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Emerging van der Waals magnets advance the development of valleytronics

With the support from the National Science and Technology Council and, a joint research team led by Prof. Chang-Hua Liu, Po-Wen Chiu and Horng-Tay Jeng at the National Tsing Hua University has successfully developed a viable route to electrically control the valley degree of freedom in transition metal dichalcogenides. The result provides a crucial step towards modern optoelectronics and information processing applications, and the work was published in the Journal “Nature Nanotechnology” in May 2022.

Over the past few decades, the continuous shrinking of silicon transistors has sustained the extension of Moore’s law, which has led to significant advancement of robotics, 5G, Internet of Things, etc. But with the increase demand of computing power and downscaling of silicon nanoelectronics reaching their fundamental limits, people are currently exploring new materials and device architectures for modern information processing as well as storage technologies to extend limits of Moore's Law.

The monolayer semiconducting transition metal dichalcogenides (TMDs) have been considered as the promising candidates for such demands. The essential reason is that these materials own the exotic excitonic and spin–valley properties. Electrons in TMDs have an extra valley degree of freedom (i.e., a charge carrier's momentum index), which can be used to encode and process quantum information. To electric control of the valley degree of freedom, a few research works have exploited conventional ferromagnetic contacts, such as diluted magnetic semiconductor and permalloy, to inject spin-polarized carriers into the specific valley in TMDs. However, such devices either require a high external magnetic field or complicated epitaxial growth steps. Thus, the practical applications of such devices are limited. 

In this novel study, we demonstrate the electric control of valley-dependent polarization in TMDs by using van der Waals (vdW) magnets for the first time. To be specific, we develop novel vdW heterostructures that incorporate a monolayer TMD (WSe2) integrated with an ultrathin Fe3GeTe2- based ferromagnetic tunneling contact. This magnetic contact can inject spin-polarized carriers into the specific valley in WSe2, leading to the behavior of valley-dependent polarization, as confirmed by the helicity-dependent electroluminescence and reflective magnetic circular dichroism (RMCD) experiments. In addition, our results indicate that injecting spin-polarized holes from Fe3GeTe2 into WSe2 can more effectively lead to valley polarization, compared with injecting spin-polarized electrons from Fe3GeTe2. Our density functional theory (DFT) calculations reveal such phenomenon could be originated from the unique electronic structure of Fe3GeTe2, owing the strong exchange splitting of spin bands. 

It is notable that the demonstrated results from this study could provide further insights into the physical properties of FGT-based magnetic contact. Furthermore, our demonstrations not only clearly address key challenges for future valleytronics development, but also highlight the promising usage of emerging 2D magnets for magneto optoelectronics applications.


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Department of Natural Sciences and Sustainable Development,
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Last Modified : 2022/08/24