The impact of metallic contacts on spin-polarized photocurrents in topological insulator $Bi2Se3$ nanowires

Recently, a quantum phase, the topological insulator, has been vividly investigated in a variety of materials. Its unique band structure allows for optical generation and control of spin-polarized currents based on the circular photogalvanic effect. In this paper, we generate and distinguish the different photocurrent contributions via the polarization of the driving light wave. We discuss the helicity-dependent spin-polarized current and the polarization-independent thermoelectric current as spatially resolved maps, focusing on the influence of the topological insulator/metallic contact interface. We observe for both current contributions a significant enhancement of the current values at the topological insulator/metallic contact interface. In the case of the thermoelectric current, the enhancement is localized at the center of the interface. The spin-polarized current reaches two extrema per contact, which differ by their sign and are localized nearby the contact edges. We discuss the general behavior of the thermovoltage as a three-material Seebeck effect and explain the enhanced values by the acceleration of the photoelectrons generated in the space charge region of the topological insulator/metallic contact interface. Furthermore, we interpret the temperature gradient together with the spin Nernst effect as a possible origin for the enhancement and spatial distribution of the spin-polarized current.