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High Tc superconductors (HTSCs) for the application of magnets and cables are usually used in a magnetic field parallel to the superconducting layers, using the intrinsic pinning of vortices in the layered structures. However, vortex states in the parallel field have not been well understood in strongly anisotropic superconductors. We have studied the vortex states by measuring the flow resistivity of Josephson vortices (JVs). From the measurements, we can determine the 3D-ordered phase of JVs, which means that JVs are well ordered and are considered not to influence the dissipation of the superconducting state. However, by the R-T measurements, we have found the excitation of pancake vortex/anti-vortex pairs to reduce the flow resistance. This limits the application of HTSC materials in the parallel magnetic fields.

Large thermal fluctuation, owing to high superconducting transition temperature, short coherence length and quasi-two-dimensionality, brings about a rich variety of vortex phases in Bi2Sr2CaCu2O8+y (Bi2212). To study how the vortex states and the transitions would be modified when vortices are confined in a small-sized crystal, we have measured the c-axis resistance using a stack of the intrinsic Josephson junctions (IJJs) of Bi2212. In tiny Bi2212 crystals of several micron, it is possible to observe the series of vorticity transitions using the c-axis resistance measurements. Combining the observation of vorticity changes and detection of a melting transition, we found an oscillating behavior of melting transition temperatures T m as a function of magnetic field (or number of vortices) in small squared Bi2212 with a lateral dimension of 5-10 μm. In the case of the square-shaped crystals, it seems that Tm is enhanced around the vortex numbers N v of i 2 (i: integer), indicating a matching of square vortex lattices in the square boundary. However, the frustration between the square boundary shape and vortex lattice that prefers a triangular lattice complicates the situation. A deformed square lattice without topological defects is probably realized at large i 2 as a geometrical matching state.

FeSr2YCu2O6+δ exhibits superconductivity around 50 K, only when it is annealed in N2 and subsequently oxidized. While N2-annealing prevents disorder of Cu and Fe, oxidizing supplies the carrier to the superconducting CuO2 sheets. Although this compound has the similar structure to Ba2YCu3O6+δ, the substitution effect is different from Ba2YCu3O6+δ. The substitution of lanthanoid with larger ionic radius for Y causes the disorder of Cu and Fe, and the introduction of interstitial oxygen. Therefore, since the CuO2 sheets becomes disordered, Tc decreases. Meanwhile, the Co-substituted compound has the order of Cu and Fe before the N2-annealing. However, it does not exhibit superconductivity by the oxidizing because the oxidizing is not effective on the supply of oxygen.

FeSr2YCu2O6+? compounds with various oxygen contents, 6 + ? = 7.02 ~ 7.55 and FeSr2Y(Cu0.95Zn0.05)2O7.55 compound were synthesized and studied by the magnetization measurement. FeSr2YCu2O7.55 compound shows superconductivity below TC = 56 K and a broad peak at 20 K, which corresponds to the magnetic ordering of Fe ion, in the magnetization measurement. The Curie constants and the asymptotic Curie temperature strongly depend on the oxygen content. Increasing the oxygen contents, the Curie constants and the absolute values of the asymptotic Curie temperature are increased. Since the asymptotic Curie temperature is negative value, the interaction between Fe ions is mainly antiferromagnetic. A hysteresis is observed in the magnetization curve of non-superconducting FeSr2Y(Cu0.95Zn0.05)2O7.55 compound. It implies that the magnetism of Fe ion is not simple antiferromagnetism.

We have performed scanning tunneling spectroscopy (STS) measurements on slightly over doped Bi2Sr2Ca(Cu1-xZnx)2O8+?(x = 0.02) to investigate spatial correlation between the impurity stats and the local gap value. In addition to several impurity states, the spatial variation of energy gap has been observed. The gap values are spatially distributed from 20 meV to 55 meV with the length scale of about 5 nm. The impurity states appear only in the regions whose gap values are less than 50 meV. Furthermore, the number of Zn impurity states estimated from STS results is slightly less than that of doped Zn atoms estimated by inductively-coupled-plasma-mass-spectroscopy. Our results insist that there exist the impurity sites without the resonant states and those impurities are located on the region with the gap value of more than 50 meV.

We have studied the dynamical motion of Josephson vortices (JVs) in the Josephson junctions, using high Tc superconductor (HTSC) Bi2Sr2CaCu2O8+y, which have intrinsically stacked Josephson junctions (IJJs), As we have observed periodical oscillations in the flow resistance of JVs against parallel magnetic field to the superconducting layers, we have introduced pancake vortices to control the motion of JVs, which have a role for pinning of JVs. Applying a time-asymmetric input current with biharmonics, we have observed a reversible "ratchet" effect with changing the phase and the frequency difference in the biharmonics. The output signal is much larger than that using the single JJ. Utilizing a multiple-function of IJJs, new types of the junctions will open the new functional devices of HTSCs.

Using high Tc superconductor (HTSC) Ei2 Sr2 CaCu2 O8+y, which have intrinsically stacked Josephson junctions (IJJs), we observed periodical oscillations in the flow resistance of JVs against parallel magnetic field to the superconducting layers. We have introduced pancake vortices to control the dynamical motion of JVs, which have a role for pinning of JVs. Applying a time-asymmetric input current with biharmonics, we have observed a reversible rectification effect with changing the phase and the frequency difference in the biharmonics. The output signal is much larger than that using the single JJ. Utilizing a multiple-function of IJJs, new types of the junctions will make the new functional devices of HTSCs possible.