The nuclear contribution to the specific heat of single crystals of YBa2Cu3O7 and Bi2Sr2CaCu2O8

Specific heat data below 1 K of Bi₂Sr₂CaCu₂O₈ and YBa₂Cu₃O₇ are analyzed. For YBa₂Cu₃O₇ the nuclear specific heat, C_N, amounts to 38T⁻² μJ/mol K. C_N for Bi₂Sr₂CaCu₂O₈ exceeds that of YBa₂Cu₃O₇ by a factor of 15. The nuclear quadrupolar specific heat contribution alone is insufficient to explain the data for YBa₂Cu₃O₇, while lack of NQR data does not allow such a comparison in Bi₂Sr₂CaCu₂O₈ to be made. The contribution to C_N from nuclear spins coupled via the contact hyperfine interaction with correlated magnetic spins (in the CuO₂ plane) is derived as a function of the correlation length. This contribution can be treated independently from the quadrupolar term. We show that the excess specific heat in YBa₂Cu₃O₇ likely originates in a few percentage of an impurity (oxygen deficient) phase with a strong hyperfine field on the Cu nuclei.     

Angle-resolved photoemission study of nonsuperconductive Bi2Sr2Ca0.4Y0.6Cu2O8

A comparative angle-resolved photoemission measurement has been performed on nonsuperconductive Bi₂Sr₂Ca_0.4Y_0.6Cu₂O₈ and superconductive Bi₂Sr₂CaCu₂O₈ to study the nature and origin of the electronic states near the Fermi level. It was found that hole-doping does not cause a rigid shift of the density of states relative to the Fermi level, but creates new electronic states in the vicinity of the Fermi level.     

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.     

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.     

O KVV Auger emission versus resonant photoemission at the O K edge of high-Tc superconductors

Photoelectron spectroscopy results on single crystals of the superconductors Bi₂Sr₂CaCu₂O₈,Bi₂Sr₂CuO₆, Ba_0.6K_0.4BiO₃ and the semiconductor Ba_0.9K_0.1BiO₃ are reported for the photon energy region around the O K absorption threshold. The development of the O-KVV Auger structure has been carefully monitored as a function of photon energy. A non-monotonic behavior displaying a feature at a constant binding energy of about 14 eV was found for Bi₂Sr₂CaCu₂O₈ and Bi₂Sr₂CuO₆ in a narrow photon energy region of 1 eV at the main edge of the O K absorption spectrum around 530 eV. The corresponding enhancement, connected with the autoionization of O 2p states, is absent in Ba_1−xK_xBiO ₃ in contrast to Bi₂Sr₂CaCu₂O₈ and Bi₂Sr₂CuO₆. The resonant enhancement is more pronounced for Bi₂Sr₂CuO₆ as compared to Bi₂Sr₂CaCu₂O₈, which can be explained by a lower charge carrier concentration in the former case, leading to a more localized nature of intermediate O 2p states. The model parameters Cu d–d and O p–p Coulomb interactions and the charge transfer energy Δ are estimated from the experiments.     

Evidence for pinning by (Sr,Ca)2CuOy particles in partial-melting processed bulk Bi2Sr2CaCu2O8+δ ceramics

Flux distributions of partial-melting processed Bi₂Sr₂CaCu₂O_8+δ ceramics are obtained using magneto-optic imaging. In remanent states (μ₀H_a=0 T), large amounts of trapped flux are observed along (Sr,Ca)₂CuO_y particles embedded in the Bi₂Sr₂CaCu₂O_8+δ matrix. Despite the relatively large size of these particles (up to 30 μm), the pinning effect is similar to that of Y₂BaCuO₅ particles in melt-processed YBa₂Cu₃O_7−δ. Furthermore, we discuss how the pinning capability of non-superconducting particles of different sizes and densities will show up in magneto-optic images.     

Observation of the flux line lattice in high Tc-superconductor Bi2.2Sr2CaCu2OX and Ti2Ba2CaCu2OX single crystals

The flux-line lattice (FLL) has been observed on the (001) face of high T_c Bi_2.2Sr₂CaCu₂O_X and Ti₂Ba₂CaCu₂O_X single cristals using the technique of decorating the sample with small ferromagnetic particles. Strong pinning of the Abrikosov vortices by plane defects along (100) and (010) planes has been found out on Bi_2.2Sr₂CaCu₂O_X. The triangular FLL with a long-range order has been observed on the perfect Ti₂Ba₂CaCu₂O_X single crystal. The penetration depth for a magnetic field parallel to (001) axis has been evaluated as ≤0.2μm at 4.2K for both materials.     

