Physics

Viewpoint: The “Sound” of Weyl Fermions


Binghai Yan, Division of Condensed Matter Physics, Weizmann Institute of Science, Rehovot, Israel

June 17, 2019• Physics 12, 67

A prediction of a brand new heat-transport mechanism—referred to as chiral zero sound—could clarify latest observations of a “big” thermal conductivity in Weyl semimetals.

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Determine 1: In a typical steel, electrons will transfer from a scorching area to a chilly one, carrying each warmth and cost. In Weyl semimetals, nonetheless, the electrons close to the Weyl factors kind quasiparticles that, in a magnetic subject, can contribute to warmth conduction with out transporting cost [1, 2]. This charge-less transport arises as a result of the contribution of cost from two pairs of Weyl factors cancel one another out (inset).In a typical steel, electrons will transfer from a scorching area to a chilly one, carrying each warmth and cost. In Weyl semimetals, nonetheless, the electrons close to the Weyl factors kind quasiparticles that, in a magnetic subject, can contribute to warmth conduction w… Present extra

Figure caption

Determine 1: In a typical steel, electrons will transfer from a scorching area to a chilly one, carrying each warmth and cost. In Weyl semimetals, nonetheless, the electrons close to the Weyl factors kind quasiparticles that, in a magnetic subject, can contribute to warmth conduction with out transporting cost [1, 2]. This charge-less transport arises as a result of the contribution of cost from two pairs of Weyl factors cancel one another out (inset).×

Warmth in a strong is especially carried by lattice vibrations (phonons) and conducting electrons. These mechanisms of warmth conduction are so dominant over different varieties that in an atypical materials they’re usually assumed to be the one ones that matter. However as reported in a pair of papers, an unique “vibration” of electrons would possibly present a 3rd, vital solution to conduct warmth at low temperatures [1, 2]. This impact happens in a household of supplies referred to as Weyl semimetals [3, 4], and it is likely to be harnessed to information warmth via a cloth.

A Weyl semimetal is a cloth that hosts particles often known as Weyl fermions. Initially conceived by German theoretician Herman Weyl in 1929 as a brand new kind of elementary particle with cost however no mass [5], a “true” Weyl fermion has by no means been noticed in nature. However fermions that may be described by Weyl’s principle do seem as particle-like excitations (quasiparticles) in sure solids—specifically, these with electron vitality bands which are linear and cross at factors (Weyl factors) in momentum house close to the Fermi vitality [6–8]. Weyl factors are available in pairs which have reverse (+ or –) chirality. Electrons with momentum close to to one in every of these factors kind quasiparticles with an identical chirality, the place the + or – denotes whether or not the quasiparticle propagates parallel or antiparallel to its spin.

Due to this intrinsic chirality, Weyl quasiparticles behave in another way than electrons in atypical metals or semiconductors. Notably, they exhibit an uncommon phenomenon referred to as the chiral magnetic impact (CME), which happens when a Weyl materials is positioned in a magnetic subject [9]. Right here, the sector generates a present of + and – Weyl quasiparticles that transfer parallel and antiparallel to the sector, respectively. In thermodynamic equilibrium, the online present is zero as a result of + and – quasiparticles exist in equal numbers. However the steadiness is damaged and a web quasiparticle present flows if one applies an electrical subject parallel to the magnetic subject. This impact is dubbed the chiral anomaly as a result of the variety of particles with a given chirality isn’t conserved [10].

A signature of the chiral anomaly is unfavourable magnetoresistance—a reducing of resistance with growing magnetic subject, which has been noticed in Weyl supplies by a number of teams. In one of many new papers, theorists Zhida Track and Xi Dai of the Hong Kong College of Science and Know-how predict that the CME must also result in a collective impact referred to as chiral zero (CZ) sound [1]. Zero sound is a purely quantum-mechanical impact that was first proposed by Lev Landau within the context of Fermi-liquid principle [5] and later noticed in helium-Three. Like common sound, zero sound comes from vibrations, however as a substitute of air molecules, the vibrating “medium” is the momentum distribution of electrons close to the Fermi degree. Track and Dai have proven that zero sound vibrations exist in Weyl semimetals when the fabric is in a magnetic subject.

These vibrations are related to some uncommon results in a Weyl semimetal with two or extra pairs of Weyl factors. If a temperature imbalance existed in a traditional steel, electrons would transfer from the new area to the chilly one, carrying each warmth and cost. An analogous temperature gradient in a Weyl semimetal would trigger CZ sound waves to circulate from scorching to chilly. However these waves would solely carry warmth and no cost as they propagate via the fabric due to a type of cost cancellation impact between Weyl-point pairs (Fig. 1). These heat-carrying waves could be excited with little or no vitality (their vitality spectrum is gapless.) Due to this fact CZ sound waves could make a major contribution to a cloth’s thermal conductivity. A CZ sound wave can also be distinct from the zero sound wave conceived by Landau: it propagates solely alongside the magnetic subject course, and its velocity is strongly modulated by this subject. As Track and Dai clarify of their paper, an experimental signature of a cloth that transmits CZ sound is that its particular warmth and thermal conductivity could be managed by a magnetic subject.

