Yasutoshi Kotaka

Carrier doping and interlayer coupling in HTSC single crystals

Abstract Experimental results of the effect of carrier doping on the irreversibility lines in (La, Sr) 2 CuO 4−δ and Bi 2 Sr 2 CaCu 2 O 8+δ single crystals are summarized. As a function of Sr or oxygen contents, systematic and dramatic widening of the irreversible regions in the B − T phase diagram was observed in both systems. The present study suggests the critical importance of carrier concentration which directly affects the interlayer coupling strength and dimensionality of the flux line lattice in all the layered HTSC compounds as a universal feature.

Crossover from the first-order vortex phase transition to the peak effect in Bi2Sr2CaCu2Oy having different oxygen contents

Abstract 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.

Doping state and transport anisotropy in Bi2212 single crystals

Abstract 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.

(Pb, La)(Zr, Ti)O3 Film Grain-Boundary Conduction with SrRuO3 Top Electrodes

The relationship between Sr interdiffusion from a Pt/SrRuO3 [SRO] top electrode into (Pb, La)(Zr, Ti)O3 [PLZT] ferroelectric thin films with different excess Pb contents (2–10%) and leakage properties was studied. Secondary ion mass spectrometry and energy dispersive X-ray spectroscopy revealed significant Sr diffusion into the PLZT film after annealing, which increased with increasing excess Pb content. The diffused Sr existed in the PLZT crystalline grain boundaries, but not in the grains. The high leakage was attributed to a low resistance compound Sr–Pb–O in the PLZT grain boundaries. To obtain low leakage SRO/PLZT capacitors, it is necessary to eliminate the Sr–Pb–O grain boundary phase by lowering the excess Pb content to 7% or less.

Doping level dependence of magnetization anomalies and heat capacity of Bi2Sr2CaCu2O8 + δ in the mixed state

Abstract 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.

Phase transition in the mixed state of Bi2Sr2CaCu2Oy observed by local and macroscopic magnetometry

In Bi2Sr2CaCu2Oy under magnetic fields along the c-axis, both the local and the macroscopic magnetization show clear steps indicating the first order phase transition (FOPT) in the vortex lattice (VL). Local measurements have revealed that the FOPT occurs from the center of the sample owing to the field inhomogeneity originated from the geometrical effect. The heights of the steps are strongly suppressed in local measurements by the demagnetization effect. With decreasing temperature, the irreversibility in the magnetization just below the step begins to increase and is smoothly connected to the second peak at low temperatures.

High Precision Magnetization and Heat Capacity Measurements of Bi2Sr2CaCu2Oy Single Crystals in the Mixed State

We measured a bulk magnetization and heat capacity of Bi2Sr2CaCu2Oy single crystals with different oxygen stoichiometry in the mixed state. The samples show a magnetization step, indicating a first order transition in high temperature region. With lowering temperature, the step changes into some depression-and-kink structure, then vanishes when the second peak appears. The first order transition line is found to become steeper with increasing oxygen content. It is also found that the transition line seems to be terminated by the second peak field. We also measured heat capacity of the same samples. However, neither latent heat nor any other anomaly was detected, possibly because of the non-uniformity of the field distribution inside the plate-like samples.

Dimensionality Transition of the Vortex State in Bi2Sr2CaCu2O8+δ

The second peak effect in the magnetisation of Bi2Sr2CaCu2O8+δ has been investigated for both under-, near optimally-, and over-doped single crystals. The size and position of the peak depend strongly on the doping, and hence anisotropy, showing a large enhancement in the overdoped sample. For all states, though, the peak persists to high temperatures, with no evidence for an intersection of the curve describing peak position in the H-T plane with the irreversibility line. The temperature dependence can be described with recent theories of a 3D-2D transition in the vortex system, allowing derivation of values of the anisotropy γ. This interpretation is supported by μSR experiments on the same samples which indicate a reduction of vortex dimensionality at crossover fields matching the peak fields.

Magnetic and HRTEM studies of iodine intercalated Bi2Sr2CaCu2Oy

Abstract Magnetic irreversibility data is presented for iodine intercalated Bi 2 Sr 2 CaCu 2 O y single crystals. Iodine transfers charge both to the CuO 2 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, H irr , suggesting either that Bi 2 Sr 2 CaCu 2 O y is firmly in the 2-dimensional limit, or that any increase in the c-axis coupling is balanced by larger separation of the CuO 2 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 I 3 chains in the lattice.