Multiterminal Diffusive Conductors

We prove the universality of shot-noise in multiterminal diffusive conductors of arbitrary shape and dimension for purely elastic scattering as well as for hot electrons. Using a Boltzmann-Langevin approach we reduce the calculation of shot-noise correlators to the solution of a diffusion equation. We show that shot-noise in multiterminal conductors is a non-local quantity and that exchange effects can occur without quantum phase coherence even at zero electron temperature. Concrete numbers for shot-noise are given that can be tested experimentally.Comment: 4 double-column pages, REVTeX, 1 eps figure embedded with eps

Split-gate quantum point contacts with tunable channel length

We report on developing split-gate quantum point contacts (QPCs) that have a tunable length for the transport channel. The QPCs were realized in a GaAs/AlGaAs heterostructure with a two- dimensional electron gas (2DEG) below its surface. The conventional design uses 2 gate fingers on the wafer surface which deplete the 2DEG underneath when a negative gate voltage is applied, and this allows for tuning the width of the QPC channel. Our design has 6 gate fingers and this provides additional control over the form of the electrostatic potential that defines the channel. Our study is based on electrostatic simulations and experiments and the results show that we developed QPCs where the effective channel length can be tuned from about 200 nm to 600 nm. Length-tunable QPCs are important for studies of electron many-body effects because these phenomena show a nanoscale dependence on the dimensions of the QPC channel