Local Structure and Superconducting Properties of Bi 2 Te 3 -Doped YBa 2 Cu 3 O 7 − δ

The samples of (Bi₂Te₃) _x (YBa₂Cu₃ O _7?δ) _1?x (x = 0, 2.5 %) have been synthesized at normal pressure, and the effect of the doping of topological insulator Bi₂Te₃ on the superconductor YBa₂Cu₃ O _7?δ is studied. The diamagnetism properties are investigated by zero-field cooling DC magnetization measurement. In the Bi₂Te₃-doped sample, an enhancement of superconducting fractions is observed, and the superconducting transition temperature (T _c ) increases. For the sample with x= 2.5 %, the phase transition of the tetragonal phase to the orthorhombic phase is observed by X-ray diffraction, and the increase of the degree of local structure disorder is confirmed by extended X-ray absorption fine structure, so it can be concluded that the superconducting fraction enhancement is attributed to the phase transition induced by the addition of topological insulator Bi₂Te₃. Scanning electron microscopy images show that the added Bi₂Te₃ is found to adhere to the surface of YBa₂Cu₃ O _7?δ grains, and it is considered that the strain induced by grains’ interaction leads to the phase transition which enhances the degree of local structure disorder at the same time. An absorption edge shift to the high-energy side is observed by the X-ray absorption near-edge structure. The edge shift can be attributed to the electron transfers from YBa₂Cu₃ O _7?δ to the additives. It is widely accepted that the hole intensity is closely related with T _c , so in YBa₂Cu₃ O _7?δ the added holes by the transition may lead to the enhancement of T _c . Thus, it can be concluded that the topological insulators can produce structural and electronic effect on the YBCO superconductors in the composite materials.     

d-Hole localization and the suppression of superconductivity in YBa2(Cu1-xZnx)3O7−y

The temperature and Zn concentration dependence of the electrical resistivity, specific heat, magnetic susceptibility, and electron paramagnetic resonance (EPR) spectra of YBa₂(Cu_1?x Zn _x )₃O_7?y withy～0.1 has been measured forx≤0.16. In addition, the temperature and field dependence of the magnetization has been measured for 2<T<300K and 0<H<9.0T, along with the temperature and quasihydrostatic pressure dependence of the electrical resistivity for selected samples for 0<P<13 GPa. The substitution of Zn for Cu in YBa₂Cu₃O_7?y causes a rapid and nearly linear depression of the superconducting transition temperature,T _c , withT _c going to 0 K forx≥ 0.10. YBa₂(Cu_1?x Zn _x )₃O_7?y retains the YBa₂Cu₃O_7-y orthorhombic structure forx≤0.16 for both the superconducting and nonsuperconducting samples. Initially, the unit cell volume increases nearly linearly with Zn content; however, an abrupt change occurs in the vicinityx=0.8–0.10. Forx<0.10, the temperature dependence of the electrical resistivity,ρ(T), is metallic-like (dρ/dT>0) andρ increases gradually with increasing Zn content. However, forx≥ 0.10,ρ(T) becomes semiconductor-like, with a very rapid increase of the resistivity with increasingx. The electrical resistivity, magnetic susceptibility, EPR spectra, and specific heat all indicate that thed-holes associated with the Cu ions become localized in the nonsuperconducting phase,x>-0.10.     

Solution-sol-gel processing of superconductors

Bulk materials and thin films of pure and homogeneous YBa₂Cu₃O_7−x and Bi₂Sr₂CaCu₂O_8+x compounds were prepared by a nanocomposite solution-sol-gel (SSG) method. The superconducting oxides of YBa₂Cu₃O_7−x and Bi₂Sr₂CaCu₂O_8+x were prepared at very low temperatures i.e. 750°C and 850°C, respectively by SSG method. Pellets sintered from these nanophasic sol powders showed sharp resistivity drops atT _c ∼ 90°K for YBa₂Cu₃O_7−x andT _c∼67°K for Bi₂Sr₂CaCu₂O_8+x . Thin films were prepared using triphasic sol of Y, Ba, Cu and tetraphasic sol of Bi, Sr, Ca and Cu on MgO and SrTiO₃ substrates. The triphasic sol coated on SrTiO₃ substrates and calcined at 800°C for 12h showed the formation of superconducting phase, YBa₂Cu₃O_7−x with preferred orientation along theC-axis. X-ray diffraction patterns of the Bi₂Sr₂CaCu₂O_8+x films on MgO substrate showed the formation of the superconducting phase with preferential orientation along the C-axis and the microwave absorption data as a function of temperature of this film revealed the onset temperature to be 90°K.     

