Comparison of 2-D Quantum and Thermal Critical Fluctuations of Underdoped BiSrCaCuO with Ultrathin YBaCuO

This paper addresses two issues. Underdoped Bi-2212 has been shown to exhibit two-dimensional quantum fluctuations, with vs. (0) scaling qualitatively similar to ultrathin YBCO films that are 2-D by construction. The present paper shows that the quantitative difference in scaling is resolved by growing Bi-2212 films that are much smoother than previously used. Also, the absence of 2-D thermal critical behavior from the superfluid density of Bi-2212 films that exhibit 2-D quantum scaling is not likely due to an inhomogeneous smearing out the 2-D thermal transition. Finally, we emphasize that there is as yet no consensus on the physics that drives the -dependence of at low to intermediate temperatures.