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Energy versus momentum depiction of different conditions that give rise to a Kohn anomaly in ordinary metals (at left), versus a topological material called a Weyl semimetal (at right).
Energy versus momentum depiction of different conditions that give rise to a Kohn anomaly in ordinary metals (at left), versus a topological material called a Weyl semimetal (at right).
Image courtesy of Mingda Li, Massachusetts Institute of Technology

The Science

Topological quantum materials comprise protected electronic states against external disturbance. Theoretical research has shown that the topology of the electronic states in a Weyl semimetal (a topological material) can leave fingerprints on their phonon properties. Phonons are quantum mechanical vibrations of atoms in a material. This effect happens because of a type of electron-phonon interaction called the Kohn anomaly that impacts how electrons screen phonons through a material. This instability can lead to many new electronic properties in materials.

The Impact

For the first time, Kohn anomaly has been theoretically predicted in a topological material and experimentally observed in a Weyl semimetal. This research revealed how the topological electronic states can alter the phonon spectra in materials, which hold promise for future quantum applications. The novel methodology developed that combines advanced neutron and X-ray scattering and theory will be broadly useful for the discovery and characterization of topological materials. These materials could help bring the low-temperature phenomena such as superconductivity to higher temperatures and to create new quantum devices.


Topological materials contain robust electronic states that are protected against perturbation. They therefore have promising applications in non-dissipative electronics and quantum computers. However, probing the topological materials has been technically challenging. A team of researchers developed a novel approach that combine theory and experiment to probe a topological material called Weyl semimetal. They show that the topology in the electronic states can be reflected in the phonon spectra, through an exotic phenomenon called the Kohn anomaly. This phenomenon is driven by topological singularities that is distinct from the conventional Kohn anomalies in metals that are driven by the Fermi surface. Theory provided conditions required to observe the Kohn anomaly in a topological material.  For the first time, this is validated by inelastic neutron scattering measurements on single crystals of tantalum phosphide at the High-Flux Isotope Reactor and the National Institute for Standards and Technology’s Center for Neutron Research, as well as X-ray scattering experiments at the Advanced Photon Source. This research demonstrated the viability of neutron and X-ray scattering techniques as probes to characterize electronic topology in materials via the measurement of phonon spectra. The novel approach can be extended to other solid-state materials for the discovery of materials for future quantum devices.


Mingda Li
Massachusetts Institute of Technology


This work was mainly supported by the U.S. Department of Energy (DOE) Office of Science, Office of Basic Energy Sciences, including support of the High Flux Isotope Reactor in Oak Ridge National Lab and Advanced Photon Source in Argonne National Lab, both DOE Office of Science user facilities.


Nguyen, T., et al., Topological Singularity Induced Chiral Kohn Anomaly in a Weyl Semimetal, Phys. Rev. Lett. 124, 236401 (2020) with Editors' Suggestion. [DOI: 10.1103/PhysRevLett.124.236401]

Related Links

MIT news: Newly observed phenomenon could lead to new quantum devices