Nature of the quantum insulator to superconductor and superconductor to normal state transitions in cuprate superconductors
Using the scaling theory of quantum critical phenomena we present a detailed analysis of various superconducting properties of La$_{2-x}$Sr$_{x}$CuO$_{4}$, Y$_{0.8}$Ca$_{0.2}$-123, Tl-1212 and Tl-2201. Our results include: the experimental data are fully consistent with a 2D insulator to superconductor (ISQ) transition in the underdoped limit. The quantum critical properties appear to be equivalent to those of a dirty 2D bosonic system with long range Coulomb interaction. Together with previous evidence, this transition appears to be a generic feature of cuprate superconductors. The 3D superconductor to normal state quantum transition (SNQ) turns out to be less unique. More extended experimental data is needed to settle the nature of this transition unambiguously.