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news & views Giulio Biroli Published online: 6 April 2020 2. Widmer-Cooper, A., Harrowell, P. & Fynewever, H. Phys. Rev. https://doi.org/10.1038/s41567-020-0873-1 Lett. 93, 135701 (2004). Laboratoire de Physique de l’Ecole Normale 3. Sussman, D. M., Schoenholz, S. S., Cubuk, E. D. & Supérieure, ENS, Université PSL, CNRS, Sorbonne Liu, A. J. Proc. Natl Acad. Sci. USA 114, 10601–10605 Université, Université de Paris, Paris, France. References (2017). 4. Ninarello, A., Berthier, L. & Coslovich, D. Phys. Rev. X 7, 021039 e-mail: [email protected] 1. Bapst, V. et al. Nat. Phys. https://doi.org/10.1038/s41567-020- 0842-8 (2020). (2017). TOPOLOGICAL MATERIALS Dynamical anomaly Using the coupling between electrons and phonons in a Weyl semimetal allows the detection of the dynamical chiral magnetic efect. Xi Dai he chiral anomaly is one of the two Weyl nodes have different chemical When an additional static magnetic field measurable properties of materials potential from each other. is applied, the chiral magnetic effect will Twith Weyl nodes in their band In solid state materials, the first evidence generate an oscillating current along the structure. In the past, various experiments of the chiral anomaly and chiral magnetic field, which can couple to the incoming light have claimed to show the static version effect came from the negative longitudinal and make such a phonon mode visible. of this effect, but so far, no consensus has magnetoresistance, where the charge The field induced optical activity developed around whether alternative pumping from one Weyl point to another observed in NbAs is the first evidence in explanations can account for the data. Now, caused by the chiral anomaly results in Weyl semimetals for the dynamical chiral writing in Nature Communications, Xiang a steady state with imbalanced chemical magnetic effect. After this, many follow-up Yuan and collaborators have demonstrated a potential between the two Weyl points. studies can be carried out along this dynamical anomaly1. This gives substantial This induces an additional current along direction. Compared to the chiral magnetic evidence that Weyl semimetals do show the magnetic field leading to negative effect in the DC limit, the dynamical these anomalies, and can couple to magnetoresistance. Although the chiral effect is less sensitive to the relaxation electromagnetic fields in unusual ways. anomaly and chiral magnetic effect provide processes — which are anisotropic in the When the mass term in the Dirac a natural explanation for the longitudinal crystal environment — and in principle the equation vanishes, two independent negative magnetoresistance discovered in effect could be more isotropic. Therefore, solutions appear. They are chiral fermions these materials, there are still many features experimental studies on the dependence with opposite handedness called Weyl that remain unclear, for example the angle of field direction should be explored. And fermions. Although they have not been dependence of the field. similar effects should also appear in many discovered as a type of fundamental In their paper, Yuan and collaborators other Weyl semimetals, which are yet to be particle within the Standard Model, from Fudan University in Shanghai observe revealed. Another important aspect is that quasiparticles with these properties a dynamical version of the chiral magnetic the dynamical effect does not suffer from were discovered about five years ago in effect and chiral anomaly for the first time the potential issues with current jetting that condensed matter systems called Weyl through a phenomenon called field-induced disrupts transport measurements of the semimetals. The chirality of the Weyl optical activity. The authors used light static effect4, making it much more certain fermion comes from the fact that the with its polarisation parallel to the surface that materials such as NbAs do indeed host direction of its pseudospin is always locked to excite a particular phonon mode in a topological Weyl fermions. ❐ to the direction of the translational motion typical Weyl semimetal material, NbAs. and is either parallel (positive chirality) or Such a mode usually cannot be seen by the Xi Dai anti-parallel (negative chirality) to it. particular setup in their experiment, but it Physics Department, Hong Kong University of Science The chiral electronic structure leads became ‘visible’ when the sample was placed and Technology, Clear Water Bay, Hong Kong. to particular charge dynamics: the chiral in a static external magnetic field. This effect e-mail: [email protected] anomaly and chiral magnetic effect. The has been theoretically proposed2,3 based on chiral anomaly refers to the charge pumping the chiral magnetic effect and it is caused by Published online: 6 March 2020 effect from one Weyl point to the other electron–phonon coupling, through which https://doi.org/10.1038/s41567-020-0844-6 when electric and magnetic fields are applied a particular phonon mode can couple to the References parallel to each other. The corresponding energy levels of the different Weyl points. 1. Yuan, X. et al. Nat. Commun. https://doi.org/10.1038/s41467-020- pumping rate is proportional to the product When oscillating, the phonon mode will 14749-4 (2020). of the magnitudes of the two fields. On push down the energy of the Weyl points 2. Song, Z., Zhao, J., Fang, Z. & Dai, X. Phys. Rev. B 94, 214306 the other hand, the chiral magnetic effect of one chirality and lift up those with the (2016). 3. Hui, A., Zhang, Y. & Kim, E.-A. Phys. Rev. B 100, refers to the appearance of additional charge opposite chirality, leading to an oscillating 085144 (2019). current along the magnetic field when the chemical potential difference between them. 4. dos Reis, R. D. et al. New J. Phys. 18, 085006 (2016). 374 NATURE PHYSICS | VOL 16 | APRIL 2020 | 373–379 | www.nature.com/naturephysics.

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