Spin-charge locking and tunnelling into a helicalmetal (Vol. 42, No. 3)

Spintronics aims at exploiting the electron spin for new device functionalities. In recent years a new spintronic paradigm, based on the spin-orbit interaction, has been proposed aiming to gain spin control by electric fields. In this respect, topological insulators (TI) appear as a very promising opportunity. At the surface of aTI gapless excitations occur with extraordinarily strong spinorbit coupling: a given surface momentum is associated with a single spin direction, such that the states on the Fermi surface have a well-defined helicity.

In this paper we present a theoretical study of the dynamics of the electrons moving on the surface of a three-dimensional TI, i.e. in a two-dimensional helical metal (HM). When the HM is brought into contact with a ferromagnet, there arises an unconventional magnetoresistance. The origin of the effect is the spin-orbit coupling: since the electron momentum is connected to a single spin state, a current flow creates a nonequilibrium spin polarization. This current-induced spin polarization increases or decreases the spin dependent voltage difference between the helicalmetal and themajority or minority carriers in the FM and thus modifies the tunnelling current. By reversing the flow of the current in the helical metal the two spin species exchange their role. In the ideal case the tunnelling current between the FM and the HM can be switched on and off depending on the relative orientation of the magnetization of the FM with respect to the direction of the current flow in the HM.