Possible manifestations of the chiral anomaly and evidence for a magnetic field induced topological phase transition in the type-I Weyl semimetal TaAs

We studied the magnetoresistivity and the Hall effect of the type-I Weyl semimetal TaAs to address the controversy surrounding its anomalous transport properties in relation to its bulk topological character. For fields and currents along the basal plane, we observe a very pronounced planar Hall effect (PHE) upon field rotation with respect to the crystallographic axes at temperatures as high as T=100K. Parametric plots of the PHE signal as a function of the longitudinal magnetoresistivity (LMR) collected at T=10K lead to concentric traces as reported for Na3Bi and GdBiPt. This would suggest that the negative LMR and the PHE observed in TaAs are intrinsically associated with the axial anomaly among its Weyl nodes. For fields nearly along the a axis we observe hysteresis as one surpasses the quantum limit, where the magnetic torque indicates a change in regime as the field increases, i.e., from paramagnetism and diamagnetism due to Weyl fermions above and below the Weyl node(s), respectively, to a paramagnetic one associated with the field-independent n=0 Landau level. Hysteresis coupled to the overall behavior of the torque would be consistent with a topological phase transition associated with the suppression of the Weyl dispersion at the quantum limit. This transition leads to the suppression of the negative LMR confirming that it is intrinsically associated with the Weyl dispersion. The Hall effect for fields along the c axis reveals two successive changes in slope, or two successive decrements in carrier mobility, one at the quantum limit and a second one at the critical field where a phase transition toward an insulating state was recently reported. This suggests the possibility of two successive phase transitions as function of the field with the higher-field one involving solely the n=0 Landau level. Finally, for both field orientations we observe Shubnikov–de Haas like oscillations beyond the quantum limit hence involving quasiparticles at fractional filling factors.