We report the superconducting transition temperature $T_c$ vs. thickness $d_F$ of Ferromagnet/Superconductor (F/S) bilayers, where F is a strong $3d$ ferromagnet (Ni, Ni$_{0.81}$Fe$_{0.19}$ (Permalloy), Co$_{0.5}$Fe$_{0.5}$) and S = Nb, taken from su
perfluid density measurements rather than resistivity. By regrouping the many physical parameters that appear in theory, we show that the effective exchange energy is determined from the F film thickness $d_F$ where $T_c$ vs. $d_F$ begins to flatten out. Using this rearranged theory we conclude: 1) the effective exchange energy, $E_{ex}$, is about 15 times smaller than measured by ARPES and 5 times smaller than deduced in previous studies similar to ours; 2) the dirty-limit coherence length, $xi_{F}$, for Cooper pairs in F is larger than the electron mean free path, $ell_F$; and 3) the $3d$-F/Nb interface is enough of a barrier that Cooper pairs typically must hit it several times before getting through. The Py/Nb and CoFe/Nb interfaces are more transparent than the Ni/Nb interface.
Evidence of two-dimensional (2D) quantum critical fluctuations is observed in the superfluid density ns(T) propto $lambda$ -2(T) of deeply underdoped Bi2Sr2CaCu2O8+x (Bi-2212) films, indicating that quantum fluctuations play a dominant role in underd
oped cuprates in general. 2D fluctuations are expressed by the linear scaling, Tc propto ns(0). 2D scaling in Bi-2212 contrasts with 3D scaling seen in the much less anisotropic YBa2Cu3O7-x. Quantum critical fluctuations could also account for the absence of thermal critical behavior in lambda^{-2}(T) of strongly underdoped Bi-2212 samples, Tc < 48 K.