In truth, Track and Dai’s work was impressed by a set of experiments from Peijie Solar of the Chinese language Academy of Sciences in Beijing and colleagues [2]. The researchers, who reported their ends in a 2018 preprint, had targeted on TaAs, a Weyl semimetal with a number of pairs of Weyl factors. The group had measured the fabric’s thermal conductivity and its electrical conductivity whereas ramping up the power of an utilized magnetic subject. In metals, the place conducting electrons dominate the thermal transport, the thermal and electrical conductivities are anticipated to oscillate up and down in a magnetic subject; these oscillations happen as a result of the sector induces quantized vitality ranges (Landau ranges) that transfer out and in of alignment with the Fermi degree. Furthermore, metals are anticipated to obey the Wiedemann-Franz regulation, which says that the thermal conductivity is proportional to the electrical conductivity multiplied by a elementary fixed and divided by the temperature.

Solar’s crew, nonetheless, noticed strikingly totally different conduct in TaAs. Analyzing the thermal conductivity alongside the course of the magnetic subject, they discovered that the thermal conductivity oscillated with an amplitude that was about 100 instances larger than that predicted by the Wiedemann-Franz regulation. Against this, the oscillation in thermal conductivity perpendicular to the sector was a lot weaker. As well as, the crew noticed that the oscillations of the thermal conductivity and electrical conductivity had been fully out of section.

By means of cautious experiment, Solar and colleagues excluded a number of attainable sources of the “big” thermal conductivity oscillations, comparable to phonons and bipolar electron-hole excitations. They’ve now concluded that the probably clarification is the CZ sound proposed by Track and Dai. In supplies the place this impact exists, a powerful violation of the Wiedemann-Franz regulation is feasible as a result of warmth and cost are not carried by the identical particles.

Discovering a brand new impact in solids is at all times intriguing to physicists. However the discovery of CZ sound may additionally have some sensible worth. In precept, one may use a magnetic subject to information (or block) the conduction of warmth in a sure course. That functionality would possibly, for instance, permit warmth to be routed round or away from some object—although the Weyl semimetal’s temperature must be low sufficient for long-lived CZ sound waves, which for TaAs is under 10 Okay. Within the quick future, nonetheless, researchers are more likely to search for CZ sound within the rising household of supplies that include Weyl factors.

This analysis is printed in Bodily Evaluation X and posted on the arXiv.

References

Z. Track and X. Dai, “Hear the sound of Weyl fermions,” Phys. Rev. X. 9, 021053 (2019).J. Xiang et al., “Large magnetic quantum oscillations and chiral anomaly within the thermal conductivity of a Weyl semimetal,” arXiv:1801.08457.N. P. Armitage, E. J. Mele, and Ashvin Vishwanath, “Weyl and Dirac semimetals in three-dimensional solids,” Rev. Mod. Phys. 90, 015001 (2018).B. Yan and C. Felser, “Topological supplies: Weyl semimetals,” Annu. Rev. Cond. Mat. Phys. eight, 337 (2017).H. Weyl, “Elektron und gravitation. I,” Z. Phys. 56, 330 (1929).S. Murakami, “Part transition between the quantum spin Corridor and insulator phases in 3D: Emergence of a topological gapless section,” New J. Phys. 9, 356 (2007).X. Wan, A. M. Turner, A. Vishwanath, and S. Y. Savrasov, “Topological semimetal and Fermi-arc floor states within the digital construction of pyrochlore iridates,” Phys. Rev. B 83, 205101 (2011).A. A. Burkov and L. Balents, “Weyl semimetal in a topological insulator multilayer,” Phys. Rev. Lett. 107, 127205 (2011).Okay. Fukushima, D. E. Kharzeev, and H. J. Warringa, “Chiral magnetic impact,” Phys. Rev. D 78, 074033 (2008).H. B. Nielsen and M. Ninomiya, “The Adler-Bell-Jackiw anomaly and Weyl fermions in a crystal,” Phys. Lett. B 130, 389 (1983).

Concerning the Creator

Image of Binghai Yan

Binghai Yan is an Assistant Professor on the Weizmann Institute of Science in Israel. After finishing his Ph.D. at Tsinghua College in 2008, he labored as a postdoc at Bremen College and later at Stanford College. He was a gaggle chief within the Max Planck Institute for Chemical Physics from 2012 to 2016, assuming his present place at Weizmann in 2017. He was awarded the ARCHES Prize in Germany in 2013 and the Israel Bodily Society Prize for Younger Scientists in 2017. He’s a theoretical physicist and at the moment inquisitive about solid-state supplies that exhibit topological properties.

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