Observations of Vortex Chains in Bi2Sr2CaCu2O8+y

Vortex states in Bi₂Sr₂CaCu₂O_8+y under tilted fields were studied by means of magneto-optical imaging and Bitter decoration. One-dimensional arrangements of pancake vortices (vortex chains) interacting with underlying Josephson vortices are visualized. From the spacing of vortex chains we directly determined the variation of the anisotropy parameter of the electronic mass () from 800 in slightly underdoped to 180 in highly overdoped samples. These results indicate that the crossing-lattices model for highly anisotropic superconductors remains relevant at least for 180, contrary to what was predicted theoretically by Dodgson [Matthew J. W. Dodgson, Physica C 369, 182 (2002).].     

Comparative Studies of the Superconducting Properties of Bi2Sr2CaCu2O8+δ and Bi1.7Pb0.3Sr2CaCu2O8+δ/Ag Random Composites

We report the results of the study of the resistive transition and the screening effect on similar random composites made of a silver matrix containing sintered superconducting grains of bismuth phase with two different stoichiometries. We show that most of the observed behaviors are compatible with a phenomenological approach in which the intergrain junctions behave as weak links whose the state depends strongly on temperature and field. We find that these links a much weaker in one of the series of samples.     

Vortex Phase Diagram of Crossing-Lattices State in Bi2Sr2CaCu2O8+y

Phase diagram of a novel vortex state realized under tilted fields, crossing-lattices state, is extensively studied using Hall probe magnetometry. Precise measurements of angular dependence of second magnetization peak and the vortex penetration fields clarify the transition from the tilted lattice state to the crossing-lattices state. Under large tilted magnetic fields, irreversible magnetization and AC local permeability show several anomalies. With all these observations we construct a phase diagram identifying three boundaries with nearly constant c-axis fields and one with nearly constant in-plane field.     

Localization effects in transition-metal doped Bi2Sr2CaCu2O8+y high-temperature superconductors

Journal of Superconductivity and Novel Magnetism - Doping Bi2Sr2Ca1Cu2O8+y with Co causes a superconductor-insulator transition. We study correlations between changes in the electrical resistivity...     

Transport properties of textured Bi2Sr2CaCu2O8+y,thin films

Superconducting films of the high-T _c compound Bi₂Sr₂CaCu₂O_8+y , have been grown on (111)-oriented gadolinium gallium garnet substrates by a liquid-phase technique. The films show a very high degree of preferential orientation with thec-axis perpendicular to the substrates. The onset of the resistive transition was 85 K while zero resistance was obtained at 78 K. Results concerning the critical current properties of the films are described. Measurements of the paraconductivity effects on the electrical resistivity above the superconducting transition due to thermodynamic fluctuations are also reported.     

The Pseudogap and the Superconducting Order Parameter in Inhomogeneous Bi2Sr2CaCu2O8+δ

Recent experiments on Bi₂Sr₂CaCu₂O_8+δ (Bi2212) have provided compelling evidence that the samples are inhomogeneous on a nanoscale, with spatially separated superconducting gap (SG) and nonsuperconducting, pseudogap (PG) regions that arise from hole–hole and particle–hole pairings, respectively. We compare and contrast the nonphase sensitive experiments that cannot distinguish these orderings from the phase-sensitive experiments that can. Although the wave vector dependence of the pseudogap is highly anisotropic, we conclude that in Bi2212, the c-axis quasiparticle tunneling is incoherent and that the c-axis supercurrent arises solely from a rather isotropic s-wave superconducting order parameter (OP) for T < T_c.     

Vortex coupling in Bi2Sr2CaCu2O8+y,using controlled sample geometry

The flux transformer geometry has been useful for studying vortex coupling in Bi₂Sr₂CaCu₂O_8+y crystals but is limited by the difficulty of controlling sample size. We have used an argon ion mill to etch two channels into the top face of a Bi₂Sr₂CaCu₂O_8+y crystal to reproduce the effects of the flux transformer geometry with more precise control over effective sample dimensions. The temperature dependence of the transformer voltage in such a crystal shows a pronounced peak between 87 K and 89 K which is linear with current. Application of a magnetic field (H c-axis) sharply suppresses the peak, broadens it, and shifts it to lower temperatures. The peak in the transformer voltage and its current and field dependence are consistent with interlayer Josephson coupling of thermally excited vortices.     

Superconductivity of barium-doped Bi2Sr2CaCu2O y

Sintered ceramic samples of Bi₂Sr_2?x Ba _x CaCu₂O_y with nominal barium fraction 0≤x≤0.3 have been prepared by the solid-state reaction method. WDS studies verified that barium enters the superconducting phase. For slowly cooled samples, the midpointT _c of the superconducting transition is significantly increased by barium doping, whereas for quenched samplesT _c is little affected. The increase ofT _c with increasing barium fraction is consistent with a decrease in the hole concentration in the superconducting layers.     

