Josephson vortex motion in stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+x

We have investigated the motion of Josephson vortices in stacks of intrinsic Josephson junctions in Bi₂Sr₂CaCu₂O_8+x (BSCCO). The stacks were realized as mesa structures on top of BSCCO single crystals. In external magnetic fields up to 5 T, oriented parallel to the layers, the current–voltage (I–V) characteristics exhibit a field dependent branch. By microwave emission measurements we show that this branch is due to a collective motion of Josephson vortices. For magnetic fields larger than 17 kOe we find a pronounced flux-flow step in the I–V characteristic that corresponds to a Josephson frequency of 1.5 THz. In this regime, we observe an intensive non–Josephson emission signal. We propose that this signal is due to Cherenkov radiation emitted from Josephson vortices moving faster than the lowest mode velocity of the stack. Numerical simulations using the coupled sine-Gordon model demonstrate downconversion of Cherenkov radiation due to the mixing of different modes.     

高温超导体Bi2Sr2CaCu2O8+δ单晶解理面的AFM研究

用ＡＦＭ在室温和大气下对高温超导体Ｂｉ２Ｓｒ２ＣａＣｕ２Ｏ８＋δ（Ｂｉ２２１２）单晶解理面的形貌和结构进行了系统的研究。观测到解理面绝大部分是原子级平滑的晶面。在晶面的边缘观测到高度为结构单元（ｃ／２）整倍数的台阶，这与该晶体是以层状模式生长的机制相一致的。进而证明了解理位置和解理分布。在解理面的局部区域，观测到高度为１．２５ｎｍ，面积约为０．０５μｍ２的平台。Ｘ－射线微区分析证明这些小平台正是在Ｂｉ２２１２单晶生长中常存在的少量Ｂｉ２２０１相。我们的研究表明ＡＦＭ不但在研究高温超导体单晶的形貌特征方面有重要用途，而且在对晶体的质量进行分析和结构特征研究中也有重要应用。     

Precipitation and pinning in Ca and Sr-Rich High-Tc superconducting “Bi2Sr2CaCu2O8” ceramics

From a consideration of the phase equilibrium diagram of the system Bi₂O₃-SrO-CaO-CuO, a simple annealing procedure was developed to precipitate Bi_2+xSr_2+xCuO_6+d, Sr₁₄Cu₂₄O_41−x, and Bi₂Sr₃O₆ in high-temperature superconducting Sr-rich “Bi₂Sr₂CaCu₂O₈” ceramics and Ca₂CuO₃ and a liquid in Ca-rich “Bi₂Sr₂CaCu₂O₈” ceramics. The transformation results in an increase of the critical current density of which is believed to express improved pinning properties of the superconducting crystals, in particular an increased pinning energy, which reduces the probability for thermally activated depinning. Possible pinning centers which were introduced during precipitation of the second phases are the surface of the precipitates.     

Recent status on high temperature superconducting Bi2Sr2CaCu2O8+x wire development at NYSIS: 1-90 meter length Jcs and 3 meter diameter ring furnace design

The status of long length, Bi₂Sr₂CaCu₂O_8+x (Bi-2212) wire development at the New York State Institute on Superconductivity (NYSIS) is reviewed and updated. Transport J_cs (4.2K, 0 T) of Bi-2212/Ag oxide powder-in-tube singlefilamentary tapes have reached 70,000-80,000, 50,000-60,000, and 30,000–40,000 A/cm² for 1, 4–15, and 40–90 meter length tapes, respectively. The decrease in J_c as the tape length was increased from 15 to 90 meters was attributed to the (measured) sensitivity of J_c to temperature nonuniformities (±3‡C) in the box-type furnace used for annealing. To reduce this problem, a ringtype high-temperature furnace (∼3 meter diameter) was designed and constructed which provides a large-volume (∼13^w × 10^h × 1000¹ cm) processing zone with expected excellent temperature uniformity (±0.5‡C). The advantages of the ring-type furnace for processing of kilometer-length conductors are described.     