Structural studies on Fe-substituted Y-Ba-Cu-O superconductors by Mössbauer spectroscopy

In this paper we report preparation and structural aspects of YBa₂Cu_2·97Fe_0·03O_7?x with different oxygen stoichiometry. The samples were prepared by standard solid-state reaction. The samples were characterized by X-ray powder diffraction and oxygen contents were estimated from iodometric titration method. The superconducting transition temperatureT _c is measured using the standard four-probe technique. Mössbauer studies of YBa₂Cu_2·97Fe_0·03O_7?x with different oxygen stoichiometry were carried out and the results show that Fe substitutes both Cu(2) and Cu(1) sites and clear asymmetry in the doublets except for the tetragonal case. Also our results show the progressive oxygenation inb-axis as oxygen stoichiometry is increased.     

Oxygen-enrichment of YBa2Cu3 YBa2Cu3O7-δ using the fluidization techniqueusing the fluidization technique

The oxygen-deficient phase of the highT _c superconductor, YBa₂Cu₃O₇, was oxygen-enriched using the fluidization technique to give good superconducting properties. The normal method of oxygen treatment at 900°C for 24 h and at 600°C for 24 h has been reduced to just one treatment at 600°C for 12 h by the fluidization technique to achieve almost the same strength of superconducting signal for the YBa₂Cu₃O₇ powder, which establishes the attractiveness of the latter route for the large-scale preparation of superconducting material. The particle sizes were in the range 0–90, 90–180 and 180–420 μm. The fluidized particles were crystalline with orthorhombic distortion.T _c ^onset , estimated using the a.c. magnetic susceptibility method, was 91·3 K. The volume fraction of superconducting material in the product was 83·7–85·3%, one of the highest values reported so far for YBa₂Cu₃O₇.     

Molten Salt Synthesis of BaPb1−x Bi x O3 Powder at Low Temperatures

Pure microcrystalline BaPb_1?x Bi _x O₃ (0≤x≤1) samples were successfully synthesized by a molten salt method using alkali metal nitrates as flux. In the present process, the particle size and shape of the samples are controllable. The optimal temperature for the synthesis of pure crystals is found to be 500 °C, which is lower than that in a conventional solid-state reaction method. The BaPb_1?x Bi _x O₃ powders are indexed to be single phase and the particles show well-crystallized euhedral shapes for the samples prepared under different conditions. Superconductivity is observed for samples within the composition range 0.1≤x≤0.3, and the highest superconducting transition temperature presents to be 13 K. Although the oxidation–reduction reactions might also take place, the “dissolution–precipitation” mechanism plays a dominant role in the molten salt synthesis of BaPb_1?x Bi _x O₃.     

Local Structural Distortions and Phase Separation in Cuprates

We report evidence from Raman measurements of local lattice distortions in the cuprates, which are induced at room temperature by varying the hydrostatic pressure and correlate with changes in the superconducting transition temperature. Anomalous nonlinear pressure dependence is observed for almost all A_g phonons of the YBa₂Cu₃O_6.5, YBa₂Cu₃O_over, and YBa₂Cu₄O₈ superconducting single crystals, which occurs at pressures where the transition temperature shows also characteristic changes. The results are compared with a similar correlation found between transition temperature and spectral modifications in Bi₂Sr₂CaCu₂O₈ by pressure and in YBa₂Cu₃O_x and La_2–xSr_xCuO₄ by chemical doping. There are strong indications that the saturation of T_c with pressure or compositional doping in these cuprates is mainly related with lattice instabilities and probably also with phase separation phenomena.     

