Signatures of the Adler–Bell–Jackiw chiral anomaly in a Weyl fermion semimetal

ARTICLE Received 18 Oct 2015 | Accepted 15 Jan 2016 | Published 25 Feb 2016 DOI: 10.1038/ncomms10735 OPEN Signatures of the Adler–Bell–Jackiw chiral anomaly in a Weyl fermion semimetal Cheng-Long Zhang1,*, Su-Yang Xu2,*, Ilya Belopolski2,*, Zhujun Yuan1,*, Ziquan Lin3, Bingbing Tong1, Guang Bian2, Nasser Alidoust2, Chi-Cheng Lee4,5, Shin-Ming Huang4,5, Tay-Rong Chang2,6, Guoqing Chang4,5, Chuang-Han Hsu4,5, Horng-Tay Jeng6,7, Madhab Neupane2,8,9, Daniel S. Sanchez2, Hao Zheng2, Junfeng Wang3, Hsin Lin4,5, Chi Zhang1,10, Hai-Zhou Lu11, Shun-Qing Shen12, Titus Neupert13, M. Zahid Hasan2 & Shuang Jia1,10 Weyl semimetals provide the realization of Weyl fermions in solid-state physics. Among all the physical phenomena that are enabled by Weyl semimetals, the chiral anomaly is the most unusual one. Here, we report signatures of the chiral anomaly in the magneto-transport measurements on the first Weyl semimetal TaAs. We show negative magnetoresistance under parallel electric and magnetic fields, that is, unlike most metals whose resistivity increases under an external magnetic field, we observe that our high mobility TaAs samples become more conductive as a magnetic field is applied along the direction of the current for certain ranges of the field strength. We present systematically detailed data and careful analyses, which allow us to exclude other possible origins of the observed negative magnetoresistance. Our transport data, corroborated by photoemission measurements, first-principles calculations and theoretical analyses, collectively demonstrate signatures of the Weyl fermion chiral anomaly in the magneto-transport of TaAs. 1 International Center for Quantum Materials, School of Physics, Peking University, Beijing, China. 2 Laboratory for Topological Quantum Matter and Spectroscopy (B7), Department of Physics, Princeton University, Princeton, New Jersey 08544, USA. 3 Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China. 4 Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore 117546, Singapore. 5 Department of Physics, National University of Singapore, Singapore 117542, Singapore. 6 Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan. 7 Institute of Physics, Academia Sinica, Taipei 11529, Taiwan. 8 Condensed Matter and Magnet Science Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA. 9 Department of Physics, University of Central Florida, Orlando, Florida 32816, USA. 10 Collaborative Innovation Center of Quantum Matter, Beijing 100871, China. 11 Department of Physics, South University of Science and Technology of China, Shenzhen, China. 12 Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, China. 13 Princeton Center for Theoretical Science, Princeton University, Princeton, New Jersey 08544, USA. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to M.Z.H. (email: ) or to S.J. (email: ). NATURE COMMUNICATIONS | 7:10735 | DOI: 10.1038/ncomms10735 | www.nature.com/naturecommunications 1 ARTICLE T NATURE COMMUNICATIONS | DOI: 10.1038/ncomms10735 he principles of physics rest crucially on symmetries and their associated conservation laws. Over the past century, physicists have repeatedly observed the violations of apparent conservation laws in particle physics, each time leading to new insights and a refinement of our understanding of nature. One of the most interesting phenomena of this type is the breaking of a conservation law of classical physics by quantummechanical effects, a so-called anomaly in quantum field theory1. Perhaps the most primitive example is the so-called chiral anomaly associated with Weyl fermions2–6. A Weyl fermion is a massless fermion that carries a definite chirality. Due to the chiral anomaly, the chiral charge of Weyl fermions is not conserved by the full quantum-mechanical theory. Historically, the chiral anomaly was crucial in understanding a number of important aspects of the standard model of particle physics. The most well-known case is the triangle anomaly associated with the decay of the neutral pion p0 (refs 3,4). Despite having been discovered more than 40 years ago, it remained solely in the realm of high-energy physics. Recently, there has been considerable progress in understanding the correspondence between high-energy and condensed matter physics, which has led to deeper knowledge of important topics in physics such as spontaneous symmetry breaking, phase transitions and renormalization. Such knowledge has, in turn, greatly helped physicists and materials scientists to better understand magnets, superconductors and other novel materials, leading to important practical device applications. Here, we present the signatures of the chiral anomaly in a low-energy condensed matter Weyl system. In order to measure the chiral anomaly in a solid-state system, one needs to find a perturbation that couples differently to the two Weyl fermions of opposite chiralities. This is most naturally realized in a Weyl semimetal, in which the two Weyl cones are separated in momentum space. Recent theoretical and experimental advances have shown that Weyl fermions can arise in the bulk of certain novel semimetals with nontrivial topology7–16. A Weyl semimetal is a bulk crystal whose low-energy excitations satisfy the Weyl equation. Therefore, the conduction and valence bands touch at discrete points, the Weyl nodes, with a linear dispersion relation in all three momentum space directions moving away from the Weyl node. The nontrivial topological nature of a Weyl semimetal guarantees that Weyl fermions with opposite chiralities are separated in momentum space (Fig. 1a), and host a monopole and an antimonopole of Berry flux in momentum space, respectively (Fig. 1b). In this situation, parallel magnetic and electric fields can pump electrons between Weyl cones of opposite chirality that are separated in momentum space (Fig. 1a). This process violates the conservation of the the chiral charge, meaning that the number of particles of left and right chirality are not separately conserved5,17–26, giving rise to an analogue of the chiral anomaly in a condensed matter system. Apart from this elegant analogy and correspondence between condensed matter and high-energy physics, the chiral anomaly also serves as a crucial transport signature for Weyl fermions in a Weyl semimetal phase. Furthermore, theoretical studies have recently suggested that it has potential applications27. In this paper, we perform magneto-transport experiments on the Weyl semimetal TaAs12–14,16. We observe a negative longitudinal magnetoresistance (LMR) in the presence of parallel magnetic and electric fields, which is indicative of the chiral anomaly due to Weyl fermions. On the other hand, due to the complicated nature of the magnetoresistence28–38, an (...truncated)