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.     

Resistivity and thermoelectric power of Bi2Sr2Ca−xPrxCu2Oy system

Samples of Bi₂Sr₂Ca_1−xPr_xCu₂O_y have been characterized by resistivity and thermoelectric power measurements. All metallic samples show superconductivity with a maximum T_c = 90 K at X = 0.2. The sample of x = 0.6 shows a crossover from hopping conduction at low temperature above T_c to metallic conduction at high temperature. For the metallic samples below x = 0.6, the results of thermoelectric power are well fitted by both of a phenomenological band spectrum model and the Nagaosa and Lee model.     

Intergranular properties of YBCO and BSCCO ceramic superconductors at low fields

Isothermal low-field AC susceptibility measurements have been used to analyze the intergranular critical current density J_c(T) on sintered, non-oriented YBa₂Cu₃O_7−δ and Bi₂Sr₂CaCu₂O_8+δ ceramic samples at zero field. Below the critical temperature, potential variations, J_c(T) ≈ (1−t_j)^m with t_j = T/T_j, have been found, T_j being the onset of grain's coherence, but with different exponents, supporting that different mechanisms limit the intergranular J_c values. Moreover, the effect of texture has been also considered on Bi₂Sr₂CaCu₂O_8+δ ceramics grown by the laser floating zone method, which have stronger intergranular junctions. Their high-temperature behaviour is limited by intrinsic effects, while at low temperatures the quality of the junctions is the limiting factor. The temperature dependence of the χ′(h₀) extrapolation at zero filed has also been correlated with the evolution of the intergranular penetration depth, λ_J(T).     

Effect of Hg addition on synthesis of Bi-based superconductors

We have investigated the effect of Hg addition on the superconducting properties of BiSrCaCuO system. Polycrystalline samples with nominal composition Bi₂Sr_2−xHg_xCa₁Cu₂O_y and Bi₂Sr_2−xHg_xCa₂Cu₃O_y (x=0.3) were synthesized and used to investigate the phase evolution by XRD, superconducting behaviour by R–T measurement and the structural grain boundary effects by SEM. From these measurements, it has been noticed that the phases obtained with both types of compositions are the same as Bi2212 but the T_c values are different. With additional annealing, T_{c zero} values were raised from 60 to 72 K in Bi₂Sr_2−xHg_xCa₁Cu₂O_y and 64 to 92 K in Bi₂Sr_2−xHg_xCa₂Cu₃O_y. Also, an improved grain boundary linkage has been observed by SEM for the 92 K sample.     

Tunneling studies on single crystals of superconducting Bi2Ca1−xYxSr2Cu2O8+δ

Tunneling studies have been carried out on single crystals of Bi₂Ca_1−xSr_xCu₂O_8+δ over a wide range of compositions wherein the hole concentration varies by a factor of 2.5. The 2Δ value varies between 25 meV and 75 meV over the composition range studied, but scales with 2Δ/k_BT_c≈9.5 throughout.     

Microwave absorption and paracoherence excess conductivity studies in high-Tc superconductors

Single phase polycrystalline samples of La_1.8Sr_0.2CuO₄, YBa₂Cu₃O_7−δ and Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_y high-T_c superconductors have been studied using XRD, SEM, EDAX, (T), χ_AC and EPR techniques. Detailed investigations on low-field-dependent microwave absorption have been carried out in order to investigate the effects of field exposure, field cooling, zero field cooling and amplitude of field variation on the samples. The results have been discussed on the basis of granular model via Josephson junctions of high-T_c superconductors. Over and above, paracoherence excess conductivity analysis has been carried out in all the three samples. The critical exponent (γ) estimated for all the three samples in close vicinity to their respective transition temperatures has an excellent agreement with the theoretically predicted value for 3-dimensional XY-ferromagnet (γ = 1.33 for D = 3, N = 2).     

