SUPERCONDUCTING PROPERTIES AND STRUCTURAL PERFECTION OF EPITAXIAL YBa2Cu3O7-x THIN FILMS

High quality epitaxial YBa₂Cu₃O_7-x thin films have been succcessfully prepared by dc magnetron sputtering deposition, on (100) and (110) aligned SrTiO₃, LaAlO₃ and yttria-stabilized zirconia (YSZ) substrates. The films showed zero resistance around 90 K and had a J_c (at 77 K, H=0) over 10⁶A/cm². It was found that superconducting properties and structures of the films were strongly dependent on oxygen pressure and substrate temperature. The epitaxial structure of the films have been studied by X-ray diffraction. Rutherford backscattering and channeling spectroscopy, X-ray double-crystal diffraction and transmission election microscopy. The experimental results demonstrated that the epitaxial YBa₂Cu₃O_7-x films had excellent superconducting properties and quite perfect structure.     

Formation of Ca-doped YBa2Cu4O8 phase in thin films

Ca-doped YBa₂Cu₄O₈ (124) thin films are prepared on (100) SrTiO₃ substrates by annealing the amorphous films deposited using a pulsed laser deposition technique. The X-ray diffraction measurements show that the Ca-doped YBa₂Cu₄O₈ phase is formed by annealing below 800°C at a oxygen pressure of 1 atm. The 124 films have c-axis orientation normal to the substrates. As the Ca content increases, the proportion of the 123 impurity phase in the samples increases. The onset temperature of superconductivity of the Y(Ca)Ba₂Cu₄O₈ films increases from 79 K to 88 K with an increase Ca-substitution for 5 to 10% of Y.     

用穆斯堡尔效应研究YBa2Cu3O7-δ中的磁有序

以⁵⁷Fe作为探针,用穆斯堡尔效应研究YBa₂Cu₃O_7-δ的磁有序与超导电性的关系。实验结果可以用非声子超导机制进行定性解释。 关键词：     

Preparation and superconducting properties of YBa2Cu4O8

Samples of YBa₂Cu₄O₈ are prepared by means of a new high oxygen pressure technique employing oxygen-HIP. Both magnetization and resistivity measurements show the superconductivity transition at 82.5 K. The lower and upper critical fields of YBa₂Cu₄O₈ are obtained from the magnetization measurements. The Ginzburg-Landau parameters, ζ(0)=17 Å, λ(0)=2400 Å, are estimated from these results.     

Phase equilibria at low oxygen partial pressure in the Y---Ba---Cu---O system

The phase equilibria around YBa₂Cu₃O_7−x (123) and YBa₂Cu₄O₈ (124) phases at low oxygen partial pressure (1 atm) were investigated by X-ray diffraction and thermal analysis. The coexistence of 123 and 124 phase was confirmed under 1 atm oxygen pressure. By using the high temperature X-ray diffraction method, the univariant reaction YBa₂Cu₃O_7−x+Cu₂OY₂BaCu₂O₂+O₂ was identified. The oxygen partial pressure dependence of several univariant reactions has been investigated and the existence of two invariant reactions of L+O₂YBa₂Cu₃O_7−x+ BaCuO₂+CuO+Cu₂O and L+Y₂BaCuO₅+O₂YBa₂Cu₃O_7−x+CuO+Cu₂O was deduced to occur at 1103 K under 0.0032 atm O₂ and at 1143 K under 0.0085 atm O₂, respectively.     

EFFECT OF THERMAL TREATMENTS ON THE SUPERCONDUCTIVITY OF UNDOPED AND WO3 DOPED Y-Ba-Cu-O SYSTEM

Undoped and WO₃-doped Y-Ba-Cu-O ceramics have been slowly cooled in flowing oxygen or quenched in air from 900℃ to room temperature, after sintering at 900℃ for 3Oh in flowing oxygen. The ac susceptibility measurements show that the T_c of the quenched WO₃-doped Y-Ba-Gu-O superconductor can reach 88K, while the undoped Y-Ba-Cu-O ceramic quenched in air is a non-superconductor. X-ray diffraction data show that WO₃ does not enter into the YBa₂Cu₃O_7-x lattice but forms all impurity phase. The WO₃-doped YBCO specimens quenched in air consist of orthorhombic YBa₂Cu₃O_7-x phase and an impurity phase which increases with increasing WO₃ content. Raman spectra chow that oxygen contents are different for undoped and WO₃-doped samples. It la suggested that suitable doping with WO₃ can change the oxygen content and reduce the effect of thermal treatments on the superconductivity of Y-Ba-Cu-O system.     

