First, some background
Scientists are very interested in Weyl materials. which contain so-called Weyl electrons. This special class of electrons behave as though they have much less mass than conventional electrons and travel much faster — close to the speed of light while traveling through Weyl materials. This quantum behavior suggests possible future applications in electronics and other areas.
Initially predicted by mathematician Hermann Weyl in 1929, these electrons were not observed until 2015, when scientists saw them in tantalum arsenide using angle-resolved photoemission spectroscopy (ARPES). After that discovery, the effort to identify more Weyl materials switched into high gear.
What did scientists discover?
The scientists put a sample of the semimetal niobium arsenide (NbAs) in a very strong magnet then began to gradually increase the magnet’s field strength. They wanted to measure the magnetic torque of NbAs — how it twists in an external magnetic field — which is a function of how its electrons are arranged and interact.
They observed an exciting change in the data when the field strength reached about 25 tesla: A sudden reversal of the slope of the magnetic torque from negative to positive, followed by a sustained upward slope, as seen in the figure above. As it turned out, that abrupt inflection point on the graph is a telltale sign of a Weyl material. So scientists demonstrated that torque magnetometry in DC fields and in pulsed fields is a very useful technique for confirming if certain compounds are Weyl materials.
Why is this important?
Weyl materials are interesting because they allow researchers to study the fundamental properties of topological semimetals, properties that may find future applications.
These results establish that high-field magnetic torque measurements identify and distinguish Weyl electrons and another class of electrons called Dirac electrons both from each other and from the more conventional electron behavior taking place in our everyday electronic devices. This will help scientists identify more of these intriguing materials.
Who did the research?
Philip J. W. Moll1, Andrew C. Potter1, Nityan L. Nair1, B. J. Ramshaw2,3, K. A. Modic2,3, Scott Riggs2,3, Bin Zeng2,3, Nirmal J. Ghimire2, Eric D. Bauer2, Robert Kealhofer1, Filip Ronning2, James G. Analytis1
1University of California at Berkeley, 2Los Alamos National Laboratory, 3MagLab
Why did they need the MagLab?
In order for the torque anomaly to be observed, the experiment must access very high magnetic fields that go beyond the quantum limit of the material. For most materials, this value is much larger than the maximum magnetic fields available in typical university laboratories.
Details for scientists
- View or download the expert-level Science Highlight, Magnetic Torque Anomaly in the Quantum Limit of Weyl Semimetals
- Read the full-length publication, Magnetic torque anomaly in the quantum limit of Weyl semimetals, in Nature Communications
This research was funded by the following grants: Office of Naval Research (No. N00014-15-1-2674), Gordon and Betty Moore Foundation’s EPiQS Grant (GBMF4374), G.S. Boebinger (NSF DMR-1157490)