Infrared conductivity of LaSb : evidence for non-compensation origin of extreme magnetoresistance.

Extremely large and unsaturating magnetoresistance (XMR) is an unprecedented phenomenon discovered in topological semimetals. Vast amount of works so far attributed XMR to perfect or near-perfect compensation of electrons and holes that assumed their carrier densities are equal. Here we performed infrared reflectance measurement as well as dc-transport on single crystal LaSb. The data show two Drude peaks with drastically different scattering rates. We determine (n, γ) for the hole and electron from the two Drude peaks and use them to calculate magnetic-dependent longitudinal resistivity ρ_xx(B) which shows an excellent agreement with measured ρ_xx(B) for wide range of, -13T<B<13T, the carrier densities for hole and electron differ by about factor 2. This result shows that the large difference of scattering rates leads to XMR, and perfect charge compensation is not the exclusive origin.