Valley physics in 2D transiton metal dichalcogenides





The Bloch bands in many crystals have a degenerate set of energy extrema

in momentum space known as valleys. For band-edge carriers, the valley

index becomes a discrete degree of freedom in addition to spin. In this

talk, I will show that, when inversion symmetry is broken in a 2D

hexagonal lattice, a pair of valleys which are time-reversal of each other

are distinguishable by their opposite values of magnetic moment and Berry

curvature. These quantities give rise to circularly-polarized valley

optical transition selection rule and valley Hall effect in monolayer

group-VI transition metal dichalcogenides (TMDs). The valley optical

selection rule makes possible dynamic pumping of valley polarization, and

optical generation of excitonic valley coherence. Moreover, we find the

electrons and holes at the band edges of monolayer TMDs are described by

massive Dirac Fermions with strong spin-valley coupling. In a TMD bilayer

with AB-stacking order, this spin-valley coupling manifests as an

effective interaction between the layer pseudospin with both the spin and

the valley, giving rise to a variety of magnetoelectric effects permitting

quantum manipulation of these electronic degrees of freedom. Monolayer

TMDs also provide an unprecedented 2D platform to explore the physics of

exciton, i.e. bound state of electron and hole pair. I will show that the

exceptionally large Coulomb interaction in monolayer TMDs can strongly

couple the exciton valley pseudospin to the motion, giving rise to novel

strain-tunable Dirac spectra.