UC Riverside

The explosion of recent work suggests that the next generation of solid state devices will be built upon an understanding of topological phases and phenomena. Key to the development and application of such devices are the notions of tunability, the manipulation of device properties with some external stimulus, and interactions, the additional influences on electrons.

Tunable devices form the basis for transistors and computer technology, but tunable topological devices have not been greatly explored. Thus, here we extend a previous proposal[2], which realized a two-node Weyl semimetal with a heterostructure of alternating topological/normal insulator layers and magnetic coupling in the direction perpendicular to the layers, with the coupling placed parallel to the layers. The magnetic coupling, arising, for example, from ferromagnetic insulators, here creates a line-node semimetal and it turns out to allow for tunable features in the device which can, in principle, be measured in future experimental studies. Interestingly, the Fermi surface can be tuned to have the topology of either a sphere or a torus, a unique aspect of line nodes.

Interactions in topological devices provide additional routes for further development. A good example is the problem of dilute magnetic impurities, providing a window into the structure of topological states. Here we use monolayer transition metal group-VI dichalcogenides for a simple model of topological bands in a semiconductor. The system is hexagonal but lacks an inversion center and includes strong spin-orbit coupling from the heavy transition metal, resulting in spin-split bands in separate valleys around the K points, with finite Berry curvature, and consequently a contrasted optical circular dichroism. The hole-doped regime possesses separate Fermi surfaces, with opposite spins on opposite sides of the Brillouin zone, producing an interesting spin structure in the Kondo ground state. Furthermore, the selective absorption of circularly polarized light according to valley/spin leads to the manipulation of the spin structure directly. We extensively study the Kondo ground state resulting from the quasi-equilibrium configuration inferred from the application of circularly polarized light, a situation which involves topology, spin-orbit interactions, hybridization with a magnetic impurity, and tunability of the spin state with light.