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.     

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.     

Electronic Structures of Two-Phase Microstructures in Pb-doped Bi2Sr2CaCu2O y

We have performed scanning tunneling microscopy/spectroscopy (STM/STS) on single crystals of heavily Pb-doped Bi ₂ Sr ₂ CaCu ₂ O _y (Pb-Bi2212). The obtained STM images clearly demonstrated phase separation into the and phases with lamella-type and new zigzag-like microstructures. From the site-specified tunneling measurements, it was directly confirmed that the phase possesses higher T _c . This suggests that the phase boundaries act as strong pinning sites especially at higher temperatures.     

Dynamic Phase Diagram of Driven Vortices in Bi2Sr2CaCu2O y

The coupled study of noise spectra and ac-dc interference effects was performed to clarify the evolution of the driven vortex state of Bi₂Sr₂CaCu₂O _y single crystals. Broadband noise and interference effects were observed in different regions of the H-T-J (H is magnetic field, T is temperature, and J is driving current density) space, which was closely connected with the change of the dynamic phase of the vortices. The obtained H-T-J phase diagram of driven vortices was found to be rather different from those in conventional superconductors, and those expected by numerical simulations.     

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).].     

Preparation and Properties of Bi2Sr2CaCu2O y Coatings on Sr–Ca–Cu–O Substrates

Bi-2212 coatings on Sr_{1 – x} Ca _x CuO₂ (x = 0, 0.5) and Sr_{14 – x} Ca _x Cu₂₄O₄₁ (x = 0, 5) substrates were prepared by partial melting at 890 and 950°C. The effects of the partial-melting temperature and the temperature and duration of subsequent heat treatment in air on the phase composition of the coatings and substrates and the electrical resistance and superconducting transition temperature of Bi-2212 were studied. After partial melting and heat treatment, the T _c(R = 0) of Bi-2212 attained 85–86 K. Sr_{1 – x} Ca _x CuO₂ and Sr_{14 – x} Ca _x Cu₂₄O₄₁ substrates caused no degradation of the superconducting properties of Bi-2212, in contrast to Al₂O₃, and ensured a higher thermal shock resistance of Bi-2212.     

Pinning potential transport measurements of Bi2Sr2CaCu2O x thin films

Flux creep characteristics ofc-axis-oriented Bi₂Sr₂CaCu₂O _x thin films grown by the method of electron beam evaporation of stoichiometric target and subsequent annealing have been investigated in the absence of external magnetic field. A decrease of the pinning potential with decreasing temperature is obtained fromI–V curves. This result is explained by the presence of the spatially nonlinear pinning potential.     

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.     

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...     

Effects of First-Order Vortex Phase Transition on the Electronic States of Bi2Sr2CaCu2O y

We measured the complex surface impedance Z _s = R _s +iX _s of optimally- and over-doped Bi ₂ Sr ₂ CaCu ₂ O _y single crystals around the vortex-phase boundaries. In both doping states, the surface reactance X _s increased rapidly just above the first-order melting transition field and the so-called second magnetization peak field, while no obvious anomalies were observed in the surface resistance R _s . From the analysis of the relationship between X _s and R _s , we showed that the origin of this anomaly is not the loss of pinning but the reduction of the superfluid density above the transition.     

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.     

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.     

Site Sensitive Measurement of Local Fluctuation of Driven Vortex Density in Bi2Sr2CaCu2O y

Noise in the local vortex density was investigated in a site-sensitive manner, which means we can measure the cross-correlation function, together with the auto-correlation function, in the directions both parallel and perpendicular to the flow direction. We found that plastic flow was realized just when the vortices started moving, whereas it changes in to the coherent flow with further increasing driving current. The total features of our results, including the variation as a function of temperature and dependence of the cross-correlation on the direction relative to the flow direction, agree with a recent computer simulation by Olson et al. very well. We also measured the conduction noise in the same material, which enables us to discuss both the density-density correlation and velocity-velocity correlation in a comparative way.     

Far-infrared spectra of Bi2Sr2CaCu2O8 single crystal

The far-infrared spectra of a Bi₂Sr₂CaCu₂O₈ single crystal at 2.7 and 298 K is reported for the spectral range of 30–350 cm^?1. Several characteristic bands of Cooper pairs are found at 2.7 K in the far infrared region, and accompanied by low-energy side bands.