Abstract
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 broken1. In certain
two-dimensional electron systems such as bilayer graphene,
the intrinsic inversion symmetry can be broken simply by
applying a perpendicular electric field2,3. 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 control4,5. Here we investigate this possibility
using polarization-resolved photoluminescence of bilayer
MoS2, which has the same symmetry as bilayer graphene but
has a bandgap in the visible spectrum6,7 allowing direct optical
probing5,8–12. We find that in bilayer MoS2 the circularly polarized
photoluminescence can be continuously tuned from 15%
to 15% as a function of gate voltage, whereas in structurally
non-centrosymmetric monolayer MoS2 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.