Spin Polarized Excitonic Superfluids 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 been proposed as a candidate system for achieving a condensate state of excitonic particles. We use scanning photocurrent microscopy to study Sb doped Bi2Se3 at varying temperature, wavelength, polarization, and incident angle. This suite of measurements confirm that electrons and holes form bound states at low temperature, and demonstrate ballistic transport at temperatures up to 40 K, as evidenced by millimeter exciton transport lengths. Furthermore, photocurrent magnitude is drastically different depending on which side of the channel the photoexcitation occurs, which we attribute to spontaneous magnetization of the exciton condensates and spin momentum locking. We also explore spin polarization at the TI surface from the dependence of photocurrent distributions on the polarization of the photoexcitation at oblique-incidence-angle.