Fabrication of Bi2Sr2CaCu2O x and Bi2Sr2Ca2Cu3O x Superconductor Thick Films by Using Cu-Free Precursors on Ni Substrates

We studied the formation of Bi₂Sr₂CaCu₂O _x (Bi-2212) and Bi₂Sr₂Ca₂Cu₃O _x (Bi-2223) thick films in a heat treatment process of the Ni-sheathed Bi-Sr-Ca-Cu-O (BSCCO) system. Cu was electrodeposited initially on the Ni substrates (Cu/Ni). Well-oriented Bi-2212 superconductor thick films were formed successfully on Ni tapes by liquid reaction between Cu-free precursors and Cu/Ni tapes. However, only a small amount of Bi-2223 was formed. Thick films were prepared by screen-printing with Bi₂O₃, SrCO₃, and CaCO₃ powders on Cu/Ni tapes and heat treating them. Heat treatment was performed in the temperature range of 750–850°C in a tube furnace for several minutes to hours. The phases and the microstructures of the high temperature superconductor thick films were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Electrical properties were examined by the standard four-probe method. At the heat treatment temperature, the specimens were in a partially molten state during reaction between the oxidized copper layer and the screen-printed precursors on the Cu/Ni tapes.     

Visualization of Dopant Oxygen Atoms in a Bi2Sr2CaCu2O8+δ Superconductor

A proper amount of excess oxygen plays a significant role in hole‐doped cuprate high‐T_c superconductivity. Here, the dopant oxygen in Bi₂Sr₂CaCu₂O_8+δ is directly imaged via integrated differential phase contrast combined with state‐of‐the‐art scanning transmission electron microscopy. The location of dopant oxygen is observed to be consistent with the position inferred from local strain analysis of the incommensurate structure. The influence of dopant oxygen on the local atomic lattice and electronic structure is further explored using first‐principle calculations. The dopant oxygen atoms not only aggravate the distortions of the local atomic arrangement but also alter the electronic states by transferring charge from the BiO planes to the CuO₂ planes. The underlying mechanism of charge transfer is resolved. The results may also be applicable to other oxygen‐doped cuprates with high‐T_c superconductivity.     

On the role of Bi2Sr2Ca1Cu2O8 and Bi2Sr2Cu1O6 on the weak links and critical currents in (Bi,Pb)2Sr2Ca2Cu2O10/Ag superconducting tapes

Silver-sheated (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) superconducting tapes with different Bi₂Sr₂CaCu₂O₈ (Bi2212) and Bi₂Sr₂Cu₁O₆ (Bi2201) concentrations, were prepared by using a two-step sintering processing and by varying cooling rates in the fabrication of the superconductors. The effect of residual Bi2212 and Bi2201 phases on weak links and critical currents of the Bi2223/Ag tapes was investigated. It was found that residual Bi2201 caused weak links at grain boundaries and limited the current-carrying capacity of the tapes. Comparatively, the residual Bi2212 phase had much less influence on both weak links and critical currents. Elimination of Bi2201 by sintering tapes at a low temperature in the final thermal cycle, or by cooling the tapes slowly, increased critical current by a factor of two. Flux pinning property was also improved by removing the residual Bi2201 phase.     

Far infrared response of thin film Bi2Sr2CaCu2O8 using a free electron laser

The far infrared response of granular thin-film Bi₂Sr₂CaCu₂O₈ superconductor has been investigated using long (≈5 μs) but sharply truncated free electron laser pulses in the frequency range between 50 cm^?1 and 125 cm^?1. Under constant current bias, a fast response and a slow bolometric signal component could be identified in this energy range, which is below the BCS energy gap (≈ 200 cm^?1). Measurements of the power dependences of the signal voltages showed that both the fast and the thermal responses are consistent with the predictions of the resistively shunted Josephson junction model.     

