An international team of researchers have uncovered a hidden spin-polarisation of bulk electronic states in the semiconductor WSe2, which could hold the key to faster more energy-efficient electronics.
In the race for ever faster, smaller, and more energy-efficient devices, scientists around the world have been trying to develop ways to exploit the electron’s tiny magnetic moment known as its spin, a.k.a. spintronics. This requires engineering electronic states in solids which are spin polarised. For decades, this has been thought to require breaking certain symmetries, either by making the material magnetic, or by breaking a global structural (so-called 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), has observed spin-polarised states residing in a semiconductor, WSe2, where these symmetries remain intact.
Using an exquisitely sensitive probe of electronic structure based on the photoelectric effect, the researchers had been measuring 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 fundamental symmetries that this material possesses”, said 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 because the electronic states are spatially localised in sub-units of the bulk crystal structure where locally, 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, Sweden and Diamond Light Source, UK, respectively, the researchers could selectively probe only one such 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 thought must have only spin-degenerate energy bands can in fact locally host spin-polarised states,” remarked King. “Controlling this could bring fantastic new opportunities for spintronics, and a whole arsenal of new materials in which we can achieve this.”