Correlated insulators, semimetals, and superconductivity in twisted trilayer graphene

TL;DR

This paper investigates the complex correlated phases, including insulators, semimetals, and superconductivity, in mirror-symmetric twisted trilayer graphene near the magic angle, using analytical and numerical methods to map out the phase diagram.

Contribution

It provides an asymptotically exact analysis of many-body ground states at all integer fillings and explores the emergence of superconductivity, especially triplet pairing, in this system.

Findings

Identification of exact correlated ground states at all integer fillings.

Construction of a phase diagram as a function of filling and displacement field.

Prediction of dominant triplet superconductivity possibly explaining experimental observations.

Abstract

Motivated by recent experiments indicating strong superconductivity and intricate correlated insulating and flavor-polarized physics in mirror-symmetric twisted trilayer graphene, we study the effects of interactions in this system close to the magic angle, using a combination of analytical and numerical methods. We identify asymptotically exact correlated many-body ground states at all integer filling fractions $\nu$ of the flat bands. To determine their fate when moving away from these fine-tuned points, we apply self-consistent Hartree-Fock numerics and analytic perturbation theory, with good agreement between the two approaches. This allows us to construct a phase diagram for the system as a function of $\nu$ and the displacement field, the crucial experimental tuning parameter of the system, and study the spectra of the different phases. The phase diagram is dominated by a correlated semimetallic intervalley coherent state and an insulating sublattice-polarized phase around charge neutrality, $\nu=0$, with additional spin-polarization being present at quarter ($\nu=-2$) or three quarter ($\nu=+2$) fillings of the quasi-flat bands. We further study the superconducting instabilities emerging from these correlated states, both in the absence and in the additional presence of electron-phonon coupling, also taking into account possible Wess-Zumino-Witten terms. In the experimentally relevant regime, we find triplet pairing to dominate, possibly explaining the observed violation of the Pauli limit. Our results have several consequences for experiments as well as future theoretical work and illustrate the rich physics resulting from the interplay of almost flat bands and dispersive Dirac cones in twisted trilayer graphene.