Observation of Quantum Hall Effect and Charge Density Wave in CaCu­₄As₂ single crystal

Quantum Hall effect (QHE) is usually observed in 2D systems due to the quantization of electron gas at sufficiently large applied magnetic fields. Whereas, in a 3D system, the charge carriers are free to move in all special directions which essentially prevents the quantization of Hall conductance.  In search of new materials, we have grown single crystals of CaCu₄As₂ which crystallizes in the rhombohedral structure with three septuple layers in one unit cell. From thermal, electrical transport and temperature dependent single crystal x-ray diffraction measurements, we observe a transition at 52 K attributed to the structural and/or charge density wave (CDW) transition. The Shubnikov-de Haas (SdH) oscillation reveals a unique frequency at 255 T, and the angular dependence of this frequency confirms the 2D nature of the charge carriers. The inverse Hall resistance vs 1/B plot exhibits clear steps like features. Our detailed analysis confirms that these steps are due to the QHE. Our experimental results indicate that the QHE may be originated from a large number of parallel conduction channels and CDW enhances the 2D nature of the charge carrier.  Thus, CaCu₄As₂ provides a new platform to understand the coexistence of both CDW and QHE in a 3D system.