Electronic structure of lattice relaxed alternating twist tNG-multilayer graphene: from few layers to bulk AT-graphite

TL;DR

This paper investigates the electronic properties of alternating twist multilayer graphene systems, revealing how lattice relaxation, electric fields, and layer number influence flat bands, magic angles, and quantum Hall effects.

Contribution

It provides a comprehensive analysis of the electronic structure of tNG multilayer graphene, including lattice relaxation effects, electric field responses, and mapping to bulk AT-graphite.

Findings

Lattice relaxations reduce magic angles and enhance electron-hole asymmetry.

Perpendicular electric fields split Dirac bands and induce gaps depending on layer number.

Nearly flat bands and quantum Hall effects are observed at high magnetic fields.

Abstract

We calculate the electronic structure of AA'AA'...-stacked alternating twist N-layer (tNG) graphene for N = 3, 4, 5, 6, 8, 10, 20 layers and bulk alternating twist (AT) graphite systems where the lattice relaxations are modeled by means of molecular dynamics simulations. We show that the symmetric AA'AA'... stacking is energetically preferred among all interlayer sliding geometries for progressively added layers up to N=6. Lattice relaxations enhance electron-hole asymmetry, and reduce the magic angles with respect to calculations with fixed tunneling strengths that we quantify from few layers to bulk AT-graphite. Without a perpendicular electric field, the largest magic angle flat-band states locate around the middle following the largest eigenvalue eigenstate in a 1D-chain model of layers, while the density redistributes to outer layers for smaller magic twist angles corresponding to higher order effective bilayers in the 1D chain. A perpendicular electric field decouples the electronic structure into $N$ Dirac bands with renormalized Fermi velocities with distinct even-odd band splitting behaviors, showing a gap for N=4 while for odd layers a Dirac cone remains between the flat band gaps. The magic angle error tolerance estimated from density of states maxima expand progressively from $0.05^{\circ}$ in t2G to up to $0.2^{\circ}$ in AT-graphite, hence allowing a greater flexibility in multilayers. Decoupling of tNG into t2G with different interlayer tunneling proportional to the eigenvalues of a 1D layers chain allows to map tNG-multilayers bands onto those of periodic bulk AT-graphite's at different $k_z$ values. We also obtain the Landau level density of states in the quantum Hall regime for magnetic fields of up to 50~T and confirm the presence of nearly flat bands around which we can develop suppressed density of states gap regions by applying an electric field in N > 3 systems.