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.
@article{arxiv.1906.11116,
title = {Fermi Level Dependent Spin Pumping from a Magnetic Insulator into a Topological Insulator},
author = {Hailong Wang and James Kally and Cuneyt Sahin and Tao Liu and Wilson Yanez and Eric J. Kamp and Anthony Richardella and Mingzhong Wu and Michael E. Flatte and Nitin Samarth},
journal= {arXiv preprint arXiv:1906.11116},
year = {2019}
}