Investigation of superconductivity effects in single-phase and multiphase Bi2Sr2CaCu2O8 single-crystals at 2-MM wavelengths

Experimental investigations of superconductivity effects in single-phase and multiphase Bi₂Sr₂CaCu₂O₈ single-crystals have been carried out at 142 GHz frequency by means of the standing wave profile method [1]. Josephson harmonic generation has been observed to be responsible for the appearence of additional peaks on the standing wave profile of the open dielectric resonator loaded with a properly orientated multiphase high-T_c superconductor specimen. This leads to the conclusion that most of the Josephson junctions in multiphase crystals are located in certain crystallographic planes. The investigations of temperature dependencies showed that sharp resonant peaks of conductivity observed earlier [2] at 60 GHz could also be observed at 142 GHz.     

Effect of site-selective substitution in bulk superconducting Tl2Ba2CaCu2O8

Magnetization measurements were carried out on bulk Tl₂Ba₂CaCu₂O₈ (referred to as Tl-2212) and on various site-selective substituted Tl-2212 samples. At 5K between 0 and 4.5 T, the 5 at. % Mg-doped Tl-2212 (Tl,Mg-2212) samples displayed enhanced pinning as demonstrated by a field dependent increase of the magnetic critical-current density J_c by 18 to 25 percent over that of pristine Tl-2212. Excess Mg (10–15 at. %), however, is deleterious. Rietveld refinement of the x-ray diffraction pattern showed Mg on the Tl sites. Auger electron spectroscopy analysis showed part of the Mg on grain boundaries. The flux-creep activation energies are higher for flux expulsion than for flux penetration in both Tl-2212 and Tl,Mg-2212 samples; the latter displays higher individual values. Our results demonstrate an increase in the number density of flux lines as a result of increased density of atomic-size-structural, defects by Mg (5 at. %) doping. In the Tl_2−yBa₂(Ca_1−zY_z)Cu₂O_8−x(z=0–0.3; single phase; x and y represent oxygen and thallium vacancies) system also studied, the T_c decreases as z increases. At z=0.3, the sample becomes an antiferromagnetic semiconductor.     

Electrical and infrared study of Bi2Sr2Ca1Cu2O8 in semiconducting,superconducting ceramic and superconducting glass ceramic state

We have studied the electrical and infrared properties of Bi₂Sr₂Ca₁Cu₂O₈ compound in three states. Electrical and IR measurements show that the pure powder state sample is a semiconductor, the ceramic Bi₂Sr₂Ca₁Cu₂O₈ sample prepared after annealing at 820°C for 240 hours shows a T_c of 85 K, whereas Bi₂Sr₂Ca₁Cu₂O₈ sample prepared through glassy route, i.e. melting at 1250°C and annealing at 820°C for 240 hours shows a drop of T_c by 5 K. The infrared spectra of superconducting ceramic and glass ceramic states in the available frequency range of measurement reveals the presence of three phonons. Since the vibrational mode around 595 cm^?1 is due to CuO₂ layers and as the CuO₂ layers are responsible for T_c in the ceramic superconductors, any change in these layers will affect the T_c. The shifting of the 595 cm^?1 mode towards lower frequencies in the glass ceramic due to different preparation process, indicates that there is a change in CuO₂ layers resulting in a change of T_c, which is confirmed by Four probe dc measurements.     

Internal friction of superconducting YBa2Cu3O6+x wires at 200 kHz under in situ heat treatment

Using piezoelectric LiNbO₃ crystal oscillator method at 200 kHz, we have measured internal friction and modulus of YBa₂Cu₃O_6+x short wires with 1 mm-diameter and about 5mm length under in situ cyclic heating-cooling treatment at temperatures of 300–1020K. Polycrystalline YBa₂Cu₃O_6+x wires show anelastic behavior as well as orthorhombic-to-tetragonal (O-T) phase transition within present temperature range. There appears a strong internal friction peak below the O-T transition temperature, corresponding to the atomic site relaxation between oxygen and a vacancy in the Cu-O plane of oxygen defective YBa₂Cu₃O_6+x. A novel peak is observed after the cyclic heat treatments at around 700K. This relaxation is attributed to the hopping movement of oxygen in the defective local structure of YBa₂Cu₃O_6+x.     