Fabrication, microstructure and critical current density of screen-printed Ag-(Bi, Pb)2Sr2Ca2Cu3Ox superconducting tapes

The influence of the sintering conditions on the microstructure and critical current density J_c has been studied on screen-printed Ag-(Bi, Pb)₂Sr₂Ca₂Cu₃O_x tapes with a ceramics mono-layer core. Three kinds of fabrication processes, which consist of a combination of cold working (rolling and/or pressing) and sintering, are applied. Four times repetition of pressing and sintering after the pre-sintering produces the highest c-axis alignment and achieves J_c= 1.5 × 10⁴ A/cm² (77 K, 0 T). The J_c versus θ data with an angle θ between B and the c-axis elucidate the relation between the anisotropy ratio γ=J_c(Bc)/J_c(B|c and the half-height angular width Δθ of a peak for Bc. This is related to both grain alignment and the J_c value. An increase in J_c, which comes from an improvement for grain alignment, enhances γ and narrows Δθ. The J_c versus θ data are fitted to the expression J_c(B, θ)=J _c(B, 90°)/[(γ−1)|cos θ|ⁿ+1] by regarding both γ and n as adjustable parameters. Fabrication of screen-printed tapes with multilayers (1≤N≤5) is presented, where the critical current increases from 8.0 A to 30.2 A at 77 K and 0 T as N increases.     

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.     

Effects of Pb-ion irradiation on the vortex pinning in melt-textured YBa2Cu3Ox

Melt-textured YBa₂Cu₃O_x quasicrystalline samples were studied for the effects of Pb-ion irradiation on the vortex pinning. The strength of the pinning was monitored by measurements of the saturation remanent magnetization (M_R) as a detailed function of temperature from 4.2 K up to T_c (≈92 K). Our results show that M_R is greatly enhanced by the Pb-ion irradiation at all temperatures below T_c. Specifically, when compared to a similar but nonirradiated sample, the M_R of an optimally irradiated sample is raised by a factor of 3.4 at 4.2 K and, more strikingly, by 12.7 at 77 K. Contrastingly, in an earlier study of a γ-ray irradiated Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀ sample, the vortex pinning was seen to be enhanced but mainly near 4.2 K, suggesting that the γ-ray irradiation produces highly localized pinning sites. Hence, our present results imply conversely that the Pb-ion irradiation creates extended structural defects that are effective as vortex pinning sites up to relatively high temperatures, including that of liquid nitrogen.     

Magnetic properties of the Bi1.95Sr2.05−xLaxCuOy (Bi-2201) superconducting phase

We have investigated the reversible mixed-state magnetization M of three lanthanum substituted Bi_1.95Sr_2.05−xLa_xCuO_y (Bi-2201) ceramic samples having different critical temperatures T_c ranging from 20.0 to 35.5 K. As for the Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) phase, we found that anisotropy of Bi-2201 is large. A manifestation of this anisotropy is the field independent magnetization M^* observed at a temperature T^*. In the framework of the London model, and including thermal fluctuations of vortices, we found for the temperature dependence of the penetration depth λ_ab(T) = λ_ab(0)[1 − (T/T_c0)ⁿ]^−1/2, with n 1.7 and λ_ab (T = 0) 4000 Å. The estimated upper critical fields μ₀H_c2,c are of the order of 10 T. We observe a peculiar negative slope M/T at low temperature and sufficiently high external magnetic field. This feature seems to be a characteristic of the Bi-2201 phase. However, we do not know whether it is associated with the superconducting mixed-state. A small amount of magnetic impurities could also be responsible for this behavior. Finally, the behavior of the reversible magnetization of the Bi-2201 samples investigated, which are situated at the optimal and in the overdoped region, did not indicate any unusual temperature dependence for the upper critical field H_c2,c.     

Trapping mechanism of superconductivity suppression induced by Pr substitution in the Ho1−xPrxBa2Cu3O7−δ system

Pr concentration dependence of the superconducting transition temperature T_c in the Ho_1−xPr_xBa₂Cu₃O_7−δ system is determined from measurements of DC electrical resistance. This dependence coincides with that for the parallely studied Y_1−xPr_xBa₂Cu₃O_7−δ reference system. Both systems have the same value of the critical concentration x_c=0.58, in accordance with nearly equal ionic radii of Ho³⁺ and Y³⁺ ions. It has been shown that the T_c(x) curve can be described with a single mechanism based on a decreasing number of sheet holes trapped by Pr^IV-ions, if one takes also into account that the number of these ions changes with x.     

