Giant anisotropic band flattening in twisted $\Gamma$ valley semiconductor bilayers

TL;DR

This paper develops a theory for anisotropic band flattening in moiré systems at the $B3$ valley, predicting giant anisotropic bands in twisted bilayer black phosphorus and exploring correlated phases like non-Fermi liquids.

Contribution

It introduces a general theory linking effective mass anisotropy to band flattening in twisted bilayers and predicts giant anisotropic moiré bands in black phosphorus with potential for correlated physics.

Findings

Giant anisotropic flattened moiré bands in twisted bilayer black phosphorus.

Large parameter space for non-Fermi liquid phases due to Coulomb interactions.

Effective mass anisotropy is enhanced in twisted bilayers, leading to one-dimensional physics.

Abstract

We propose a general theory of anisotropic band flattening in moir\'e systems at the $\Gamma$ 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 effectively. We predict twisted bilayer black phosphorus (tBBP) has giant anisotropic flattened moir\'e bands ($\eta\sim10^4$) from ab initio calculations and continuum model, where the low energy physics is described by the weakly coupled array of one-dimensional 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.