Weyl fermion-phonon coupling and magnetic-field-induced large thermopower in TaP

Weyl semimetals (WSM) have massless chiral fermions, the so-called Weyl Fermions. The chiral anomaly in its magneto-transport is one of the signatures for the existence of the plus and minus Weyl nodes which always appear in a pair. Recent theoretical predictions discussed the possibility to detect chiral anomaly based on phonon spectra, where the exotic electronic degrees of freedom in a WSM are playing an increasingly important role to influence the phonon structure.
We measured the phonons via inelastic X-ray scattering (IXS) on the TaP crystals near the Weyl points, where an ambipolar behavior is observed when moving away from the Weyl points. This is a reliable indicator of the existence and the strength of Weyl fermion-phonon coupling. We are further using the magnetic field to break the time-reversal-symmetry for observing the central phenomenon of chiral anomaly. We also observed an unexpected magnetic-field-induced large thermopower in TaP. We consider the large thermopower of the Weyl semimetal induced by magnetic field subjected to a quantizing magnetic field. The thermopower of the Weyl semimetal grows linearly with the field without saturation and can reach extremely high values. This sheds lights on the achievement of a record-high thermoelectric figure-of-merit.