Critical current densities and irreversibility fields of high-Tc superconductors AuBa2Can−1CunO2n+3 (n=3,4) prepared under high pressure

Critical current densities (J_c) and irreversibility fields (B_irr) of high-T_c superconductors AuBa₂Ca_n−1Cu_nO_2n+3 [Au-12(n−1)n, n=3,4] prepared under high pressure were determined from the hysteresis in DC magnetization loop. Both the J_c and B_irr values of the Au-1234 phase were larger than those of Hg- and Bi-based cupurate superconductors, but smaller than those of the Y-123 superconductor. This result suggests that the electronic structure of Au-1234 is less anisotropic compared with the Hg- and Bi-phases. The Au-1223 phase showed very small J_c values corresponding to its small superconducting volume fraction.     

Crystal structure and anisotropy of iodine-intercalated Bi2Sr2Ca n−1Cu n O x

The electronic state and structural configuration of the intercalated iodine species in stage-1, I-Bi₂Sr₂Ca _n?1Cu _n O _x (n = l, 2), have been studied through polarization-resolved Raman and¹²⁹I Mössbauer spectroscopy. The polarization dependence of the Raman spectra and the Mössbauer measurement confirmed the dominant species to be triiodide ions, I ₃ ^? , with alignment of these linear molecules either along thea- orb-axis in the host crystals. Transport measurements such as thermoelectric power and Hall coefficient clearly indicated that hole carriers are doped into the CuO₂ planes upon intercalation, by whichT _c of the host superconductor is changed. Furthermore, based on resistivity measurements in a magnetic field, we suggest that the iodine intercalation leads to a decrease of the anisotropy both in normal and superconducting states, suppressing the extremely two-dimensional character of the Bi₂Sr₂Ca _n?1Cu _n O _x systems.     

Artificially layered Bi2Sr2Ca n−1Cu n O2n+4 (n≥3) films by sequential sputter deposition

Uniformly layered mixture of the succeeding members in the structure series was found in artificially layered Bi₂Sr₂Ca _n?1Cu _n O_2n+4 films synthesized by a three-target sequential sputter deposition technique. The intergrowth structure was quantitatively evaluated by X-ray analysis technique. An averagedc-axis (half) unit length and a plane spacing d₀～-0.31 nm $\left( { \simeq d_{CaCuO_2 } } \right)$ in the intergrowth structure are considered as a modulation wavelength and an average lattice of superlattice in the analysis. It is shown that the X-ray diffraction patterns observed in our films are in good agreement with that predicted by the superlattice model.     

Superstructures exhibited by oxides of the aurivillius family, (Bi2O2)2+ (An−1BnO3n+1)2−

Bi₅Ti₃FeO₁₅ and Bi₇Ti₃Fe₃O₂₁ which are n=4 and n=6 members of the family of oxides of the general formula (Bi₂O₂)²⁺(A_n−1B_nO_3n+1)²⁻ show unusual superstructures, possibly due to cation ordering.     

Static bismuth disorder in Bi2−x(CrTa)O7−y

The crystal structure of the pyrochlore Bi_2−x(CrTa)O_7−y has been refined by the Rietveld method from powder neutron and synchrotron X-ray diffraction data. Using the ideal pyrochlore structural model, anomalously high atomic displacement parameters (APD), which could not be satisfactorily explained with anisotropic ADP, of both Bi and O′ sites were observed. The structure was successfully refined with a model that incorporated static displacive disorder of both the Bi and O′ atoms. Using this model physically reasonable ADP parameters for both the Bi and O′ atoms, and improvement in the various measure of fit were obtained. The underbonding of the Bi atoms, observed in ideal pyrochlore model, but removed in the disordered structure, is believed to be the main driving force for this disorder.     

Effect of Yb-substitution on thermally activated flux creep in the Bi2Sr2Ca1Cu2−xYbxOy superconductors

In the present work, Bi ₂ Sr ₂ Ca ₁ Cu _2?x Yb _x O _y (x = 0.0, 0.05, 0.1, and 0.25) superconductors were prepared by a polymer matrix route technique, and subsequently used as feed in a laser induced directional solidification (LFZ) process. The X-ray diffraction, DC magnetization and magnetoresistance measurements were studied. All samples consist mainly of the Bi-2212 phase which indicates the performed process is adequate. However, some impurity phases like CaCuO₂ were also detected. We have calculated the hole number using the relation given by Presland, and particle size, L_hkl, using the XRD patterns results. From M–T results, the superconducting transition temperatures, T _C , of the samples were found 93, 90, 87, and 85 K, respectively. The activation energies, irreversibility fields (H_irr), upper critical fields (H_c2) and coherence lengths at 0 K (ζ(0)) were calculated from the resistivity versus temperature (R–T) curves, under DC magnetic fields up to 5 T. The thermally activated flux creep model has been studied in order to calculate the flux pinning energies. The results indicated that Hc2(0) varied from 132 to 30 T and the flux pinning energies varied from 650 to 400 meV at 0 T, with the content of Yb.     

