Thermal transport and thermodynamic properties of the Weyl monophosphide NbP

NbP is a Weyl semimetal, which exhibits a huge positive magnetoresistance (MR) exceeding 8 x 10⁵ % at 2 K for an electrical current applied along b and a magnetic field of 9 T along c. The MR is further increasing roughly linearly up to at least 60 T. This finding has been attributed to an ultrahigh charge carrier mobility. We performed thermal transport and specific heat measurements on NbP for the same configuration, namely the magnetic field B along c and the heat current along b. We find a huge change of the thermopower in magnetic fields with a maximum value of 800 μV/K at 9 T and 50 K. Such large effects have been rarely observed in bulk materials, the only example with a larger magnitude at our knowledge being the doped semiconductor InSb. We suggest that the high charge-carrier mobility held responsible for the giant magnetoresistance of NbP is also causing the large magnetothermopower. In addition, electron-phonon scattering processes may play a role, an idea which is also in line with the observation of quantum oscillations in the thermal conductivity of NbP. These are much larger than expected for the electronic contribution estimated from the Wiedemann-Franz-law. Quantum oscillations are also seen in the thermopower and specific heat data.