S.H. Han

The effect of annealing on the superconducting properties and structure of Bi-2212 single crystals

Abstract Two types of Bi-2212 single crystals, one type having a low T c and the other having a high T c were annealed in air at different temperatures over the range 450°C to 750°C followed by furnace cooling. The transition temperature of the crystals with low T c (∼ 56 K) increased with increase of annealing temperature, while the crystals with high T c (∼ 91 K) had just the opposite behaviour under the same annealing conditions. It is indicated that the quality of as-grown crystal has a significant effect on the superconducting properties of the annealed crystal. The c -axis values for both types of crystals decreased with increase of annealing temperature. According to the relationship between c -axis values and the oxygen content, our results confirmed the existence of a dome-shaped curve of T c with carrier concentration in Bi-2212 single crystal similar to that in Bi-2212 compound reported previously by Groen and de Leeuw [Physica C 159 (1989) 417]. By studying the increase of annealing time at a Fixed temperature of 650°C, T c and ΔT c were found to change very little for both types of crystals. Thus, T c and ΔT c were more sensitive to annealing temperature than to annealing time. XRD, SEM and EDS revealed that a small quantity of powder phase appeared after annealing at temperatures above 500°C for both types of crystals. This powder phase had the Bi-2212 composition, which indicated that the integrity of the Bi-2212 single crystals was significantly degraded by the annealing process.

Asymptotic power spectrum analysis of electrical noise in the vortex state of Br2Sr2CaCu2O8+x single crystals

Abstract The voltage noise of a Bi 2 Sr 2 CaCu 2 O 8+ x single crystal below T c is analyzed with an asymptotic power spectrum method. Low-frequency noise power S v ( T ) shows a peak at T = T p . Above T p , the power spectrum, S v ( f ), exhibits a power-law fall-off. Below T p , S v () dramatically deviates from the power-law behaviour, showing some wide peaks which are proposed upon a background of an exponential-law characteristic. This is qualitatively consistent with numerical simulation. It is suggested that the pinned vortex state is a deterministic motion with low-dimensional chaotic attractors while the unpinned state is an extended dissipative dynamic system with self-organized criticality.

Vortex-glass-liquid transition in Bi2Sr2CaCu2O8Bi2Sr2Ca2Cu3O10 intergrowth single crystals

Abstract The vortex-glass-liquid transition in Biue5f8Srue5f8Caue5f8Cuue5f8O (BSCCO) intergrowth single crystals in which some CuO2 trilayers are inserted into the Bi2Sr2CaCu2O8 matrix has been studied. The curvature of the log V-logI curves for CuO2 trilayers is found to change from negative to positive with increasing magnetic field and a scaling behaviour for the vortex-glass state is observed. The magnetic field dependence of the vortex-glass transition temperature, TG, qualitatively obeys the relation H(TG) ∼ (Tc − Tg)2v0, with v0 = 1.15 ± 0.2, which is larger than the 2 3 predicted theoretically for an isotropic 3D system. Our results reveal that the vortex-glass transition exists in a system constructed with CuO2 trilayers.

In-Plane Paraconductivity of Bi2Sr2CaCu2O8 Single Crystals with Different Resistivity Anisotropies

Superconducting fluctuations have been studied by systematic measurements of the in-plane paraconductivity in Bi 2 Sr 2 CaCu 2 O 8 single crystals with different resistivity anisotropies. It has been observed that the interlayer coupling strength decreases dramatically with the anisotropy of the sample, which significantly influences the dimensional behaviour of the in-plane paraconductivity. For all the samples studied, the in-plane paraconductivity is in good agreement with the Lawrence and Doniach model. For the sample of highest resistivity anisotropy, the behaviour of the in-plane paraconductivity is basically two-dimensional, but deviates significantly from two-dimensional behaviour as the resistivity anisotropy decreases.

