Quantum tornadoes in momentum space: First experimental proof of a new quantum phenomenon

 

Quantum tornado in momentum space. Credit: Think-Design | Jochen Thamm

Würzburg researchers have improved a well-established technique to experimentally demonstrate a quantum tornado for the first time.  Electrons in momentum space behave as a spinning vortex in the quantum semimetal tantalum arsenide (TaAs).  Eight years ago, a founding member of the Cluster of Excellence ct.qmat, based in Dresden, made the first prediction about this quantum event.

 Physical Review X has recently published the discovery, which was made by ct.qmat, the research network of the Universities of Würzburg and Dresden, and worldwide collaborators.

The ability of electrons to create vortices in quantum materials has long been recognized by scientists.  What is novel is the experimental confirmation of the conclusion that these small particles form tornado-like formations in momentum space.  Dr. Maximilian Ünzelmann, a group leader at the Universities of Würzburg and Dresden's ct.qmat—Complexity and Topology in Quantum Matter—led this accomplishment.

 An important turning point in the study of quantum materials has been reached with the demonstration of this quantum phenomena.  By using electrons' orbital torque to transmit information in electronic components rather than electrical charge, the team hopes that the vortex-like behavior of electrons in momentum space will open the door for new quantum technologies like orbitronics, which could reduce energy losses.

Momentum space vs. position space

A key idea in physics, momentum space characterizes the travel of electrons in terms of energy and direction rather than their precise physical location.  Position space (its "counterpart") is the domain in which well-known phenomena like hurricanes or water vortices take place.  Up until recently, position space was the sole way to view even quantum vortices in materials.

 The first three-dimensional image of a vortex-like magnetic field in the position space of a quantum material was taken a few years ago by another ct.qmat research team, which caused a stir throughout the world.

Theory confirmed

A quantum tornado might potentially emerge in momentum space, according to a theory put out by Roderich Moessner eight years ago.  The co-founder of ct.qmat, who was stationed in Dresden at the time, called the phenomena a "smoke ring" since it is made up of vortices, just like smoke rings.  But nobody understood how to measure them until recently.

 The ground-breaking tests demonstrated that orbital angular momentum, or the circular motion of electrons around atomic nuclei, is what generates the quantum vortex.  "When we first saw signs that the predicted quantum vortices actually existed and could be measured, we immediately reached out to our Dresden colleague and launched a joint project," says Ünzelmann.

Quantum tornado discovered by refining a standard method

The researchers from Würzburg improved on a popular method known as ARPES (angle-resolved photoemission spectroscopy) to identify the quantum tornado in momentum space.  "A key instrument in experimental solid-state physics is ARPES.  A material sample is exposed to light, electrons are extracted, and their energy and exit angle are measured.

 "This gives us a direct look at a material's electronic structure in momentum space," says Ünzelmann.  "We measured orbital angular momentum by ingeniously modifying this technique.  Since my dissertation, I have been using this method.

The foundation of ARPES is the photoelectric effect, which Albert Einstein originally defined and which is taught in high school physics classes. In 2021, Ünzelmann had already improved the technique and achieved reputation throughout the world for identifying orbital monopoles in tantalum arsenide. Now, the team has advanced the method to detect the quantum tornado—another significant milestone—by including a type of quantum tomography.

As with medical tomography, we examined the sample layer by layer. We confirmed that electrons form vortices in momentum space and were able to reconstruct the orbital angular momentum's three-dimensional structure by piecing together individual photos," says Ünzelmann.

Matthias Vojta, TU Dresden's Professor of Theoretical Solid-State Physics and ct.qmat's Dresden spokesperson, says, "The experimental detection of the quantum tornado is a testament to ct.qmat's team spirit."  We are able to smoothly combine theory and experiment because to our robust physics hubs in Dresden and Würzburg.

 Additionally, our network encourages collaboration between early-career scientists and top experts, which supports our study of topological quantum materials.  Naturally, practically all physics projects nowadays are international in scope, and this one is no exception.

The German Electron Synchrotron (DESY) in Hamburg's PETRA III, a significant international research facility, is where the tantalum arsenide sample was grown in the United States and examined.  A Norwegian researcher was instrumental in the experiments, and a Chinese scientist helped with the theoretical modeling.

 The ct.qmat team is investigating the possibility of using tantalum arsenide to create orbital quantum components in the future.

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