Non-Technical: The performance and capabilities of key present solid-state technologies, such as light-emitting diodes, solar cells, and other electro-optic components, are ultimately constrained by the fundamental properties of conventional semiconductors such as silicon and gallium arsenide. Monolayers of the layered transition-metal dichalcogenides (with formula MX2, such as MoS2 and WSe2) have recently been discovered to be a new class of semiconductors effectively at the atomically thin limit. They have dramatically different fundamental properties which can defy previous constraints, allowing observation and discovery of new physical phenomena and offering potential for unprecendented solid-state device possibilities. Fundamentally new properties in MX2 include the relationship between electron spin, the "valley index" specifying which of two equivalent momentum states the electron occupies in the 2D lattice, and the "layer index" specifying which layer the electron is in when two monolayers form a bilayer. The spin, valley and layer quantum numbers in this material are linked and can be manipulated jointly, as has already been demonstrated theoretically and experimentally by the investigators. The proposed work aims to study all aspects of this "spin-valley-layer coupling", ranging from its role in electrical transport to its interplay with optical cavities, with a view to novel yet practical device technologies with electrical, magnetic, and optical control. The work will be done by a mixed team of physics and engineers and will provide interdisciplinary research education to a large number of graduate and undergraduate students.
In monolayer MX2 the strong spin-orbit coupling locks the real spin at the band edges to the valley index to produce spin-valley coupling. At the same time the valleys, and hence spins, are addressable using circularly polarized optical selection rules. Additionally, in bilayers and heterostructures the valley index is further coupled to the layer index for a given valley index, leading to new magnetoelectric effects. MX2s thus provide the first solid-state system in which electric and dynamical control of combined valley pseudospin and real spin are possible, opening up a wealth of hybrid spin and valley device possibilities. The focus of the proposed work is on the studying the unique spin, valley and layer pseudospin couplings in MX2 monolayers, bilayers and heterostructures with photonic and spintronic device applications in mind. The specific goals and methods are as follows: (1) develop microscopic theories for coupled spin-valley-charge transport to guide and model the experimental efforts and synthesis of monolayers, bilayers, heterostructures, and hybrid systems; (2) develop highest quality, large-area crystal growth with controllable charged and magnetic dopants; (3) combine electrical and optical investigation of intrinsic spin and valley properties to determine the fundamental parameters of monolayers and bilayers in various device geometries for hybrid spin-valleytronics; (4) investigate and demonstrate manipulation of spin and valley polarizations in heterostructures and hybrid photonic devices combining MX2s with structures such as photonic-crystal cavities, addressing aspects including spin-valley-charge transport across interfaces, interlayer exciton physics, exciton-polaritons in integrated cavity structures, and strong photon-valley exciton interactions within a nanocavity; and (5) develop MX2 lateral junction devices for efficient, controllable, and spin and valley specific light emitters, modulators and detectors.
