Probing the chiral Fermi surface of the Weyl semimetal NbP using Transverse Electron Focusing

Abstract

Weyl fermions are massless spin-1/2 chiral fermions and, together with Dirac and Majorana fermions, are one of the possible types of elementary fermions. None of the elementary particles in the Standard Model are Weyl fermions. Still, in condensed matter physics, they can emerge as quasiparticles, hosted in the so-called Weyl semimetals (WSM), which can therefore be used to study the unique properties of Weyl fermions. However, WSM are generally characterized by a multitude of charge carriers pockets, most of which host trivial fermions that contribute in parallel to transport and hinder Weyl fermions' properties. To address this issue, we perform transverse electron focusing (TEF) in microstructured single-crystal of the Weyl semimetal niobium phosphide (NbP). TEF allows separating in real space charged quasiparticles with different momenta, and therefore to separate trivial and Weyl bulk fermions in NbP. By combining TEF with Shubnikov de Haas (SdH) experiments, we reconstruct the unique peanut-shaped Fermi surface of the Weyl electrons, which originates from the combination of two Fermi surfaces hosting carriers with opposite chirality. Our data indicate two different Fermi momenta associated to the two chiral flavors, leading to a real space separation of chiral electrons in the TEF experiment. NbP is interesting also because characterized by an exceptionally high mobility. We extract Fermi momentum, effective mass, mobility, and mean scattering time of its Weyl electrons, which, among all the other carriers, appear to be the ones with the longest mean free path, and the main responsible for the high mobility of this fascinating material.