Magnetic order in Sr x Ca1−x Cu0.99 57Fe0.01O2

Sr _x Ca_1–x Cu_0.99 ⁵⁷Fe_0.01O₂ was studied forx=0.13, 0.15, and 0.17. Mössbauer spectroscopy and magnetization measurements indicate magnetic ordering characteristic of spin glass systems withT _f70K forx=0.15 and 0.13.     

Superconductivity and magnetism in Y1−x Ca x Sr2GaCu2O7−δ

Samples of Y_1–x Ca _x Sr₂GaCu₂O_7– (x=0, 0.4) doped with⁵⁷Fe, prepared under various oxygen pressures, have been studied by magnetometry and Mössbauer spectroscopy. Most of the iron ions (Fe³⁺) enter the Ga(Cu(1)) site. For thex=0 sample, the Mössbauer spectra of the iron nuclei in the Cu(2) sublattice display magnetic order of Cu,T _N=370 K. The iron ions in the Ga site display magnetic order only at low temperatures. At temperatures above 90 K, these iron ions display a pure quadrupole doublet Mössbauer spectrum. The samplex=0.4 also displays magnetic order of the Cu(2) site,T _N370 K. A sharp drop in the hyperfine field is observed atT _N, probably associated with a first-order phase transition or two-dimensional ordering. The iron nuclei in the Ga site display paramagnetic long spin relaxation time phenomena at 4.2 K. Thex=0.4 sample prepared under 110 atm oxygen pressure, displays superconductivity,T _c50 K. The Mössbauer spectra give evidence of the presence of two phases. One displays magnetic order, the other is paramagnetic, the last is probably associated with the superconducting phase.     

Electron paramagnetic resonance of Gd3+ ions in Ca1 − x − y Y x Gd y F2 + x + y crystals

Electron paramagnetic resonance of Ca_{1 ? x ? y} Y _x Gd _y F_{2 + x + y} single crystals has revealed spectra that are not typical of gadolinium-doped CaF₂ crystals. These spectra have a nearly tetragonal symmetry and are most probably caused by Gd³⁺ ions localized in yttrium clusters. Weak spectra of tetragonal Gd³⁺ centers, whose parameters are close to those of a cubic gadolinium center caused by an isolated Gd³⁺ ion, have been also detected. These centers are attributed to isolated Gd³⁺ ions localized near octahedral rare-earth clusters or their associations.     

Lattice dynamics of the superconducting Bi2Sr2Ca(Cu,Sn)2O8−y

Phonon anomaly was observed below 140 K for the absorption area (lnA) and the isomer shift () plotted against the temperature (T) in the¹¹⁹Sn Mössbauer study of the Bi₂Sr₂Ca(Cu_0.995Sn_0.005)₂O_8–y superconductor (glass-ceramic,T _c(on)=86 K), which was prepared by heat treatment of the glassy sample. The anomaly suggests that the softening (quenching) of the lattice vibration causes the superconducting transition, as was observed in several YBa₂(Cu, Sn)₃O_7–y superconductors. Fitting of the theoretical curve for the normal phonon of the combined Debye and Einstein model to the lnA versusT and versusT plots yielded _D and _E of 240 and 140 K, respectively. These results suggest that the probe (Sn⁴⁺) was located at the interstitial site close to the Cu site, and the softening of the Cu site vibration could be detected before the superconducting transition took place. Disordered structure seems to be favorable for the detection of phonon anomaly in the Bi system.     

Peculiarities of magnetic flux penetration and trapping in monocrystalline (YBa2Cu3O7−x and Bi2Sr2CaCu2O8+x ) and polycrystalline (YBa2Cu3O7−x ) samples

A comparative analysis of the magnetic flux trapping in monocrystalline and polycrystalline HTSC samples is carried out, and the possibility of employing the dependence of the trapped magnetic flux on the external magnetic field for obtaining comparative estimates of the effect of pinning centers is determined.     

