Quantum Hall States in an Anisotropic Bulk Weyl Semimetal TaAs

Our study reports the two-dimensional (2D)-like Shubnikov-de Hass (SdH) oscillations accompanied by quantized Hall resistivity in the high mobility (multi-quantum layers) TaAs crystal with c- (001) sample surface. On the other hand, there is no quantum Hall (QH) plateau with the single crystal sample of a- (100) crystalline surface. Moreover, microwave photovoltage (PV) and photocurrent (I_ph) present distinct 2D-like quantum oscillations and QH states with beautifully features. PV and I_ph minima appear at more Landau level fillings, because the QH degeneracy is broken further. Microwave enhances the conductivity and mobility within the skin depth of the electromagnetic field, thus more QH features are observed. Thus our work shows important implications for transport studies of 3D topological materials.