Spin to charge conversion in the topological insulator HgTe and in STO-based two-dimensional electron gas*

While classical spintronics has traditionally relied on ferromagnetic metals as spin generators and spin detectors, a new approach called spin-orbitronics exploits the interplay between charge and spin currents enabled by the spin-orbit coupling in non-magnetic systems. However, the interconversion efficiency of the Hall effect is a bulk property that rarely exceeds ten percent, and does not take advantage of interfacial and low-dimensional effects otherwise ubiquitous in spintronics.

In this contribution, we first focus on strained mercury telluride, using spin pumping experiments at room temperature. We show that a HgCdTe barrier can be used to protect the HgTe topological surface states, leading to high conversion rates, with inverse Edelstein lengths up to 2.0±0.5 nm. These measurements, associated with the temperature dependence of the resistivity, suggest that these high conversion rates are due to the spin momentum locking property of HgTe surface states [1].

We then focus on the SrTiO3 (STO)-based 2D electron system, presenting experiments performed on NiFe/Al/STO heterostructures. We investigate the nature of the spin-to-charge conversion through a combination of spin pumping, magnetotransport, spectroscopy and gating experiments, finding a very highly efficient spin-to-charge conversion, with inverse Edelstein lengths beyond 20 nm. More importantly, we demonstrate that the conversion rate can be tuned in amplitude and rate by a gate voltage. We then discuss the amplitude of the effect and its gate dependence on the basis of the electronic structure of the 2DES and highlight the importance of a long scattering time to achieve efficient spin-to-charge interconversion.

[1] P. Noel et al., Phys. Rev. Lett. 120, 167201 (2018)
[2] D. C. Vaz et al., Nature Materials 1-7 (2019)