Chiral anomaly: An end to decades-long hunt

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  • Published: Sep 15, 2015
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Chiral anomaly: An end to decades-long hunt

Decades of anomaly

The phenomenon of 'chiral anomaly' in crystals was first posited in the 1940s, formalised in the 1960s, but has only now been detected with nuclear magnetic resonance spectroscopy in a compound of sodium and bismuth.

The 'chiral anomaly' was first posited in the 1960s and suggested to exist in crystalline materials in the 1980s but has only now been detected by conductivity measurements in a semi-metal compound of sodium and bismuth.

In a new class of semimetals called 3D Dirac semimetals, the electrons occupy quantum states (called Dirac nodes) that are well-described by the fundamental Dirac equation originally introduced to describe extremely high-energy electrons, positrons and neutrinos. For Dirac semimetals, each Dirac node is resolved into two Weyl nodes of opposite handedness (or chirality). According to N. Phuan Ong of Princeton University, New Jersey, USA, and colleagues writing in the journal Science, these two populations do not mix. However, they explain, in parallel electric and magnetic fields, it is predicted that a current (called chiral or axial current) will flow between the Weyl nodes. In a crystal, this is predicted to give rise to an unusual negative magnetoresistance. This chiral current is an anomaly that has been investigated in quantum field theory for the past five decades. Now, the researchers have reported what they refer to as a large, negative longitudinal magnetoresistance in the Dirac semimetal Na3Bi. The behaviour of this property is consistent with the theorised chiral anomaly in a crystalline material.    

Prediction fulfilled

"Our research fulfils a famous prediction in physics for which confirmation seemed unattainable," says Ong, who worked with Robert Cava and others. "The increase in conductivity in the crystal and its dramatic appearance under the right conditions left little doubt that we had observed the long-sought chiral anomaly."

The Princeton study builds on decades of theorising hinted at in the 1940s by Hermann Weyl at the Institute for Advanced Study in Princeton, and others. It was first theorized formally in the 1960s and predicted to occur in crystals in 1983. The fact that the demonstration also coincides with an observed increase in conductivity in the Dirac semimetal Na3Bi might also point the way to improving electrical conductivity and minimize energy consumption in future electronic devices.

Weyl's original discovery in the 1940s was that all elementary particles that have zero mass (including neutrinos, despite their having an extremely small mass) strictly segregate into left- and right-handed populations that never intermix. This phenomenon was later found to be overturned by the presence of electric and magnetic fields which can induce one population to transform into the other giving rise to experimentally observed changes in the material. In quantum field theory, this field-induced mixing is known as the chiral anomaly. It was first uncovered theoretically in 1969 by Stephen Adler of the Institute for Advanced Study, John Bell of the European Organization for Nuclear Research (CERN) and Roman Jackiw of the Massachusetts Institute of Technology, who used it to explain why neutral pions decay much faster - some 300 million times faster - than their charged counterparts. In the decades since, the chiral anomaly has played a perplexing role especially in the quest to unify the four fundamental forces of nature.

Axial awesome

In 1983, physicists Holger Bech Nielsen of the University of Copenhagen and Masao Ninomiya of the Okayama Institute for Quantum Physics suggested that the same phenomenon might exist in crystalline materials so that it can be investigated in the laboratory. Such an experiment would allow experiments in the presence of  intense magnetic fields under conditions that would be  impossible in high-energy particle colliders. The recent emergence of research on "topological" materials  has led to semimetals with electrons that mimic   massless particles that segregate into left- and right-handed populations.

The experiments on Na3Bi were carried out by graduate student Jun Xiong who cooled to cryogenic temperatures a crystal of the semimetal grown by post-doctoral researcher Satya Kushwaha from Cava's team. The cooling was carried out in the presence of a strong magnetic field that can be rotated relative to the direction of the applied electrical current in the crystal. When the magnetic field was aligned parallel to the current, the researchers found that the two chiral populations mixed together and this gave rise to an increase in conductivity, the so-called "axial current plume", which would only arise if there is a chiral anomaly in the crystal.

"One of the key findings in the experiment is that the intermixing leads to a charge current, or axial current, that resists depletion caused by scattering from impurities," Ong explains. "Understanding how to minimize the scattering of current-carrying electrons by impurities - which causes electronic devices to lose energy as heat - is important for realizing future electronic devices that are more energy-efficient. While these are early days, experiments on the long-lived axial current may help us to develop low-dissipation devices."

"The next step is to understand why the current enhancement is so large," Ong told SpectroscopyNOW. "Are the impurities (which normally degrade current flow) somehow 'blind' to the flow of the axial current?" he asks. "An important path is to explore a larger family of Dirac and Weyl semimetals to see how pervasive the chiral anomaly is." There is one drawback to Na3Bi in that it is very air sensitive. "In wet New Jersey air, it oxidizes to a white powder in a few minutes," Ong laments. He points out, however, that there are several groups - in Inst. of Physics (Beijing), Max Planck Inst (Dresden) and Brookhaven National Lab (Upton, US) -- that also uncovered similar results within months of their report. "In the coming years, I anticipate a lot of research activity worldwide around the chiral anomaly in crystalline materials."

Related Links

Science 2015, online: "Evidence for the chiral anomaly in the Dirac semimetal Na3Bi"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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