  Electron spins in edge or surface modes of topological insulators (TIs) with
strong spin-orbit coupling cannot be directly manipulated with microwaves due
to the locking of electron spin to its momentum. We show by contrast that a
resistively detected nuclear magnetic resonance (RDNMR) based technique can be
used to probe the helical nature of surface conducting states. In such
experiments, one applies a radio frequency (RF) field to reorient nuclear spins
that then couple to electronic spins by the hyperfine interaction. The spin of
the boundary electrons can thereby be modulated, resulting in changes in
conductance at nuclear resonance frequencies. Here, we demonstrate that the
conductivity is sensitive to the direction of the applied magnetic field with
respect to the helicity of the electrons. This dependence of the RDNMR signal
on angle probes the nature of the conductive edge or surface states. In the
case of 3D TI in the quantum Hall regime, we establish that the dominant
mechanism responsible for the conductance change in a RDNMR experiment is based
on the Overhauser field effect. Our findings indicate that the same physics
underlying the use of RDNMR to probe TI states also enables us to use RF
control of nuclear spins to coherently manipulate topologically protected
states which could be useful for a new generation of devices.
