Feb 20, 2021 
New phenomena for the design of future quantum devices
(Nanowerk News) Topological quantum materials comprise protected electronic states against external disturbance. Theoretical research (Physical Review Letters, "Topological Singularity Induced Chiral Kohn Anomaly in a Weyl Semimetal") 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 electronphonon 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.


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: Mingda Li, Massachusetts Institute of Technology) (click on image to enlarge)

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 Xray scattering and theory will be broadly useful for the discovery and characterization of topological materials. These materials could help bring the lowtemperature 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 nondissipative 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 HighFlux Isotope Reactor and the National Institute for Standards and Technology’s Center for Neutron Research, as well as Xray scattering experiments at the Advanced Photon Source.

This research demonstrated the viability of neutron and Xray scattering techniques as probes to characterize electronic topology in materials via the measurement of phonon spectra. The novel approach can be extended to other solidstate materials for the discovery of materials for future quantum devices.
