GdPtBi, the drosophila for Weyl
Weyl fermions, massless relativistic particles, were proposed by Hermann Weyl in the context of high-energy physics in 1929. After many decades, these Weyl particles were finally experimentally discovered in 2015, not in a typical high-energy physics experiment, but rather in a simple condensed matter physics experiment in a family of materials based on TaAs. Since this discovery Weyl Fermions have been identified in a large number of materials using spectroscopic and microscopic tools. Weyl Fermions are low energy excitations and appear at unavoidable touching points of a valence band and a conduction band in solid-state systems: these systems are “Weyl semimetals”. The touching points always come in pairs with opposite chiralities (right and left) and, moreover, act as large fictitious magnetic fields, so-called Berry fields, for electronic charge carriers.
A group of scientists from the Max Planck Institute for Chemical Physics of Solids in Dresden, in collaboration with the Technische Universität Dresden, and the high magnetic field laboratories at the HLD Rossendorf-Dresden, the HMFL Netherlands, and the PSI Switzerland, have found Weyl fermion mediated transport properties in GdPtBi and NdPtBi, two members of the Heusler family. Structurally, they consist of three interpenetrating fcc lattices such that rare-earth, platinum and bismuth atomic layers are formed along the  crystal axis. GdPtBi and NdPtBi are low charge carrier antiferromagnetic (AFM) semimetals with magnetic transitions at 9.0 K and 2.1 K, respectively. This study shows that magnetism plays a major role in creating Weyl Fermions via exchange splitting of bands.
The authors present the following key properties of Weyl physics in the Heusler compound GdPtBi:
- An extremely large chiral anomaly effect, namely a large negative magnetoresistance when current and magnetic field are parallel, due to pumping of Weyl fermions between pairs of Weyl points. This effect appears up to high temperatures and high magnetic fields.
- A large anomalous Hall effect due to a non-zero Berry curvature, which provides an intrinsic source of magnetic field. This leads to a large Hall angle of up to 23%, the highest value yet observed among Weyl semimetals.
- A p-Berry phase is observed via quantum oscillations, which is a consequence of linearly dispersing Weyl bands.
The same group of scientists also established, for the first time, that the chiral anomaly and anomalous Hall effect follow a similar trend with temperature, thereby revealing their common origin, i.e. Weyl fermions.
CS / CPfS