Elusive quasiparticle discovered in diphosphide materials

Weyl fermions are massless chiral particles that can emerge under the appropriate conditions in crystalline materials. Because of their massless character, the Weyl fermions are very efficient at conducting electric charge and could be used to create high-speed electronic circuits and even quantum computers. Using high-throughput computational methods, EPFL scientists have now predicted the presence of a new kind of Weyl fermions in two semimetals, molybdenum and tungsten diphosphide, making them easy to study experimentally. The work is published in Physical Review Letters.

Weyl fermions were predicted in 1929 as an alternative solution to the Dirac equation. However, they have never been observed by particle physicists, leaving them in the domain of theory until it was discovered last year that they can exist as quasiparticles in materials called Weyl semimetals.

In these materials, the Weyl fermions emerge in pairs of opposite chirality, at points were the conduction and valence bands touch. These “Weyl points” have attracted the attention of particle physicists, who see Weyl fermions as fundamental to the Standard Model, perhaps even acting as the building blocks of “normal” fermions such as electrons.

The lab of Oleg Yazyev at EPFL has now predicted the existence of Weyl fermions in two Weyl semimetals, molybdenum diphosphide and tungsten diphosphide. The scientists used a high-throughput computational method screening a large database of existing materials, performing electronic-structure computations for each candidate.

Their specific goal was to identify the presence of what are called “type-II Weyl fermions”, which have the added ability to break Lorentz symmetry. This refers to the feature of nature wherein the laws of physics remain unchanged for all observers that are moving with the same velocity with respect to one another.

The study revealed that the two diphosphides produce robust type-II Weyl fermions. In fact, the distance between the emerging pair of opposite-chirality Weyl fermions is large enough for experimental observation. This can be done with angle-resolved photoemission spectroscopy, an experimental technique that can measure the energy-momentum relation of electrons in materials.

“Since these particles appear in a condensed-matter system, they are quite easy to study experimentally without the need for a high-energy particle detector,” says Gabriel Autès, first author of the paper. “Moreover, because of the presence of these Weyl fermions, Weyl semimetals possess a high mobility and interesting magneto-electric properties, which makes them good candidates for electronics and spintronics applications.”

This study was led by EPFL’s Chair of Computational Condensed Matter Physics, with contributions from ETH Zürich, and St. Petersburg State University. It was funded by NCCR-MARVEL, NCCR-QSIT, the European Research Council (grants “TopoMat” and SIMCOFE), and Microsoft Research.

Reference

Autès G, Gresch D, Troyer M, Soluyanov AA, Yazyev OV. Robust Type-II Weyl Semimetal Phase in Transition Metal Diphosphides XP₂ (X=Mo, W). Physical Review Letters 117, 066402. 22 July 2016. DOI: 10.1103/PhysRevLett.117.066402