Chiral Anomalies in Organic Dirac Semimetals

Conservation laws that hold in classical theory can be violated in quantum mechanics, a phenomenon known as the “chiral anomaly.” This intriguing violation of the conservation laws has been the focal point of research in particle physics and astrophysics for many years. However, recently, physical phenomena stemming from chiral anomalies have been observed in topological materials, revealing that chiral anomalies are a universal phenomenon that also manifest in solid-state systems.

Chiral anomaly effects in solids result in pronounced responses, including transport phenomena without scattering, such as negative magnetoresistance, planar Hall effects, and anomalous Hall effects. Consequently, topological materials that exhibit chiral anomalies have garnered significant interest for potential device applications.

Broken chiral symmetry is crucial for the occurrence of chiral anomalies, and the dimensionality of electron motion is another key factor. According to quantum field theory, chiral anomalies are present in three-dimensional (3D) systems but absent in two-dimensional (2D) systems. Typically, Dirac semimetals are categorized as either 2D or 3D, with no materials bridging these dimensions yet. To explore the physics behind chiral anomaly effects, candidate materials capable of transitioning between 2D and 3D dimensionalities have been searched.

We propose that organic conductor α-(BEDT-TTF)₂I₃ under high pressures emerges as a prime candidate material. This is the first bulk (multilayered) massless Dirac fermion system identified. Although various physical phenomena within this system have traditionally been interpreted through a 2D framework, the evidence of coherent interlayer tunneling at low temperatures reveals its intrinsic 3D nature. Theoretical insights indicate that this system functions as a 3D Dirac semimetal with chiral symmetry broken by electron correlation.

In the 3D Dirac semimetal with broken chiral symmetry, a “magnetic monopole”—stemming from the topology of the wave function—is formed at the two Dirac points, K and K', analogous to the N and S poles. The observation of negative magnetoresistance and planar Hall effects, driven by a virtual magnetic field (known as the Berry curvature) emanating from the Dirac points, confirms the realization of a 3D Dirac semimetal with chiral anomalies in α-(BEDT-TTF)₂I₃ under high pressures at low temperatures. This system provides a pivotal platform for probing chiral anomalies.

(Written by N. Tajima on behalf of all authors)