Wydarzenia

Seminarium Fizyki Politechniki Wrocławskiej

Magneto-optics of Bright and Dark Excitons in Transition Metal Dichalcogenide Monolayers

dr hab. Maciej Molas, Prof. UW

Wydział Fizyki, Uniwersytet Warszawski

Monolayers (MLs) of semiconducting transition metal dichalcogenides (TMDs) MX₂, where M = Mo or W and X = S, Se, or Te, are direct band gap semiconductors. The strong spin−orbit interaction and the lack of inversion symmetry results in two subgroups of TMD MLs: “bright” (the excitonic ground state is optically active or bright) comprising MoSe₂ and MoTe₂, and “darkish” (the excitonic ground state is optically inactive or dark) composed of MoS₂, WS₂, and WSe₂.

I will start my talk by giving a concise overview of excitonic complexes in TMD MLs. In particular, I will discuss the differences between “bright” and “darkish” MLs in terms of their optical transitions. [1].

In the second part, I will discuss the effect of temperature and electrostatic doping on the brightening dynamics of dark excitons and trions in a WSe₂ MLs under an in-plane magnetic field [2,3]. Special attention will be given to the competition between magnetic activation of dark states and their thermal depopulation, as well as to the role of carrier density in controlling the relative populations of neutral and charged dark complexes. These effects lead to a significant modification of the emission spectra, including the suppression of dark-related emission and the enhancement of bright excitonic features.

The next part of my talk will be devoted to the magneto-optical properties of excitonic complexes in alloyed TMD MLs from the Mo­_xW_1-xSe₂ family [4]. In particular, I will discuss how alloying enables continuous tuning of excitonic g factors, leading to exceptionally large values for neutral excitons. While in the parent compounds the g factor of neutral excitons is typically around −4, in the alloys it can reach values as large as about −10 for compositions close to  x≈0.2. I will also address the microscopic origin of this effect, which is related to the alloy-induced mixing of electronic states from different valleys in the conduction band, resulting in a nonmonotonic dependence of the g factor on composition.