The effect of pinning centers in Zn-doped CuBa2Ca3Cu4O12−y high-temperature superconductors

CuBa₂Ca₃Cu₄O_12−y (Cu:1234) high-temperature superconductors (HTS) doped with up to 2% Zn were grown using the high-pressure synthesis technique. Magnetization loops of the samples were measured at various temperatures between 5 and 77.3 K and magnetic fields up to 14 T. Critical current densities J_c of the samples were estimated using the critical state model. The results show that Zn-induced pinning centers increase J_c of Cu:1234 several times, depending on field and temperature. From the experimentally determined field-temperature region in which a higher Zn concentration lead to a higher J_c, we suggest the existence of a cross-over from quite efficient, extended (in the c-axes direction) pinning centers to point-like (inefficient) pinning centers at a certain temperature, depending on field. This effect can be attributed to the fact that, unlike other HTS, in Cu:1234 there is a second critical temperature T_c2 of about 70–80 K (in zero field, and 50–60 K in 15 T), related to the over-doping of pyramidal basal plane (outer CuO₂ plane).     

Synthesis and characterization of (Bi2O3)1−x−y−z(Gd2O3)x (Sm2O3)y(Eu2O3)z quaternary solid solutions for solid oxide fuel cell

Sm₂O₃, Gd₂O₃, Eu₂O₃ triple-doped Bi₂O₃ based quaternary solid solutions were synthesized as a candidate electrolyte material using the solid-state reaction technique. The structural, thermal and electrical conductivity features of the ceramic samples were examined and compared by using X-ray powder diffraction (XRD), thermal gravimetry/differantial thermal analysis (TG/DTA) and the four-point probe technique (4PPT). The result of XRD measurements indicated that the (Bi₂O₃)_{(1−x−y−z)}(Gd₂O₃)_x(Sm₂O₃)_y(Eu₂O₃)_z (x = 10/y = 10/z = 5, 15, 20 mol % and x = 10/y = 5, 10, 15, 20/z = 10 mol %) samples have a stable face-centered cubic δ-phase and mixed phase crystallographic structure. The phase stability was also checked by the DTA evaluations results. The temperature dependent electrical conductivity measurements showed that the highest electrical conductivity was observed for the sample of the (Bi₂O₃)_0.75(Gd₂O₃)_0.10(Sm₂O₃)_0.05(Eu₂O₃)_0.10 system which has a stable and δ-phase was found as 6.67 × 10⁻³ (Ω cm)⁻¹ at 650 °C. This sample can be used as an electrolyte material in the solid oxide fuel cells (SOFCs) which is possible to operate at intermediate temperature ranges. The activation energy was also calculated at a low temperature range (350–650 °C) and high temperature range (above 650 °C). The values for the samples vary from 0.63 eV to 1.08 eV at low temperature and at high temperature they vary from 0.43 eV to 0.75 eV.     

EPR study on the La2/3Ca1/3Mn1−x Cu x O y system

The interaction between Mn and Cu ions is studied by measuring the resistance and electron paramagnetic resonance (EPR) at different temperatures of the Cu-doped compound La_2/3Ca_1/3Mn_1−xCu_xO_y. A new transition inR-T curve and substantial enhancement in magnetoresistance are induced by the substitution of Mn ions by Cu ions. The EPR measurement shows that two resonance signals appear at temperature lower than the spin-ordering temperature of Mn ions. A tentative interpretation for the observed phenomena is proposed by considering the interaction between the Cu/Mn ions besides the Mn³⁺/Mn⁴⁺ ions.     

Superconductivity and processing of single Bi−O layered cuprates in the (Bi,Pb)−Sr−(Ca,Y)−Cu−O system

The effects of improved materials processing on single Bi–O layered cuprates in the (Bi, Pb)–Sr–(Ca, Y)–Cu–O system have been investigatged. For Bi-1212 we have improvedT _c to 102 K. The bulk nature of superconductivity is confirmed by the presence of superconducting volume fractions ( ZFC) around 30–40%. The critical current density is 2×10⁶ Acm^–2 at 5 K and 0 T. Moreover, indications for the presence of a second phase probably Bi-1223 with a transition to superconductivity in the range of 115–150 K have been found.     