Direct white-light from core-shell-like sphere with Sr3Mg-Si2O8: Eu2+, Mn2+ coated on Sr2SiO4:Eu2+

A method of color mixture for white light is presented with Sr3MgSi2O8:Eu2+, Mn2+ shell coated on Sr2SiO4:Eu2+ core by spray pyrolysis procedure. Upon near ultraviolet (NUV) excitation, a 550 nm band emission of Eu2+ from core host combines with the simultaneous emissions of Eu2+ at 457 nm and Mn2+ at 683 nm based on energy transfer in the shell lattice to generate warm white light with color rendering index (CRI) of 91. With such a core-shell-like structure, the re-absorption of blue light from shell layer can be effectively suppressed, and the chemical stability of the phosphor is verified experimentally to be superior to that of the Sr2SiO4:Eu2+. This new proposed phosphor provides great potential in the color mixture of blending-free phosphor converted white NUV light emitting diode (LED) devices.     

Phase relations and homogeneity region of the hightemperature superconducting phase (Bi,Pb)2Sr2Ca2Cu3O10+d

For the nominal composition of Bi_2.27−xPb_xSr₂Ca₂Cu₃O_10+d, the lead content was varied from x < 0.05 to 0.45. The compositions were examined between 800 and 890‡C which is supposed to be the temperatue range over which the so-called 2223 phase (Bi₂Sr₂Ca₂Cu₃O_10+d) is stable. Only compositions between x < 0.18 to 0.36 could be synthezised in a single phase state. For x <0.36, a lead-containing phase with a stoichiometry of Pb₄(Sr,Ca)₅CuO_d with a small solubiliy of Bi is formed, for x > 0.18 mainly Bi₂Sr₂CaCu₂O_8+d and cuprates are the equilibrium phases. The temperature range for the 2223 phase was found to be 800 to 890‡C but the 2223 phase has extremely varying cation ratios over this temperaure range. Former single phase 2223 samples turn to multiphase samples when annealed at slightly higher or lower temperaures. A decrease in the Pb solubility with increasing as well as decreasing temperature with a maximum at about 850‡C was found for the 2223 phase.     

An optical investigation of electronic excitations in the high temperature superconductors YBa/sub 2/Cu/sub 3/O/sub 7- delta / and Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+ Delta /

The results of Raman scattering from superconducting gap excitations in the high T/sub c/ compounds YBa/sub 2/Cu/sub 3/O/sub 7- delta / and Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+ Delta / are compared. In the normal state, both materials exhibit strong electronic interband scattering which manifests itself as a broad background continuum having both A/sub 1g/ and B/sub 1g/ symmetry components. At low temperatures a redistribution of this electronic scattering occurs, indicative of the formation of a superconducting gap in these compounds. The two symmetries exhibit distinct redistribution, however, denoting strong anisotropy. At temperatures well below T/sub c/, both compounds exhibit residual low-energy scattering, a feature suggestive of the coexistence of normal electrons and superconducting quasi-particles.<>     

The growth of low dislocation density Sr1−x,Bax Nb2O6 crystals

The dislocation structures of both pure and Nd doped strontium barium niobate crystals, grown by the Czochralski method, were studied using an etch pit technique. It was determined that dislocations in the boule were being propagated from the seed and were confined to the center of the crystal. Typical dislocation density was 5x10⁴ cm⁻². Through the careful control of growth parameters and use of seed material cut from the dislocation free outer portion of a crystal, it was possible to grow crystals with very low dislocation densities, 1x10² cm⁻², and on occasion dislocation free crystals.     

