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The effects of oxygen content on the anisotropic resistivity and Cu 2+ localized spin moments have been studied for the Bi 2 Sr 2 CaCu 2 O 8 + x single crystals. It was found that the in-plane and out-of-plane resistivities ( ϱ ab and ϱ c ), as well as the electron spin resonance (ESR) signals decrease with increasing oxygen contents from underdoping to overdoping states. We believe that the in-plane resistivity upturn in an underdoped region is closely related to the carrier localization, and the scattering of spin fluctuations plays an important role in the charge transport properties. On the origin of c-axis resistivity, we should take into account the scattering in the `barrier' layer between CuO 2 ‘cells’ and the scattering or fluctuations in the planes.

Magnetization M and heat capacity C were measured around the first order vortex transition lines of two Bi 2 Sr 2 CaCu 2 O 8 + δ single crystals having different oxygen contents. The entropy jump per vortex per layer, Δs , at the transition was estimated from the step in M . The oxygen-content dependence of Δs was contradictory to the simple vortex lattice melting theory based on the Lindemann criterion. In both crystals, no anomalies in C were observed at the transition owing to both the small Δs and the broadening of the transition.

We have measured the magnetization and the heat capacity of over-doped (OVD) and optimally doped (OPD) Bi 2 Sr 2 CaCu 2 O y single crystals in the mixed state under magnetic fields applied along the c -axis. In both samples, a distinct magnetization step is observed in the magnetization curve at high temperatures and is found to change to a kink at low temperatures. At still lower temperatures, the peak effect dominates the magnetization curve and the kink vanishes. The origin of the step and the kink in the magnetization curve is considered to be the first-order phase transition in a quasi-three-dimensional vortex system although the heat capacity shows little anomalies at the transition because the latent heat is small. The first-order phase transition line of each sample is drawn on the H - T plane and it is found that the slope of the line of the OVD sample is steeper than that of the OPD sample. The entropy jump at the transition of the OVD sample is estimated to be larger than that of the OPD one at all temperatures and magnetic fields studied. This behavior is contradictory to the simple vortex-lattice melting picture.

Magnetic irreversibility data is presented for iodine intercalated Bi2Sr2CaCu2Oy single crystals. Iodine transfers charge both to the CuO2 planes and the BiO bilayers. The possibility of enhanced coupling along the c-axis through increased conductivity in the rocksalt block is not borne out in the irreversibility fields, Hirr, suggesting either that Bi2Sr2CaCu2Oy is firmly in the 2-dimensional limit, or that any increase in the c-axis coupling is balanced by larger separation of the CuO2 planes. Magnetic relaxation data is also discussed. Electron microscopy reveals two forms of in-plane defect associated with iodine intercalation, one resulting from faults in realignment of the BiO layers from an a/2 staggered sequence to a registered sequence, and the other related to the packing of I3 chains in the lattice.

Electrical resistivity was studied parallel to the ab -plane as well as along the c -axis of Bi2212 single crystals having both over-doped and under-doped states. The anisotropy within the ab -plane (perpendicular and parallel to the modulated b -axis directions) have been also systematically evaluated. The temperature dependence of ϱ a , ϱ b and ϱ c was strongly dependent on the oxygen composition and the anisotropy factors ϱ c / ϱ a and ϱ c / ϱ b showed a similar dependence ; the more carrier doped crystals showed the smaller anisotropies in each case, while ϱ a / ϱ b showed an opposite dependence.