Photoinduced charge and spin currents in topologically protected quantum systems

In a quantum system, the electron wave function can acquire an additional geometrical phase when its Hamiltonian is driven through an adiabatic change in parameter space. The geometrical phase can be associated with novel spin–orbit physics and spin selection rules. Importantly, exotic phases of matter such as topological insulators and Weyl-semimetals are governed by the global properties of the geometrical phase, which are robust against adi- abatic changes. Topological insulators are narrow band-gap semiconductors, which exhibit gapless states with a spin-helical Dirac dispersion at the surface. The spin-helical character renders topological surface states very promising candidates for spintronic applications based on charge to spin and spin to charge conversion. Weyl-semimetals are at the intersection between topological and normal insulator and they exhibit a very unique spin structure due to the local accumulation of geometrical phase.
In this thesis, the charge and spin transport properties in three-dimensional topological insulators and Weyl-semimetals are investigated using polarization and time-resolved opto- electronic spectroscopy techniques. We demonstrate that the topological surface states can be accessed in the topological insulator Bi2Te2Se utilizing a pulsed photoexcitation scheme. Moreover, we find that an optical excitation of the Bi2Te2Se in close vicinity of the Schottky depletion field of the metal contacts leads to the generation of THz pulses that can cou- ple into the near field of on-chip electronic waveguides. By combining polarization-resolved photoconductance experiments with Kerr-rotation microscopy, we present the first observa- tion of a spin-Hall effect, which originates from the topological bulk of three-dimensional topological insulators. Our findings reveal that the optical orientation of bulk spins results in a helicity dependent modulation of the edge conductivity in topological Bi2Te2Se. In a next step, we demonstrate that a four-terminal geometry allows us to reveal a photo-induced linear Hall effect in the type-II Weyl-semimetal WTe2. The close correspondence between in-plane anisotropy of the linear Hall effect and intrinsic breaking of crystal inversion sym- metry in the bulk WTe2 clearly indicates that the origin of the photo-induced Hall effect is of intrinsic nature closely related to the Berry-curvature dipole in the bulk band structure of WTe2. We further design an optoelectronic probe circuit which relies on the Shockley-Ramo theorem. Applied to the topological insulators Bi2Se3 and (Bi0.5Sb0.5)2Te3, the Shockley- Ramo type detection reveals an unprecedented quantization of the conductance at 1e2/h which is a clear signature of purely spin-polarized transport. The quantized response occurs at the lateral circuit edges, where the in-plane symmetry is intrinsically broken, and it can further be switched on and off via electrostatic gating with a ferroelectric backgate electrode. We discuss the results in terms of local adiabatic transport at the Fermi-level.