Tuning spin-charge interconversion with quantum confinement in ultrathin bismuth films

Spin-charge interconversion (SCI) phenomena have attracted a growing interest in the field of spintronics as a means to detect spin currents or manipulate the magnetization of ferromagnets. The key ingredients to exploit these assets are a large conversion efficiency, the scalability down to the nanometer scale, and the integrability with optoelectronic and spintronic devices. Here, we show that, when an ultrathin Bi film is epitaxially grown on a Ge(111) substrate, quantum size effects arising in nanometric Bi islands drastically boost the SCI efficiency, even at room temperature. Using x-ray diffraction, scanning tunneling microscopy, and spin- and angle-resolved photoemission, we obtain a clear picture of the film morphology, crystal, and electronic structures. We then directly probe SCI with three different techniques: magneto-optical Kerr effect to detect the charge-to-spin conversion generated by the Rashba-Edelstein effect (REE), optical spin orientation, and spin pumping to generate spin currents and measure the spin-to-charge conversion generated by the inverse Rashba-Edelstein effect (TREE). The three techniques show a sizable SCI only for 1-3-nm-thick Bi films corresponding to the presence of bismuth nanocrystals at the surface of germanium. Due to three-dimensional quantum confinement, those nanocrystals exhibit a highly resistive volume separating metallic surfaces where SCI takes place by (I)REE. As the film size increases, the Bi film becomes continuous and semimetallic leading to the cancellation of SCIs occurring at opposite surfaces, resulting in an average SCI that progressively decreases and disappears. These results pave the way for the exploitation of quantum size effects in spintronics.