Inverted valley polarization in optically excited transition metal dichalcogenides

Gunnar Berghäuser (), Ivan Bernal-Villamil, Robert Schmidt, Robert Schneider, Iris Niehues, Paul Erhart, Steffen Michaelis de Vasconcellos, Rudolf Bratschitsch, Andreas Knorr and Ermin Malic
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Gunnar Berghäuser: Chalmers University of Technology
Ivan Bernal-Villamil: Chalmers University of Technology
Robert Schmidt: University of Münster
Robert Schneider: University of Münster
Iris Niehues: University of Münster
Paul Erhart: Chalmers University of Technology
Steffen Michaelis de Vasconcellos: University of Münster
Rudolf Bratschitsch: University of Münster
Andreas Knorr: Technische Universität Berlin
Ermin Malic: Chalmers University of Technology

Nature Communications, 2018, vol. 9, issue 1, 1-8

Abstract: Abstract Large spin–orbit coupling in combination with circular dichroism allows access to spin-polarized and valley-polarized states in a controlled way in transition metal dichalcogenides. The promising application in spin-valleytronics devices requires a thorough understanding of intervalley coupling mechanisms, which determine the lifetime of spin and valley polarizations. Here we present a joint theory–experiment study shedding light on the Dexter-like intervalley coupling. We reveal that this mechanism couples A and B excitonic states in different valleys, giving rise to an efficient intervalley transfer of coherent exciton populations. We demonstrate that the valley polarization vanishes and is even inverted for A excitons, when the B exciton is resonantly excited and vice versa. Our theoretical findings are supported by energy-resolved and valley-resolved pump-probe experiments and also provide an explanation for the recently measured up-conversion in photoluminescence. The gained insights might help to develop strategies to overcome the intrinsic limit for spin and valley polarizations.

Date: 2018
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DOI: 10.1038/s41467-018-03354-1

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