Circular photogalvanic effect in two-dimensional Weyl semimetals

As a host of massless Weyl fermions, two-dimensional (2D) Weyl semimetals (WSMs) provide an ideal platform for studying exotic quantum phenomena in the emerging field of Dirac physics, including the circular photogalvanic effect (CPGE). Here, we report such behavior in a 2D WSM created in Bi_0.96Sb_0.04 thin films by a thickness-dependent topological phase transition caused by inversion symmetry breaking. Photocurrent maps and line profiles, and CPGE of lateral device structures are shown to depend on bias voltage and polarity, and to be well described by bias-dependent variations of the band profiles at the electrode/BiSb interfaces. Of particular note is the observation that the CPGE exhibits helicity-dependent behavior, indicating a counter-propagating distribution of opposite spins of the Weyl cones, which originates from reduced symmetry in the 2D film structure of WSMs despite normal incidence of the illumination. A strong thickness-dependent responsivity is also observed over a wide spectral range from ∼400 to ∼950 nm, and is attributed to the linear-dispersion of the Weyl cones. These results demonstrate manipulation of photocarrier generation, separation and transport processes in a simple 2D-WSM-based planar device using light polarization, bias voltage, and film thickness, and are promising for energy-harvesting devices.