In a work published in npj Quantum Materials, a team led by Prof. Leonardo Degiorgi in the Department of Physics at ETH Zurich studied the broadband charge dynamics (i.e., longitudinal optical conductivity) of the ferromagnetic (FM), noncentrosymmmetric PrAlGe material and reveal its electronic environment, based on correlated Weyl states, which favours an unusually large anomalous Hall conductivity (AHC) at low temperatures. They thus propose a suitable experimental approach to trace the relevant ingredients of the electronic structure deploying substantial Berry curvatures, indispensable for AHC.
The family of the noncentrosymmetric RAlGe (R = rare-earth) materials is a suitable arena in order to advance our knowledge on novel topological states, which cover all varieties of Weyl semimetals, including type I, type II, inversion and time-reversal breaking symmetry, depending on the choice of the rare-earth element. PrAlGe is of particular relevance, since it breaks both the space-inversion and time-reversal symmetry, leading to the formation of pairs of type I Weyl nodes.
The work presents measurements of the optical reflectivity, collected from the far infrared (FIR) to the ultraviolet at nearly normal incidence as a function of temperature, which is the prerequisite in order to perform reliable Kramers–Kronig transformation of the measured quantity, giving access to all optical functions. The authors' discussion is then supported by devoted first-principles calculations of the electronic band structure. They discover that electronic correlations get reinforced upon lowering temperature and induce a renormalisation of the non-trivial bands hosting the Weyl nodes. This is reflected in a sizeable reduction of the Fermi velocity with respect to the bare band value. In the FM state, the charge dynamics maps a band reconstruction, which additionally causes a reshuffling of spectral weight in the FIR. This indicates the empowered matrix element of dipole-active excitations related to non-trivial states for which a large effective Berry curvature may be predicted.
This work was carried out in collaboration with groups at the Max Planck Institute for Chemical Physics of Solids, Dresden, Germany and RIKEN, Wako, Japan; at the University of Fribourg; at Brookhaven National Laboratory and Stony Brook University, Stony Brook, US; and at the Chinese Academy of Sciences and the University of the Chinese Academy of Sciences, Beijing, China, and the South Bay Interdisciplinary Science Center, Dongguan, China.
Note - This is a shortened version of a communication originally posted in Nature Portfolio Communities at https://physicscommunity.nature.com/posts/charge-dynamics-of-a-noncentrosymmetric-magnetic-weyl-semimetal
