The possibility of using the polarization or ‘spin’ of an electron in solid-state structures could one day make it possible to construct faster, smaller and more efficient devices. One of the goals of researchers developing ‘spintronics’ is to find ways to generate a pure, fully polarized spin current without relying on external magnetic fields. This can be achieved at nanometer scales by quantum effects due to confinement in a thin volume coupled with surface phenomena. Iwao Matsuda and colleagues from the University of Tokyo in Japan in collaboration with scientists in Italy1 have now elucidated the electronic properties of a highly spin-polarized nanometer-thick film.

Fig. 1: Schematic illustration of electronic band hybridization (right) of spin-splitting surface states (SSs) in the atomic surface layer, and quantum well states (QWSs) in the ultrathin (1–3 nm) silver film.

The team used angle-resolved and spin-resolved photoemission spectroscopy to study the energy band structure of electrons in an ultrathin film of silver coated with an atomically ordered bismuth/silver alloy (Fig. 1). The results show that electrons in the silver film can move freely in the film plane, independent of their spin, but their motion in the cross-film direction is spin-quantized due to edge effects. This gives rise to electronic bands known as quantum well states (QWSs). The electronic band structure of the alloy surface, on the other hand, consists of a series of highly spin-polarized bands known as surface states that form due to an effect known as ‘Rashba splitting’.

Studying how the electronic bands of the two layers hybridize when placed together, Matsuda and his colleagues observed a spin-dependence in the band mixing: QWSs only mix with surface states when their spins are parallel. A careful analysis showed that this behavior reflects the type of wavefunction of the electrons. For example, a surface state with an ‘sp’ wavefunction can only hybridize with a QWS with a wavefunction of the same symmetry.

“Our work demonstrates that the spin properties of non-magnetic quantum films can be controlled by properly designing the spin orientation and symmetry of the electronic states in the interface atomic layer,” says Matsuda. In principle, the properties of QWSs and surface states can be controlled by varying the film thickness or the concentration of heavy elements, allowing for a great variety of structures to be studied.