Invited Paper GR+EM+NS+SS+TF-ThA3
Topological Valleytronics in Monolayers of Group-VI Dichalcogenides

Thursday, October 31, 2013, 2:40 pm, Room 101 A

In many crystals the Bloch bands have inequivalent but degenerate energy extrema in the momentum space, known as valleys. The valley index constitutes a well-defined discrete degree of freedom for low-energy carriers that may be used to encode information. This has led to the concept of valleytronics, a new type of electronics based on manipulating the valley index of carriers. In this talk, we show that, when inversion symmetry is broken, a pair of valleys which are equivalent by time-reversal are distinguishable by their magnetic moment and Berry curvature. These quantities give rise to valley Hall effect and circularly-polarized valley optical selection rule both in graphene and in monolayer group-VI transition metal dichalcogenides. Moreover, in monolayer dichalcogenides, we find that the electrons and holes at the band edges are described by massive Dirac Fermions with strong spin-valley coupling, which further results in valley and spin dependent optical selection rule, and coexistence of valley Hall and spin Hall effects. These phenomena pave the way towards dynamic control of valley and spin by electric and optical means for device applications in monolayer dichalcogenides. We will report photoluminescence studies on dichalcogenide thin films, which show the first evidence on valley optical selection rule and optical valley pumping.