Anomalous doping dependence of the fluctuation-induced diamagnetism in superconductors of YBCO family

SQUID magnetization measurements in oriented powders of Y1-xCaxBa2Cu3Oy from the underdoped to the overdoped regime are discussed in the light of an anomalous precursor fluctuating diamagnetism occurring above the bulk superconducting transition temperature. It is emphasized how both in the underdoped as well as in the overdoped YBCO compounds, the fluctuating magnetisation differs strongly from the predictions based on the Ginzburg-Landau anisotropic functional, at variance of what occurs in optimally doped YBCO. A theoretical description based on the extension of a recent theory for fluctuations of the order parameter in a layered systems of vortices, is briefly outlined. It is shown that such an approach, essentially assuming droplets with a local superconducting temperature above the bulk TC, accounts for most of the experimental findings, including irreversibility effects.