Field-tunable superconductor to insulator transition in twisted-trilayer graphene

When dimensionality is reduced, strong disorder-driven quantum fluctuations manifest in superconductor to insulator transition (SIT), destroying the superconducting (SC) long-range correlation. In contrast, for weak disorder, quasi-long-range SC order can survive locally within an insulating state. This produces an intermediate mixed state, known as Quantum Griffith phase (QGP), characterized by Cooper pair localization and divergent scaling exponents. While the experimental observation of QGP is so far limited to clean 2D SCs, the nature of localization and its connection to the Bose-insulating state in disordered 2D superconductor is not clear. We present experimental evidence of the QGP in magnetic field-tuned mirror-symmetric twisted tri-layer graphene near the magic angle. The QGP is characterized by a divergent scaling exponent and large broadening of the critical resistance at low temperature in perpendicular magnetic fields. The large phase-space of the QGP enables us to perform current bias dependence of the dynamic resistance, revealing the localized SC puddles. Remarkably, the QGP disappears in a parallel magnetic field configuration, indicating that the phase fluctuations are field orientation tunable. Furthermore, we show that applying a displacement field provides access to field-induced vortices and localization effects. Our results provide new insights into Cooper pair localization in clean 2D superconductors.