An international team of researchers, led by a physicist from the City University of Hong Kong (CityU), discovered that a new metallic crystal exhibits unexpected electrical activity on its surface due to the crystal’s unique atomic structure. Their results suggest that this material might be used to create quicker and smaller microelectronic devices.
The substance under investigation is a newly found “kagome” metal compound composed of three elements: gadolinium (Gd), vanadium (V), and tin (Sn). The ratio of the three metal elements found in the GdV6Sn6 crystal is indicated by the designation “1-6-6.” The atoms are organised in a complicated yet regular geometric structure, which results in exceptional surface properties.
Negatively charged electrons in atoms normally move around within distinct energy bands at varying distances from the positively charged nuclei. However, the top layers of exposed atoms on the surface of GdV6Sn6 are projected to interact with one another and distort the topology, or shape and location, of the energy bands. This deformation might theoretically create a novel and stable electrical property that has yet to be observed in GdV6Sn6 or any other kagome metal.
The first instance of anomalous surface electrical activity in a kagome metal has been seen.
“For the first time, our team unequivocally demonstrated that a kagome metal may display changed electronic energy-band structures of the sort known as ‘topologically non-trivial Dirac surface states,'” explains Dr Ma Junzhang, Assistant Professor in CityU’s Department of Physics.
“Electrons generate their magnetic field due to their inherent spin and charge, and they function like miniature gyroscopes with both rotation and an inclined tilt that points in a certain direction. In GdV6Sn6, surface electrons get reordered or spin-polarized,’ and their tilts rearrange themselves around a common axis perpendicular to the surface.”
The ordered alignment of electrons around a common axis is known as “spin chirality,” and it may be clockwise or anticlockwise. More critically, the study team was able to effectively reverse the spin chirality by modifying the crystal surface physically. “Because we discovered that the spin chirality of the spin-polarized electrons is readily reversible,” Dr Ma continues, “this material has enormous promise for use in next-generation spintronics transistors.”
The work, which will be published in Science Advances on September 21, 2022, was driven by theoretical predictions of unique surface electronic band topologies based on GdV6Sn6 kagome crystal special characteristics. Layers of repeated V3Sn subunits, for example, are placed between alternating layers of Sn and GdSn2.
Furthermore, several V3Sn subunits are geometrically organised in a “kagome layer,” whose repeating pattern of six equilateral triangles enclosing a hexagon is similar to the kagome lattice observed in Japanese bamboo basket weaving. Finally, the V3Sn kagome layer is not magnetic, but the GdSn2 layer is.
First, the researchers created GdV6Sn6 crystals by heating and gently cooling Gd, V, and Sn metals. The next split a crystal through the stacked layers and studied the exposed surface using angle-resolved photoemission spectroscopy, or ARPES, after validating the chemical composition and structure using single-crystal X-ray diffraction. The results showed that the cleft surface had rearranged energy band structures, and subsequent examination confirmed the clockwise spin character.
Finally, the researchers demonstrated that by covering the surface with potassium atoms, the surface energy bands could be substantially distorted, a process known as electron doping. As a consequence, as the doping level increased, the electron spin chirality shifted from clockwise to anticlockwise.
Potential uses in information transmission enhancement and beyond
Researchers’ ability to intentionally invert the spin chirality of surface electrons on the GdV6Sn6 crystal makes it a potential candidate material for a wide range of practical electronic applications.
“In the future, we may be able to use a local voltage, or electrostatic ‘gate,’ to directly edit or adjust the electronic band structure, and therefore alternate the electron spin chirality on the surface of 1-6-6 kagome metals,” Dr Ma explains.
“Controlling the direction of electron spin-polarization is an appealing alternative to typical binary digital coding based on the presence or absence of electrical charge, which is somewhat slow and may cause issues such as device heating. When combined with superconductors, our approach has the potential to greatly improve efficiency in digital information transport while generating less heat.”
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