Spin orbitronics at a topological insulator-semiconductor interface

Abstract

Topological insulators (TIs) hold great promise for new spin-related phenomena and applications thanks to the spin texture of their surface states. However, a versatile platform allowing for the exploitation of these assets is still lacking due to the difficult integration of these materials with the mainstream Si-based technology. Here, we exploit germanium as a substrate for the growth of Bi 2 Se 3 , a prototypical TI. We probe the spin properties of the Bi 2 Se 3 /Ge pristine interface by investigating the spin-to-charge conversion taking place in the interface states by means of a nonlocal detection method. The spin population is generated by optical orientation in Ge and diffuses toward the Bi 2 Se 3 , which acts as a spin detector. We compare the spin-to-charge conversion in Bi 2 Se 3 /Ge with the one taking place in Pt in the same experimental conditions. Notably, the sign of the spin-to-charge conversion given by the TI detector is reversed compared to the Pt one, while the efficiency is comparable. By exploiting first-principles calculations, we ascribe the sign reversal to the hybridization of the topological surface states of Bi 2 Se 3 with the Ge bands. These results pave the way for the implementation of highly efficient spin detection in TI-based architectures compatible with semiconductor-based platforms.

Helicity dependent photoresistance measurement vs. beam-shift thermal gradient

“…Here, we characterize this additional effect, and we find a solution for correcting the results. Our findings are relevant for experiments where the circularly polarized focused beam is used to probe locally the helicitydependent properties [19][20][21][22][23][24]26,29,31,34 , as the thermal gradients create similar signals. Furthermore, for experiments using a fixed circularly polarized beam 18,25,27,28,30,32,33,[36][37][38] , the beam deflection associated to the helicity reversal may slightly modify the illumination conditions and also influence the results.…”

“…It has been shown that circularly polarized 15 light can induce the magnetization reversal 5,16 as well as produce photovoltaic or photoresistive effects 17,18 . The changes of photoresistance [19][20][21][22][23][24][25] or photocurrent 24,[26][27][28][29][30][31][32][33][34][35][36] have been attributed to the interaction between light and spin accumulation. This type of effect appears to be "universal", as it has been observed in semiconductors 24,27 , semimetals 25,37 , topological insulators [29][30][31][32][33][34][35][36] , normal metals 19,20 and even complex metalorganics structure 38 .…”

“…The anisotropic index of refraction causes a different retardation of the orthogonal light polarizations, creating a relative phase shift that defines the resulting circular polarization. The left hand and right hand circularly polarized light is obtained by varying the angle between the optical axis of the birefringent crystal and the polarization plane of the linearly-polarized input light, either mechanically by rotating a quarter-wave plate (λ/ 4) 22,25,[27][28][29]31,[34][35][36]38 or electrically by applying a high voltage to a photoelastic modulator [19][20][21]23,24,26,30,32,37 . The spin-related signals are extracted from the difference of the results obtained for the two helicities of the circularly polarized light.…”

Electric field modulation of spin transport

“…In particular, we report on the drift-diffusive spin-transport regime in lightly n-doped Ge at room temperature. We exploit the optical orientation technique 28,29 taking place below the edges of Pt stripes [30][31][32] to locally generate spin-polarized electrons in Ge. The spin detection is performed by measuring the voltage drop given by the inverse spin-Hall effect (ISHE) in a thin Pt pad 33,34 grown on Ge and spatially separated from the point where spin is generated.…”

Mapping the evolution of Bi/Ge(111) empty states: From the wetting layer to pseudo-cubic islands

“…21 In addition, a complicated "spin texture" is found at the surfaces of either bulk Bi 18,22,23 or Bi alloys. 24,25 In semi-metallic Bi, spin-related effects involving electron transport are enhanced due to the limited contribution to the electrical conductivity coming from bulk electronic states, making these systems very attractive for applications in the field of spintronics. 26 Spin-resolved photoemission spectroscopy (SR-PES) is the technique of choice to map the electronic structure of Bi surfaces and thin films, being also sensitive to the peculiar spin texture stemming from the presence of SOC effects.…”