Needle-like microstructure in Bi2Sr2CaCu2O8+x

We have obtained superconducting Bi₂Sr₂CaCu₂O_8+x ceramic compounds with a grain structure that consists of needle-like particles of about 1.5 mm in length and 100 μm in diameter, which seem to be formed by flake-like small grains. The Bi(2212) compound has been prepared following the polymer solution synthesis method. The thermal treatment used is a simplified version of the melt-textured growth process (MTG), with the sample placed in a zone of a simple tube furnace where the temperature gradient was about 15°C/cm. We have performed scanning electron microscopy (SEM) observations on the samples, before and after thermal treatment. In addition, we have measured the ac susceptibility of both samples to confirm their superconducting nature and critical temperatures of about 91 K.     

Superconductivity in Na-doped Bi2Sr2CaCu2O y system

Samples with the nominal composition Bi₂Sr₂Ca_1−x Na _x Cu₂O _y (x=0, 0·1, 0·2 and 0·3) were prepared by solid-state reaction of the individual compounds. X-ray diffraction patterns indicate that the samples have a majority 2212 phase with 2223 also being present. From the DC four-probe resistance data, we have observed that the furnace-cooled samples show metallic behaviour while the quenched samples show superconductivity up to 97 K.     

Incommensurate fluctuations in Bi2Sr2CaCu2O8

A special neutron scattering technique has been used to discover an incommensurate fluctuation in Bi₂Sr₂CaCu₂O₈ that appears below T_c. The fluctuation is identified as a dynamic charge density wave since its scattering intensity appears to increase with increasing momentum transfer. The fluctuation is found at a wavevector near 2k_F and could be associated with a dynamic stripe phase.     

Studies of Both Intralayer and Interlayer Electrodynamics of Bi2Sr2CaCu2O y in the Superconducting State

We studied the intralayer and interlayer electrodynamics of the superconducting Bi ₂ Sr ₂ CaCu ₂ O _y single crystals in the microwave and millimeter wave regions. The c-axis superfluid density fraction _s ^c shows a more rapid growth than _s ^ab just below T _c and a weaker temperature dependence at low temperatures. These features suggest the anisotropic growth mechanism of _s . The temperature dependence of _ab ¹ shows a large increase below T _c with a broad peak around 20 K. However, to discuss the detailed behavior of ₁ ^c or the anisotropy of conductivity _ab ¹ / _c ¹ in the superconducting state, much more careful measurement is required in the c-axis direction.     

Characterization of epitaxial Bi2Sr2CaCu2O8+δ thin films

We report a dc sputtering method for the fullin situ preparation of Bi₂Sr₂CaCu₂O_8+δ thin films on SrTiO₃ and LaAlO₃.T _c values of more than 90 K can be achieved by oxidizing annealing below the melting point, followed by a reducing anneal at 500°C. The structural properties of the films are revealed by X-ray diffraction in Bragg-Brentano geometry (strongc-axis orientation with FWHM (0 0 10)=0.3) and also byΦ scans (epitaxy within the substrate plane). Rutherford backscattering and channeling confirmed the correct composition of the cations while the minimum yield,χ _min, is 23%. Depth profiles by SNMS show a very homogeneous distribution of the cations with no detectable loss of bismuth near the surface. The surface morphology of the films was studied by SEM and by STM. Patterning of the films in lateral geometry can be performed by photolithographic techniques without degradation ofT _c .     

Oxygen stoichiometry and properties of Pb-substituted Bi2Sr2CaCu2O8 + δ

The effect of partial Pb substitution for Bi on the oxygen stoichiometry and superconducting properties of Bi₂Sr₂CaCu₂O_8+δ was studied. The Pb content and heat-treatment conditions were optimized to attain the highest critical parameters. The observed changes in the superconducting properties were shown not to correlate with oxygen stoichiometry. Superconducting properties were found to persist in the absence of Cu in the formal oxidation state 3+. A reduction in the concentration of excess oxygen to about the lowest possible level is accompanied by a significant increase inT _c ^{R = 0} (96 K) and critical current density. The possible mechanisms underlying the effect of Pb substitution on the properties of 2212 are discussed.     

Magnetic field dependence of the Josephson coupling energy along the c-axis in Bi2Sr2CaCu2O8+y

The magnetic field dependent c-axis dissipation in Bi₂Sr₂CaCu₂O_8+y single crystals is shown to agree with that of a series stack of Josephson tunnel junctions. We extract the Josephson coupling energy E_j from the measured the c-axis resistance as a function of the angle between the c-axis and the magnetic field B using an extended Ambegaokar-Halperin model. For B > 0.5T, E_j increases continuously with both decreasing temperature and decreasing magnetic field. At T = 78K, E_j exhibits a B^–0.5 behavior.     

Thermomagnetic Flux-Jump Instabilities and Second Magnetization Peak in Bi2Sr2CaCu2O8 High-T c Superconducting Crystals

Measurements of the magnetization hysteresis loop performed on Bi ₂ Sr ₂ CaCu ₂ O ₈ high-T_c superconducting single crystals reveal the occurrence of pronounced jumps of the irreversible magnetization at T > 40 K. In this work we demonstrate their thermomagnetic origin. It is also shown that the low-field hollow in the magnetization hysteresis loops measured at T < 40 K and leading to the so-called second magnetization peak, is the manifestation of a thermomagnetic instability effect. It is suggested that the plastic vortex motion triggers the magnetic instabilities.