Chiral Weyl Pockets and Fermi Surface Topology of the Weyl Semimetal TaAs

Tantalum arsenide is a member of the noncentrosymmetric monopnictides, which are putative Weyl semimetals. In these materials, three-dimensional chiral massless quasiparticles, the so-called Weyl fermions, are predicted to induce novel quantum mechanical phenomena, such as the chiral anomaly and topological surface states. However, their chirality is only well defined if the Fermi level is close enough to the Weyl points that separate Fermi surface pockets of opposite chirality exist. In this Letter, we present the bulk Fermi surface topology of high quality single crystals of TaAs, as determined by angle-dependent Shubnikov–de Haas and de Haas–van Alphen measurements combined with ab initio band-structure calculations. Quantum oscillations originating from three different types of Fermi surface pockets were found in magnetization, magnetic torque, and magnetoresistance measurements performed in magnetic fields up to 14 T and temperatures down to 1.8 K. Of these Fermi pockets, two are pairs of topologically nontrivial electron pockets around the Weyl points and one is a trivial hole pocket. Unlike the other members of the noncentrosymmetric monopnictides, TaAs is the first Weyl semimetal candidate with the Fermi energy sufficiently close to both types of Weyl points to generate chiral quasiparticles at the Fermi surface.

Figure 1

Top: ab initio band structure of TaAs. Bottom: Scheme of the bands close to the Fermi energy along the line connecting the Weyl nodes. $n_0$ marks the charge neutral point, $E_F$ the Fermi energy as determined by quantum oscillations. The gray shaded area corresponds to the energy window in which separate Fermi surface pockets of opposite chirality exist.

Figure 2

(a) Magnetic field dependence of the magnetization at 2 K. The inset shows the de Haas–van Alphen oscillations plotted over the inverse magnetic field normalized by the lowest quantum oscillation frequency. A linear background fitted to the low field magnetization was subtracted. Curves are offset for clarity. (b) Left panel: transverse magnetoresistance for magnetic fields applied within the (100) plane in steps of 20° measured at 2 K (data have been offset by $0.4\mathrm{\Omega }$ each for clarity). Right panel: corresponding Fourier transformations of the background subtracted Shubnikov-de Haas signals over a magnetic field range of 1 to 14 T. The dashed lines are guides to the eye showing the angular dependence of $F_{\alpha }$, $F_{\beta }$, and $F_{\gamma }$.

Figure 3

(a) Angular dependence of the quantum oscillation frequencies $F$ of TaAs for magnetic fields applied within the (100), (110), and (001) plane. Black triangles, dots, and stars represent frequencies determined from electrical transport, magnetic torque, and magnetization, respectively. The solid and dashed lines show the theoretical angular dependence according to the ab initio band structure at a Fermi energy of $+6\mathrm{meV}$. (b) and (c) First Brillouin zone and Fermi surface topology of TaAs according to the angular dependence in graph (a). The dashed rings show the nodal lines as explained in the text.

Figure 4

Temperature dependences of the de Haas–van Alphen oscillation amplitude as determined from magnetization measurements in the magnetic field interval of 1 to 7 T along the high symmetry axes. Lines are fits to the Lifshitz-Kosevich temperature reduction term $\tilde{M}\propto x/\sinh (x)$ with $x={\pi }^2{m}^{*}{k}_{B}T/{\mu }_{B}B$ [21, 26, 27].