Chiral magnetic e ect in ZrTe5

The chiral magnetic e ect is the generation of an electric current induced by chirality imbalance in the presence of a magnetic field. It is a macroscopic manifestation of the quantumanomaly1,2 in relativisticfield theoryof chiral fermions (massless spin 1/2 particles with a definite projection of spin on momentum)—a remarkable phenomenon arising from a collective motion of particles and antiparticles in the Dirac sea. The recent discovery3–6 of Dirac semimetals with chiral quasiparticles opens a fascinating possibility to study this phenomenon in condensed matter experiments. Here we report on the measurement of magnetotransport in zirconium pentatelluride, ZrTe5, that provides strong evidence for the chiral magnetic e ect. Our angle-resolved photoemission spectroscopy experiments show that this material’s electronic structure is consistent with a three-dimensional Dirac semimetal. We observe a large negative magnetoresistance when the magnetic field is parallel with the current. The measured quadratic field dependence of the magnetoconductance is a clear indication of the chiral magnetic e ect. The observed phenomenon stems from the e ective transmutation of aDirac semimetal into a Weyl semimetal induced by parallel electric and magnetic fields that represent a topologically non-trivial gauge field background. We expect that the chiral magnetic e ect may emerge in a wide class of materials that are near the transition between the trivial and topological insulators. The recent discovery of the three-dimensional (3D) Dirac semimetals Cd3As2 and Na3Bi (refs 3–6) has enabled experimental studies of the quantum dynamics of relativistic field theory in condensed matter systems. The relativistic theory of charged chiral fermions in three spatial dimensions possesses the so-called chiral anomaly1,2—non-conservation of chiral charge induced by external gauge fields with non-trivial topology, for example, by parallel electric and magnetic fields. The existence of chiral quasiparticles in Dirac and Weyl semimetals opens the possibility to observe the effects of the chiral anomaly7. Of particular interest is the chiral magnetic effect (CME; ref. 8)—the generation of electric current in an external magnetic field induced by the chirality imbalance9. This phenomenon is at present under intense study in relativistic heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) at BNL and at the Large Hadron Collider (LHC) at CERN, where it was predicted10 to induce fluctuations in hadron charge asymmetry with respect to the reaction plane. The experimental data from the STAR (ref. 11) Collaboration at RHIC and the ALICE (ref. 12) Collaboration at LHC indicate the fluctuations are consistent with the theoretical expectations. Closely related phenomena are expected to play an important role in the Early Universe, possibly causing the generation of primordial magnetic fields13–17. However, the interpretation in all these cases is under debate owing to lack of control over the chirality imbalance produced. The most prominent signature of the CME in Dirac systems in parallel electric and magnetic fields is a positive contribution to the conductivity that has a quadratic dependence onmagnetic field8,18,19. This is because the CME current is proportional to the product of the chirality imbalance and the magnetic field, and the chirality imbalance in Dirac systems is generated dynamically through the anomaly with a rate that is proportional to the product of electric and magnetic fields. As a result, the longitudinal magnetoresistance becomes negative18,19. Let us explain how this mechanism works in Dirac semimetals in more detail. In the absence of external fields, each Dirac point initially contains leftand right-handed fermions with equal chemical potentials, μL =μR = 0. If the energy degeneracy between the leftand right-handed fermions gets broken, we can parameterize it by introducing the chiral chemical potential μ5≡(μR−μL)/2. The corresponding density of chiral charge is then given by8