Modulating unpolarized current in quantum spintronics: Visibility of spin-interference effects in multichannel Aharonov-Casher mesoscopic rings

The conventional unpolarized current injected into a quantum-coherent semiconductor ring attached to two non-magnetic external leads can be modulated from perfect conductor to perfect insulator limit via electrically tunable Rashba spin-orbit (SO) coupling, thereby avoiding the usage of any ferromagnetic elements or external magnetic fields. This requires that ballistic propagation of electrons, whose spin precession is induced by the topological Aharonov-Casher phase accumulated by the spin wave function during a cyclic evolution, takes place through a single conducting channel ensuring that electronic quantum state remains a pure separable one in the course of transport. We study the fate of such spin-sensitive quantum interference effects as more than one orbital conducting channel becomes available for quantum transport. Although the conductance of multichannel rings, in general, does not go all the way to zero at any value of the SO coupling, some degree of current modulation survives. We analyze possible scenarios that can lead to reduced visibility of the destructive spin interference effects that are responsible for the zero conductance at particular values of the Rashba interaction: (i) the transmitted spin states remain fully coherent, but conditions for destructive interference are different in different channels; (ii) the transmitted spins end up in partially coherent quantum state arising from entanglement to the environment composed of orbital degrees of freedom of the same particle to which the spin is attached.