Effect of additives on the morphology and superconductivity of YBa2Cu3O6+x ceramic materials

The effect of the addition of 0–15 wt.% alumina powder on the grain size, morphology and superconducting properties of sintered YBa₂Cu₃O_6+x was investigated as a function of alumina concentration in the composites. The results suggest that the grain size of YBa₂Cu₃O_6+x decreases with an increase in the additive concentration. Some surface erosion of the YBa₂Cu₃O_6+x grains was also noticeable in Al₂O₃/YBa₂Cu₃O_6+x composites. The results also indicate that the additive tends to stabilize the nonsuperconducting tetragonal phase of the YBa₂Cu₃O_6+x at the expense of the superconducting orthorhombic crystal phase.     

Influence of Boron Doping on Transport Properties of?YBa2Cu3O7?y HTS

The nominally pure and boron-doped YBa₂Cu₃B _x O_7?y samples with B-doping level x varying between 0 and 0.15 were prepared by the solid state reaction. X-ray diffraction analysis shows that all the obtained compounds are single YBa₂Cu₃O_7?y phase. The small additives of boron in YBa₂Cu₃B _x O_7?y (x=0.025 and 0.05) do not essentially affect the critical temperature T _c and it remains near 90 K. The higher-level boron doping causes degradation of T _c and tail remains on the ??(T) curve for x=0.15 at 65 K. Lowest-level boron doping applied leads to a significant improvement in J _c compared to the undoped sample (from 100 A?cm^?2 for a control sample to 147 A?cm^?2 for a B-doped sample with x=0.025). Grain boundary critical current density and superconducting volume fraction decrease with increasing amount of added B₂O₃. The obtained results indicate possibility of boron dopant being entered into the lattice-sites.     

Microstructure and dielectric properties of Ca1–3/2xBixCu3Ti4O12 (x = 0, 0.05, 0.10, 0.15 and 0.20) ceramics

Influence of bismuth substitution on calcium site in CaCu₃Ti₄O₁₂ has been investigated. Compositions of Ca_1-3/2xBi_xCu₃Ti₄O₁₂ (x = 0, 0.05, 0.10, 0.15 and 0.20) were fabricated by solid-state sintering method. Crystal structure is remained cubic. X-ray diffraction indicates the presence of secondary phase of CuO in CCTO ceramics. Bismuth doping restrains the formation of CuO phase apparently. The grain size of CaCu₃Ti₄O₁₂ ceramics was greatly decreased by Bi³⁺ doping, resulting from the ability of bismuth to inhibit the grain growth. The dielectric and electric properties of CCTO ceramics were found to be influenced by bismuth doping. The fitting results of the complex impedance spectra showed an increase of the resistance of grain and grain boundary by bismuth substitution. Ca_0.70Bi_0.20Cu₃Ti₄O₁₂ showed the highest dielectric constant in the low frequency range. A modest composition such as Ca_0.85Bi_0.10Cu₃Ti₄O₁₂ expressed the optimized dielectric properties of higher dielectric constant (1.3 × 10⁴) and lower dielectric loss (0.06) than pure CCTO. The low and high temperature dielectric loss spectra demonstrate the interfacial polarization of the initial and secondary oxygen ionization, relating with the grain and grain boundary (the electrode contact for Ca_0.70Bi_0.20Cu₃Ti₄O₁₂) respectively.     

151Eu Mössbauer studies on Zn-doped EuBa2Cu3O7−y

¹⁵¹Eu Mössbauer studies have been performed on the compounds EuBa₂(Cu_1?x Zn _x )₃O_7?y withx=0·0, 0·025, 0·05, 0·075 and 0·1. The parent compound, EuBa₂Cu₃O_7?y is superconducting with a transition temperature (T _c ) of 88 K.T _c is depressed as Zn is substituted for Cu in this system and the compounds withx>0·05 do not show superconductivity down to 12 K.¹⁵¹Eu Mössbauer studies at 295 K show a single Mössbauer line in all the compounds (whether superconducting or not) with isomer shift value typical of Eu^{3 +} ion. Further, the isomer shift values are nearly independent ofx and the temperatures down to 10 K. These observations imply that the Cu-O network responsible for superconductivity is very weakly coupled to the Eu sublattice.     

