We propose a general theory of anisotropic band flattening in moiré systems at the $\mathrm{\Gamma }$ valley. For a two-dimensional semiconductor with a rectangular unit cell of $C_{2z}$ or mirror symmetries, we find that a larger effective mass anisotropy $\eta =m_y/m_x$ of the valence or conduction bands in the monolayer will have a stronger tendency to be further enhanced in its twisted bilayer. This gives rise to strong anisotropic band flattening and correlated physics in one dimension (1D) effectively. We predict twisted bilayer black phosphorus (tBBP) has giant anisotropic flattened moiré bands ($\eta \sim {10}^4$) from ab initio calculations and the continuum model, where the low-energy physics is described by the weakly coupled array of 1D wires. We further calculate the phase diagram based on the sliding Luttinger liquid by including the screened Coulomb interactions in tBBP and find a large parameter space may host the non-Fermi liquid phase. We thus establish tBBP as a promising and experimentally accessible platform for exploring correlated physics in low dimensions.

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• Electronic structure
• 2-dimensional systems
• Strongly correlated systems
• Twisted heterostructures
• Density functional theory
• Luttinger liquid model