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Generating spin currents in germanium using spin-orbit coupling - Matthieu Jamet

SPINTEC, Grenoble

LPS Bât 510 Moyen Amphi - 14H (Attention horaire inhabituel ! )

In this presentation, I will discuss about the possibility to use spin-orbit coupling (SOC) to generate and detect spin currents in germanium. We first focused on the Fe/Ge(111) interface where a strong Rashba SOC is predicted. For this purpose, we have grown thin Fe films by molecular beam epitaxy (MBE) at room temperature on Ge(111) substrates and used the ferromagnetic resonance-spin pumping technique to study the spin-to-charge conversion at the Fe/Ge(111) interface. We could demonstrate the very efficient conversion of a pure spin current into a 2D charge current within Fe/Ge(111) Rashba states [1]. Then, we have grown by MBE very thin films of Bi on Ge(111). Indeed, MBE gives the opportunity to grow metastable allotropic phases of Bi or to induce strain into bulk Bi which may give topological properties to this material [2]. By spin-momentum locking at the surface of such topological phases, one could generate 100% spin-polarized currents at the surface of Bi and transfer it to the germanium film. In this study, we have grown a bismuth wedge (0-15 nm) on Ge(111). Using structural characterization (RHEED and x-ray diffraction), we found a critical thickness of ≈5 nm below which Bi exhibits an allotropic pseudocubic phase. A careful angle-resolved and spin-resolved photoemission spectroscopy study using synchrotron radiation (ELETTRA, Trieste, Italy) showed that the pseudocubic phase exhibits surface states with a linear band dispersion and a characteristic helical spin texture. Moreover, low temperature magnetotransport measurements demonstrated the 2D character of electron transport into these surface states. We have then investigated the spin-to-charge interconversion into Bi surface states using 3 different techniques : magneto-optical Kerr effect to probe the spin Hall effect (SHE), inverse SHE using optical spin orientation in the Ge film beneath and finally spin pumping from a ferromagnetic layer grown on top of Bi separated by an Al spacer. In all three cases, we found a clear signature of the strong spin-to-charge interconversion in surface states.


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