Newtype large Rashba splitting in quantum well states induced by spin chirality in metal/topological insulator heterostructures

In a two-dimensional system without inversion symmetry, such as a surface or interface, the potential-asymmetry-induced electric field can create the Rashba effect, which is spin-band splitting caused by spin–orbit coupling (SOC). On the basis of Rashba splitting, several promising spintronic devices, such as the Datta-Das spin transistor, have been designed for manipulating spin precession in the absence of a magnetic field. Rashba splitting can be created in several metal quantum well states (QWSs), such as in Pb/Si; however, the effect is usually moderate because the itinerant carriers within the metal film strongly screen out the electric field. A large Rashba splitting can be found in the unoccupied QWSs of the Bi monolayer on Cu(111); however, the metallic substrate limits the diversity of its applications. To achieve strong Rashba splitting in metal thin film based on an insulating substrate, which is most desirable, we propose the normal metal (NM)/topological insulator (TI) heterostructure for manufacturing Rashba-type splitting in NM QWSs. In such a hybrid system, the TI spin-momentum-locking Dirac surface state associated with SOC strongly modifies the penetration depth of the NM QWS into the TI substrate according to the spin orientation, leading to strong Rashba-type splittings in the NM QWS. Combining ab initio calculations and analytical modeling, the momentum separation of the Rashba splitting in the NM QWS can be as large as 0.18 Å−1, which is the largest ever found in metal-film/non-metallic substrate systems. Furthermore, the induced spin polarization in the Rashba band is nearly 100%, much higher than the typical value of 40–50% in the TI surface state itself. This remarkably large Rashba splitting and the high spin polarization in the NM QWS evoked by the spin chirality of the TI surface state confer great potential for the development of spintronic devices. Computational researchers have identified a thin-film device with an order-of-magnitude improved control over current in spin transistors. Recent advances in two-dimensional materials have produced systems that can manipulate spin without magnetic fields, thanks to ‘Rashba effects’ that split interfacial spin bands. Using first-principles calculations and analytical modelling, Horng-Tay Jeng from National Tsing Hua University in Taiwan and co-workers now propose a new mechanism to produce giant Rashba splitting by exploiting the asymmetric properties of spin states at interfaces between a topological insulator and a thin metal. Their analysis revealed that two-dimensional surface states of topological insulators such as bismuth selenide interact significantly with quantum well states in nanometre-thin silver layers. The strong coupling induces a ‘giant’ Rashba splitting on silver layers, which could be used to produce nanoscale spin transistors operable at room temperature. To achieve the room-temperature Datta-Das spin transistor at the nanoscale, we propose a new mechanism to produce giant Rashba splittings in the quantum-well-state (QWS) of normal metal (NM) thin films by exploiting the asymmetric properties of spin states at the topological insulator (TI) interface based on the NM/TI heterostructure. In such a hybrid system, the TI spin–momentum-locking Dirac-surface states strongly modify the NM QWSs, leading to extremely large Rashba splittings in the NM QWSs. The giant Rashba splitting as well as the nearly 100% spin polarizations in the NM QWSs manifest great potential for manipulating spin without magnetic fields in spintronic devices.