高温超导Bi2Sr1.6La0.6CuO6外延薄膜生长模式的AFM研究

利用ＡＦＭ对在（１００）和与（１００）有６°切偏角的ＳｒＴｉＯ３基片上用射频溅射方法制备的高温超导Ｂｉ２Ｓｒ１．６Ｌａ０．４ＣｕＯ６＋δ（Ｂｉ２２０１）薄膜的生长模式进行了系统地研究。对应以上两类不同切割的基片,实验观察到两种不同的薄膜生长模式。对切偏角小于０．４°的（１００）ＳｒＴｉＯ３基片,本征的生长模式是梯田岛模式（Ｖｏｌｍｅｒ－Ｗｅｂｅｒ模式）,每层的厚度为ｃ／２（１．２５ｎｍ）;在切偏角为６°的衬底上沉积的Ｂｉ２２０１薄膜则以台阶流模式（Ｓｔｅｐ－ｆｌｏｗｍｏｄｅ）生长。Ｂｉ系高温超导体的本征的二维特性决定了薄膜的生长模式。     

Effect of TiO2 Ratio on the Phase and Microwave Dielectric Properties of Li2ZnTi3+x O8+2x Ceramics

Low-loss materials Li₂ZnTi_3+x O_8+2x (LZT) (x = 0, 0.10, 0.17, 0.25, 1.00) were prepared by the conventional solid-state route. The effect of TiO₂ ratio on phase composition, microstructure, and the microwave dielectric properties of Li₂ZnTi_3+x O_8+2x ceramics were investigated using x-ray diffraction, scanning electron microscopy, energy-dispersive x-ray spectroscopy, and Vector Network Analyzer. The results revealed that a two-phase system Li₂ZnTi₃O₈-TiO₂ was formed. The appropriate content of TiO₂ ratio can effectively adjust the temperature coefficient of the resonant frequency (τ _f) value from ?14.5 to 0 ppm/ °C without obvious degradation of the microwave dielectric properties. The microwave dielectric properties of the Li₂ZnTi_3+x O_8+2x materials were characterized at microwave frequencies. Typically, the Li₂ZnTi_3+x O_8+2x (x = 0.17) ceramic sintered at 1,160 °C for 5 h showed excellent microwave dielectric properties with ε _r = 28.51, Q × f = 58,511 GHz, and τ _f = + 2.3 ppm/ °C.     

Atomic displacements in the modulated structure of Bi2Sr2(Ca(1-x)Pr(x))Cu2O(8+delta) and their effect on HRTEM reverse contrast image

The amplitude of transverse atomic displacement wave along the direction of incident electron beam exerts a significant effect on the brightness of spots in a high-resolution transmission electron microscope (HRTEM) reverse contrast image. The bright spot is associated with the projection of the modulated atomic chain along the beam direction. In general, the spot brightness is roughly inversely proportional to the amplitude of the displacement wave in an appropriate region of defocus, specimen thickness and amplitude. Monotonic decrease in the spot brightness with increasing amplitude can be attributed to the decrease in the degree of interference between scattered electrons. A complicated domain configuration has been found by the analysis of the spot brightness modulation observed in a [0 0 1] HRTEM reverse contrast image.     

Giant Pressure-Induced Spectral Shift in Cyan-Emitting Eu2+-Activated Sr8Si4O12Cl8 Microspheres for Ultrasensitive Visual Manometry

To address the unsatisfactory pressure sensitivity of luminescent manometers, Eu²⁺-activated supersensitive microspheres operating in the visible range are developed. A series of Eu²⁺-doped Sr₈Si₄O₁₂C_l8 materials are synthesized as microspheres, and their structural and spectroscopic properties are studied theoretically and experimentally. Excited at 350 nm, the samples emit a bright cyan luminescence at ambient conditions that, upon pressure, changes to green emission and finally to yellow light above 7 GPa. Most importantly, a huge red-shift of the emission band from 497.3 to 568.8 nm is observed as the pressure increases, leading to an ultrahigh-pressure sensitivity of 9.69 nm/GPa, which is the highest sensitivity ever reported. The designed microspheres with polychromatic emissions and high-pressure sensitivity are suitable for visual optical pressure sensing, and the applied strategy provides some important guidelines for the development of new optical manometers, allowing pressure monitoring with unprecedented accuracy.