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Van der Waals engineering of ferromagnetic semiconductor heterostructures for spin and valleytronics

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Science Advances  31 May 2017:
Vol. 3, no. 5, e1603113
DOI: 10.1126/sciadv.1603113
  • Fig. 1 Ultrathin CrI3/WSe2 heterostructure and observation of spontaneous magnetization.

    (A) Schematic of van der Waals heterostructure formed by monolayer WSe2 and ferromagnetic-layered semiconductor CrI3 and encapsulated by h-BN. (B) Top and side views of CrI3 crystal structure. (C) Optical microscope image of device 2. The WSe2/CrI3 heterostructure is sandwiched by optically transparent h-BN. Scale bar, 5 μm. (D) Spin-valley locking effect and valley-dependent optical selection rules in monolayer WSe2. Dashed (solid) lines indicate the band edges before (after) exchange field coupling. Black arrows denote spins. (E) Circularly polarized PL spectra above TC (65 K, left) and below TC (5 K, right) in the absence of an applied magnetic field. It is evident that the valley degeneracy is lifted at 5 K because of the magnetic proximity effect.

  • Fig. 2 Ferromagnetic substrate control of spin and valley pseudospin dynamics.

    (A) Maps of the total PL intensity as a function of emission energy and applied magnetic field for left circular (L) and right circular (R) excitation. The black arrow indicates the applied magnetic field sweeping direction. (B) RR and LL spectra at selected magnetic fields [indicated by the white arrows in (A)]. See text for definition of RR and LL. a.u., arbitrary units. (C) Valley splitting and (D) normalized PL intensity difference between RR and LL (ρ) as a function of applied magnetic field sweeping up (orange) and down (green). Insets in (D) are zoomed-in plots of hysteresis curves. (E) Schematic depicting the spin orientation–dependent charge hopping between WSe2 and CrI3, which leads to the excitation helicity-dependent PL intensity in (A). See text for detailed description. (F) PL spectral linewidth (blue) and intensity (purple) versus applied magnetic field (sweeping from positive to negative) for the LL condition. Broad width is always associated with weak PL intensity.

  • Fig. 3 Polarization-resolved micro-PL imaging of domain structures.

    Each panel is a spatial map of ρ (see text for definition) at the indicated applied magnetic field. Left and right columns are arranged in a time-reversal manner corresponding to increasing (left) and decreasing (right) applied magnetic field, respectively. The blue arrow indicates a domain in which the sign of ρ flips three times by sweeping the field, whereas the red arrow points to a domain that flips the sign of ρ only once. Scale bar, 3 μm.

  • Fig. 4 Position-sensitive ferromagnetic domain dynamics.

    (A) Spatial map of ρ from Fig. 3 (−0.5 T, sweep down) with blue, gray, brown, and black circles indicating the spots of selected magnetic field sweeps of ρ in (B), (C), (D), and (E).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/5/e1603113/DC1

    section S1. Electronic structure of CrI3/WSe2 bilayer

    section S2. Linear polarization

    section S3. Peak parameter extraction

    section S4. Comparison of valley splitting between bare WSe2 and WSe2/CrI3

    section S5. Power dependence of valley splitting

    section S6. Voigt geometry

    section S7. Helicity-independent differential reflection at the excitation energy

    section S8. Valley polarization and intensity modulation parameter

    section S9. Linewidth difference between polarizations

    section S10. Rapid switching of heterostructure PL

    section S11. Magnetic field sweep rate dependence

    section S12. Spatial maps of valley splitting

    section S13. Temperature dependence

    section S14. Model of strong and weak domains

    movie S1. Circularly polarized PL in a changing external magnetic field.

    fig. S1. The atomic structure and the electronic band structure of the CrI3-WSe2 bilayer.

    fig. S2. Linearly polarized excitation and detection.

    fig. S3. Peak parameter extraction.

    fig. S4. Valley splitting in bare WSe2 and WSe2/CrI3.

    fig. S5. Power dependence of valley splitting.

    fig. S6. PL measurements in Voigt geometry.

    fig. S7. Differential reflection spectrum of WSe2/CrI3.

    fig. S8. Valley polarization and intensity modulation parameter.

    fig. S9. Linewidth difference between polarizations.

    fig. S10. Rapid switching of PL in WSe2/CrI3.

    fig. S11. Sweep rate dependence.

    fig. S12. Spatial maps of valley splitting.

    fig. S13. Temperature dependence of CrI3 magnetization.

    fig. S14. Strong- and weak-domain modeling.

    table S1. The free energy and its second-order derivatives with respect to the angles of the two domains.

    References (4450)

  • Supplementary Materials

    This PDF file includes:

    • section S1. Electronic structure of CrI3/WSe2 bilayer
    • section S2. Linear polarization
    • section S3. Peak parameter extraction
    • section S4. Comparison of valley splitting between bare WSe2 and WSe2/CrI3
    • section S5. Power dependence of valley splitting
    • section S6. Voigt geometry
    • section S7. Helicity-independent differential reflection at the excitation energy
    • section S8. Valley polarization and intensity modulation parameter
    • section S9. Linewidth difference between polarizations
    • section S10. Rapid switching of heterostructure PL
    • section S11. Magnetic field sweep rate dependence
    • section S12. Spatial maps of valley splitting
    • section S13. Temperature dependence
    • section S14. Model of strong and weak domains
    • Legend for movie S1
    • fig. S1. The atomic structure and the electronic band structure of the CrI3-WSe2 bilayer.
    • fig. S2. Linearly polarized excitation and detection.
    • fig. S3. Peak parameter extraction.
    • fig. S4. Valley splitting in bare WSe2 and WSe2/CrI3.
    • fig. S5. Power dependence of valley splitting.
    • fig. S6. PL measurements in Voigt geometry.
    • fig. S7. Differential reflection spectrum of WSe2/CrI3.
    • fig. S8. Valley polarization and intensity modulation parameter.
    • fig. S9. Linewidth difference between polarizations.
    • fig. S10. Rapid switching of PL in WSe2/CrI3.
    • fig. S11. Sweep rate dependence.
    • fig. S12. Spatial maps of valley splitting.
    • fig. S13. Temperature dependence of CrI3 magnetization.
    • fig. S14. Strong- and weak-domain modeling.
    • table S1. The free energy and its second-order derivatives with respect to the angles of the two domains.
    • References (4450)

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    Other Supplementary Material for this manuscript includes the following:

    • movie S1 (.mp4 format). Circularly polarized PL in a changing external magnetic field.

    Files in this Data Supplement:

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