Quenching of Intervalley Exchange Coupling in the Presence of Momentum-Dark States in TMDCs

Monolayers of transition metal dichalcogenides are promising materials for valleytronic applications, since they possess two individually addressable excitonic transitions at the non-equivalent $K$ and $K'$ points with different spins, selectively excitable with light of opposite circular polarization. Here, it is of crucial importance to understand the elementary processes determining the lifetime of these optically injected valley excitons. In this study, we perform microscopic calculations based on a Heisenberg equation of motion formalism to investigate the efficiency of the intervalley coupling in the presence (W based TMDCs) and absence (Mo based TMDCs) of energetically low lying momentum-dark exciton states. While we predict a valley exciton lifetime on the order of some hundreds of fs in the absence of low lying momentum-dark states we demonstrate a strong quenching of the valley lifetime in the presence of such states

Radio-frequency discharges in Oxygen. Part 1: Modeling

In this series of three papers we present results from a combined experimental and theoretical effort to quantitatively describe capacitively coupled radio-frequency discharges in oxygen. The particle-in-cell Monte-Carlo model on which the theoretical description is based will be described in the present paper. It treats space charge fields and transport processes on an equal footing with the most important plasma-chemical reactions. For given external voltage and pressure, the model determines the electric potential within the discharge and the distribution functions for electrons, negatively charged atomic oxygen, and positively charged molecular oxygen. Previously used scattering and reaction cross section data are critically assessed and in some cases modified. To validate our model, we compare the densities in the bulk of the discharge with experimental data and find good agreement, indicating that essential aspects of an oxygen discharge are captured.Comment: 11 pages, 10 figure

Ein Blick in die wissenschaftliche Begründung der Homöopathie; drei Vorträge gehalten zu Stuttgart

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Loss mechanisms of negative oxygen ions in an inductively coupled rf discharge

A high fraction of negative ions (approx. 90 %) is observed in a pulsed oxygen rf discharge (13.56 MHz, 10 Pa). At the end of a discharge pulse of 1 ms duration both the axial and the radial density profiles of the negative ions coincide in the centre of the discharge with the density profile of the positive charge carriers. The dominant loss reactions -in particular of the negative ions -can be found from measurements of the temporal decay of the positive and negative charge carriers in the afterglow. Recombination with positive oxygen ions and collisions with atomic oxygen dominate the decay of the negative ions. These observations are consistent with the determination of the atomic oxygen density and determinations of the ion species (plasma monitor). Probe measurements indicate a production of electrons during this late phase. This can be explained by collisions of negative oxygen ions with atoms, whereby oxygen molecules are formed

Particle-in-cell Monte Carlo and fluid simulations of argon-oxygen plasma: Comparisons with experiments and validations

This article was published in the journal, Physics of plasmas and is also available at: http://pop.aip.org/pop/top.jsp or http://dx.doi.org/10.1063/1.2179430Particle-in-cell Monte Carlo collision (PIC-MCC) and fluid simulations of argon-oxygen plasmas in capacitively and inductively coupled plasma reactors are presented. Potential profiles and electron/ ion kinetic information such as electron/ion energy distributions and temperatures are compared with experimental data as well as with other analytical and numerical results. One-dimensional PIC-MCC simulations compare favorably with experimental data obtained in capacitively coupled reactors over a wide range of pressure and power. Two-dimensional fluid simulations of capacitive discharges differs from the results of PIC-MCC simulations as nonlocal effects play an important role in these discharges. Fluid simulations as nonlocal inductively coupled plasmas, however, agree favorably with experimental observations