Electric field tunable coupling strength and quantum metric hot spots in a moiré flatband superconductor

Alternating twisted multilayer graphene presents a compelling multiband system, with a coexistence of Dirac bands and flat bands, for exploring superconductivity. However, the roles of flat bands and Dirac bands played in determining the superconductivity remain elusive. Here, we focus on the alternating twisted quadralayer graphene to reveal unconventional superconducting behaviors. We disentangle Dirac bands and flat bands, revealing a Coulomb interaction-induced band broadening effect. We further quantify the electric-field-dependent evolution of the critical temperature and coherence length, and estimate the flat-band Fermi velocity and superfluid stiffness via critical current measurements. Our results demonstrate an electric-field-tunable coupling strength within the superconducting phase, revealing unconventional properties with vanishing Fermi velocity and large superfluid stiffness. Combined with our theoretical analysis, these observations support a picture in which displacement-field-driven hybridization between flat and Dirac-like bands enhances the quantum-metric contribution to superconductivity, offering new insight into multiband flat-band superconductivity in moiré systems.