  Topological spintronics aims to exploit the spin-momentum locking in the
helical surface states of topological insulators for spin-orbit torque devices.
We address a fundamental question that still remains unresolved in this
context: does the topological surface state alone produce the largest values of
spin-charge conversion efficiency or can the strongly spin-orbit coupled bulk
states also contribute significantly? By studying the Fermi level dependence of
spin pumping in topological insulator/ferrimagnetic insulator bilayers, we show
that the spin Hall conductivity is constant when the Fermi level is tuned
across the bulk band gap, consistent with a full bulk band calculation. The
results suggest a new perspective, wherein "bulk-surface correspondence" allows
spin-charge conversion to be simultaneously viewed either as coming from the
full bulk band, or from spin-momentum locking of the surface state.
