Quasi-two-dimensional fluctuations in the magnetization of L a1.9 C a1.1 C u2 O6+δ superconductors

We report the results of magnetization measurements with the magnetic field applied along the c axis on superconducting La1.9Ca1.1Cu2O6+δ single crystals processed under ultrahigh oxygen pressure. Strong fluctuation effects were found in both low- and high-field regimes. Scaling analysis of the high-field magnetization data near the critical temperature (Tc=53.5K) region reveals the characteristics of critical fluctuation behavior of quasi-two-dimensional (2D) superconductivity, described by Ginzburg-Landau theory using the lowest Landau level approximation. Low-field magnetic susceptibility data can be successfully explained by the Lawrence-Doniach model for a quasi-2D superconductor, from which we obtained the ab plane Ginzburg-Landau coherence length of this system, ξab(0)=11.8±0.9Å. The coherence length along the c axis, ξc(0), is estimated to be about 1.65 Å, which is in between those of 2D cuprate systems, such as Bi2Sr2Ca2Cu3O10 and Bi2Sr2CaCu2O8, and quasi-three-dimensional (3D) cuprate systems, such as overdoped La2-xSrxCuO4 and YBa2Cu3O7-δ. Our studies suggest a strong interplay among the fluctuation effects, dimensionalities, and the ratios of the interlayer Cu-O plane spacing, s, to the c-axis coherence lengths. A high s/ξc(0) was observed in the high-pressure oxygenated La1.9Ca1.1Cu2O6+δ, and that apparently drives this system to behave more like a quasi-2D superconductor.