Electrical tuning of valley magnetic moment through symmetry control in bilayer MoS ₂

Crystal symmetry governs the nature of electronic Bloch states. For example, in the presence of time-reversal symmetry, the orbital magnetic moment and Berry curvature of the Bloch states must vanish unless inversion symmetry is broken. In certain two-dimensional electron systems such as bilayer graphene, the intrinsic inversion symmetry can be broken simply by applying a perpendicular electric field. In principle, this offers the possibility of switching on/off and continuously tuning the magnetic moment and Berry curvature near the Dirac valleys by reversible electrical control. Here we investigate this possibility using polarization-resolved photoluminescence of bilayer MoS ₂ , which has the same symmetry as bilayer graphene but has a bandgap in the visible spectrum allowing direct optical probing. We find that in bilayer MoS ₂ the circularly polarized photoluminescence can be continuously tuned from-15% to 15% as a function of gate voltage, whereas in structurally non-centrosymmetric monolayer MoS ₂ the photoluminescence polarization is gate independent. The observations are well explained as resulting from the continuous variation of orbital magnetic moments between positive and negative values through symmetry control.