  The superconducting ground state in a large number of two-dimensional (2d)
systems can be created and destroyed through quantum phase transitions (QPTs)
driven by non-thermal parameters such as the carrier density or magnetic field.
The microscopic mechanism of QPTs has not been established in any 2d
superconductor, in part due to an emergent scale-invariance near the critical
point, which conceals the specific processes driving the transitions. In this
work, we find that the pair-breaking mechanism causing the suppression of the
Cooper pair density gives a unifyingly consistent description of
magnetic-field-driven QPTs in amorphous MoGe, Pb and TaN films, as well as in
quasi-2d high-temperature superconductor La$_{1.92}$Sr$_{0.08}$CuO$_{4}$. This
discovery was facilitated by the development of a novel theoretical approach,
one which goes beyond the standard determination of critical exponents and
allows for the extraction of a microscopic seeding length scale of the
transitions. Remarkably, for the materials studied, and also for MoGe
nanowires, this scale matches the superconducting coherence length. Further,
this approach has been successfully applied to many other complex,
non-superconducting systems.
