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(1) Presentation(s)

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Mar. 31/01/2017 14:00 Grande Ourse, Bâtiment 13, Etage 1 (à confirmer)

Colloquium
PLOCHOCKA Paulina (Laboratoire National des Champs Magnétiques Intenses (LNCMI))
Toulouse
Excitons in mono and tri–layers transition metal dichalcogenides

Sommaire:

Monolayer transition metal dichalcogenides (TMDs) have recently emerged as a new material system in which coupled spin-valley physics can be explored. In this talk I will discuss the optical and magneto optical properties of excitons in mono and tri – layer transitions metal dichalcogenides (TMDs).
First, I will show that optical spectroscopy in high magnetic fields B < 70 T reveals the very different nature of carriers in monolayer and bulk transition metal dichalcogenides. In mechanically exfoliated monolayer WSe2, the exciton emission shifts linearly with the magnetic field and exhibits a splitting which originates from the magnetic field induced valley splitting. The monolayer data can be described using a single particle picture with a Dirac-like Hamiltonian for massive Dirac fermions, with an additional term to phenomenologically include the valley splitting. In contrast, in bulk WSe2 where the inversion symmetry is restored, transmission measurements show a distinctly excitonic behavior with absorption to the 1s and 2s states. Magnetic field induces a spin splitting together with a small diamagnetic shift and cyclotron like behavior at high fields, which is best described within the hydrogen model. Additionally, in the monolayer material valley polarization is induced by magnetic field for a charged exciton demonstrating it’s inter valley character [1]. Furthermore I will compare the optical and mechanical properties of the exfoliated and large area single layer grown by Chemical vapor deposition. The results of the magneto optical studies suggests, surprisingly, that the 0.2% tensile strain, which is naturally present in large area CVD grown Mo based TMDs on sapphire, does not represent any serious impediment for device applications [2]. Finally, I will show our very recent results of the optical properties of the trilayer structures such as MoS2/MoSe2/MoS2 obtained by a combination of CVD growth and mechanical transfer. First, our results demonstrate a new, simple, way to encapsulate monolayer TMDS toward better optical/electrical properties. The optical properties of MoSe2 improve dramatically after embedding it into MoS2. Furthermore, while optically pumping in MoS2, the charge transfer between the layers is observed in tri layer structures leading to valley polarization of MoSe2. These results demonstrate that the spin is preserved during the charge transfer [3]. In the end, the measurements of the valley polarization of MoSe2 obtained for resonant excitation as close as 10 meV suggest that inter valley scattering is dominated by exchange interaction rather than by phonons. The experimental results are further supported by a simple model [4].


[1] A. Mitioglu, PP et al, Nano Letters 15, 4387 (2015)
[2] A. Mitioglu, PP et al, PRB 93, 165412 (2016)
[3] A. Surrente, PP et al under preparation
[4] M. Baranowski, PP et al submitted


Pour plus d'informations, merci de contacter Rousseau E.