| Mar 25, 2025 |
First experimental proof of a new quantum phenomenon.
(Nanowerk News) A previously unseen quantum phenomenon first predicted several years ago has been experimentally demonstrated by an international research team, including DESY scientists, at PETRA III. The team observed a tornado-like quantum structure in a semimetal compound, tantalum arsenide (TaAs), wherein electrons behave like a swirling vortex in momentum space.
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The discovery, a collaborative effort between the Würzburg–Dresden Cluster of Excellence for Complexity and Topology in Quantum Matter (ct.qmat) and several other partners, has now been published in the journal Physical Review X (“Imaging Orbital Vortex Lines in Three-Dimensional Momentum Space”). The new phenomenon potentially opens the door for applications in novel quantum technologies.
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| Quantum tornado in momentum space: Electrons form vortices in the quantum material tantalum arsenide (TaAs). Momentum space is a concept in physics that describes the motion of electrons in terms of energy and direction. (Image: think-design | Jochen Thamm)
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It has long been known that electrons can form vortices in quantum materials. However, these vortices were always seen in a context called position space, literally only their physical location. Several years ago, a member of ct.qmat, a joint Cluster of Excellence between the University of Würzburg and TU Dresden, posited that the vortices could also exist in momentum space – in other words, in terms of their energy and momentum. This means as well that the vortex is linked to quantum properties of the electrons, beyond their physical location. Experimentally verifying this phenomenon is a milestone in the development of quantum materials research.
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The breakthrough experiments revealed that the quantum vortex is created by orbital angular momentum – electrons’ circular motion around atomic nuclei. To detect the quantum tornado in momentum space, the experimental core team, led by Maximilian Ünzelmann (University of Würzburg) and Hendrik Bentmann (Norwegian University of Science and Technology in Trondheim), enhanced a well-known technique called ‘Angle-Resolved Photoemission Spectroscopy’ (ARPES).
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“ARPES is a fundamental tool in experimental solid-state physics,” explains Maximilian Ünzelmann, a group leader at ct.qmat and the University of Würzburg. “It involves shining light on a material sample, extracting electrons, and measuring their energy and exit angle. This gives us a direct look at a material’s electronic structure in momentum space. By adapting this method, we were able to measure orbital angular momentum.”
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The experiments have been conducted using the ASPHERE III measurement station at the Variable Polarization XUV beamline P04 at PETRA III at DESY. The accurate circular polarization of the P04 beam, together with the angle and energy resolving precision of the ASPHERE III instrument, were crucial to the effort. The instrument was built in a long-term collaboration between the Würzburg researchers and a team from Kiel University and DESY as part of the ErUM-Pro programme of the German Federal Ministry of Education and Research (BMBF).
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The momentum space of the sample was analysed layer by layer, similar to how medical tomography works. By stitching together individual images, the team was able to reconstruct the three-dimensional structure of the orbital angular momentum and confirm that electrons form vortices in momentum space.
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The scientists next want to explore whether tantalum arsenide can be used to develop orbital quantum technological components for a novel form of electronics called orbitronics. These would rely on the orbital torque of the electrons to transfer information instead of relying on electrical charge, which could be a more energy efficient form of electronic component.
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“The next level of ARPES measurements would be to observe how the electrons move in such devices during operation,” says co-author Kai Rossnagel, professor of physics at Kiel University and lead scientist at DESY. “The coherent X-ray radiation of PETRA IV will allow us to do this in unprecedented detail.”
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