Optical tuning of electronic valleys
15:00 - 16:00
Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA
CFEL (Bldg. 99)
Seminar Room IV, O1.111
James McIver / Andrea Cavalleri
Monolayer transition-metal dichalcogenides such as MoS₂ and WS₂ are prime examples of atomically thin semiconducting crystals that exhibit remarkable electronic properties. They have a pair of valleys that can serve as a new degree of freedom, but these valleys are energetically degenerate, protected by time-reversal symmetry.
In this seminar, I will show that femtosecond laser pulses can be used to break this symmetry and lift the valley degeneracy in a coherently controlled manner. The resulting energy splitting is extremely large, comparable to what would be obtained using a hundred Tesla of magnetic field. We will discuss the physics of our observation in two separate mechanisms, one that is mediated by excitons and another by intervalley biexcitons. Finally, we will explore the prospect of using this technique to create a new phase of matter called a valley Floquet topological insulator.