Local structure,magnetism, and superconductivity in Sr analogs of Fe-doped YBa2Cu3−y Fe y O6+x

The local structure, magnetism, and superconductivity of Y_1–z Ca _z Sr₂Cu_3–y Fe _y O_6+x have been investigated by magnetic susceptibility,⁵⁷Fe Mössbauer absorption spectroscopy, X-ray powder diffraction, and iodometric titration. By modification of the hole distribution between the Cu(1)O _x chains and Cu(2)O₂ sheets, we induced bulk superconductivity with a maximum superconducting critical temperatureT _c near 20 K in Y–zCa _z Sr₂Cu_2.5Fe_0.5O_6+x (0.05z0.18). We interpret the development of a broad maximum inT _c and the Meissner fractionV _f versusz nearz=0.12 to be due to an increasing transfer of holes from the Cu(1)O _x chains to the Cu(2)O₂ sheets induced by a shorterc-axis and reduced Coloumb repulsion of the positively charged Y_1–z Ca _z plane. The increase in hole transfer from the Cu(1)O _x chains to the Cu(2)O₂ sheets also modulates the magnetic ordering temperatureT _m. We show thatT _m of YSr₂Cu₂FeO_7.26 is approximately 65 K, which is much lower than that observed in YBa₂Cu_2.31Fe_0.69O_7.21 (T _m400 K).     

Spin dynamics in Gd x Y1−x Ba2Cu3O6.85 investigated by ESR of Gd spin probes

Electron spin resonance investigations of Gd³⁺ ions in YBa₂Cu₃O_7?δ give distinct indications for an exchange coupling of these paramagnetic ions to the doped charge carriers and allow the corresponding interaction parameters, as well as the mutual Gd-Gd exchange coupling, to be extracted.     

Superconducting energy gap in Bi2Sr2Ca2Cu3O10+δ (Bi2223) single crystals

Current-voltage characteristics of S-I-S tunnel break junctions fabricated from pure undoped Bi2223 single crystals (T _c =110 K) were measured. High quality of the crystals enabled production of good tunnel junctions with a low or almost zero leakage current and well developed gap structure in the tunneling spectra. The peak-to-peak energy gap values 2Δ_p-p in different crystals and the tunnel junctions ranged from 80 to 105 meV. The tunneling conductance in the superconducting state was normalized to that in the normal state and compared to a smeared BCS density of states. A simple fit of the data gave the average value of Δ=38.5 meV and reduced gap 2Δ/kT _c ?8, consistent with a very strong coupling mechanism.     

Structure and dielectric properties of Bi6 − x Sr x Ti2 − x Nb2 + x O18 (x = 0–2) solid solutions

The structural and electrophysical characteristics of a series of solid solutions of layered perovs-kite-like oxides Bi_{6 ? x} Sr _x Ti_{2 ? x} Nb_{2 + x} O₁₈ (x = 0, 0.25, 0.5, 1.0, 1.5, 2.0) have been studied. The temperature dependences of the relative permittivity ?/?₀(T) and dielectric loss tangent tanδ have been measured. The dependences of the maximum of the permittivity ?/?₀, Curie temperature T _C, lattice parameters, and the unit cell volume on x have been obtained. The structural parameter a, which corresponds to the polar direction, and the value of the orthorhombic distortion of the unit cell have been found to demonstrate noticeable negative deviations from the Vegard’s law. It has been established that the variations of the orthorhombic distortion correlate with the variations of the permittivity maximum; however, they do not markedly influence the Curie temperature that varies linearly over entire range of changes in x.     

μSR study of vortex structure mobility and penetration depth in Bi2Sr2CaCu2O2-δ textured sample

The giant mobility of the magnetic flux in FrFC and ZFC conditions have been studied in textured sample Bi₂Sr₂CaCu₂O_2- bySR. The comparison of results obtained in both cases showed that it is difficult to explain this phenomenon by only thermally activated flux creep and the melting of vortex lattice must be considered. The magnetic field dependence of the melting temperature was obtained for H<90mT. It is shown that the possibility of vortex motion does not affect the results of FC-measurements.     

