Kekulé spirals and charge transfer cascades in twisted symmetric trilayer graphene

Abstract: We study the phase diagram of magic-angle twisted symmetric trilayer graphene in the presence of uniaxial heterostrain and interlayer displacement field. For experimentally reasonable strain, our mean-field analysis finds robust Kekul\'e spiral order whose doping-dependent ordering vector is incommensurate with the moir\'e superlattice, consistent with recent scanning tunneling microscopy experiments, and paralleling the behaviour of closely-related twisted bilayer graphene (TBG) systems. Strikingly, we identify a new possibility absent in TBG: the existence of $\textit{commensurate}$ Kekul\'e spiral order even at zero strain for experimentally realistic values of the interlayer potential in a trilayer. Our studies also reveal a complex pattern of charge transfer between weakly- and strongly-dispersive bands in strained trilayer samples as the density is tuned by electrostatic gating, that can be understood intuitively in terms of the `cascades' in the compressibility of magic-angle TBG.