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Tuesday, 22 December 2020

A new route to spin transport control in 2D materials revealed

Researchers from the ICN2 Theoretical and Computational Nanoscience group, led by ICREA Prof Stephan Roche, the National University of Singapore, and the University of Grenoble Alpes (France) have predicted interesting properties of the low-symmetry structural configuration of a semimetal material (tungsten ditelluride monolayer, WTe2), which can provide new tools to manipulate spin. These results are relevant to the development of new nanodevices based on all-electrical spin control.

The ability to control and manipulate spin in materials is at the basis of spintronics, a field of research and branching sector of electronics that aims at using spin to carry and propagate information. Spin related phenomena are studied in order to take advantage of peculiar characteristics exhibited by some materials. Among them, transition metal dichalcogenide (TMDs) monolayers are particularly interesting. These two-dimensional materials exhibit a three atomic interlayered structure that can assume distinct configurations, called phases. Different properties emerge in each phase, such as unusual spin-charge interconversion and topological characteristics, which are extremely relevant to their application.

Researchers from the ICN2 Theoretical and Computational Nanoscience group, led by ICREA Prof. Stephan Roche, the National University of Singapore, and the University of Grenoble Alpes (France) have predicted unique features of a low-symmetry structural phase of the tungsten ditelluride (WTe2) monolayer – a material of the transition metal dichalcogenide family – which suggests alternative ways to manipulate spin information. As explained in a paper recently published in Physical Review Letters, quantum transport simulations and modelling showed that the structure of this material leads to an unconventional quantum spin Hall (QSH) effect, which is a spin transport phenomenon observed in some two-dimensional semiconductors.

Typically, the strong symmetry in 2D materials’ structure compel the spins to align with one of the crystalline directions. In absence of such constrains, spins can assume arbitrary orientations. This is what happens in the so-called distorted octahedral phase (1T’) of the WTe2 monolayer, exhibiting a low-symmetry structure. While the traditional spin Hall effect is usually associated with spin polarization pointing perpendicularly to the conducting plane, here a so-far-unique QSH effect defined by an oblique spin polarization axis is predicted. The orientation of this axis is prescribed by the spin-orbit coupling parameters, which are tuneable via a number of means: strain, electrostatic gates, substrates choice, or pressure.

Remarkably, the reliability of these predictions has been confirmed just a few months after the completion of this study. In fact, while the paper was undergoing the peer-review process, two experimental works were revealed (in pre-print version), whose results are in quantitative agreement with the present one. This research opens a new avenue for spintronics, in which spins can be controlled through the electronic environment, instead of by conventional magnetic means. 

This research has been partially supported by the European Union Horizon 2020 Programme through the Graphene Flagship project and the TOCHA project (led by the ICN2). The paper in which this work is reported is signed by Dr José H. Garcia and PhD student Marc Vila, from Prof. Roche’s group, as first authors. Vila carried out a part of this study during a two-month stay at the Centre for Advanced 2D Materials (Ca2DM) of the National University of Singapore, which was made possible by a grant from the Graphene Flagship.

 

Article reference:

Jose H. Garcia, Marc Vila, Chuang-Han Hsu, Xavier Waintal, Vitor M. Pereira, and Stephan Roche, Canted Persistent Spin Texture and Quantum Spin Hall Effect in WTe2. Phys. Rev. Lett. 125, 256603 – 18 December 2020. DOI: 10.1103/PhysRevLett.125.256603