Study of magnetic flux pinning in granular YBa2Cu3O7 − y /nanoZrO2 composites

In this work, the effect of ZrO₂ nanoparticles prepared in a low-pressure arc discharge plasma on magnetic flux pinning of granular YBa₂Cu₃O_{7 ? y} /nanoZrO₂ composites has been studied. It has been shown that the ZrO₂ nanoparticles do not change the superconducting transition and the microstructure of superconductors. At a temperature of 5 K, the addition of 0.5 and 1 wt % of ZrO₂ nanoparticles may lead to the additional effect of magnetic flux pinning and the increase in the critical current density J _c. The J _c value for composites with 1 wt % is two times larger than that for the reference sample. The fishtail effect is observed for YBa₂Cu₃O_{7 ? y} /nanoZrO₂ composites at the temperatures of 20 and 50 K. The problems associated with the additional effect of magnetic flux pinning of granular YBa₂Cu₃O_{7 ? y} /nanoZrO₂ composites and the appearance of the fishtail effect have been discussed.     

Preparation and superconductivity of (B1−xCx)(Sr1−yBay)2Ca2Cu3Oz

Preparation of (B_1−xC_x)(Sr_1−yBa_y)₂Ca₂Cu₃O_z has been studied using high-pressure synthesis technique to improve sample quality. Samples were prepared from various starting compositions of 0.2≤x≤0.8 and 0≤y≤1. Nearly single-phased samples were obtained for (B_0.6C_0.4)(Sr_1−yBa_y)₂Ca₂Cu₃O_z (0≤y≤0.75) and (B_1−xC_x)-(Sr_0.25Ba_0.75)₂Ca₂Cu₃O_z (0.3≤x≤0.6). We have found that the partial substitution of Ba is effective to improve the sample quality as well as to enhance the T_c. The C substitution was also demonstrated to affect the sample preparation and the physical properties. Based on the substitution study, a maximum T_c of 120 K was observed for the sample with a starting composition of (B_0.65C_0.35)(Sr_0.3Ba_0.7)₂Ca₂Cu₃O_9+δ. Critical current density (J_c) and irreversibility field (H_irr) were estimated from magnetization measurements. The J_c at 77 K in a field of 1 T was about 1.1×10⁴ A/cm³ and the H_irr at 77 K was about 2.5 T. The H_irr was well-described by H_irr=a(1−T/T_c)ⁿ with a=39.1 and n=2.38.     

p(x) and Tc(x) characteristics in the Y 1−b(Ca)bBa2Cu3O6+x cuprate family

A model has been proposed to calculate the $p\left(x\right)$ and $T_c\left(x\right)$ dependences in the Y _1?b(Ca)_bBa₂Cu₃O_6+x high-$T_c$ cuprate family and applied to $b=0$, $b=0.1$, and $b=0.2$ cases, for which experimental data exist in the literature. The results obtained imply that the Ca efficiency to provide holes is independent of the basal plane oxygen concentration, which is consistent with a view that electrons from CuO₂ layers would go primarily to Ca since it is twice closer than oxygen (in addition, the chain oxygen is screened by a layer made up of Ba and O(4) ions). It is shown that, in fully oxygenized compounds ($x=1$) the average efficiency, $\chi$, of a chain oxygen to attract an electron from the two nearby layers is reduced by the Ca insertion, though not because the charge transfer mechanism is in itself weakened by Ca, but because a part of electrons that are otherwise available in CuO${}_2$ layers has already been removed by the substitution of Y ³⁺ with Ca²⁺. It has been found that the $b$-dependence of the average oxygen doping efficiency can be fairly accurately described by the following relation: $\chi \left(b\right)=0.39\times (1?0.78b)$. The calculated $p\left(x\right)$ and $T_c\left(x\right)$ dependences are in very good agreement with the available experimental data.