Novel phase diagram and quantum phase transitions in 2D superconducting systems

Xiaofu Zhang (UZH)

Superconductivity in normal state is generally characterized by broken
Cooper pairs. However, the superconducting state is described an order
parameter, including a Cooper density related amplitude and a phase
parameter. As a result, the breakdown of superconductivity can be
classified into two distinct path: (a) superconductor to Fermi insulator
quantum phase transition induced by suppressing the amplitude to zero;
(b) superconductor to Bose insulator quantum phase transition induced by
the loss of global coherence, in which Cooper pairs are localized. We
experimentally demonstrate that a two dimensional superconducting system
is firstly driven into a Bose insulating state at a lower critical
field, and finally became a Fermi insulator at higher field. Such phase
transitions are successfully described by a finite size scaling theory.