掺杂Y2O3和BaCeO3提高MOD-YBCO超导性能的研究

本文通过在前驱液中添加过量钇盐和铈的有机盐,采用三氟乙酸盐-金属有机沉积法(TFA-MOD) 在铝酸镧单晶基体上制备了含有纳米氧化钇和纳米铈酸钡的YBCO薄膜. 与纯YBCO薄膜相比,掺杂Y₂O₃/BaCeO₃的YBCO膜的临界转变温度几乎保持不变,为91 K左右. 而掺杂Y₂O₃/BaCeO₃的YBCO膜的临界电流密度达到5.0 MA/cm² (77 K, 0T), 是纯YBCO膜临界电流密度的1.5倍.薄膜中的Y₂O₃和BaCeO₃可能在YBCO内部起到了 有效的钉扎磁通作用. 关键词： 钇钡铜氧薄膜 2O₃和纳米BaCeO₃')" href="#">纳米Y₂O₃和纳米BaCeO₃ 磁通钉扎 三氟乙酸盐-金属有机沉积     

Inducing superconductivity and structural transformations by fluorination of reduced YBCO

Bulk superconductivity with T_c up to 94 K has been induced by fluorination of non-superconducting YBa₂Cu₃O_6.11 using XeF₂ as a fluorination agent. Strong changes on X-ray patterns were found after fluorination of reduced YBCO. High resolution electron microscopy of superconducting samples showed the presence of a new phase with c ≈ 13 Å which exists as noticeable areas included within a matrix of the 123 structure or which occurs as isolated defects with a limited extension. All fluorinated compounds exhibited a strong disorder along the c-direction resulting in a ‘loss’ of c-parameter on X-ray patterns. The present results demonstrate that fluorine indeed enters the YBCO structure with a significant structural rearrangement for the high level of fluorination. The structure of the fully fluorinated YBa₂Cu₃O₆F₂ phase, possibly responsible for superconductivity, has been deduced from high resolution electron microscopy.     

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.     

Superconductivity in La1.85Sr0.15Cu1-xZnxO4

Based on the multiple exchange-mediated interaction mechanism of high-T_c superconductivity and the Abrikosov-Gorfiov theory of magnetic impurity superconductors, this letter quantitatively explains the Zn-substitution experiment of La_1.85Sr_0.15Cu_1-xZn_xO₄     

POSITRON LIFETIME STUDIES OF Y1-xCaxBa2Cu3O7-δ AND Y1-xCaxBa2Cu3-xFexO7-δ

In this paper it is reported that the measurement of the bulk positron lifetime as a function of substitution content x in the temperature range from 70 to 220K was performed in two high-T_c superconducting systems, Y_1-xCa_xBa₂Cu_3-xFe_xO_7-δ and Y_1-xCa_xBa₂Cu₃O_7-δ. It was found that τ_B of both systems decreases significantly with x and the temperature dependence of τ_B is very weak in normal state. In lower temperature region (Tc), a dramatic x-dependent temperature variation of τ_B was observed in the Ca-substituted system: from a decrease of τ_B with decreasing T to an increase of τ_B. With increasing x, the temperature dependence of τ_B remains weak in the Ca- and Fe- substituted systems. Compared with the experimental data of positron lifetime in other substituted systems and the calculation of the positron density distribution, the authors suggest that positron bulk lifetime spectra behaviour can be interpreted by the physical model based on the transfer of electron density between the CuO₂ planes and Cu-O chains. Therefore, the study of positron lifetime spectra provides a useful means to detect the local charge density and to study the correlation between the electronic structure and the high-T_c superconductivity.     

Electron concentration in YBa2Cu3O6.9 single crystals determined from field electron emission spectroscopic measurements

Results of field electron emission spectroscopic investigations of YBa₂Cu₃O_6.9 single crystal specimens oriented perpendicular to the c-axis are presented. It is shown that the shift of the total energy distribution of field-emitted electrons from YBa₂Cu₃O_6.9 along the energy scale with increasing electric-field strength at the surface of the specimen is due to the penetration of the electric field into the subsurface region of the YBa₂Cu₃O_6.9 specimen. The electron concentration in YBa₂Cu₃O_6.9 found from spectroscopic measurements is n=(2−4) × 10²¹ cm⁻³. Field electron emission spectroscopy and microscopy of YBa₂Cu₃O_6.9 allows one to draw the conclusion that the electron concentration in the bulk of the material does not change with cooling of specimens from 300 K to 115 K.     

Optical conductivity in superconducting layered cuprate oxides

Various properties of the cuprate oxides are explained by a layer model in which normal and superconducting layers are alternately stacked and the bulk superconductivity is maintained by the proximity effect. By using this model, the optical conductivity of the cuprate oxides is calculated. The result for YBa₂Cu₃O₇ is that the optical conductivity for the light polarized parallel to the CuO chain has finite values even for frequencies below the gap, although the conductivity for the light perpendicular to the chain vanishes below the gap, as in the conventional superconductors. The character of the optical conductivities is seen in the results of the optical conductivities measured by Schlesinger et al. using untwinned YBa₂Cu₃O₇ single crystals.     

Theoretical and experimental investigation of the linear optical response of YBa2Cu4O8 The relevance of LDA calculations

We report results of ellipsometric measurements of the dielectric tensor of YBa₂Cu₄O₈ as well as calculations employing density functional theory in the local density approximation (LDA). For the computation of the one-electron eigenvalues and states the linear-muffin-tin-orbital method (LMTO) was used. Measured and calculated results show good agreement. In particular, the calculation predicts different band structures for YBa₂Cu₃O₇ and YBa₂Cu₄O₈ in the vicinity of the Fermi energy which lead to significant differences in the optical spectra. One such difference is a region of small ε₂ in the near infrared which is a fingerprint of the high (relative to the top of the highest fully occupied band) Fermi energy as compared to the situation in YBa₂Cu₃O₇. Our experimental results confirm the theoretical findings.     

