While modern electronics rely on electron charge for data transfer, future technologies may harness other properties of electrons to process information in a more environmentally friendly way. Until now, the leading contender in this realm has been spintronics, which uses electron spin. However, the field of orbitronics, which uses the orbital angular momentum of electrons, is emerging as a promising new avenue, particularly for creating energy-efficient memory devices.
A key challenge for orbitronics is finding the right materials to facilitate OAM flows. Now, an international research team led by scientists from PSI and Max Planck Institutes in Germany has demonstrated that chiral topological semi-metals, first discovered at PSI in 2019, have properties ideally suited for generating OAM currents.
Chiral Topological Semi-Metals: Ideal Candidates for Orbitronics
Conventional materials such as titanium have already shown some promise for orbitronics. However, the helical atomic structure of chiral topological semi-metals, discovered just five years ago, makes them particularly intriguing. Their unique structure naturally supports the formation of OAM patterns and textures, enabling the flow of OAM without the need for external stimuli.
"This offers a significant advantage to other materials because you don't need to apply external stimuli to get OAM textures - they're an intrinsic property of the material," explained Michael Schuler, co-lead of the study and group leader at the Center for Scientific Computing, Theory and Data at PSI. Schuler, also an assistant professor at the University of Fribourg, emphasized the potential to create stable and efficient OAM currents under normal conditions.
The Potential of OAM Monopoles
One of the most captivating prospects in chiral topological semi-metals is the existence of OAM monopoles. These monopoles radiate OAM uniformly in all directions, like spikes on a hedgehog. "This is a very useful property as it means flows of OAMs could be generated in any direction," Schuler added.
However, despite their theoretical appeal, OAM monopoles had not been experimentally observed until now. The team turned to Circular Dichroism in Angle-Resolved Photoemission Spectroscopy (CD-ARPES), a technique using circularly polarized X-rays, to reveal the elusive OAM textures.
Bridging the Gap Between Theory and Experiment
In past experiments, the gap between theory and data interpretation hindered OAM monopole detection. Previous assumptions that CD-ARPES data directly correlated with OAMs proved inaccurate. Schuler and his colleagues painstakingly examined CD-ARPES data from palladium-gallium and platinum-gallium semi-metals. They realized the CD-ARPES signal rotated around the monopoles as photon energy changed, uncovering the monopoles by testing their assumptions with rigorous theory and complex analysis.
Expanding Orbitronics Potential
This breakthrough allows researchers to not only visualize OAM monopoles but also manipulate their polarity by using materials with mirror image chirality. "This is a very useful property, as orbitronics devices could potentially be created with different directionality," Schuler noted.
With this newfound ability to explore OAM textures in various materials, the discovery could lead to significant advancements in orbitronics, offering a path toward energy-efficient technologies.
Research Report:Controllable orbital angular momentum monopoles in chiral topological semimetals
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