Axion particle spotted in solid-state crystal

Max Planck Institute for Chemical Physics of Solids

Illinois Physics Professor Barry Bradlyn. Photo by Siv Schwink/Illinois Physics
Illinois Physics Professor Barry Bradlyn. Photo by Siv Schwink/Illinois Physics
Scientists at the Max Planck Institute for Chemical Physics of Solids in Dresden, Princeton University, the University of Illinois at Urbana-Champaign, and the University of the Chinese Academy of Sciences have spotted the fingerprint of an elusive particle: The axion—first predicted 42 years ago as an elementary particle in extensions of the standard model of particle physics.

The team found signatures of axion particles composed of Weyl-type electrons (Weyl fermions) in the correlated Weyl semimetal (TaSe4)2I. At room temperature, (TaSe4)2I is a one-dimensional crystal, in which electrical current is conducted by Weyl fermions. However, by cooling (TaSe4)2I down below -11 °C, these Weyl fermions themselves condense into a crystal—a so called "charge density wave"—which distorts the underlying crystal lattice of the atoms. The initially free Weyl fermions are now localized and the initial Weyl semimetal (TaSe4)2I becomes a non-magnetic axion insulator. Similar to the existence of free electrons in metallic atomic crystals, the Weyl semimetal-based charge-density-wave crystal hosts axions that can conduct electrical current. However, such axions behave quite differently from the more familiar electrons. When exposed to parallel electric and magnetic fields, they produce an anomalous positive contribution to the magnetoelectric conductivity.

Based on predictions from Illinois Physics Professor Barry Bradlyn and Princeton Physics Professor Andrei Bernevig's group, the group of Chemical Physics Professor Claudia Felser at Max Planck in Dresden produced the charge density wave Weyl metalloid (TaSe4)2I and investigated the electrical conduction in this material under the influence of electric and magnetic fields. It was found that the electric current in this material below -11 °C is actually carried by axion particles.

“Finding these signatures of axion electrodynamics in a correlated material like (TaSe4)2I shows that there is still so much to discover about topological materials,” notes Bradlyn.

The results of the experiments were published online in the journal Nature on October 7, 2019.

"It's very surprising that materials that we think we know are suddenly showing such interesting quantum particles," notes Felser, one of the lead authors of the paper.

Scheme of a Weyl-semimetal-based axion insulator.

Scheme of a Weyl-semimetal-based axion insulator. © MPI CPfS
Examining the novel properties of axion particles in table-top experiments could not only allow scientists to better understand the mysterious realm of quantum particles, but also to open up the field of highly correlated topological materials.

"Another building block to my lifelong dream of realizing ideas from astronomic and high-energy physics with table-top experiments in solids," says Johannes Gooth, also a lead author of the paper.

This research was supported in part by the National Science Foundation, the US Department of Energy, the National Science Foundation of China, and the Simons and Packard Foundations. The conclusions presented are those of the researchers and not necessarily those of the funding agencies.


Recent News

  • In the Media
  • Student News
  • Atomic Molecular and Optical Physics
  • Quantum Information Science

When it comes to furthering our overall understanding of the physical world, ultracold quantum gases are awfully promising. As the famous physicist Richard Feynman argued, to fully understand nature, we need quantum means of simulation and computation. Ultracold atomic systems have, in the last 30 years, proven to be amazing quantum simulators. The number of applications for these systems as such simulators is nothing short of overwhelming, ranging from engineering artificial crystals to providing new platforms for quantum computing. In its brief history, ultracold atomic experimental research has enhanced physicists’ understanding of a truly vast array of important phenomena.

  • Research
  • Condensed Matter Physics

A Majorana particle is a fermion that is its own anti-particle. Majorana particles were postulated to exist by Ettore Majorana in a now famous paper written in 1937. However, such particles have not  been discovered in nature to date.  The possible realization of Majorana particles in condensed matter systems has generated much excitement and revived interest in observing these particles, especially because the condensed matter realization may be useful for topological quantum computation. A new paper by Illinois Physics Professor Vidya Madhavan and collaborators recently published in Science shows the first evidence for propagating 1D Majorana modes realized at 1D domain walls in a superconductor  FeSexTe1−x

  • In the Media

Albert Einstein was right again. More than 100 years ago, his calculations suggested that when too much energy or matter is concentrated in one place, it will collapse in on itself and turn into a dark vortex of nothingness. Physicists found evidence to support Einstein’s black hole concept, but they’d never observed one directly. In 2017, 200-plus scientists affiliated with more than 60 institutions set out to change that, using eight global radio observatories to chart the sky for 10 days. In April they released their findings, which included an image of a dark circle surrounded by a fiery doughnut (the galaxy Messier 87), 55 million light years away and 6.5 billion times more massive than our sun. “We have seen what we thought was unseeable,” said Shep Doeleman, leader of what came to be known as the Event Horizon Telescope team. The team’s name refers to the edge of a black hole, the point beyond which light and matter cannot escape. In some ways, the first picture of a black hole is also the first picture of nothing.

Institute for Condensed Matter Theory in the Department of Physics at the University of Illinois at Urbana-Champaign has recently received a five-year grant of over $1 million from the Gordon and Betty Moore Foundation. The grant is part of the Gordon and Betty Moore Foundation’s Emergent Phenomena in Quantum Systems (EPiQS) Initiative, which strives to catalyze major discoveries in the field of quantum materials—solids and engineered structures characterized by novel quantum phases of matter and exotic cooperative behaviors of electrons. This is the second 5-year EPiQS grant awarded to the ICMT by the Moore Foundation. The two awards establish an EPiQS Theory Center at the Institute for Condensed Matter Theory.