LAUSR

Superconductivity occurs in metals when attractive interactions between electrons promote the formation of Cooper pairs which get locked in a phase-coherent state, leading to superfluid behavior. In situations where the phase stiffness falls below the pairing energy, the superconducting transition temperature ${T}_{c}$ is driven by phase fluctuations and pairing can continue to survive above ${T}_{c}$. Such a behavior has long been thought of as the hallmark of unconventional superconductivity. Here we combine sub-K scanning tunneling spectroscopy, magnetic penetration depth measurements, and magnetotransport measurements to show that in ultrathin amorphous MoGe ($a$-MoGe) films the superfluid density is strongly suppressed by quantum phase fluctuations at low temperatures for thickness below 5 nm. This is associated with a rapid decrease in the superconducting transition temperature ${T}_{c}$ and the emergence of a pronounced pseudogap above ${T}_{c}$. These observations suggest that even in conventional superconductors strong disorder and low dimensionality will ultimately trigger a Bosonic route for the destruction of superconductivity.