Signature of Instability of Chiral Fermion in Ultraquantum Weyl Semimetal TaAs

In the compound of Tantalum Monoarsenide (TaAs), the novel Weyl fermions are hosted. In our high mobility single crystal samples, the Shubnikov-de Haas oscillations are distinct below the quantum limit (B \textasciitilde 10 T). Beyond the quantum limit, within the configuration-B (electric field E // B-fields), we observed the novel quantum phase transitions in the $\rho _{zz}$-measurements, which may come from the nesting of the vector over a wide range of B-T plane. Due to the similarity to those observed density wave phases or the excitonic states in three-dimensional graphite semimetal, the novel phase may from many body effect beyond the quantum limit. However, in the other measurements (R$_{xx}$ in configuration-A: E $\bot $ B-fields), the novel quantum phase transition does not exist. In general, our results on the density wave phase support its non-trivial topological order.