Theory of spin and orbital charge conversion at the surface states of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Bi</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msu

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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>Co</a:mi><a:mtext>/</a:mtext><a:msub><a:mi>Bi</a:mi><a:mrow><a:mn>1</a:mn><a:mo>−</a:mo><a:mi>x</a:mi></a:mrow></a:msub><a:msub><a:mi>Sb</a:mi><a:mi>x</a:mi></a:msub></a:mrow></a:math> 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 <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:msub><b:mi>Bi</b:mi><b:mrow><b:mo>.</b:mo><b:mn>85</b:mn></b:mrow></b:msub><b:msub><b:mi>Sb</b:mi><b:mrow><b:mo>.</b:mo><b:mn>15</b:mn></b:mrow></b:msub></b:mrow></b:math> 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. Published by the American Physical Society 2024