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The equilibrium magnetization of grain-aligned HgBa 2 Ca 4 Cu 5 O 12+y ͑Hg1245͒ with T c Ӎ 108 K was measured. The Ginzburg-Landau parameter ͑͒, obtained by using the Hao-Clem model, increased unphysically with increasing temperature for T / T c ജ 0.7, which indicates that the fluctuation is greatly enhanced. The two-dimensional scaling behavior of the magnetization and the temperature variation of the irreversibility field H irr ͑T͒ from a power-law behavior at low fields to an exponential behavior at high fields indicate that Hg1245 is highly anisotropic, which seems to be affected by the appearance of antiferromagnetism at the inner CuO 2 planes while blocking the coupling of the superconductivity between the outer CuO 2 layers.

To understand the effect of carbon doping on the superconductivity in MgB(2), we obtained the angle- and temperature-dependent upper critical fields [H(c2)(θ) and H(c2)(T)] for Mg(B(1-x)C(x))(2) single crystals (x = 0.06 and 0.1) from resistivity measurements while varying the temperature, the field, and the direction of the field. The detailed values of the diffusivity for two different directions for each σ-band and π-band were obtained to explain both the temperature- and the angle-dependent H(c2) by using the dirty-limit two-gap model. The induced impurity scattering of the σ-band and the π-band for both the ab-plane and the c-direction is studied.

We report the reversible magnetization of the Ca 2−x Na x CuO 2 Cl 2 ͑x = 0.18͒ single crystal for H ʈ c up to 1.2 T to study the effect of oxygen replacement by chlorine at the apical site. The superconducting transition temperature T c ͑ϳ27 K͒ of this single crystal is the highest reported in this family. Various thermodynamic parameters, such as the penetration depth ab ͑0͒, the coherence length ab ͑0͒, the critical field H c ͑0͒, the upper critical field H c2 ͑0͒ and the Ginzburg-Landau parameter , are also reported. Compared to the iso-structural compound La 1.82 Sr 0.18 CuO 4 with apical oxygen, Ca 1.82 Na 0.18 CuO 2 Cl 2 has two significant differences: ͑1͒ ab ͑0͒ is larger, so the Cooper pair density is quite reduced, and ͑2͒ the anisotropic nature is more enhanced. From these observations, chlorine at the apical site is thought to be less effective than oxygen in supplying charge carriers to the CuO 2 planes; as a result, the interlayer coupling between the CuO 2 planes is weakened.

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