Magnetic field induced polarization enhancement in monolayers of tungsten dichalcogenides: Effects of temperature

Optical orientation of localized/bound excitons is shown to be effectively enhanced by the application of magnetic fields as low as 20 mT in monolayer WS$_2$. At low temperatures, the evolution of the polarization degree of different emission lines of monolayer WS$_2$ with increasing magnetic fields is analyzed and compared to similar results obtained on a WSe$_2$ monolayer. We study the temperature dependence of this effect up to $T=60$ K for both materials, focusing on the dynamics of the valley pseudospin relaxation. A rate equation model is used to analyze our data and from the analysis of the width of the polarization deep in magnetic field we conclude that the competition between the dark exciton pseudospin relaxation and the decay of the dark exciton population into the localized states are rather different in these two materials which are representative of the two extreme cases for the ratio of relaxation rate and depolarization rate

1-Methyl-1-azonia-3,5-diaza-7-phosphatricyclo­[3.3.1.13,7]decane tetra­fluoro­borate

The title compound, C7H15N3P+·BF4 − or [PTA-Me][BF4], is the N-methyl­ated derivative of the well known water-soluble amino­phosphine 1,3,5-triaza-7-phosphaadamantane (PTA). The asymmetric unit consists of a cage-like cation [PTA-Me]+ and a disordered tetra­fluoro­borate anion; two F atoms are disordered equally over two sites. A network of weak inter­molecular C—H⋯F hydrogen bonds results in a three-dimensional supra­molecular assembly

1-Methyl-1-azonia-3,5-diaza-7-phospha­tricyclo­[3.3.1.1]decane 7-oxide triiodide

The title compound, C7H15N3OP+·I3 −, is a derivative of the well known water-soluble amino­phosphine 1,3,5-triaza-7-phosphaadamantane (PTA). The crystal structure is composed of a cage-like 1-methyl-1-azonia-3,5-diaza-7-phospha­tricyclo­[3.3.1.1]decane 7-oxide cation and a triiodide anion. The N-methyl­ation of the PTA cage results in a slight elongation of the corresponding C—N bonds, while the oxidation of the P atom leads to a slight shortening of the C—P bonds in comparison with those of PTA. In general, most of the bonding parameters are comparable with those reported for related compounds bearing the PTA core. Two inter­molecular C—H⋯O hydrogen bonds between methyl­ene groups and the P=O group are responsible for the linkage of neighbouring cations into linear one-dimensional hydrogen-bonded chains