Spin–layer locking effects in optical orientation of exciton spin in bilayer WSe2

Journal name:
Nature Physics
Volume:
10,
Pages:
130–134
Year published:
DOI:
doi:10.1038/nphys2848
Received
Accepted
Published online

Coupling degrees of freedom of distinct nature plays a critical role in numerous physical phenomena1, 2, 3, 4, 5, 6, 7, 8, 9, 10. The recent emergence of layered materials11, 12, 13 provides a laboratory for studying the interplay between internal quantum degrees of freedom of electrons14, 15. Here we report new coupling phenomena connecting real spin with layer pseudospins in bilayer WSe2. In polarization-resolved photoluminescence measurements, we observe large spin orientation of neutral and charged excitons by both circularly and linearly polarized excitation, with the trion spectrum splitting into a doublet at large vertical electrical field. These observations can be explained as a locking of spin and layer pseudospin in a given valley15, where the doublet implies an electrically induced spin splitting. The observed distinctive behaviour of the trion doublet under polarized excitation further provides spectroscopic evidence of interlayer and intralayer trion species, a promising step towards optical manipulation in van der Waals heterostructures16 through interlayer excitons.

At a glance

Figures

  1. Coupled spin, valley and layer degrees of freedom in bilayer WSe2.
    Figure 1: Coupled spin, valley and layer degrees of freedom in bilayer WSe2.

    a, AB stacking order in bilayer TMDCs corresponds to 180° rotation of the lattice between layers, leading to an effective layer pseudospin σz. Tz and Γ indicate the valley index and first Brillouin zone centre, respectively. b, Cartoon depicting excitation/emission processes in the K valley of bilayer WSe2. Spin configuration is indicated by (↑) for holes (electrons). The same for the −K valley is obtained by time reversal. c, Depiction of spin-down (-up) hole states localized in the upper (lower) layer in the K valley.

  2. Photoluminescence and differential reflectivity versus gate.
    Figure 2: Photoluminescence and differential reflectivity versus gate.

    a, Photoluminescence intensity as a function of gate voltage and photon energy with labelled neutral exciton (Xo), and negative (X) and positive (X+) trion emission. b, Corresponding differential reflectance showing relationship of X/X+ emission to the absorption feature. c, Photoluminescence spectra extracted at +60, 0 and −60V, showing excitonic peaks X, Xo and X+, respectively. Red lines in the 0V spectrum show Lorentzian peak fits.

  3. Optical orientation of spin and gate-induced peak splitting in trions.
    Figure 3: Optical orientation of spin and gate-induced peak splitting in trions.

    a, Normalized photoluminescence spectra versus photon energy at selected gate voltages for σ+-polarized excitation and σ+ (black curve) and σ (red curve) detection. Blue lines in plots for Vg>50V show Lorentzian peak fits of σ+ detection data. b, Trion photoluminescence circular polarization, ησ, as a function of gate voltage, where black (red) data indicate σ+ (σ) excitation. For Vg>50V, the trion peak splits and black (blue) data correspond to ησ of peak I (II) under σ+ excitation. Inset: zoom-in showing enhancement of ησ with applied gate voltage for both σ+ (black) and σ (red) excitation. c, Trion peak splitting as a function of applied gate voltage for circularly (black) and linearly (red) polarized excitation. Error bars indicate standard deviation of peak position from fitting. d, Schematic depiction of the formation of excitons in both K and −K valleys under σ+ excitation in unbiased bilayer WSe2, resulting in no net valley polarization. Hollow (solid) circles denote holes (electrons). Grey circles denote photo-excited electron–hole pairs. e, Schematic of electric-field-induced band shifts and electron spin relaxation pathways (green arrows). Emission from the upper and lower layers is at ω1 and ω2, respectively, whose splitting originates in the difference between the conduction (Δc) and valence (Δv) band energy shifts with gate electric field.

