Exotic Carriers from Concentrated Topology: Dirac Trions as the Origin of the Missing Spectral Weight in Twisted Bilayer Graphene

TL;DR

This paper identifies Dirac trions as the origin of missing spectral weight in twisted bilayer graphene, revealing their role in the material's exotic topological and spectral properties near superconductivity.

Contribution

It introduces and characterizes Dirac trions as key excitations explaining spectral weight loss and connects them to the emergence of the intervalley Kekulé spiral state.

Findings

Missing spectral weight is due to Dirac trion excitations.

Dirac trions form a massless Dirac cone at charge neutrality.

Trions evolve into quasiparticles in the Kekulé spiral state.

Abstract

The nature of charge carriers in twisted bilayer graphene (TBG) near $\nu = -2$, where superconductivity emerges, remains mysterious. While various symmetry-broken ground states have been proposed, experimental evidence of significant entropy persisting to low temperatures suggests that the disordered thermal state is a more natural starting point for understanding the normal state physics. Our previous work proposed that this thermal state in TBG hosts nearly decoupled nonlocal moments and an exotic Mott semimetal at charge neutrality. This evolves into a spectrally imbalanced Mott state at non-zero integer fillings. Notably, at $\nu = -2$, the quasiparticle residue vanishes at the top of the valence band, precisely where superconductivity develops. The vanishing quasiparticle residue naturally leads to the following question: Which excitation accounts for the missing spectral weight, and how are they related to electrons? In this work, we demonstrate that the missing spectral weight corresponds to a trion excitation, which we explicitly construct and characterize. Remarkably, despite being composed of heavy particles away from the Gamma point, this trion is unexpectedly light. At charge neutrality, the electron and trion hybridize with a momentum-dependent phase that winds around zero, forming a massless Dirac cone. At finite doping $\nu < 0$, the Dirac cone acquires a mass with the valence (conduction) band becoming trion (electron)-like near the $\Gamma$ point. Finally, we show that such trion excitations evolve into the quasiparticles of the intervalley Kekul\'e spiral (IKS) state at finite doping, where they represent electrons dressed by the IKS order parameter. More broadly, our work highlights the unusual spectral properties and excitations that emerge when fermions interact within topological bands with concentrated quantum geometry and topology.