Superfluid density measurements of high-temperature superconducting films

We have constructed a precision two-coil inductive system suitable for measuring the superfluid density of superconducting films. With this system we measure several intrinsic properties of YBa2Cu3O 7-x (YBCO) and Bi2Sr2CaCu2O 8+delta (Bi-2212) films. Two competing models of the pseudogap are compared to the temperature dependence of the superfluid density as a function of doping in Bi-2212. Near zero temperature, a maximum in the superfluid density as a function of hole doping is observed at approximately 0.19 holes per unit Cu, in agreement with a model of the pseudogap with competing order. At temperatures close to the Tc, the superfluid density exhibits critical phase fluctuations described by an XY model. These phase fluctuations are compared to a model of the pseudogap described by preformed pairs. In contrast to previous reports on YBCO films, we find that the phase transition in a YBCO film is described by 3D-XY critical fluctuations. In heavily underdoped Bi-2212, we find a drop in superfluid density that is approximately described by a Kosterhtz-Thouless-Berezinskii (KTB) transition. However, in Bi-2212 films near optimal and critical doping, we find evidence for 3D-XY static and dynamic fluctuations. The dynamic critical exponent, z, in thin optimally-doped Bi-2212 films is directly observed as diffusive (z = 2) at Tc when the correlation length exceeds the film thickness (D = 2). In the 3D regime we find evidence for z ≈ 1.5, indicative of a propagating mode.