Superconductive and Magnetic Properties of Bi2Sr2Ca2Cu3O10+�� Ceramics Doped by Pb

Among the superconducting phases of bismuth-based Bi?CSr?CCa?CCu?CO, compound Bi₂Sr₂Ca₂Cu₃O_10+?? (Bi-2223) is the most interesting because of its relatively high critical temperature (T _C =95?C110 K) and numerous applications. However, this phase is also known for its low stability and the difficulty of purifying parasites phases including the Bi-2212. To this end, the Pb used in relatively high proportions can stabilize, purify, and improve the further enhancement of T _C . The influence of Pb on structural, superconducting and magnetic properties has been extensively investigated in polycrystalline Bi_2?x Pb _x Sr₂Ca₂Cu₃O_10+?? ceramics (0<x<1). For low Pb amounts, structural analysis shows that the Bi-2223 phase is difficult to achieve without the Bi-2212 phase, and for high Pb content a large fraction of secondary phases containing Pb is detected. Our results confirm that the optimal Pb content for obtaining a Bi-2223 single phase is x=0.3, 0.4.     

Mössbauer studies of Gd-doped bismuth-based ceramic superconductors

The results of¹⁵⁵Gd Mössbauer studies (above 1·5K) of the oxides, Bi_3·5Gd_0·5Ca₃Sr₃Cu₄O _y and Bi₂Sr₂Ca_1?x Gd _x Cu₂O _y (x=0·25 and 0·5) are reported. The observed electric quadrupolar split resonance patterns suggest that Gd ions prefer to occupy Ca site in both series. The spectral features give evidence for paramagnetic relaxation of isolated Gd ions at low temperatures.     

Geometrical model and experimental evidence for the growth of Ag-doped YBa2Cu3O7−x thin films

The improvement in the properties of laser-ablated Ag-doped YBa₂Cu₃O_7?x (YBCO) thin films in both normal and superconducting states has been interpreted using a growth model. The model is simple and is based on the widely accepted characteristics of Ag such as its flux action at high temperature and nonreactivity with YBCO phase.     

Effect of annealing and quenching on superconductivity in Pb and Sb-doped Bi2Sr2Ca2Cu3O x

Samples of the series Bi_1·9−x Pb _x Sb_0·1Sr₂Ca₂Cu₃O _y withx=0, 0·1, 0·2, 0·3 and 0·4 were prepared by the solid-state route. The X-ray and d.c. electrical resistivity data on furnace-cooled and quenched samples are presented. Though the starting composition is 2223, the end products were multiphase with 4334 as the major phase. A superconducting transition withT _c=100K was observed in the pure 2223 sample after quenching. The furnace-cooled samples were metallic, while samples withx=0·1, 0·2 and 0·3 were superconducting after quenching. The amount of the 4334 phase decreases with increasing Pb content. Quenching seems to be favourable for the formation of the 4334 phase.     

The phase conversion of precursor powders for powder melting process YBa2Cu3O7−x superconductors

The effects of heating rate and holding time on the formation of YBa₂Cu₃O_7−x phase in precursor powders for YBa₂Cu₃O_7−x superconducting bulks prepared by powder melting process have been investigated. The phase conversion of the precursor powders is studied by X-ray diffraction and found to be different for different heating rates during heating. The YBa₂Cu₃O_7−x phase is formed during heating to peritectic temperature at 100 and 400 °C/h, but not at 6,000 °C/h. The longer the holding time, the more the amount of YBa₂Cu₃O_7−x phase between 880 °C and about 950 °C. The results are useful for understanding the mechanism of powder melting process and controlling the process conditions.     

