Ultrafast helicity control of surface currents in topological insulators with near-unity fidelity

In recent years, a class of solid state materials, called three-dimensional topological insulators, has emerged. In the bulk, a topological insulator behaves like an ordinary insulator with a band gap. At the surface, conducting gapless states exist showing remarkable properties such as helical Dirac dispersion and suppression of backscattering of spin-polarized charge carriers. The characterization and control of the surface states via transport experiments is often hindered by residual bulk contributions yet at cryogenic temperatures. Here, we show that surface currents in Bi2Se3 can be controlled by circularly polarized light on a picosecond time scale with a fidelity near unity even at room temperature. We re-veal the temporal separation of such ultrafast helicity-dependent surface currents from photo-induced thermoelectric and drift currents in the bulk. Our results uncover the functionality of ultrafast optoelectronic devices based on surface currents in topological insulators.Comment: 19 pages, 4 figures, supplementary informatio

Theory of a two-level artificial molecule in laterally coupled quantum Hall droplets

We present a theory of laterally coupled quantum Hall droplets with electron numbers (N1,N2) at filling factor $\nu=2$. We show that the edge states of each droplet are tunnel coupled and form a two-level artificial molecule. By populating the edge states with one electron each a two electron molecule is formed. We predict the singlet-triplet transitions of the effective two-electron molecule as a function of the magnetic field, the number of electrons, and confining potential using the configuration interaction method (CI) coupled with the unrestricted Hartree-Fock (URHF) basis. In addition to the singlet-triplet transitions of a 2 electron molecule involving edge states, triplet transitions involving transfer of electrons to the center of individual dots exist for $(N1 \geq 5, N2 \geq 5)$.Comment: 5 pages, 10 figure