Synthesis and investigation of the complex copper oxides Bi2Sr2R2−xCexCu2O10+δ (R=rare earth)

Bi-2222 phases were prepared by solid state reactions and were studied using X-ray diffraction, chemical and EDS-analysis. Compounds with composition Bi₂Sr₂R_1.33Ce_0.67Cu₂O_10+δ were obtained for R=Pr, Nd, Sm-Er. The phases Bi₂Sr₂Cu₂O_10+δ with similar structure but without Ce were prepared for R = Pr, Sm-Dy. For Bi₂Sr₂Eu_2−xCe_xCu₂O_10+δ the solubility limit was determined and the effect of treatment under different oxygen pressures was investigated. Weak diamagnetic signals (<3%) were detected for some of these samples.     

Synthesis and characterization of the Aurivillius phases Bi2 − xPbxSr1 − xNdxNb2O9

The n = 2 Aurivillius phase Bi_{2 − x}Pb_xSr_{1 − x}Nd₂O₉ was successfully synthesized as a ceramic material over the whole range of simultaneous, charge compensated substitution x = 0–1.0. Structural investigations were performed by Rietveld refinement applying different space groups Fmmm and A2₁am, and additionally by X-ray absorption spectroscopy (EXAFS) on the Nd L_III-edge, confirming the accommodation of Nd on the atomic sites of Sr, which implies the substitution of Bi³⁺ by the isoelectronic Pb²⁺. The ferroelectric transition temperature T_c = 270 °C of the substituted powders with x = 0.4 and 1.0 is distinctly reduced compared to the unsubstituted sample with T_c = 450 °C. In temperature resolved powder X-ray diffraction patterns no structural phase transition could be detected.     

Temperature dependence of the exafs spectrum in YBa2Cu3O7−δ compounds

The temperature dependence of the extended X-ray absorption fine structure (EXAFS) is studied in the high T_c superconductors, YBa₂Cu₃O_7−δ. The measurements were done at the Cu K-edge for samples of two orthorhombic phases (T_c≈90 K and ≈58 K, respectively) and a nonsuperconducting tetragonal phase. Interatomic distances and mean square relative displacements σ² for Cu-O bonds are determined by the least squares refinement. The results indicate that values of σ² increase near T_c for both the orthorhombic samples. It is concluded that this anomalous behavior related to T_c is caused by an anomalous vibration of oxygen atoms in the Ba-O layer. Changes in the Cu-O distances from 300 to 20 K are not found.     

The effects of substrate temperature on the superconducting properties of Tl2Ca2Ba2Cu3O10 films sputter-deposited from stoichiometric oxide targets

The annealing characteristics and the superconducting properties of Tl₂Ca₂Ba₂Cu₃O₁₀ thin films sputter-deposited onto yttrium- stabilized ZrO₂ substrate at up to 500°C from two stoichiometric oxide targets are reported. The films deposited at 400–500°C were found to require a lower post-annealing temperature than the films deposited at lower temperatures to attain the highest T_c superconducting state, due to a more pronounced Ba diffusion toward the substrate as indicated by their secondary ion mass spectrometry depth profiles. The highest T_c achieved tends to degrade with increasing substrate temperatures, a zero resistance T_c of 121 and ≈90 K, respectively, being observed for the films deposited at -ambient temperature and at 500°C. The formation of the highest T_c phase (Tl₂Ca₂Ba₂Cu₃O₁₀) generally is associated with a sheet type of crystal growth morphology with smooth and aligned surfaces which can be obtained only from the films capable of sustaining prolonged annealing at 900°C. Annealing at lower temperatures (≈860°C) results in the formation of rod or sphere type of morphologies with rough and randomly oriented crystals and the lower T_c phases such as Tl₂Ca₁Ba₂Cu₂O₈.     