Synthesis and Characterization of Doped YBa2Cu 4 O8 Superconductor by Cd+2

In this study, the effect of yttrium substitution by cadmium in the YBa₂Cu₄O₈ superconductor was investigated. A series of superconducting oxide ceramics of (Y_1?x Cd _x )Ba₂Cu₄O₈ with a different Cd content (x=0.00–0.18 mol) were prepared by an aqueous sol-gel method using aqueous mixtures of the metal acetates. Homogeneous precursor gels were obtained by complexing copper ions with tartaric acid. The obtained gel powders and end products were characterized by X-ray powder diffraction analysis, infrared spectroscopy, thermal gravimetric analysis, differential thermal analysis, scanning electron microscopy, energy of dispersive X-ray analysis, DC-electrical resistivity, and magnetic susceptibility measurements. The gel synthesized was amorphous and highly homogeneous. Single-phase bulk samples were obtained after annealing at 1108 K for 100 h, in an atmosphere of flowing O₂ gas. XRD measurements indicated that the crystal structure remains orthorhombic (up to x=0.12 mol). With a further increase of the cadmium substitutional level, peaks of Y-124 phase disappeared. Doping the YBa₂Cu₄O₈ phase with cadmium showed a strong effect on the superconducting properties increasing the critical temperature (T _c) from 80 K (for the nonsubstituted sample) to 92 K (for the (Y_0.88Cd_0.12)Ba₂Cu₄O₈), and T _c decreases with a further increase of the cadmium substitutional level. Also prepared was a series of (Y_1?x Cd _x Ba₂Cu₄O₈) samples with (0.0≤x≤0.08 mol), via the ceramic method and annealed at 1123 K for 132 h in an atmosphere of flowing O₂ gas. The characterization of the final products was conducted by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray analysis. We have not observed any trace of the superconducting Y-124 phase in any of our Cd doped samples.     

Comparison of Y-Ba-Cu-O compounds prepared from BaCuO2 and Ba2Cu3O5+δ

Series of Y-Ba-Cu-O compositions were prepared from barium cuprates and Y₂O₃, using a two-step synthesis route. It has been shown that Ba₂Cu₃O_5+δ is essential in formation of the YBa₂Cu₃O_7?δ superconducting phase while BaCuO₂ is not an appropriate starting material. A wide composition range fromx=2 to 17 was prepared from Ba₂Cu₃O_5+δ in the YBa _x Cu_1.5x O _z series without disappearance of macroscopic superconductivity atT _c>77 K. Resistivity measurements hint at the existence of two superconducting phases. ESR investigations revealed a baseline hysteresis, depending on the actual value ofx.     

Correlation Between Ferromagnetism and Superconductivity at Interfaces of La2/3Ca1/3MnO3/YBa2Cu3O7−δ /La2/3Ca1/3MnO3 Trilayers Grown by dc Sputtering

In this work, we study the magnetoelectrical properties of La_2/3Ca_1/3MnO₃/YBa₂Cu₃O_7?δ /La_2/3Ca_1/3MnO₃ heterostructures grown by means of high oxygen-pressure dc sputtering onto (001)-oriented SrTiO₃ substrates. The test heterostructures are composed of ferromagnetic layers of constant thicknesses (～280 unit cells, ～110 nm) and superconducting interlayers with thicknesses ranging between 5 (～6 nm) and 15 unit cells (～17 nm). Transport measurements show a strong suppression of the superconducting properties when the thickness of the YBa₂Cu₃O_7?δ interlayer is reduced to a value below 10 unit cells. Magnetic measurements show superconducting transition in junctions with YBa₂Cu₃O_7?δ interlayers with thicknesses larger than ～15 unit cells. The observed difference in the values of the superconductivity onset when determined by electrical and magnetic measurements might be related with the presence of a spontaneous vortex phase in the temperature range around the superconducting transition.     

The Underdoped Region of the Phase Diagram of YBa2Cu3O6+ x

Here we present a reviewed phase diagram of the high-T _c superconducting YBa₂Cu₃O_{6+ x} compound, finely mapped in the strongly underdoped region (0?<?x?<?0.5), from the pure antiferromagnetic state to the superconducting regime. The Neèl and spin freezing temperatures have been measured by μSR experiments while the hole density per Cu atom in the CuO₂ planes has been determined from the resistive T _c and from Seebeck coefficients at 290 K. The phase diagram is discussed in comparison to those of La_{2? x} Sr _x CuO₄ and Y_{1? x} Ca _x Ba₂Cu₃O₆ cuprate systems.