Coexistence of Charge Density Wave and Quantum Hall Effect in Bulk 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, charge carriers in 3D systems are free to move in all special directions which makes the quantization of Hall resistance difficult. In the search for new materials, we have grown CaCu₄As₂ single crystal which crystallized in rhombohedral structure. It contains three-septuple layers in one conventional unit cell. Thermodynamic, electrical transport and temperature dependent single crystal x-ray diffraction confirm structural or charge-density-wave (CDW) transition at 51 K. Shubnikov-de Haas (SdH) oscillation of 255 T has been observed in magnetoresistance, and angular SdH studies depict a quasi-2D character of charge carriers contributing to the bulk transport. Furthermore, the electronic structure reveals its 2D like nature where the hole bands form corrugated cylindrical Fermi pockets and the electron bands constitute open sheet Fermi pockets. Interestingly, in the inverse Hall resistance <i>vs</i> <i>1/B</i> plot, quantized Hall plateaus are observed. These all suggest- the observation of CDW and quantum Hall effect (QHE) in CaCu₄As₂. Regarding the QHE in CDW system, Halperin proposed that the intrinsic instability of the electronic structure, for example, CDW opens a gap that can lead to quantization in Hall ^[1]. But our experimental results indicate that the QHE may be originated from a large number of parallel conduction channels present in CaCu₄As₂, and CDW enhances the 2D charge carrier confinement. Thus, CaCu₄As₂ provides a new platform to understand the coexistence of both CDW and quantum Hall effect.<br/>^[1] B. I. Halperin, “Possible states for a three-dimensional electron gas in a strong magnetic field,” Jpn J. Appl. Phys. 26, 1913 (1987).