Synopsis: Quantum Hall Anomaly in 3D

A novel quantized Hall effect is likely observable in a known ferromagnetic compound.
Synopsis figure
G. Xu et al., Phys. Rev. Lett. (2011)

Topological phases in 2D insulators are characterized by an integer called the Chern number. In the context of the quantum Hall effect, it equals the number of chiral states at the edge of the 2D system. Researchers who have been looking for a 3D analog of the Chern number have reasons to expect that it will—in 3D scenarios that are more complicated than those one would get by merely stacking 2D states—result in a semimetal, in which topological constraints force the two energy bands near the Fermi level to overlap.

The band structure of this Chern semimetal state is reminiscent of graphene and topological insulators but substantively different. One manifestation of this difference is that the representative topological objects, which determine the transport characteristics, are not Dirac but Weyl fermions.

In their paper, Gang Xu and co-workers at the Beijing National Laboratory for Condensed Matter Physics report first-principles calculations that tell us that the known ferromagnetic compound HgCr2Se4 happens to exhibit the various characteristics of a Chern semimetal; in particular, they find a pair of Weyl fermions in momentum space. An additional benefit, as it were, is the possibility of observing, in a thin film of HgCr2Se4, the quantum Hall effect that arises (from topological properties) in the absence of an external magnetic field—the quantized anomalous Hall effect. This particular ferromagnetic material points to an alternative to pyrochlore-based compounds, another scenario in which theorists expect to see a topological semimetal. – Sami Mitra


More Announcements »

Subject Areas

MesoscopicsTopological Insulators

Previous Synopsis

Nuclear Physics

Benchmarking the Standard Model

Read More »

Next Synopsis

Atomic and Molecular Physics

The Simplest Molecular Anion

Read More »

Related Articles

Viewpoint: Exorcising Maxwell’s Demon

Viewpoint: Exorcising Maxwell’s Demon

A pair of connected single-electron devices functions as a Maxwell’s demon that operates without external control. Read More »

Viewpoint: High Chern Numbers in Photonic Crystals

Viewpoint: High Chern Numbers in Photonic Crystals

A two-dimensional photonic crystal has multiple topologically protected channels, which can be used to transmit and reroute light without scattering losses. Read More »

Viewpoint: Quantum Squeezing of Micromechanical Motion

Viewpoint: Quantum Squeezing of Micromechanical Motion

The act of a quantum measurement reduces the uncertainty in the motion of a vibrating membrane below the fundamental quantum limit. Read More »

More Articles