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.     

The effect of gallium substitution on the superconductivity of the BiSrCaCuO system

Magnetic susceptibility, X-ray diffraction and resistivity measurements of the system Bi_1.4Pb_0.6Sr₂Ca_2−xGa_xCu₃O_y are reported for 0 x 2. The high-T_c 2223 phase with a T_c of 107 K for x = 0 exists for x 0.3. The low-T_c 2212 phase with a T_c of 75 K for x = 0 exists for the full range of x. The highest values of the critical temperature and the largest volume fraction of the low-T_c phase in compounds with Ga occur for x = 0.5 ± 0.1. The identification of CaO by X-ray diffraction for x 0.6 indicates that Ga replaces Ca in the compound.     

Effective activation energy Ue(T,J,H) in textured Bi1.84Pb0.4Sr2Ca2Cu3Oy silver-clamped thick films

We have measured the resistivity of textured Bi_1.84Pb_0.4Sr₂Ca₂Cu₃O_y silver-clamped thick films as a function of temperature, current density ranging from 10 to 1×10³ A/cm² and magnetic field up to 0.3 T. We find that the effective activation energy U_e follows U_e(T,J,H)=U₀(1−T/T_p)^mln(J_c0/J)H⁻ with m=1.75 for Hab-plane and 2.5 for Hc-axis and =0.76 for Hab and 0.97 for Hc, for the current density regime above 100 A/cm², where T_p is a function of applied magnetic field and current density. This result suggests the effective activation energy U_e be correlated with the temperature, current density and magnetic field. The possible dissipative mechanisms responsible for the temperature, current density and magnetic field dependence of the effective activation energy are discussed.     

Origin of intrinsic Josephson coupling in the cuprates and its relation to order parameter symmetry: An incoherent hopping model

Experiments on the cuprate superconductors demonstrate that these materials may be viewed as a stack of Josephson junctions along the direction normal to the CuO₂ planes (the c-axis). In this paper, we present a model which describes this intrinsic Josephson coupling in terms of incherent quasiparticle hopping along the c-axis arising from wave-function overlap, impurity-assisted hopping, and boson-assised hopping. We use this model to compute the magnitude and temperature T dependence of the resulting Josephson critical current j_c(T) for s- and d-wave superconductors. Contrary to other approaches, d-wave pairing in this model is compatible with an intrinsic Josephson effect at all hole concentrations and leads to j_c(T) T at low T. By parameterizing our theory with c-axis resistivity data from YBa₂Cu₃O_7−δ (YBCO), we estimate j_c(T) for optimally doped and underdoped members of this family. j_c(T) can be measured either directly or indirectly through microwave penetration depth experiments, and current measurements on Bi₂Sr₂CaCu₂O₈ and La_2−xSr_xCuO₄ are found to be consistent with s-wave pairing and the dominance of assisted hopping processes. The situation in YBCO is still unclear, but our estimates suggest that further experiments on this compound would be of great help in elucidating the validity of our model in general and the pairing symmetry in particular.     

Crystal structure of Tl0.5Pb0.5Sr2Ca2Cu3O9 at 300 K and around Tc (118 K)

The crystal structure including the cation distribution, of a polycrystalline sample of nominal composition Tl_0.5Pb_0.5Sr₂Ca₂Cu₃O₉ with T_c = 118.2 K has been determined using resonant synchrotron X-ray diffraction data collected at the Cu K, Tl L_III and Sr K edges and time-of-flight powder neutron diffraction data. No oxygen deficiency was observed, but cation disorder at all the non copper sites according to the formula (Tl_0.60Pb_0.40)(Sr_1.60Ca_0.40)(Ca_1.93Tl_0.07) Cu₃O₉ gives a mean hole concentration of 0.18(1) per Cu atom for the three CuO₂ planes, consistent with the high T_c for this material. Analysis of five time-of-flight powder neutron diffraction data between 80 and 150 K have revealed a possible discontinuity in the variation of the c lattice parameter at T_c, due to an anomaly in the position of the apical oxygen atoms.