Search

We investigate theoretically the noise and the full counting statistics of electrons that are emitted from a superconductor into two spatially separated quantum dots by the splitting of Cooper pairs and further on collected in two normal-state electrodes. With negatively-biased drain electrodes and a large superconducting gap, the dynamics of the Cooper pair splitter can be described by a Markovian quantum master equation. Using techniques from full counting statistics, we evaluate the electrical currents, their noise power spectra, and the power-power correlations in the output leads. The current fluctuations can be attributed to the competition between Cooper pair splitting and elastic cotunneling between the quantum dots via the superconductor. In one regime, these processes can be clearly distinguished in the cross-correlation spectrum with peaks and dips appearing at characteristic frequencies associated with elastic cotunneling and Cooper pair splitting, respectively. We corroborate this interpretation by analyzing the charge transport fluctuations in the time domain, specifically by investigating the $g^{(2)}$-function of the output currents. Our work identifies several experimental signatures of the fundamental transport processes involved in Cooper pair splitting and provides specific means to quantify their relative strengths. As such, our results may help guide and interpret future experiments on current fluctuations in Cooper pair splitters., Comment: 15 pages, 6 figures

By means of an electron hole rate equation model we explain the phase dynamics of a quantum dot semiconductor optical amplifier and the appearance of different decay times observed in pump and probe experiments. The ultrafast hole relaxation leads to a first ultrafast recovery of the gain, followed by electron relaxation and, in the nanosecond timescale, radiative and non-radiative recombinations. The phase dynamics is slower and is affected by thermal redistribution of carriers within the dot. We explain the ultrafast response of quantum dot amplifiers as an effect of hole escape and recombination without the need to assume Auger processes.

We propose a scheme of rate equations for stable dimers that permits the description quite satisfactorily of the evolution of the number of islands in the entire range of surface coverage. The characteristic time for coalescence and the capture number are computed through a stochastic approach for both random and nonrandom arrangements of nuclei. Rate equations are applied to describe kinetic Monte Carlo simulations previously published, in the whole range of surface coverages. It is found that to reproduce the simulation, the effect of the nonrandomicity of the nuclei distribution must be taken into account.

Multiphoton ionisation and dissociation of NO2 has been studied experimentally at 375 nm for laser pulse widths of 10 ns and 50 fs. The parent NO2 ion peak is not seen in the ns data. In all spectra, the main peak observed is due to the ionisation of the NO molecule which results from the dissociation of excited NO2 formed after absorbing a 375 nm photon. The intensity dependencies of both NO and NO2 ion peaks have also been measured. The data has been analysed within the context of a rate equation model using published cross-sections and dissociation rates except for the two-photon ionisation cross-section for NO2 which was chosen to reproduce the NO2/NO ion signal ratios at 50 fs. The rate equation model provides a good description of the complete set of data. Indirectly, it may be concluded that coherence effects do not play an important role in the multiphoton excitation/ionisation of NO2. The data also rules out the importance of above-ionisation dissociation in NO2 - a conclusion which is consistent with previous data at 496 and 248 nm for laser pulse widths greater than or equal to 300 fs. Chemical Physics Letters