Letter | Published:

Magnetic control of valley pseudospin in monolayer WSe2

Nature Physics volume 11, pages 148152 (2015) | Download Citation

Abstract

Local energy extrema of the bands in momentum space, or valleys, can endow electrons in solids with pseudospin in addition to real spin1,2,3,4,5. In transition metal dichalcogenides this valley pseudospin, like real spin, is associated with a magnetic moment1,6 that underlies the valley-dependent circular dichroism6 that allows optical generation of valley polarization7,8,9, intervalley quantum coherence10 and the valley Hall effect11. However, magnetic manipulation of valley pseudospin via this magnetic moment12,13, analogous to what is possible with real spin, has not been shown before. Here we report observation of the valley Zeeman splitting and magnetic tuning of polarization and coherence of the excitonic valley pseudospin, by performing polarization-resolved magneto-photoluminescence on monolayer WSe2. Our measurements reveal both the atomic orbital and lattice contributions to the valley orbital magnetic moment; demonstrate the deviation of the band edges in the valleys from an exact massive Dirac fermion model; and reveal a striking difference between the magnetic responses of neutral and charged valley excitons that is explained by renormalization of the excitonic spectrum due to strong exchange interactions.

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References

  1. 1.

    , , & Spin and pseudospins in layered transition metal dichalcogenides. Nature Phys. 10, 343–350 (2014).

  2. 2.

    , , , & Field-induced polarization of Dirac valleys in bismuth. Nature Phys. 8, 89–94 (2012).

  3. 3.

    , & Valley filter and valley valve in graphene. Nature Phys. 3, 172–175 (2007).

  4. 4.

    et al. Valley susceptibility of an interacting two-dimensional electron system. Phys. Rev. Lett. 97, 186404 (2006).

  5. 5.

    , , & Quantized conductance in an AlAs two-dimensional electron system quantum point contact. Phys. Rev. B 74, 155436 (2006).

  6. 6.

    , , , & Coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides. Phys. Rev. Lett. 108, 196802 (2012).

  7. 7.

    , , , & Valley polarization in MoS2 monolayers by optical pumping. Nature Nanotech. 7, 490–493 (2012).

  8. 8.

    et al. Valley-selective circular dichroism of monolayer molybdenum disulphide. Nature Commun. 3, 887 (2012).

  9. 9.

    , , & Control of valley polarization in monolayer MoS2 by optical helicity. Nature Nanotech. 7, 494–498 (2012).

  10. 10.

    et al. Optical generation of excitonic valley coherence in monolayer WSe2. Nature Nanotech. 8, 634–638 (2013).

  11. 11.

    , , & The valley Hall effect in MoS2 transistors. Science 344, 1489–1492 (2014).

  12. 12.

    , & Unconventional quantum Hall effect and tunable spin Hall effect in Dirac materials: Application to an isolated MoS2 trilayer. Phys. Rev. Lett. 110, 066803 (2013).

  13. 13.

    et al. Valley-splitting and valley-dependent inter-Landau-level optical transitions in monolayer MoS2 quantum Hall systems. Phys. Rev. B 90, 045427 (2014).

  14. 14.

    et al. Spin-layer locking effects in optical orientation of exciton spin in bilayer WSe2. Nature Phys. 10, 130–134 (2014).

  15. 15.

    et al. Electrical tuning of valley magnetic moment through symmetry control in bilayer MoS2. Nature Phys. 9, 149–153 (2013).

  16. 16.

    et al. Carrier and polarization dynamics in monolayer MoS2. Phys. Rev. Lett. 112, 047401 (2014).

  17. 17.

    et al. Valley dynamics probed through charged and neutral exciton emission in monolayer WSe2. Phys. Rev. B 90, 075413 (2014).

  18. 18.

    et al. Non-linear optical spectroscopy of excited exciton states for efficient valley coherence generation in WSe2 monolayers. Preprint at (2014)

  19. 19.

    , & Exciton binding energy of monolayer WS2. Preprint at (2014).

  20. 20.

    , , , & Direct imaging of band profile in single layer MoS2 on graphite: Quasiparticle energy gap, metallic edge states, and edge band bending. Nano Lett. 14, 2443–2447 (2014).

  21. 21.

    et al. Probing excitonic dark states in single-layer tungsten disulphide. Nature 513, 214–218 (2014).

  22. 22.

    et al. Exciton binding energy and nonhydrogenic Rydberg series in monolayer WS2. Phys. Rev. Lett. 113, 076802 (2014).

  23. 23.

    et al. Tightly bound excitons in monolayer WSe2. Phys. Rev. Lett. 113, 026803 (2014).

  24. 24.

    et al. Giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor. Nature Mater. 13, 1091–1095 (2014).

  25. 25.

    , , , & Dirac cones and Dirac saddle points of bright excitons in monolayer transition metal dichalcogenides. Nature Commun. 5, 3876 (2014).

  26. 26.

    , , , & Three-band tight-binding model for monolayers of group-VIB transition metal dichalcogenides. Phys. Rev. B 88, 085433 (2013).

  27. 27.

    et al. Monolayer MoS2: Trigonal warping, the Γ valley, and spin–orbit coupling effects. Phys. Rev. B 88, 045416 (2013).

  28. 28.

    et al. Valley Zeeman effect in elementary optical excitations of a monolayer WSe2. Preprint at (2014).

  29. 29.

    et al. Valley degeneracy breaking by magnetic field in monolayer MoSe2. Preprint at (2014).

Download references

Acknowledgements

We thank X. Li for helpful discussions. This work is mainly supported by the DoE, BES, Materials Sciences and Engineering Division (DE-SC0008145). Z.G. and W.Y. were supported by the Croucher Foundation (Croucher Innovation Award) and the RGC of Hong Kong (HKU705513P, HKU9/CRF/13G). D.C. is supported by US DoE, BES, Materials Sciences and Engineering Division (DE-SC0002197). J.Y. and D.G.M. were supported by US DoE, BES, Materials Sciences and Engineering Division. R-L.C. and C.Z. are supported by ARO (W911NF-12-1-0334) and AFOSR (FA9550-13-1-0045). X.X. acknowledges a Cottrell Scholar Award. Device fabrication was performed at the University of Washington Microfabrication Facility and NSF-funded Nanotech User Facility.

Author information

Affiliations

  1. Department of Physics, University of Washington, Seattle, Washington 98195, USA

    • G. Aivazian
    • , Aaron M. Jones
    • , David Cobden
    •  & X. Xu
  2. Department of Physics and Center of Theoretical and Computational Physics, University of Hong Kong, Hong Kong, China

    • Zhirui Gong
    •  & Wang Yao
  3. Department of Physics, The University of Texas at Dallas, Richardson, Texas 75080, USA

    • Rui-Lin Chu
    •  & Chuanwei Zhang
  4. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

    • J. Yan
    •  & D. G. Mandrus
  5. Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA

    • J. Yan
    •  & D. G. Mandrus
  6. Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA

    • D. G. Mandrus
  7. Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, USA

    • X. Xu

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Contributions

X.X. and W.Y. conceived the project; G.A. performed the experiment, assisted by A.M.J., under the supervision of X.X.; G.A. and X.X. analysed the data; Z.G. and W.Y. provided the theoretical explanation, with input from R-L.C. and C.Z.; J.Y. and D.G.M. synthesized and characterized the bulk WSe2 crystals; G.A., X.X., W.Y., D.C. and Z.G. wrote the paper. All authors discussed the results.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Wang Yao or X. Xu.

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DOI

https://doi.org/10.1038/nphys3201

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