Phonon-assisted Photoluminescence from Dark Excitons in Monolayers of Transition Metal Dichalcogenides

The photoluminescence (PL) spectrum of transition metal dichalcogenides (TMDs) shows a multitude of emission peaks below the bright exciton line and not all of them have been explained yet. Here, we study the emission traces of phonon-assisted recombinations of momentum-dark excitons. To this end, we develop a microscopic theory describing simultaneous exciton, phonon and photon interaction and including consistent many-particle dephasing. We explain the drastically different PL below the bright exciton in tungsten- and molybdenum-based materials as result of different configurations of bright and dark states. In good agreement with experiments, we show that WSe$_2$ exhibits clearly visible low-temperature PL signals stemming from the phonon-assisted recombination of momentum-dark excitons

Impurity intrusion in radio-frequency micro-plasma jets operated in ambient air

Space and time resolved concentrations of helium metastable atoms in an atmospheric pressure radio-frequency micro-plasma jet were measured using tunable diode laser absorption spectroscopy. Spatial profiles as well as lifetime measurements show significant influences of air entering the discharge from the front nozzle and of impurities originating from the gas supply system. Quenching of metastables was used to deduce quantitative concentrations of intruding impurities. The impurity profile along the jet axis was determined from optical emission spectroscopy as well as their dependance on the feed gas flow through the jet.Comment: Journal of Physics D: Applied Physics (accepted), 6 page

Toward seamless environments for dispute prevention and resolution

Given the evolution of the Information Technology society, it is now rather simple to acquire products or services in a foreign country. This practice may conduct to the event of conflicts whenever a consumer detects some fault or malfunction in services or products he/she had bought. A situation that may worsen if at the time of the uncovering of the defect, the shopper is already in a different geographical arena. There is thus the need to develop computational tools that may prevent these disputes from even happening. In this work it is proposed the development of seamless intelligent environments for dispute resolution that will surround the user, independently of his/her location. It is described the implementation of a prototype that may provide contextualized real-time information and legal support to consumers. The objective is to decrease the number of disputes due to a poor understanding in relation to the The Law and make justice more personalized and closer to people.The work described in this paper was developed under the TIARAC - Telematics and Artificial Intelligence in Alternative Conflict Resolution Project (PTDC/JUR/71354/2006), which is a research project supported by FCT (Science & Technology Foundation), Portuga

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