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| chiral magnetic effect | |
|---|---|
| Name | Chiral magnetic effect |
| Field | Quantum chromodynamics; Condensed matter physics |
| Discovered | 2004 |
| Discovered by | Dmitri Kharzeev; H. J. Warringa; Kenji Fukushima |
| Experimental systems | Relativistic heavy ion collisions; Weyl semimetals; Dirac semimetals |
| Related | Chiral anomaly; Axial anomaly; Berry phase; Chiral vortical effect |
chiral magnetic effect
The chiral magnetic effect is a predicted quantum phenomenon in which an imbalance of chirality (handedness) among fermions in the presence of a magnetic field induces an electric current along the field direction. Proposed in the context of Quantum chromodynamics and later connected to Condensed matter physics realizations, it links topological properties of gauge fields and band structures to macroscopic transport. The effect unites concepts from anomalies, topology, and relativistic quantum field theory with experimental platforms such as Relativistic heavy ion collisions and Weyl semimetal materials.
The chiral magnetic effect arises when a nonzero chiral chemical potential or axial charge exists alongside a magnetic field, producing an electric current proportional to the product of the axial chemical potential and the magnetic field. Originating in studies of Quantum chromodynamics under extreme conditions, it was motivated by considerations of topological charge changing transitions like Instantons and Sphalerons in high-temperature plasmas. The idea has been extended to electronic systems where quasiparticles emulate relativistic Weyl or Dirac fermions, allowing tests in materials such as TaAs, Na3Bi, and Cd3As2.
At its core the effect is rooted in the Chiral anomaly (also called the Axial anomaly), a quantum violation of axial current conservation first elucidated in perturbative calculations by Adler, Bell and Jackiw. In Quantum chromodynamics the anomaly connects chirality to topological gluon configurations like Instantons and Sphaleron transitions studied by Gerard 't Hooft and Mikhail Shifman. In condensed matter, the Berry curvature and band topology described by Michael Berry and formalized in Topological insulator theory by Charles Kane and Shoucheng Zhang provide an analogous source of chiral imbalance. Field-theoretic analyses by Kenji Fukushima, Dmitri Kharzeev, and H. J. Warringa derived the current using finite-temperature quantum field theory and argued for a nondissipative contribution linked to topological charge.
Searches for the chiral magnetic effect have proceeded along two main experimental lines. In Relativistic heavy ion collision experiments at facilities such as the Relativistic Heavy Ion Collider and the Large Hadron Collider, collaborations including STAR and ALICE have reported charge-dependent correlations consistent with CME expectations but complicated by background effects like local charge conservation and elliptic flow. In condensed matter, positive signatures consistent with CME-induced magnetotransport have been observed in materials such as TaAs, ZrTe5, Na3Bi, and Cd3As2 by groups associated with institutions including Max Planck Institute for Chemical Physics of Solids and Princeton University, often measured as negative longitudinal magnetoresistance under parallel electric and magnetic fields.
Beyond testing quantum field theory, the chiral magnetic effect has implications for several domains. In Quantum chromodynamics it bears on the physics of the quark–gluon plasma produced in Relativistic heavy ion collisions and possibly on the matter–antimatter asymmetry via mechanisms contemplated in Electroweak baryogenesis studies by V. A. Rubakov and Mikhail Shaposhnikov. In astrophysics, chiral transport may influence magnetic field evolution in compact objects considered in work by Alexander Vilenkin and Maxim Dvornikov. In condensed matter, CME-like responses inform design principles for Topological semimetal electronics and may enable low-dissipation devices discussed in the context of Spintronics and Valleytronics research groups.
Debate persists over the interpretation of experimental signals. In heavy-ion collisions, separating a genuine CME current from flow-driven backgrounds remains contentious, with competing analyses by collaborations such as STAR and theorists including Edward Shuryak and Dima Kharzeev. In materials, questions remain about contributions from current jetting, sample inhomogeneity, and trivial band effects versus true topological chiral transport; groups at Columbia University and Stanford University have published contrasting interpretations. Open theoretical questions include the role of interactions, finite-frequency responses, and non-equilibrium axial charge generation in realistic systems.
The minimal expression for the chiral magnetic current is J = (e^2/2π^2) μ5 B, where e is the charge, μ5 the chiral chemical potential, and B the magnetic field; this follows from anomaly-related triangle diagrams first analyzed by Stephen Adler and John Bell with extensions by Roman Jackiw. More complete treatments employ the generating functional of anomalous currents in Quantum field theory and hydrodynamic frameworks developed by Dam Thanh Son and P. Surowka, incorporating constitutive relations with anomaly-induced transport coefficients. In condensed matter, semiclassical equations of motion with Berry curvature corrections introduced by D. Xiao, M.-C. Chang, and Qian Niu reproduce analogous expressions for momentum-space monopoles at Weyl nodes studied by F. D. M. Haldane.
Phenomena closely connected to the chiral magnetic effect include the Chiral vortical effect predicted in rotating fluids and analyzed by Dam Thanh Son and Pavel Kovtun, the Anomalous Hall effect observed in Ferromagnetism research by Karplus and Luttinger, and the Chiral separation effect which produces axial currents in magnetic fields. In condensed matter, related concepts appear in Fermi arc surface states of Weyl semimetals and in Axion electrodynamics invoked by Frank Wilczek.
The theoretical lineage traces from studies of anomalies in the 1960s by Stephen Adler, John Bell, and Roman Jackiw to nonperturbative QCD work by Gerard 't Hooft in the 1970s. Connections to heavy-ion phenomenology and topological charge fluctuation driven currents were articulated in the 1990s and early 2000s by researchers including Dmitri Kharzeev and Edward Shuryak, with the specific chiral magnetic effect formulated in 2008 by Kenji Fukushima, Dmitri Kharzeev, and H. J. Warringa. The 2010s saw rapid cross-fertilization with condensed matter after the experimental discovery of Weyl semimetals by groups including Zhong Fang and Shuang Jia, prompting material studies and transport measurements that continue to shape the field.