High-resolution superconducting quantum interference device isothermal magnetization measurements in Al-doped Sm-based superconducting cuprates are presented, at the aim to analyze the properties of the underdoped, pseudogapped phase in regards of the superconducting fluctuations (SF) above Tc. In the optimally doped compound, the SF are well described by the conventional Ginzburg-Landau (GL) free-energy functional for three-dimensional anisotropic systems. On the contrary, in the underdoped compounds, obtained by Al for in-chain Cu substitution at constant oxygen content, dramatic differences are detected. The isothermal curves Mdia above Tc show an upturn field Hup, where |Mdia| starts to decrease on increasing field. Hup is found to increase on increasing temperature. The experimental data on the field dependence of the diamagnetic magnetization above Tc can be justified by transforming the GL-Lawrence-Doniach functional into the one for a layered system of vortices with frozen amplitude of the order parameter but with strong phase fluctuations. It is argued that this behavior is characteristic of the underdoped phase of the cuprates, thus providing insights on the pseudogapped phase as accompanied by fluctuations in the phase of order parameter.
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