Letter | Published:

Valley Zeeman effect in elementary optical excitations of monolayer WSe2

Nature Physics volume 11, pages 141147 (2015) | Download Citation

Abstract

A monolayer of a transition metal dichalcogenide such as WSe2 is a two-dimensional direct-bandgap valley-semiconductor1,2 having an effective honeycomb lattice structure with broken inversion symmetry. The inequivalent valleys in the Brillouin zone could be selectively addressed using circularly polarized light fields3,4,5, suggesting the possibility for magneto-optical measurement and manipulation of the valley pseudospin degree of freedom 6,7,8. Here we report such experiments that demonstrate the valley Zeeman effect—strongly anisotropic lifting of the degeneracy of the valley pseudospin degree of freedom using an external magnetic field. The valley-splitting measured using the exciton transition deviates appreciably from values calculated using a three-band tight-binding model9 for an independent electron–hole pair at ±K valleys. We show, on the other hand, that a theoretical model taking into account the strongly bound nature of the exciton yields an excellent agreement with the experimentally observed splitting. In contrast to the exciton, the trion transition exhibits an unexpectedly large valley Zeeman effect that cannot be understood within the same framework, hinting at a different contribution to the trion magnetic moment. Our results raise the possibility of controlling the valley degree of freedom using magnetic fields in monolayer transition metal dichalcogenides or observing topological states of photons strongly coupled to elementary optical excitations in a microcavity10.

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Acknowledgements

We acknowledge many enlightening discussions with W. Yao, particularly regarding the role of the intracellular current and electron–hole exchange in optical excitations in TMDs. This work is supported by NCCR Quantum Science and Technology (NCCR QSIT), a research instrument of the Swiss National Science Foundation (SNSF).

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Affiliations

  1. Institute of Quantum Electronics, ETH Zurich, CH-8093 Zurich, Switzerland

    • Ajit Srivastava
    • , Meinrad Sidler
    •  & A. Imamoğlu
  2. Electrical Engineering Institute, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Zurich, Switzerland

    • Adrien V. Allain
    • , Dominik S. Lembke
    •  & Andras Kis

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Contributions

A.S. and M.S. carried out the optical measurements. A.V.A., D.S.L. and A.K. prepared the samples. A.S. and A.I. supervised the project and analysed the experimental data. All authors contributed extensively to this work.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Ajit Srivastava or A. Imamoğlu.

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DOI

https://doi.org/10.1038/nphys3203

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