Annealing experiment on high- superconductor Bi-2212 single crystals

Two different types of as-grown (Bi-2212) single crystals grown by a directional solidification and floating zone methods have been annealed in air and 1 atm of flowing oxygen at for 72 h followed by different cooling processes. Experimental results show that there are no change in composition or phase segregation caused by the annealing. Annealing of the single crystals grown by the directional solidification method increases the superconducting transition temperature from 70 K (as grown) to 83 K (annealed), with the superconducting transition width (5-95% of complete superconducting transition) decreasing from 36 K to 12 K. Annealing of the single crystals grown by the floating zone method depresses from 91 K (as grown) to 83 K (annealed), with increasing from 4 K to 7 K. Further, the annealing of the as-grown crystals results in increase of the superconducting volume fraction. The results show that oxygen content and distribution in the crystals play a key role in the superconductivity of these crystals. The quality of the as-grown crystals also affects the results of the annealing experiments.

Resistance anomaly in the superconducting fluctuation region in Bi2Sr2CaCu2O8 single crystals

Abstract A resistivity anomaly near the superconducting transition temperature is observed in the ab -plane of as-grown Bi 2 Sr 2 CaCu 2 O 8 (Bi-2 2 1 2) single crystals. The resistivity at the peak is much larger than the normal state resistivity around 100 K. The peak vanished gradually with increasing the applied magnetic field. Iue5f8V curves of the Bi-2 2 1 2 samples also exhibit a peak close to the transition steps, vanishing gradually with increasing intrinsic granularity of high- T c superconductors.

Correlative motion of vortices near the vortex lattice melting point in single crystals

Voltage noise of the single crystal below is analysed by an asymptotic power spectrum method. The low-frequency noise power, , shows a peak at . This peak is found to be closely related to the vortex lattice melting temperature. Near the peak the power spectrum, , exhibits a power-law fall-off, while at much lower temperatures dramatically deviates from the power-law behaviour, showing some broad peaks which are superposed upon a background with an exponential-law characteristic. The estimated correlation dimension of the noise data reveals that the vortex motion is highly correlated at the early stage of melting, then develops into an extended dissipative dynamic system with self-organized criticality, showing a behaviour. The correlated motion disappears when the lattice completely melts.

Flux pinning in melt-textured YBa2Cu3O7−y with different sizes of secondary phase particles

Abstract Flux pinning force and irreversible line of the melt-textured-growth (MTG) YBa2Cu3O7−y (Y-123) crystals with various sizes of secondary phase particles have been investigated by measuring the magnetization of the samples under a series of magnetic fields up to 55 kOe. The magnetic Jc and the pinning force are found to increase as the particle size decreases. A linear relationship between Jc and 1/D has been obtained where D is the average size of the precipitates. The highest Jc value has reached 4×104 A/cm2 at 77 K and 40 kOe in a sample with a high density of the precipitates in nanometre size. Two peaks are observed in the Fp(H) curve in which the position of the first peak is independent of the particle size. The height and the location of the second peak is sensitive to the particle size, indicating that the peak is associated with the small particles.

Vortex lattice melting and dimensionality crossover in Bi2Sr2CaCu2O8+x

Abstract Differential resistivity, I–V characteristics, and voltage noise have been measured to study the vortex dynamics in Bi 2 Sr 2 CaCu 2 O 8+x single crystals. The sample shows a crossover from three to two dimensional behaviour as the magnetic field is higher than a crossover field of 365 Oe. Below the crossover field, the sample show a vortex lattice melting in pinned vortex state as well as in the flux flow state. During the melting, a remarkable voltage noise was observed. The I–V curve measurements reveal that the irreversibility line does not overlap with the melting ling in H-T plane. The separation between these two lines is greatly enhanced when the applied field is over the crossover field.

Vortex-glass superconductivity of CuO2 trilayers in BSCCO single crystals

Abstract Superconductivity of BSCCO single crystals in which some CuO 2 trilayers are inserted in the Bi 2 Sr 2 CaCu 2 O 8 matrix is studied. The curvatures of the I-V curves for CuO 2 trilayers change from negative to positive with increasing field, and the scaling behaviour of a vortex-glass state is observed. I c is found to obey the relation I c = I c0 (1 − T / T c ) α with α = 1.2. Our results reveal that the vortex-glass transition exists in the system constructed with CuO 2 trilayers.