  4. Linearly polarized excitation of interlayer and intralayer trions.
    Figure 4: Linearly polarized excitation of interlayer and intralayer trions.

    a, Normalized photoluminescence spectra versus photon energy at selected gate voltages for vertically polarized excitation and vertical ( , black) and horizontal ( , red) detection. Blue lines in plots for Vg>50V show Lorentzian peak fits of data for vertically polarized detection. b, Polar plots showing normalized magnitude of trion peak height as a function of detection angle, at Vg=0. Green arrow indicates incident polarization direction. c, Trion linear polarization, , as a function of gate voltage for vertically polarized excitation (centre), with corresponding depictions of interlayer (top) and intralayer (bottom) trions. For Vg>50V, the trion peak splits. Black and red data correspond to of peak I (interlayer trions) and peak II (intralayer trions), respectively. d, Cartoons depicting a superposition of valley trion configurations giving rise to trion peaks I (top) and II (bottom). Owing to negligible exchange interactions (Jeh, Jee), surviving inter-valley coherence for interlayer trion configurations leads to large linear polarization at ω1 (top), whereas the presence of finite exchange interactions for intralayer configurations eliminates linear polarization at ω2 (bottom).

References

  1. Kato, Y. K., Myers, R. C., Gossard, A. C. & Awschalom, D. D. Observation of the spin Hall effect in semiconductors. Science 306, 19101913 (2004).
  2. Wunderlich, J., Kaestner, B., Sinova, J. & Jungwirth, T. Experimental observation of the spin-Hall effect in a two-dimensional spin–orbit coupled semiconductor system. Phys. Rev. Lett. 94, 047204 (2005).
  3. Hasan, M. Z. & Kane, C. L. Colloquium: Topological insulators. Rev. Mod. Phys. 82, 30453067 (2010).
  4. Qi, X-L. & Zhang, S-C. Topological insulators and superconductors. Rev. Mod. Phys. 83, 10571110 (2011).
  5. Fu, L. & Kane, C. Superconducting proximity effect and Majorana fermions at the surface of a topological insulator. Phys. Rev. Lett. 100, 096407 (2008).
  6. Mourik, V. et al. Signatures of Majorana fermions in hybrid superconductor–semiconductor nanowire devices. Science 336, 10031007 (2012).
  7. Lin, Y-J., Jiménez-Garcı´a, K. & Spielman, I. B. Spin–orbit-coupled Bose–Einstein condensates. Nature 471, 8386 (2011).
  8. Cheong, S-W. & Mostovoy, M. Multiferroics: A magnetic twist for ferroelectricity. Nature Mater. 6, 1320 (2007).
  9. Žutić, I. & Das Sarma, S. Spintronics: Fundamentals and applications. Rev. Mod. Phys. 76, 323410 (2004).
  10. Pesin, D. & MacDonald, A. H. Spintronics and pseudospintronics in graphene and topological insulators. Nature Mater. 11, 409416 (2012).
  11. Novoselov, K. S. et al. Two-dimensional atomic crystals. Proc. Natl Acad. Sci. USA 102, 1045110453 (2005).
  12. Mak, K. F., Lee, C., Hone, J., Shan, J. & Heinz, T. F. Atomically thin MoS2: A new direct-gap semiconductor. Phys. Rev. Lett. 105, 136805 (2010).
  13. Splendiani, A. et al. Emerging photoluminescence in monolayer MoS2. Nano Lett. 10, 12711275 (2010).
  14. Xiao, D., Liu, G-B., Feng, W., Xu, X. & Yao, W. Coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides. Phys. Rev. Lett. 108, 196802 (2012).
  15. Gong, Z. et al. Magnetoelectric effects and valley-controlled spin quantum gates in transition metal dichalcogenide bilayers. Nature Commun. 4, 15 (2013).
  16. Geim, A. K. & Grigorieva, I. V. Van der Waals heterostructures. Nature 499, 419425 (2013).
  17. Cao, T. et al. Valley-selective circular dichroism of monolayer molybdenum disulphide. Nature Commun. 3, 887 (2012).
  18. Mak, K. F., He, K., Shan, J. & Heinz, T. F. Control of valley polarization in monolayer MoS2 by optical helicity. Nature Nanotech. 7, 494498 (2012).
  19. Zeng, H., Dai, J., Yao, W., Xiao, D. & Cui, X. Valley polarization in MoS2 monolayers by optical pumping. Nature Nanotech. 7, 490493 (2012).
  20. Li, X., Cao, T., Niu, Q., Shi, J. & Feng, J. Coupling the valley degree of freedom to antiferromagnetic order. Proc. Natl Acad. Sci. USA 110, 37383742 (2013).
  21. Kormányos, A. et al. Monolayer MoS2: Trigonal warping, the Γ valley, and spin–orbit coupling effects. Phys. Rev. B 88, 045416 (2013).
  22. Castro, E. et al. Biased bilayer graphene: Semiconductor with a gap tunable by the electric field effect. Phys. Rev. Lett. 99, 216802 (2007).
  23. Min, H., Borghi, G., Polini, M. & MacDonald, A. Pseudospin magnetism in graphene. Phys. Rev. B 77, 041407 (2008).
  24. San-Jose, P., Prada, E., McCann, E. & Schomerus, H. Pseudospin valve in bilayer graphene: Towards graphene-based pseudospintronics. Phys. Rev. Lett. 102, 247204 (2009).
  25. Feldman, B. E., Martin, J. & Yacoby, A. Broken-symmetry states and divergent resistance in suspended bilayer graphene. Nature Phys. 5, 889893 (2009).
  26. Maher, P. et al. Evidence for a spin phase transition at charge neutrality in bilayer graphene. Nature Phys. 9, 154158 (2013).
  27. Zhao, W. et al. Evolution of electronic structure in atomically thin sheets of WS2 and WSe2. ACS Nano 7, 791797 (2013).
  28. Ross, J. S. et al. Electrical control of neutral and charged excitons in a monolayer semiconductor. Nature Commun. 4, 1474 (2013).
  29. Jones, A. M. et al. Optical generation of excitonic valley coherence in monolayer WSe2. Nature Nanotech. 8, 634638 (2013).
  30. Zeng, H. et al. Optical signature of symmetry variations and spin–valley coupling in atomically thin tungsten dichalcogenides. Sci. Rep. 3, 1608 (2013).
  31. Song, Y. & Dery, H. Transport theory of monolayer transition-metal dichalcogenides through symmetry. Phys. Rev. Lett. 111, 026601 (2013).