Studies of synthesizing behaviors and superconductivity of sol-gel YBa2Cu3O7−x samples in flowing oxygen atmosphere

Systematic studies of synthesizing behaviors of sol-gel YBa₂Cu₃O_7?x samples in flowing oxygen atmosphere and their superconductivity have been performed. A set of high temperature ρ-T curves has been obtained for the whole synthesizing process. After four rounds of synthesizing, the resistivity of the sample was around ρ=1.00×10^?3Ω · cm at room temperature. The ρ-T curve of the fourth round shows that the orthorhombic to tetragonal phase transformation of the sample occurs around 600 °C, which is lower than that of the YBa₂Cu₃O_7?x sample prepared by conventional solid-state reaction method. Other measurements, such as X-ray diffraction, SEM measurement and low temperature R-T and M-T measurement, were also performed. And the R-T and M-T measurement results suggest that during the synthesizing process, there exist some state at which the sample has better superconductivity than the other states. Moreover, we found screw dislocations presenting on the sample broken surface from the SEM images. This will change the concept that the screw dislocations can only grow on the surface of the YBCO thin films and single crystals.     

基于低温光声光谱技术的YBa2Cu3O7-δ超导转变过程

利用低温光声系统实时采集高Tc超导材料超导转变过程光声信号,是高Tc超导材料超导机理研究的一种新的实验方法。选取YBa₂Cu₃O_7-8超导体作为试样,分别构建常态→超导态（试样降温过程）和超导态→常态（试样升温过程）,得到转变过程光声信号对比图,并依据光声光谱图对试样失超现象作出可能的解释。     

EFFECTS OF THE QUALITY OF SUBSTRATE ON THE STRUCTURE AND SUPERCONDUCTING PROPERTIES OF Yba2Cu3O7-x THIN FILM

We have systematically investigated the structures, morphologies, microwave surface resistance, DC performances and the substrate qualities of three YBa₂Cu₃O_7-x(YBCO) thin films 30mm in diameter. The three samples deposited by the same process have been proved of different properties. It has been shown that the structure and superconducting properties of the thin film depend strongly on the quality of substrate.     

EFFECT OF THE Ba VACANCY ON THE FLUX PINNING IN YBaxCu3O7-δ SYSTEM

A series of Y123 single phase samples with various Ba vacancy concentration was prepared by making their Ba contents deviate from the stoichiometric composition. The measurements of their structure, superconductivity and flux pinning behaviour were systematically carried out. It is found that, compared with YBa₂Cu₃O_6.96 sample, the strength of the flux pinning in YBa_xCu₃O_7-δ(1.8≤x<2.0) samples is increased, and that there is an optimum value of Ba vacancy concentration for the maximum flux pinning force density. The possible origin of the flux pinning centers to determine the flux pinning behavior at higher field is discussed in detail. We suggest that the flux pinning effect at lower field may stein from the interaction between the vortex and the surfaces of grains, and that the flux pinning mechanism at higher field belongs to the core interaction.     

Realization and properties of ramp-type YBa2Cu3O7−δ/Au/Nb junctions

All-thin-film ramp type Josephson junctions between YBa₂Cu₃O_7−δ and Nb have been fabricated. This procedure allows connections between high-T_c and low-T_c superconductors at different crystal sides of the high-T_c superconductor on one chip, which is of great interest for novel phase devices. A thin Au layer is incorporated as a chemical barrier to avoid oxygen transfer from the YBa₂Cu₃O_7−δ to the Nb. Critical current densities up to 600 A/cm² are obtained at T=4.2 K, with typical R_nA values of 0.8 μΩ cm². The variation of the magnetic field dependence of the critical current with the angle between the junction barrier and the YBa₂Cu₃O_7−δ crystal axes is explained by considering a predominant d_x²−y² order parameter symmetry of the YBa₂Cu₃O_7−δ. The successful fabrication of these junctions allows the implementation of novel superconducting electronics, such as complementary Josephson circuitry or proposed qubit concepts, using the unconventional order parameter symmetry of the high-T_c superconductor.     

Effect of lead addition to Bi-based 2222 superconducting cuprates Bi2-xPbxSr2(Y, Ce)2Cu2O10+δ

Bi_2-xPb_xSr₂(Y, Ce)₂Cu₂O_10+δ, a new family of lead containing Bi-2222 compounds has been synthesized and effects of annealing in high pressure of oxygen studied. Lead-free Bi₂Sr₂(Y, Ce)₂Cu₂O_10+δ synthesized under flowing oxygen is known to be non-superconducting, but annealing under high oxygen pressure (100 bar) at 500°C, induced superconductivity with T_c=25 K. On substitution of lead (x>0.4), the superconductivity appears even in samples synthesized under flowing oxygen (1 bar). T_c increases with oxygen annealing pressure for all compositions, and it also increases with Pb content at a given oxygen pressure. These findings are discussed.