Quantum tornadoes: Physicists discover electron vortex in semimetal

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Electrons can form vortex structures in quantum materials, but it was previously unclear whether these could also occur in momentum space. An international research team led by Maximilian Ünzelmann from the Cluster of Excellence ct.qmat at the Universities of Würzburg and Dresden has now provided experimental proof of this theory. The researchers observed the vortex-like structures in a semimetal, which could become the basis for a new type of quantum technology. The results of the study were published in the journal "Physical Review X".

Vortices in momentum space

Momentum space is a physical concept that describes the movement of particles based on their energy and direction of movement. It is therefore an alternative to the more familiar location space, which describes the location of the particles. Phenomena familiar from everyday life, such as water vortices and tornadoes, are usually described in local space.

The Dresden-based solid-state physicist Roderich Moessner already theorized in 2017 that vortex-like structures are possible not only in the local space, but also in the momentum space of semimetals. At the time, he compared this phenomenon to a smoke ring.

The Würzburg-Dresden team has now been able to demonstrate this behavior in the semimetal tantalum arsenide (TaAs). Tantalum arsenide exhibits these vortex-like structures in momentum space due to its intrinsic electronic properties. The challenge was to prove this experimentally.

The method with which the research team was able to provide evidence is called ARPES, which stands for Angle Resolved Photo Emission Spectroscopy. "ARPES is part of the standard repertoire of experimental solid-state physics," explains Ünzelmann. "This involves irradiating material samples with light, extracting electrons and measuring their energy and exit angle. This provides a direct view of the electronic material structure in the momentum space."

The team also combined the method with a special form of tomography. "We examined the sample layer by layer, as is known from medical tomography. The individual images were strung together. This allowed us to see the three-dimensional structure of the orbital angular momentum and prove that the electrons form vortices in the momentum space," says Ünzelmann.

Further development of orbitronics

The experimental proof of this phenomenon not only confirms the theory of Moessner, a founding member of ct.qmat, but also demonstrates the potential of the extended ARPES technique to make complex quantum phenomena visible.

The researchers hope that their findings can contribute to the development of orbitronics. Instead of the electrical charge, the orbital angular momentum of the electrons is used to transmit information in electronic components. This could significantly reduce energy losses during data transmission in the future.

International cooperation

Julius-Maximilians-Universität Würzburg and Technische Universität Dresden have been jointly supporting the Cluster of Excellence ct.qmat since 2019. The acronym stands for "Complexity and Topology in Quantum Matter". More than 300 scientists are researching topological quantum materials that reveal unique phenomena under extreme conditions such as ultra-low temperatures, high pressure or strong magnetic fields.

International researchers contributed to the current results. A US group grew the semimetal tantalum arsenide under investigation. This was then examined at the international large-scale research facility PETRA III of the German Electron Synchrotron (DESY) in Hamburg. A scientist from China was also involved in the theoretical modeling and a researcher from Norway led the experiment.

(wpl)