Sequential superconductor-Bose insulator-Fermi insulator transitions in two dimensional a-WSi

A zero-temperature magnetic-field-driven superconductor to insulator transition (SIT) in quasi-two-dimensional superconductors is expected to occur when the applied magnetic-field crosses a certain critical value. A fundamental question is whether this transition is due to the localization of Cooper pairs or due to the destruction of them. Here we address this question by studying the SIT in amorphous WSi. Transport measurements reveal the localization of Cooper pairs at a first critical field B_C¹ (Bose-insulator), with a product of the correlation length and dynamical exponents zν~1.33 near the quantum critical point (QCP). Beyond B_C¹ superconducting fluctuations still persist. Above a second critical field B_C²>B_C¹, the Cooper pairs are destroyed and the film becomes a Fermi-insulator. The different phases all merge at a tricritical point with zν=0.67. Our results suggest sequential phase transitions (superconductor - Bose insulator - Fermi insulator) in the zero-temperature limit, which differs from the conventional scenario involving a single quantum critical point.