In the race for ever faster, smaller and more energy-efficient devices, scientists around the world have been trying to develop ways to take advantage of the electron’s tiny magnetic movement, known as its spin, in a field of study called spintronics. This requires the engineering of electronic states in solids, which are spin-polarised. For decades this has been thought to require the breaking of certain symmetries, either by magnetising the material, or by breaking a global structural inversion symmetry of the crystal. Now, an international team of researchers led by Dr. Phil King at the University of St Andrews, in collaboration with researchers from Norway (NTNU), Japan (Tokyo), Denmark (Aarhus), Sweden (MAX-lab), the UK (Diamond Light Source), and Thailand (Suranaree), have observed spin-polarised states residing in a semiconductor, WSe2, where these symmetries remain intact.
Using an extremely sensitive probe of electronic structure, based on the photoelectric effect, the researchers were able to measure the electronic structure of WSe2.
“When we measured the spin polarisation of some of its bulk electronic states, we were surprised to discover really strong values – almost 100% spin-polarised – which seemed to contradict the fundamental symmetries of this material”, says Jon Riley, first author of the study.
Through a combination of systematic photon energy-dependent experiments and first-principles theoretical calculations, the researchers showed how this occurs as the electronic states are spatially localised in sub-units of the bulk crystal structure where inversion symmetry is not present, allowing them to develop huge spin polarisations. Using surface-sensitive spin- and angle-resolved photoemission spectroscopy at MAX-lab synchrotron, researchers from Sweden and Diamond Light Source, UK, were each able to selectively probe a single sub-unit of the material, uncovering the surprising intrinsic spin polarisation of these states.
“This is exciting because it reveals that a whole new class of materials, which we previously believed to only have spin-degenerate energy bands, can in fact locally host spin-polarised states,” says King, and continues. “Controlling this could bring fantastic new opportunities for spintronics, and a whole arsenal of new materials in which we can achieve this.”
The study is available as an Advanced Online Publication in Nature Physics (DOI: 10.1038/nphys3105).