Superconducting diamagnetic fluctuations in Sm-based underdoped cuprates studied via SQUID magnetometry

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 under-doped, pseudogapped phase in regards of the superconducting fluctuations (SF) above T-c. 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 M-dia above T-c show an upturn field H-up, where vertical bar M-dia vertical bar starts to decrease on increasing field. H-up is found to increase on increasing temperature. The experimental data on the field dependence of the diamagnetic magnetization above T-c 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.