Angular dependence of the magnetization curves and interlayer Josephson coupling in Bi2Sr2CaCu2O8

Magnetization curves were measured for Bi₂Sr₂CaCu₂O₈ crystals which were irradiated with 5.8 GeV Pb ions along the c direction or at 45° with respect to it. Each ion produces continuous columnar that crosses the sample and yields a unidirectional pinning center. The density of the defects in the various samples ranges from 10¹⁰ to 2.5 X 10¹¹ ions/cm². In all the irradiated samples the width of the magnetization curves is found to be the largest for fields along the defects indicating that when the field is at other directions the vortices are tilted away from the defects. This manifests the important role of the Josephson coupling between the CuO₂ planes. We also report on accelerated flux escape around zero magnetization and around zero field, particularly in samples with a high dose of irradiation.     

Bi2Sr2CaCu2O8的晶体结构与超导性能

本文研究了Bi-Sr-Ca-Cu-O体系中理想成份为Bi₂Sr₂CaCu₂O₈化合物的超导性能和晶体结构。名义成份为BiSrCaCu₂O_5.5零电阻超导转变温度T_c(0)=81.5K。用X射线粉末衍射方法测定了Bi₂Sr₂CaCu₂O₈的晶体结构,其基本结构属体心四方晶系,空间群为D_4h¹⁷-l4/mmm,点阵常数a=3.825?,c=30.82?。每单胞化合式单位为2.2Ca占据2(a)等效点系,4Sr,4Bi和4Cu占据三组4(e)位置,其原子参数z分别为0.110,0.302和0.445,16O分别占据8(g),z=0.445和二组4(e),z=0.210和0.380。Bi₂Sr₂CaCu₂O₈晶体结构可认为是阳离子沿z轴的(00z)和((1/2)(1/2)z)交错排列,由Aurivillius相导生出来的。讨论了在Bi-Sr-Ca-Cu-O体系中可能存在的其它沿z轴不同堆垛层数的超导相。 关键词：     

Bi2Sr2CaCu2O8单晶a-b平面热电势率的各向异性

测量了Bi₂Sr₂CaCu₂O₈单晶a方向的热电势率S_a和b方向的热电势率S_b与温度T的关系,发现在a-b平面内热电势率存在各向异性。认为S_a与S_b的差异来源于b方向的调制结构,并用扩散热电势率S_d,声子曳引热电势率S_g和相关跳跃热电势率S_h三种贡献,对实验结果进行了 关键词：     

Modelling of the reactive interface formation on Bi2Sr2CaCu2O8

We have applied the phenomenological model of diffused interfaces, developed originally to describe the Fe growth on Bi₂Sr₂CaCu₂O₈, to interpret quantitatively the core photoemission spectra of two reactive interfaces, namely Cr and Ge on Bi₂Sr₂CaCu₂O₈. At room temperature the interfaces with Fe and Cr have the same kinetics of growth suggesting similar adatom condensation, while the semiconductor-superconductor interface grows layer-by-layer according to the Franck-van der Merwe kinetics. Differences in the parameters of the model resulting from the simultaneous fit of the core-level decays are related to differences in the microscopic processes occurring during the growth.     

Bi2Sr2CaCu2O8+y掺Na样品的超导机制

用熔融法制备了Bi₂Sr₂CaCu₂O_8+y掺Ｎａ多晶样品，转变温度Ｔ_c提高到９０Ｋ以上，最大的零电阻温度达到９２．５Ｋ。在流动氧气退火后，未掺Ｎａ样品Ｔ_c显著降低，而掺Ｎａ样品Ｔ_c几乎不变，说明二者Ｔ_c提高具有不同的机制。据此我们认为Ｎａ替代了Ｂｉ而使Ｂｉｏ层电荷失去平衡，Ｃｕｏ面上电子转移到Ｂｉｏ层而形成空穴，空穴和电子的吸引相 关键词：     

Low-temperature AC conductivity of Bi2Sr2CaCu2O8+δ

We report measurements of anamolously large dissipative conductivities, σ₁, in Bi₂Sr₂CaCu₂O_8+δ at low temperatures. We have measured the complex conductivity of Bi₂Sr₂CaCu₂O_8+δ thin films at 100–600 GHz as a function of doping from the underdoped to the overdoped state. At low temperatures there exists a residual σ₁ which scales with the T=0 superfluid density as the doping is varied. This residual σ₁ is larger than the possible contribution to σ₁ from a thermal population of quasiparticles (QP) at the d-wave gap nodes.     

