Origin of the Extreme and Anisotropic Magnetoresistance in the Weyl semimetal NbP

Abstract

The fascination with semimetals, especially Dirac and Weyl semimetals, is given by their surprisingly strong response to magnetic fields. In particular, the extremely large magnetoresistance (XMR), i.e., the change in electrical resistivity as a function of the applied magnetic field, has attracted interest because of its deviation by several orders of magnitude from the behavior of normal metals, and its potential for technological applications. To date, it is unclear if the XMR in topological semimetals is inherently correlated to the very high electron mobility and electron-hole compensation, or to other exotic mechanisms. Here, we show that the relativistic and topological nature of charge carriers of the Weyl semimetal niobium phosphide (NbP) are only indirect causes of the XMR. Instead, the XMR can be explained by the very long mean free path ${l_e} (4 K) ≈ 8 μm$ in combination with the small cyclotron orbits emerging in the presence of a magnetic field ${r_c} (9 T) ≈ 20 nm$ of the NbP’s Weyl electrons. More precisely we find $MR = c {l_e}/{r_c},$ where c is a parameter independent of temperature and angle between the magnetic field and the crystal. To demonstrate, we use temperature and angle-dependent magnetoresistance measurements, and extract the mean free path and cyclotron radius from an analysis of the Shubnikov–de Haas oscillation.