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Sounding out Weyl fermions

24 Jun 2019 Isabelle Dumé
Weyl semimetal topological phases. Credit: iStock Lee Yiu Tung

Researchers have put forward a completely new heat transport mechanism – known as chiral zero sound – in Weyl semimetals. The new mechanism could explain recent observations of higher-than-predicted oscillations of the thermal conductivity in these materials along the direction of an applied magnetic field and may even be exploited to control the thermal current through certain types of electronics devices using this field.

A Weyl semimetal is a recently discovered class of topological material (one that can be insulating in the bulk but has conducting surface states due to symmetry-protected topological order). In these materials, electronic excitations behave as massless, Weyl, fermions. These particles were first predicted in 1929 by the theoretical physicist Herman Weyl as a solution of the Dirac equation.

The chiral magnetic effect

Fermions that can be described by Weyl’s theory can appear as quasiparticles in solids that have linear electron energy bands crossing at (Weyl) points near the Fermi energy. These quasiparticles behave quite differently to electrons in ordinary metals or semiconductors in that they show the chiral magnetic effect (CME). This occurs when a Weyl metal is placed in a magnetic field, which generates a current of positive and negative Weyl particles that move parallel and antiparallel to the field.

The net current flow is usually zero because the positive and negative particles are present in equal amounts. This thermodynamic balance changes, however, when an electric field is applied parallel to the magnetic field. In this case, a net quasiparticle current flows and this effect, known as the chiral anomaly, can manifest itself as negative magnetoresistance – a lowering of the resistance with increasing magnetic field.

Chiral zero sound

Zhida Song and Xi Dai of the Hong Kong University of Science and Technology are now saying that the CME should also lead to a collective effect called chiral zero sound (CZS).

Zero sound is a quantum mechanical effect first put forward by the theoretical physicist Lev Landau in the context of Fermi-liquid theory. Like normal sound, zero sound comes from vibrations, but instead of being carried by air molecules, the vibrating medium in the case of zero sound is in fact the momentum distribution of electrons near the Fermi Level.

The Hong Kong team has now indeed shown that these zero sound vibrations exist in Weyl semimetals when a magnetic field is applied and that they significantly contribute to the material’s thermal conductivity. The chiral sound wave is quite different to the zero sound proposed by Landau though because it is an electronic acoustic mode and only propagates along the applied magnetic field direction, says Dai. “What is more, we can strongly modulate its velocity using the magnetic field, which is quite exotic because only the thermal conductivity parallel to the field oscillates.”

The researchers say that the CZS described in their work could be directly measured in various types of experiments. These include: by a pump and probe optical measurement technique; by detecting the possible formation of polariton modes (quasiparticles that are part matter and part light) formed by the hybridization of CZS and optical phonon modes (vibrations of the crystal lattice); and by using supersonic measurements.

“Completely new sound mode”

“We have proposed a completely new sound mode carried by Weyl fermions under a magnetic field,” Dai tells Physics World. “Until now, we could only ‘see’ these fermions using techniques like angle-resolved photoemission, but now we could also ‘hear’ them.”

Since, the CZS can conduct thermal current only along the direction of the applied magnetic field, it might be exploited in some special devices that would allow us to control thermal current flow using a magnetic field, he adds.

The researchers say they established the basic theory of CZS in a previous paper but they still need to perform quantitative calculations for the CZS velocity for each particular Weyl semimetal they have studied. “We have some clear ideas on how to conduct the first principles calculations for calculating this velocity and have already begun related computational work,” reveals Dai.

The work is detailed in Physical Review X and is also published on the arXiv server.

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