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：     

SURFACE PHASE DECOMPOSITION IN Bi2Sr2CaCu2O7-y SINGLE CRYSTALS UNDER NITROGEN IONS IRRADIATION

The influence of irradiation by 30 keV nitrogen ions with a fluence 1×10¹⁸N⁺·cm^-2 on the crystal structure of single crystal Bi₂Sr₂CaCu₂O_7-y was investigated by means of X-ray photoelectron spectroscopy and X-ray diffraction. The irradiation caused a transformation from Bi₂Sr₂CaCu₂O_7-y (2212 phase) to Bi₂Sr₂CuO_5-x (2201 phase). It was observed that. a small amount of metallic bismuth with an average thickness of about 6.3nm appeared after the irradiation. The possible reaction mechanism under nitrogen-ion irradiation was discussed.     

Mechanism of characteristic shape of resistive transition in Bi2Sr2CaCu2O8 films made by laser evaporation

We measured the temperature dependence of resistivity, (T, H), of Bi₂Sr₂CaCu₂O₈ thin films in magnetic fields. These films have a preferential orientation of the c-axis perpendicular to a surface of the substrate. We could not observe any meaningful anisotropy of the characteristic shape of the resistive transition between two cases of the current flow parallel and perpendicular to the magnetic field in the basal plane. Therefore, it is very difficult to ascribe the broadness to any origin due to the Lorentz force produced by an external current.     

Zero-field current-voltage characteristics in high-temperature superconductors

We have observed the anomalous magnetization of Bi₂Sr₂CaCu_2−xNi_xO₈ (x = 0 and 0.02) single crystals. Anisotropy decreases with iodine intercalation although it expands the space between CuO₂ layers. Iodine intercalation seems to suppress the magnetization anomaly for Ni = 1% crystals, but not for Ni = 1% substituted crystals. We have discussed these results in terms of the increase of anisotropy by Ni substitution and the dimensional crossover of flux lines. Effects of both oxygen concentration and substitution of a magnetic element for the Cu site on the anisotropy of Bi₂Sr₂CaCu₂O₈ crystals show the same tendency as the case of the YBa₂Cu₃O₇ superconductor.     

Structure and properties of Bi2Sr2CaCu2Ox/Bi2Sr2CuOx superlattices prepared by RF magnetron sputtering

We have prepared Bi₂Sr₂CaCu₂O_x/Bi₂Sr₂CuO_x (2212/2201) superlattices with various stacking periodicity by multitarget RF magnetron sputtering. Their crystal structure and electrical properties were investigated. The superlattice was found to be successfully constructed from the data of XRD patterns, AES, etc. The critical temperature increased almost linearly with increasing thickness of the 2212 phase up to a thickness of 6 unit cells. On the other hand, it did not decrease as the thickness of the 2201 layers was increased. The unit cell/unit cell superlattice showed T_{c zero} of 30 K.     

Oxygen nonstoichiometry effect on structural modulation in Bi-2212 cuprate superconductors

The X-ray diffraction patterns for Bi₂Sr₂Ca_1−xY_xCu₂O_8+y sintered in air or Ar indicated that the reduction in the number of excess oxygen ions induced a structural change from orthorhombic to pseudotetragonal. When the oxygen contents decreased with the reducing treatment, the period of modulation in Bi₂Sr₂Ca_1−xY_xCu₂O_8+y increased. This result supported the model of excess oxygen ions in the Bi---O sheet. However, it was difficult to understand the change of modulation period with increasing Y contents using the model of excess oxygen ions in the Bi---O layers. Also, it is included that the Y substitution for Ca and La substitution for Sr in the Bi₂Sr₂CaCu₂O_8+y system result in parallel effects for the modulation structure.     

