Thickness and magnetic field-tuned superconductor-insulator transitions in a-Nb₁₅Si₈₅

Results from a study of amorphous superconducting Nb_0.15Si _0.85 thin films are presented. These compounds are subject to a thickness-tuned superconductor-insulator transition at zero-field and, for each given thickness, a magnetic-field tuned superconductor-insulator transition is observed. The field-tuned transition is characterized by an isobestic point (H_c,R_c) that indicates the quantum critical nature of this superconductor-insulator transition and the absence of an intermediate metallic state. Similarly, we find that the thickness-tuned superconductor-insulator transition in zero-field is characterized by a critical thickness d_c. We were thus able to obtain the phase diagram in the (H,d) plane for Nb _0.15Si_0.85.