Download references

Author information

Affiliations

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

    • Aaron M. Jones,
    • Philip Klement &
    • Xiaodong Xu
  2. Department of Physics and Center of Theoretical and Computational Physics, University of Hong Kong, Hong Kong, China

    • Hongyi Yu &
    • Wang Yao
  3. Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, USA

    • Jason S. Ross &
    • Xiaodong Xu
  4. Department of Physics, Justus-Liebig-University, Giessen 35392, Germany

    • Philip Klement
  5. Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA

    • Nirmal J. Ghimire &
    • David G. Mandrus
  6. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

    • Nirmal J. Ghimire,
    • Jiaqiang Yan &
    • David G. Mandrus
  7. Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA

    • Jiaqiang Yan &
    • David G. Mandrus

Contributions

X.X. and W.Y. conceived the experiments. A.M.J. performed the measurements. J.S.R. fabricated the devices, assisted by A.M.J. and P.K. H.Y., W.Y., A.M.J. and X.X. analysed the results. The WSe2 crystals were synthesized by N.J.G., J.Y. and D.G.M., who also performed characterization measurements of bulk crystals. A.M.J., X.X., H.Y. and W.Y. co-wrote the paper. All authors discussed the results and commented on the manuscript.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to:

Author details

Supplementary information

PDF files

  1. Supplementary Information (692KB)

    Supplementary Information

Additional data