Surface morphology and electrical properties of Bi---Sr---Ca---Cu---O thin films deposited by reactive sputtering from multiple targets

Smooth, superconducting films of Bi₂Sr₂CaCu₂O_8+° have been prepared by reactive sputtering from elemental targets in the presence of ozone. The influence of substrate temperature, deposition rate, and ozone pressure on the resulting films are discussed. Films deposited on SrTiO₃ substrates are c-axis oriented and featureless for substrate temperatures below 710°C. Above this temperature, small inclusions of CuO appear. Films on MgO exhibit mixed a-axis and c-axis orientation below about 710°C, and inclusions of CuO above this temperature. The temperature at which this transition occurs increases with increasing deposition rate. T_c increases and then decreases sharply with decreasing oxygen content. The oxygen partial pressure corresponding to the maximum T_c of 77 K is well above the thermodynamic stability limit for Bi₂Sr₂CaCu₂O_8+°, suggesting that an optimum carrier concentration has been achieved for these films.     

Effects of redox thermal treatments on the transverse resistance peak at the edge of the superconducting transition in Bi2Sr2CaCu2O8+x

The growth of the transverse resistivity just above the transition in Bi₂Sr₂CaCu₂O_8+x (BSCCO 2212) expitaxial films is studied in the framework of the recent fluctuation model proposed by Ioffe, Larkin, Varlamov and Yu Lu. The effects of different redox thermal treatments on the transverse resistive peak is investigated. The fit of the experimental curves with the theory is good and allows one to estimate the values of some physical parameters included in the theory itself (namely the in-plane Fermi velocity ν_F, the carrier lifetime τ and the hopping integral w). It is shown that the oxidizing thermal treatment gives rise to a strong decrease in the transverse resistance peak just above T_c. Such effect is ascribed to the increase in ν_F due to the growth of the carrier concentration induced by the oxidizing treatment. Effects on the carrier lifetime and the hopping integral are hindered by the large variation of ν_F.     

Control of carbon impurities in 2212 superconducting phase

Bi₂Sr₂CaCu₂O₈ superconductor has been synthesized by the so-called liquid mix technique under different conditions. The influence of both carbon content and thermal treatments on the superconducting properties is discussed.     

The resonance peak in the high-Tc cuprates

I present a theory in which the resonance peak observed in inelastic neutron scattering experiments on YBa₂Cu₃O_6+x and Bi₂Sr₂CaCu₂O_8+x arises from a dispersing spin mode. I argue that it is heavily damped in the normal state and becomes visible in the superconducting state due to a drastic decrease in the spin damping. I show that a spin-fermion model correctly describes the doping dependence of the peak position and of its integrated intensity. Finally, I derive a criterion for the existence of the resonance peak in other cuprate superconductors.     

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.     

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.     

Electrical transport and superconductivity in the Al2O3-Bi2Sr1.8Ca1.2Cu2Oy and MgO-Bi2Sr1.8Ca1.2Cu2Oy composites

We have measured the resistivities of Al₂O₃-Bi₂Sr_1.8Ca_1.2Cu₂O_y and MgO-Bi₂Sr_1.8Ca_1.2Cu₂O_y composites with the nominal Bi₂Sr_1.8Ca_1.2Cu₂O_y volume fraction, ₂₂₁₂, ranging from 0.15 to 1.00. For the Al₂O₃-Bi₂Sr_1.8Ca_1.2Cu ₂O_y composites, we find for the samples with ₂₂₁₂≥0.6 that the superconducting transition temperature, T_c, is not disturbed by the addition of Al₂O₃. For ₂₂₁₂<0.3, no zero-resistivity state is observed. For the MgO-Bi₂Sr_1.8Ca_1.2Cu₂O_y composites, T_c is barely disturbed for the samples with ρ₂₂₁₂≥0.7. No superconducting state is observed for the samples with ρ₂₂₁₂<0.35. The variation of (300 K) with ρ₂₂₁₂ indicates a three-dimensional percolating Bi-Sr-Ca-Cu-O matrix occurring at ρ₂₂₁₂≈0.19 and ≈0.15 in Al₂O₃-Bi₂Sr_1.8Ca_1.2 Cu₂O_y and MgO-Bi₂Sr_1.8Ca_1.2Cu₂O_y, respectively. Both resistivity and magnetization measurements suggest that the reactions of Bi₂Sr_1.8Ca_1.2Cu₂O_y with MgO are weaker than with Al₂O₃.