Theory of spin and orbital charge conversion at the surface states of Bi1-xSbx topological insulator

Topological insulators are quantum materials involving time-reversal protected surface states making them appealing candidates for the design of the next generation of highly efficient spintronic devices. The very recent observation of large transient spin-charge conversion and subsequent powerful THz emission from Co/Bi1-xSbx bilayers clearly demonstrates such potentiality and feasibility for the near future. Among the exotic properties appearing in and at the surface of such quantum materials, spin-momentum locking and Rashba-Edelstein effects remain as key ingredients to effectively convert the spin degree of freedom into a charge or a voltage signal. In this work, we extend our analyses to the quantification of orbital momentum-locking and related orbital charge conversion effects in Bi.85Sb.15 via orbital Rashba-Edelstein effects. In that sense, we will provide some clear theoretical and numerical insights implemented by multiorbital and slab tight-binding methods to clarify our recent experimental results obtained by THz spectroscopy in the time domain.