Loss of interface spin transparency from conductivity mismatch in topological insulator based spintronic terahertz emitters

Terahertz emissions from magnetic heterostructures comprising Fermi-level engineered 3D topological insulators, Bi 2 Se 3 , are compared to assess how tuning the chemical potential absent any compositional change affects the resultant emission properties. Despite an enhancement in spin-orbit torque efficiency exhibited by bulk-insulating Bi 2 Se 3 heterostructures, an ∼5-fold reduction in the terahertz emission amplitude is observed as compared to its bulk-conducting counterpart. Such discrepancy between reciprocal measurements of spin-charge conversion is understood to result from a loss of interface transparency to spin currents due to the low carrier density and high sheet resistance of bulk-insulating Bi 2 Se 3 , resulting in a pronounced conductivity mismatch between the metallic ferromagnet and topological insulating thin films that constitute our spintronic terahertz emitters.