2021/11/22 by Patrick J. Ledwith, Eslam Khalaf, Ledwith, Patrick J. +10
Materials Science · Physics and Astronomy · #FOS: Physical sciences #Graphene research and applications #Mesoscale and Nanoscale Physics (cond-mat.mes-hall) #Quantum and electron transport phenomena #Strongly Correlated Electrons (cond-mat.str-el) #Topological Materials and Phenomena
paper · pdf · doi:10.48550/arxiv.2111.11060
openalex publication_date 2021/11/22 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
The emergence of alternating twist multilayer graphene (ATMG) as a generalization of twisted bilayer graphene (TBG) raises the question - in what important ways do these systems differ? Here, we utilize ab-initio relaxation and single-particle theory, analytical strong coupling analysis, and Hartree-Fock to contrast ATMG with n=3,4,5,… layers and TBG. I: We show how external fields enter in the decomposition of ATMG into TBG and graphene subsystems. The parallel magnetic field has little effect for n odd due to mirror symmetry, but surprisingly also for any n > 2 if we are are the largest magic angle. II: We compute the relaxation of the multilayers leading to effective parameters for each TBG subsystem. We find that the second magic angle for n=5, θ= 1.14 is closest to the "chiral" model and thus may be an optimal host for fractional Chern insulators. III: We integrate out non-magic subsystems and reduce ATMG to its magic angle TBG subsystem with a screened interaction. IV: We perform an analytic strong coupling analysis of the external fields and corroborate it with Hartree-Fock. An in-plane magnetic field in TBG drives a transition to a valley Hall state or a gapless "magnetic semimetal." A displacement field V has little effect on TBG, but induces a gapped phase in ATMG for small V for n = 4 and above a critical V for n = 3. For n≥ 3, we find the superexchange coupling - believed to set the scale of superconductivity in the skyrmion mechanism - increases with V at angles near and below the magic angle. V: We complement our strong coupling approach with a weak coupling theory of ATMG pair-breaking. While for n=2 orbital effects of the in-plane magnetic field can give a critical field of the same order as the Pauli field, for n>2 we expect the critical field to exceed the Pauli limit.