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 .     

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).].     

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.     

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...     

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.     

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.     

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.     

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.     

Preparation and properties of Tl2Ba2CaCu2O8 thin films

Anex situ process has been developed to produce thin superconducting Tl₂Ba₂CaCu₂O₈ films. The properties of films grown on different substrates using different annealing regimes were studied. Critical temperatures of 103–107 K were measured on films prepared in a broad range of annealing temperatures on SrTiO₃, LaAlO₃, and Y-ZrO₂ substrates. A critical current density,J _c, of 2×10⁶ A/cm² at 77 K was measured on LaAlO₃. Film morphology was studied by SEM, AFM, and STM.     

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.     

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.     

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.     

HREM surface profile images of Bi2Sr2CaCu2O8

Surface profile images of Bi₂Sr₂CaCu₂O₈ have been obtained using high-resolution electron microscopy. The cleaved (001) surface of the crystals terminates with a single Bi-O atomic layer. The modulated structure developed in this surface atomic layer was observed directly. The (hk0) surfaces were found to decompose in air into an amorphous coating layer. This coating layer was unlikely recrystallized into the original structure under electron beam irradiation. The amorphous layer on the (hk0) surface formed in pure Ar atmosphere was relatively thin and could be recrystallized into some secondary phases in which a Bi loss was observed. The original (001) surface might also be covered by an amorphous-like layer. This disordered layer could be recrystallized under electron beam irradiation into BiSr₂Ca₂Cu₃O₉, BiSr₂CuO₅, etc. which intergrow with the parent crystal perfectly on the (001) planes.     

Magnetic-field-induced non-linear effects of Josephson coupled superconductor Bi2Sr2CaCu2O8+δ

The dc current-voltage (I-V) relation along the c-axis of single crystalline Bi₂Sr₂CaCu₂O_8+δ has been measured in magnetic fields parallel and perpendicular to the c-axis. In zero field a clear and sharp jump with large hysteresis in theI-V curve was observed, indicative of the dc-Josephson effect. In magnetic field below a characteristic fieldB ^*≈0.4 T (at T=0 K) parallel to the c-axis the magnetic field suppresses the hysteresis and reduces the critical currentI _c drastically, whereas aboveB ^* theI-V curve becomes broad and featureless behavior. The characteristic field scaleB ^* can be interpreted as an energy scale of the Josephson coupling between superconducting layers in Bi₂Sr₂CaCu₂O_8+δ and is argued with emphasis on the correlation length of pancake vortices in this system.     

紫外激光脉冲沉积法制备高温超导（Bi,Pb）2Sr2CaCu2O8薄膜的研究

用紫外脉冲激光沉积技术在单晶衬底上制备高温超导(Bi,Pb)2Sr2CaCu2O8薄膜,系统研究了不同镀膜参数对薄膜结晶质量的影响,获得了优化的镀膜工艺。并在较低温沉积、高温退火,以及高温沉积两种工艺下均获得了具有c轴取向、无杂相及无挥发(分解)的(Bi,Pb)2Sr2CaCu2O8薄膜。相比于高温沉积,低温沉积、高温退火制备的薄膜结晶质量更好。     

Enhancement of Interlayer Phase Coherence in the Vortex Liquid State of Bi2Sr2CaCu2O8+y with Columnar Defects

We studied the phase coherence in the vortex liquid state of Bi ₂ Sr ₂ CaCu ₂ O _8+y with columnar defects (CDs) using Josephson plasma resonance. The phase coherence as a function of magnetic field is nonmonotonic in samples with higher density of CDs, and we observe multiple resonance peaks in a limited temperature range. The characteristic field H ^* for the enhancement of the phase coherence normalized by the matching field (B ) is constant between 1/5 and 1/3 for samples with larger B , while H ^* is constant in samples with smaller B . In samples with CDs introduced at angles from the c-axis, H ^* is determined only by the density of CDs in the CuO ₂ layers. In a sample with CDs at 80° from the c-axis, a large enhancement of the phase coherence was observed when the field is applied parallel to the direction of CDs and clear triple resonance peaks are observed.