Nonlocal Helicity Dependent Photocurrent Response in Topological Insulators

Topological insulators (TIs) are a remarkable class of materials due to their linear dispersion relation and exotic spin textures at the surface and have recently shown millimeter-long diffusion lengths with a highly tunable Fermi level. We use scanning photocurrent microscopy to study Sb doped Bi2Se3 as a function of temperature, wavelength, gate voltage, and angle to construct spatial maps that illustrate a nonlocal helicity-dependent photocurrent response. Remarkably, these maps display a spin diffusion length of more than 100 μm. We compare the polarization-independent photocurrent response to this observed helicity-dependent response to shed light on the deeper nature of the carriers responsible for these remarkable observations, which may be due to the carrier states forming an exciton condensate.