Temperature dependent water adsorption on Bi2Sr2CaCu2O8 single crystal

Temperature dependent infrared study of water adsorption on the bc-plane of a Bi₂Sr₂CaCu₂O₈ (Bi2212) single crystal reveals that water is molecularly physisorbed, and forms hydrogen-bonded clusters in the temperature range of 85-150 K. Although dissociative adsorption is expected on surfaces terminated with Ca-O and Cu-O groups, no dissociated species is detected. The main absorption bands occur between 3200 and 3500 cm⁻¹ and are assigned to the O-H stretching and the overtone of O-H bending. For exposures higher than 1.0 L, the sticking coefficients are similar for different temperatures. The features in the O-H stretch region for water clusters red-shift as a function of temperature between 85 and 140 K. Water clusters have an amorphous structure between 85 and 140 K, and form a crystalline structure at 150 K.     

Bi2Sr2CaCu2O8单晶中的反常尖锋效应

使用牛津震动样品磁强计(VSM)研究了Bi₂Sr₂CaCu₂O₈单晶的磁滞回线.在20到40K温度之间发现了反常的尖锋效应,随样品O含量的增加,发生尖锋效应的外场也相应提高.可以认为在尖峰效应处发生了由涡漩物质的有序固态到无序固态的相变,在有少量点缺陷存在的BSCCO单晶相图上,B_sp线终止于20K温度处,在20K以下温区没有发生准格子到涡漩玻璃的相变,涡漩固相始终以准格子形式存在;可以认为尖峰效 关键词： 2Sr₂CaCu₂O₈单晶')" href="#">Bi₂Sr₂CaCu₂O₈单晶 磁滞回线 尖锋效应 相变     

Intrinsic Josephson junctions in Bi2Sr2CaCu2O8+δ single crystals

We report on thec-axis superconducting energy gap parameter Δ _c (T) of intrinsic Josephson tunnel junctions inBi ₂ Sr ₂ CaCu ₂ O _8+δ (Bi2212) single crystals. Δ _c (4.2K)≈10−13 meV, which is approximately a factor of two smaller than reported in the majority of tunneling experiments. Δ _c (T) deviates strongly from the BCS temperature dependence. These observations may be explained by a multilayer model of Bi2212 which assumes that theBi−O layers are superconducting due to the proximity effects. The Josephson tunneling then takes place between adjacentBi−O layers while there is a strong proximity coupling betweenBi−O andCu−O layers. The work is supported by Swedish Supercon-ductivity Consortium and NUTEK, and, in part, by Russian Foundation for Basic Research, grant #95-02-04307     

Scanning tunneling spectroscopy of a superlattice superconductor: Bi2Sr2CaCu2O8

It is clear that many of the cuprate superconductors are truly superlattices, composed of sheets whose individual superconducting critical temperatures may approach bulk values. Such a cuprate is Bi₂Sr₂CaCu₂O₈, often referred to as BSCCO-2212. Scanning tunneling spectroscopy (vacuum tunneling) applied toa-bBiO cleavage planes ofT_c≈90 K BSCCO single crystals under liquid helium simultaneously provides topography and local dI/dVspectra (superconducting density of states: DOS). The spectra, which are similar to those obtained from angle-resolved photoemission spectroscopy, confirm a large gap parameter Δ(x,y) on the uppermost layer. The dI/dVspectra do not unequivocally select order parameter symmetry, but are probably consistent with d-wave or anisotropic s-wave states. Spatial variations of Δ on a 100 Å scale are attributed to variation in BiO metallicity, originating in oxygen stoichiometry variations. A model is presented to explain the different dI/dVspectra which are seen, and associated with different local oxygen concentrations. This model, based on the superconducting proximity effect, assumes that in some regions the BiO uppermost layer is insulating and in other regions it is metallic, in the latter case induced superconductivity by proximity to the CuO₂planes. Our STM measurements appear to sample only the uppermost half cell of the crystal, and contain no obvious superlattice features. Recent measurements have confirmed Josephson radiation from voltage biasedc-axis pillars of BSCCO. From the point of view of the present work, the superconducting systems which weakly couple along thec-direction to create Josephson junctions are probably half-cell slabs of height 15.4 Å, each containing two CuO₂and two BiO layers, which act as single composite electrodes for the Josephson junctions.     

Interlayer tunnelling in Bi2Sr2CaCu2O8+δ single crystals

We present measurements of the intrinsic quasi-particle conductivity along the c-axis of 2212-BSCCO single-crystal mesa structures in the superconducting and normal states. Direct measurement of the mesa temperature enables corrections to be made for self-heating and permits the acquisition of reliable I–V characteristics over a wide range of temperatures and voltages. Unlike a conventional superconductor, there is no evidence for any change in the quasi-particle conductivity at T_c, consistent with precursor pairing of electrons in the normal state. At low temperatures the initial low-voltage linear conductivity exhibits a T² dependence, approaching a limiting value at zero temperature.     

