The rapidly advancing field of spintronics, which involves the manipulation of electron spins in device technology, has led to big improvements in magnetic storage. However developing spintronic analogues of active electronic devices has proved much more challenging. A collaboration involving experimentalists and theorists from seven different countries in Europe and Asia have now shown that the semiconductor WSe2 exhibits spin-polarised electronic states despite retaining bulk inversion symmetry.
Photoemission spectroscopy measurements that led to a recent publication in Nature Physics were collected at the angle-resolved photoemission spectroscopy (ARPES) beamline (I05) at Diamond Light Source, and at MAX-III in Sweden. This research opens up the potential for a new class of materials in which spins can be controlled for possible logic applications.
PhD student and first author of the study, Jon Riley, working on I05.
In semiconductor systems, spin-orbit coupling is a crucial factor that concerns spin transport and relaxation properties. A key objective in spintronics is to use spin-orbit coupling to generate spin-polarised electronic states in non-magnetic solids. This enables manipulation of electron spins, and if spin polarisations can be controlled there is huge potential for new quantum technologies to be developed. It is generally accepted that to generate spin-polarised states, it is necessary to break the global structural inversion symmetry that many materials possess.
However, using spin- and angle-resolved photoemission spectroscopy, scientists have reported the observation of spin-polarised bulk states in the transition metal dichalcogenide 2H-WSe2 that has just such an inversion symmetry of its bulk crystal structure. Surprisingly, very strong spin polarisations were detected by spin-resolved ARPES at MAX-lab, showing some of the material’s electronic states to be almost 100% spin polarised. Further measurements were therefore required to confirm whether these observations resulted from the bulk electronic structure of the material.
Angle-resolved photoemission spectroscopy (ARPES) measurements of the electronic structure of WSe2 along the (K^' ) ̅-Γ ̅-K ̅ direction, and corresponding theoretical calculations (coloured lines). The layer-specific spin texture extracted from spin-resolved ARPES measurements is shown schematically for the upper valence bands.
Dr Phil King of the University of St. Andrews describes the experiments at Diamond: “We could perform detailed measurements of the material’s electronic structure using the new ARPES set-up at I05. In our first beamtime on I05, we were already able to determine the full three-dimensional electronic dispersion relations of our samples. The experimental measurements showed an almost quantitative agreement to theoretical calculations of the ideal bulk electronic structure of WSe2.” The observations convinced the team that they were measuring the bulk electronic structure of the material, and indicated that this non-magnetic material indeed hosts spin-polarised bulk states, which seems contrary to the centrosymmetric nature of its crystal structure.
The ARPES measurements, together with theoretical simulations, also revealed how the wavefunctions of the upper pair of valence band states, which exhibited the strongest spin polarisations, were confined within a single WSe2 layer of the crystal structure. Within one of these layers, there is a local breaking of inversion symmetry. This allows spin polarised states to emerge due to the strong relativistic spin-orbit coupling promoted by the heavy tungsten atoms within each layer. The full crystal structure is created by stacking such layers together, but with a 180° rotation between neighbouring layers. This results in opposite spin orientations in each neighbouring layer, ensuring that there is an overall spin degeneracy as required for the global centrosymmetric system, but permitting strong spin polarisations to develop locally.
The experiments and calculations carried out demonstrate a strong coupling between the spin and the layer degree of freedom in these materials. The results open up a wealth of new opportunities, with the overall goal of applying this knowledge to developing spintronic systems. Beyond this lies further potential in the novel field of valleytronics, as Phil King explains: “Materials such as WSe2 have sets of band extrema located at different points in the Brillouin zone, called valleys. In valleytronics, the idea is to use these valleys to encode information, and several groups have made great progress recently using single layers of transition-metal dichalcogenides for this, where they found the spin becomes locked to the valley degree of freedom. The spin texture we measured in bulk WSe2 inherits this spin-valley locking, but it further becomes entangled with the layer pseudospin.”
He adds “If you could exploit this in a device, you might be able to do energy efficient computations.” The ability to probe and control spin and valley polarisation is thus essential to the developing fields of spintronics and valleytronics, and the discovery of such behaviour in bulk solids is a promising step along the route to new electronic materials optimised for such applications.
Riley J. M., Mazzola F., Dendzik M., Michiardi M., Takayama T., Bawden L., Granerød C., Leandersson M., Balasubramanian T., Hoesch M., Kim, T. K. Takagi H., Meevasana W., Hofmann Ph., Bahramy M. S., Wells J. W. and King P. D. C. Direct observation of spin-polarized bulk bands in an inversion-symmetric semiconductor. Nature Physics 10, 835 (2014) DOI: 10.1038/nphys3105
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