Magnetoelectric response of the time-reversal invariant helical metal

We derive compact analytical expressions for the coupled spin-charge susceptibility of a clean helical metal at the surface of a three-dimensional topological insulator (TI). These expressions lead to unconventional noncollinear Ruderman-Kittel-Kasuya-Yoshida interactions between two impurity magnetic moments placed on the surface of a TI and predict the generation of electric currents by time-dependent magnetic moments. We determine the influence of gate and bias voltages on the interlayer exchange coupling between two singledomain ferromagnetic monolayers deposited on top of a TI.