The a---b anisotropy in Bi2Sr2CaCu2O8 superconductor: An investigation by Raman spectroscopy

In the present paper, we report a study of the Raman active phonons of Bi₂Sr₂CaCu₂O₈ for frequencies extending down to 20 cm⁻¹ and temperatures ranging from 77 to 673 K. In spite of important changes in the spectra occurring under heating, we show that the a---b anisotropy, exemplified by Raman spectroscopy in Bi₂Sr₂CaCu₂O₈ and obvious on the macroscopic scale through the crystal morphology, still persists at 673 K. These changes are interpreted in terms of disordering effects in the BiO planes and speak in favor of an order-disorder transition around 500 K.     

Bi2Sr2CaCu2-xSnxO8+δ系列 超导体的XRD和XPS研究

实验研究了Bi₂Sr₂CaCu_2-xSn_xO_8+δ 的X射线衍射(XRD)和光电子能谱(XPS).实验发现随着掺杂(Sn)量的增加,晶格参数a 和c都有所变化,O1s和Cu2p芯能级谱也发生了变化.实验结果表明： 在低掺杂量时,Sn主要 呈二价态;而在高掺杂浓度时呈四价态;掺Sn对超导电性的影响与其他元素的掺杂不同.这 些实验结果支持化学环境在高温超导样品的电子结构中起着重要作用的结论. 关键词： Bi系超导体 掺杂 X射线衍射 光电子能谱     

ARPES studies of c-axis intracell coupling in Bi2Sr2CaCu2O8+δ

Using angle-resolved photoemission spectroscopy we have performed a detailed study of bilayer splitting in Bi₂Sr₂CaCu₂O_8+δ as a function of doping level and temperature. In heavily overdoped samples where the splitting is the clearest, we extract an intracell coupling t_⊥∼55 meV. As a function of photon energy the intensity ratio of the bonding and antibonding bands varies, allowing us to detect the bilayer splitting effect in the optimal and underdoped regimes. Surprisingly, with reduced doping the intracell coupling is not measurably reduced. Upon cooling to the superconducting state, a gap Δ opens in both bands yet the magnitude of the splitting remains unchanged.     

Carrier concentration dependence and impurity effects of microwave response in Bi2Sr2CaCu2O8+ y

We report the surface impedance (Z _s ) measurements in high quality single crystals of Bi₂Sr₂CaCu₂O_8+y . At relatively low oxygen content, the change of the penetration depth, Δλ(T) ≡λ(T)-λ(0), of the pure single crystals exhibits linear temperature dependence both parallel to the CuO₂ planes and in thec direction. In contrast to this behavior, by further oxygenation or 0.6% Zn substitution, theT-linear dependence is disrupted andT ² dependence of Δλ is observed. We also found that 0.9% Ni-substitution induces no pairbreaking effect. The present results suggest that the low-lying excitation spectrum of quasiparticles depends on the carrier concentration and is easily changed by a small amount of Zn substitution.     

Impurity-state-like nature of Fermi-liquid states in Bi2Sr2CaCu2O8 observed by photoemission and x-ray absorption

Photoemission and X-ray absorption spectroscopy have been performed on Bi₂Sr₂Ca_1−xY_xCu₂O₈ in which the hole concentration is controlled by the ratio between Ca and Y atoms. It was found that the density of electronic states at the Fermi level gradually increases as the hole concentration with almost no rigid shift of the electronic structure with respect to the Fermi level. This suggests that the electronic states at the Fermi level (Fermi-liquid states) would not be simple one-electron states but be a kind of impurity states produced through the strong hybridization between doped O2p hole orbitals and empty Cu3d orbitals.     

Pinning model derived from resistive measurements on melt processed Bi2Sr2CaCu2O8

On melt processed samples of the 86 K superconductor Bi₂Sr₂CaCu₂O₈ we have performed resistive measurements in the low field limit B0.13 T and 40 KT77 K. The voltage drop is found to rise exponentially with current E ∝exp j/j₀, which is interpreted in terms of thermal activation of pinned flux lines. An activation energy U₀(T)90 meV is derived from the transition width j₀(T) and is related to a plausible core pinning interaction of flux lines with normal conducting precipitates. This reproduces the measured j_c(B, T) values in the whole regime investigated. We conclude that pinning centers must have a minimum size in order to control flux creep. Finally we demonstrate that conventional summation of the single site pinning forces cannot account for the observed macroscopic depinning current density.     

Lock-in transition in Bi2Sr2CaCu2O8+δ observed through 10GHz dissipation

We observe the lock-in transition in Bi₂Sr₂CaCu₂O_8+δ single crystals, by measuring the 10 GHz dissipation for various orientations of the magnetic field; various vortex lattice structures are revealed by different dissipation regimes.