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International Research Team Report Giant Spin-splitting In WSe2

Discovery paves way for new approaches to ultrafast and energy efficient spin-based electronics

Scientists may be a step closer to super-fast energy efficient computers and smartphones, according to a study led by Philip King of the University of St Andrews just published in Nature Physics.

Physicists for many years have been trying to develop ways to exploit the electron's tiny magnetic moment in electronic devices, a scheme termed spintronics. While this has already led to enormous advances in computer hard drives, now scientists are working to create active electronic devices, like the transistors at the heart of every computer, tablet and smartphone, that also capitalise on the quantum-mechanical property of an electron's spin.

This requires making the underlying electronic band structure, the fundamental relations that dictate how electrons travel in solids, to be spin-dependent, and to be able to switch this property on and off in a device. To date, though, such 'spin splittings' have been too small for practical applications.

The international team of researchers from the University of St Andrews, the Norwegian University of Science and Technology, the Universities of Tokyo, Aarhus, and Suranaree, the Max-Planck Institute for Solid State Research, MAX-lab and Diamond Light Source, now report the observation of a giant spin splitting in the semiconductor tungsten selenide (WSe2), a transition-metal dichalcogenide (an emerging class of semiconductors).

Jon Riley (pictured above), a PhD student at the University of St Andrews, and first author of the study, explains: "We were measuring the electronic structure of WSe2 using a sensitive technique based on the photoelectric effect. When we did this in a spin- sensitive mode, we were surprised to discover really strong signals - the bands were almost 100 percent spin polarised - which seemed to contradict fundamental symmetries that this material possesses."

Above: Experimental and calculated electronic structure  of WSe2, and extracted layer specific spin texture.    

Combining detailed experiments and theoretical calculations, the researchers showed how this arises because of symmetry breaking at atomic length scales. Philip King, lead researcher at the University of St Andrews, explains: "You can think of these materials as being built up of two repeating layers, with each second layer rotated 180degrees relative to the first. Within these individual atomically-thin building blocks, a special structural symmetry called inversion is not present. A relativistic effect then allows electronic states that are strongly confined within one of the layers to develop huge spin polarisations. When you take the whole crystal together, however, inversion symmetry is restored."

He added:  "We found that the sign of the spin polarisation for states confined in one layer is exactly compensated by that of the equivalent states in the rotated layer, and so even though the states are strongly spin-polarised, no fundamental constraints of the global symmetry are violated."

By carefully tuning the measurement parameters, the researchers could select conditions that selectively probed just the top layer of the crystal, allowing the first direct observation of these hidden spin polarisations.

King adds: "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. Controlling this could bring fantastic new opportunities for spintronics, and a large arsenal of new materials in which we can achieve this."

The paper 'Direct observation of spin-polarized bulk bands in an inversion-symmetric semiconductor' by JM Riley et al, appears in Nature Physics (2014) doi:10.1038/nphys3105

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