Quantifying spin in WTe2 for future spintronics

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Nov 03, 2021

(Nanowerk News) A RMIT-led, worldwide collaboration printed this week has noticed massive in-plane anisotropic magnetoresistance (AMR) in a quantum spin Hall insulator and the spin quantization axis of the sting states could be well-defined (Nano Letters, “Spin-momentum locking induced anisotropic magnetoresistance in monolayer WTe2). A quantum spin Hall insulator (QSHIs) is a two-dimensional state of matter with an insulating bulk and non-dissipative helical edge states that show spin-momentum locking, that are promising choices for growing future low-energy nano-electronic and spintronic units. The FLEET collaboration of researchers at RMIT, UNSW and South China Normal University (China) affirm for the primary time the existence of enormous in-plane AMR in monolayer WTe2 which is a novel QSHI with increased vital temperatures. By permitting electrical conduction with out wasted dissipation of vitality, such supplies might kind the idea of a brand new future technology of ultra-low vitality electronics.

Fabricating monolayer WTe2 units

The rise of topological insulators has supplied important hope for researchers searching for non-dissipative transport, and thus an answer to the already noticed plateauing of Moore’s legislation. Unlike previously-reported quantum-well programs, which might solely exhibit quantized edge transport at low temperatures, the latest commentary of quantized edge transport at 100 Okay in a predicted massive band-gap QSHI, monolayer WTe2 , has shed extra gentle on the purposes of QSHI. Left: monolayer WTe2 system (scale bar = 5µm). Right: Gate-dependent conductance at various temperatures. (© Nano Letters) “Although we had gained much experience in stacking van der Waals (vdW) heterostructures, fabricating monolayer vdW devices was still challenging for us,” the research’s first writer Dr Cheng Tan says. “Because monolayer WTe2 nanoflakes are difficult to obtain, we firstly focused on a more mature material, graphene, to develop the best way for fabricating monolayer WTe2 vdW devices” says Cheng, who’s a FLEET Research Fellow at RMIT University in Melbourne. As the monolayer WTe2 nanoflakes are additionally very delicate to the air, protecting ‘suits of amours’ made from inert hBN nanoflakes ought to be utilized to encapsulate them. Additional, the meeting was carried out in an oxygen- and water-free glove field earlier than collection of assessments outdoors. After some effort, the crew then efficiently fabricated the monolayer WTe2 units with gate electrodes and noticed typical transport behaviours of gated monolayer WTe2. “For materials to be used in future spintronic devices, we need a method to determine spin characteristics, in particular the direction of spin,” says Dr Guolin Zheng (additionally at RMIT). “However, in monolayer WTe2, spin-momentum locking (an essential property of QSHI) and whether spin quantization axis in its helical edge states could be determined had yet to be experimentally demonstrated.” Anisotropic magnetoresistance (AMR) is an efficient transport measurement methodology to disclose the connection between the electrons’ spin and momentum when the present is spin-polarized. Considering that the sting states of a QSHI solely enable the transport of spin-polarized electrons, the crew then used AMR measurements to discover the potential spin-momentum locking within the edge states of monolayer WTe2. “Fortunately, we found the proper method to deal with the monolayer WTe2 nanoflakes,” says co-author Dr Feixiang Xiang (UNSW). “So then we performed angular-dependent transport measurements to explore the potential spin features in the edge states.”

Performing anisotropic magnetoresistance and defining the spin quantization axis

However, the topological edge states should not the one doable trigger for spin-momentum locking and in-plane AMR results in a QSHI. Rashba splitting might additionally generate related results, which can make the experimental outcomes unclear. “Fortunately, topological edge states and Rashba splitting induce very different gate-dependent in-plane AMR behaviours, because the band structure under these two situations are still very different.” says co-author Prof Alex Hamilton (additionally at UNSW). “Most of the samples show that minimum of in-plane AMR happens when the magnetic field is nearly perpendicular to the edge current direction.” says Cheng. Further theoretical calculations by collaborators at South China Normal University additional confirmed that electrons’ spins within the edge states of monolayer WTe2 ought to be all the time perpendicular to their propagation instructions, so-called ‘spin-momentum locking’. “The amplitudes of the in-plane AMR observed in monolayer WTe2 is very large, up to 22%” says co-author A/Prof Lan Wang (additionally at RMIT). Quantifying spin in WTe2 for future spintronics When monolayer WTe2 system (left) is tilted in in-plane course, AMR (proper) varies by angle of tilt, proven at various magnetic discipline, and reaches a minimal worth when the magnetic discipline is perpendicular to the sting present course. (© Nano Letters) “While the previous amplitudes of in-plane AMR in other 3D topological insulators are only around 1%. By AMR measurements, we can also precisely determine the spin quantization axis of the spin polarized electrons in the edge states.” “Again, this work demonstrates the promising potential of QSHI for designing and developing novel spintronic devices and prove AMR as a useful tool for the design and development of QSHI-based spintronic devices, which are one of the promising routes for FLEET to realize low-energy devices in future.”

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