Quantum breakdown of superconductivity in mirror-symmetric twisted-trilayer graphene

Two-dimensional (2D) superconductors offer a promising opportunity to investigate the interplay between localization and long-range superconducting order since both phenomena have a critical dimension of 2. In the zero-temperature limit in 2D systems, the quasi-particles are not expected to diffuse due to the enhanced effect of disorder, making a metallic state forbidden. However, in the quantum phase transition (QPT) between a superconductor and a high-field insulating state, an intermediate metallic state was observed in the earlier experiments in disordered 2D films. The nature of this field-tuned anomalous metallic state is intensely debated and discussed in the context of strong phase fluctuations and localization of the Cooper pairs. In this talk, We shall present our recent experimental observation of the magnetic field-tuned QPT in mirror-symmetric twisted tri-layer graphene near magic angle. The superconductor-to-insulator transition (SIT) is characterized by the finite size scaling analysis with a scaling exponent of 4/3, resembling the classical percolation model. Under a small magnetic field, the superconducting state transforms to an anomalous metallic phase with finite resistance which increases with increasing magnetic field and eventually switches to an insulating state at the QPT. We shall discuss these results in the context of field-induced vortices and Josephson-coupled superconducting puddles. Our results provide new insights into the interplay between disorder and superconductivity in the flat bands of moire superconductors.