Emulating twisted double bilayer graphene with a multiorbital optical lattice

Abstract

This work theoretically explores how to emulate twisted double bilayer graphene with ultracold atoms in multiorbital optical lattices. In particular, the quadratic band touching of Bernal stacked bilayer graphene is emulated using a square optical lattice with px, py, and d x 2 −y 2 orbitals on each site, while the effects of a twist are captured through the application of an incommensurate potential. The quadratic band touching is stable until the system undergoes an Anderson like delocalization transition in momentum space, which occurs concomitantly with a strongly renormalized single particle spectrum inducing flat bands, which is a generalization of the magic-angle condition realized in Dirac semimetals. The band structure is described perturbatively in the quasiperiodic potential strength, which captures miniband formation and the existence of magic-angles that qualitatively agrees with the exact numerical results in the appropriate regime. We identify several magic-angle conditions that can either have part or all of the quadratic band touching point become flat. In each case, these are accompanied by a diverging density of states and the delocalization of plane wave eigenstates. It is discussed how these transitions and phases can be observed in ultracold atom experiments.

Low energy excitation spectrum of magic-angle semimetals

“…For Dirac semimetals, the bands become flat when the Dirac velocity vanishes due to a so-called "magic-angle" condition [15]. The magic-angle effect has been shown to be quite general and applicable to a broad class of nodal and topological band structures in the presence of a incommensurate tunneling or potential [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41].…”