Severe violation of the Wiedemann-Franz law in quantum oscillations of NbP

Abstract

The thermal conductivity (\ensuremath{\kappa}) of the Weyl semimetal NbP was studied with the thermal gradient and magnetic field applied parallel to the [0 0 1] direction. At low temperatures \ensuremath{\kappa}($B$) exhibits large quantum oscillations with frequencies matching two of several determined from the Shubnikov--de Haas effect measured on the same sample with analogous electrical current and magnetic field orientation. Both frequencies found in \ensuremath{\kappa}($B$) originate from the electron pocket enclosing a pair of Weyl nodes. The amplitude of the oscillatory component of the thermal conductivity turns out to be two orders of magnitude larger than the corresponding value calculated from the electrical conductivity using the Wiedemann-Franz law. Analysis of possible sources of this discrepancy indicates the chiral zero sound effect as a potential cause of its appearance.