Highly efficient spin-to-charge current conversion at room temperature in strained HgTe surface states. (Invited)

Abstract

We report the observation of spin-to-charge current conversion in strained mercury telluride at room temperature, using spin pumping experiments. The conversion rates are found to be very high, with inverse Edelstein lengths up to $2.0 + /-0.5$nm. The influence of the HgTe layer thickness on the conversion efficiency has been studied, as well as the role of a HgCdTe barrier inserted in-between the HgTe and NiFe layers. These measurements, associated to the temperature dependence of the resistivity, allow ascribing these high conversion rates to the spin momentum locking property of HgTe surface states [1], promising electronic states for spintronics. Spintronics devices need efficient ways to transform charge current into spin current or to make the opposite conversion to detect spin current. Classical spintronics generally uses magnetic materials and exchange interaction. Such manipulation can also be achieved by harnessing the Spin Orbit Coupling (SOC) in non-magnetic materials. For instance, the Spin Hall Effect permits to convert charge currents into spin currents in the bulk of heavy metals, such as Pt or Ta [2]. Yet a more efficient conversion can be obtained in two dimensional electron gas (2DEG) at surfaces and interfaces such as Rashba Interfaces [3]and newly discovered topological insulator materials. The main interest of topological insulators lies in their surface states, which possess a linear Dirac-like dispersion, and a spin momentum locking as seen on figure 1. A flow of electric current in the 2DEG would cause a perpendicular spin accumulation. This effect is known as the Edelstein Effect [4], while the reverse spin-to charge-conversion effect is known as the Inverse Edelstein Effect (IEE). Recent results suggest that surfaces of topological insulators as Bi 2 Se 3 [5]or strained $\alpha -$Sn [6]have a strong potential for spintronics, both for the generation or detection of spin currents through direct or inverse Edelstein effects. Among these newly discovered class of material strained HgTe is a promising one. Gap opening and Topological insulator properties can be induced in HgTe by applying a tensile strain that can be achieved by epitaxy of HgTe on a substrate with a larger lattice constant, such as CdTe [7]. Moreover large mobility and mean free path has been reported. The spin to charge current conversion was studied in strained HgTe thin films by ferromagnetic resonance spin pumping in cavity as described on figure 2. The spin to charge current conversion rate, the inverse Edelstein length [8], was measured to be up to 2 nm, one to two orders of magnitude larger than in Bi-based topological insulators [9]. Such high conversion rate can be related to the large value of the mobility [10]and mean free path of the surface states of strained HgTe and the lower bulk to surface conductivity ratio at room temperature compared to Bi 2 Se 3 . Moreover the non-conventional thickness dependence of the conversion rate allows ascribing this conversion to the topological surface states.