Anisotropic Quantum Hall Transport in a Black Phosphorus Two-Dimensional Hole Gas

Intriguing many-body ground states often emerge from quantum Hall systems, driven by electron-electron interactions. Among them, states breaking translational and rotational symmetries─manifested by anisotropic magnetotransport─can be stabilized by intrinsic band anisotropy. Here, we fabricate few-layer black phosphorus devices hosting a high-mobility two-dimensional hole gas with large effective-mass anisotropy. These exceptional properties enable us to observe pronounced anisotropic transport in the quantum Hall regime. Unlike conventional anisotropic stripe states, these emergent states remain robust up to T ≈ 4 K, and their corresponding partial filling factors (ṽ) deviate from the conventional value of 1/2: ṽ approaches 1/4 in the N = 2 Landau level and increases to approximately 0.4 with increasing Landau-level index. Supported by theoretical calculations, our results point to the emergence of anisotropic quantum Hall states consistent with stripe-like order. Our work establishes few-layer black phosphorus as a unique platform for exploring emergent quantum phenomena arising from anisotropic correlated electronic states.