Extremely large magnetoresistance in the topologically trivial semimetal alpha-WP2

Extremely large magnetoresistance (XMR) was recently discovered in many nonmagnetic materials, while its underlying mechanism remains poorly understood due to the complex electronic structure of these materials. Here we report an investigation of the alpha-phase WP2, a topologically trivial semimetal with monoclinic crystal structure (C2/m), which contrasts with the recently discovered robust type-II Weyl semimetal phase in beta-WP2. We found that alpha-WP2 exhibits almost all the characteristics of XMR materials: the near-quadratic field dependence of MR, a field-induced up-turn in resistivity followed by a plateau at low temperature, which can be understood by the compensation effect, and high mobility of carriers confirmed by our Hall effect measurements. It was also found that the normalized MRs under different magnetic fields have the same temperature dependence in alpha-WP2, the Kohler scaling law can describe the MR data in a wide temperature range, and there is no obvious change in the anisotropic parameter gamma value with temperature. The resistance polar diagram has a peanut shape when the field is rotated in the ac plane, which can be understood by the anisotropy of the Fermi surface. These results indicate that both field-induced-gap and temperature-induced Lifshitz transition are not the origin of up-turn in resistivity in the alpha-WP2 semimetal. Our findings establish alpha-WP2 as a new reference material for exploring the XMR phenomena.