Enhancement in superconducting transition temperature and Jc values in Na-doped Bi2Sr2Ca1Cu2−xNaxOy superconductors

The effects of Na substitution on the properties of Bi₂Sr₂Ca₁Cu_2?xNa_xO_y were investigated. The prepared samples are characterized using X-ray powder diffraction, scanning electron microscope, DC electrical resistivity and magnetic-hysteresis loop measurements. It has been found that with increasing Na doping for Cu, the transition temperature gradually increase while crystal lattice parameters slightly change. Magnetic hysteresis measurements have shown that the hysteresis loops of doped samples are greater than the undoped sample. In addition, significant enhancement has been observed in the J_c values of Na-doped samples, which were calculated from the M–H curves by using Bean’s critical state model.     

XRD and PES Studies of the Bi2Sr2Ca1−xPrxCu2O8+δ System

The Bi₂Sr₂CaCu₂O_8+δ system samples doped with Pr on Ca sites were synthesized. Resistivity measurements, x-ray powder diffraction, and photoemission experiments were performed. The experiment results show that Pr ions entered the lattice and chemical environment plays a major role in the shift of core-level spectra and valence-band spectra.     

μ+SR Study on Bi2Sr2Ca1−x Y x (Cu1−y Zn y )2O8+δ around the Hole Concentration of \frac{1} {8} per Cu

In order to study the effect in different high-T _c oxides from the La-system, muon spin relaxation measurements were applied to the Zn-substituted Bi-2212 system, Bi ₂ Sr ₂ Ca _1–x Y _x (Cu _1–y Zn _y ) ₂ O ₈₊ , around the hole concentration p= per Cu. It has been revealed that the magnetic correlation between Cu spins is anomalously enhanced in the Zn-substituted samples at per Cu, proving the existence of the effect" in the Bi-2212 system as well.     

Enhancement of thermoelectric power in layered Bi2Sr2Co2−x Ir x O y single crystals

We report on the structural, electrical, and thermal transport properties of the single crystals of Bi₂Sr₂Co_2?x Ir _x O _y (0 ≤ x ≤ 0.2). Large-sized (centimeter-level) and good-quality single crystals were grown by a modified flux method. The substitution of Ir ions for Co ones makes the in-plane resistivity ρ _ab increase monotonically, whereas, the in-plane thermopower S _ab initially increases and then decreases as x is larger than 0.1. Therefore, among all samples, the in-plane power factor P _ab (=S _ab ² /ρ _ab) of Bi₂Sr₂Co_1.9Ir_0.1O _y would reach a maximum value of 116.4 μWm^?1K^?2 at 300 K and shows a significant improvement of about 20 % compared to that of the parent sample. Our results indicate that an appropriate doping of Ir ions at Co-sites may provide an effective way to enhance the thermoelectric performance of Bi₂Sr₂Co₂O _y system.     

Activation Energy of Bi2Sr2Ca1−x Er x Cu2O8+d Epitaxial Thin Films

The scaling behavior of the effective activation energy U(T, H) of epitaxial oriented Bi₂Sr₂Ca_1–x Er _x Cu₂O_8+d thin films with 0x0.10 has been studied as function of temperature and magnetic field. It has been found that the activation energy scales as U(H, T)=U ₀(1–t)^m H ⁿ with exponent m=1.20±0.03 and n=–1/2. The field and temperature dependence of the activation energy, as well as its overall magnitude, is consistent with a model in which U(T, H) arises from plastic deformations of viscous flux liquid above the vortex-glass transition temperature.     

Transport Properties of Heavy Fermion Compounds: YbIn1−x Cu4+x and YbIn1−y Ag y Cu4

The intermetallic compounds of Yb with In and Cu (YbIn_1–x Cu_4+x ) and Yb with In, Ag, and Cu (YbIn_1–y Ag _y Cu₄) exhibit interesting magnetic and transport properties. Of the compounds of Yb with In and Cu the compound with x=0, namely YbInCu₄, has attracted particular attention, because—while being a Curie–Weiss paramagnet—it undergoes a first-order isostructural phase transition at T _v =approx. 40 to 80 K and atmospheric pressure. Below T _v the ytterbium in this compound is in a mixed-valence state and the compound as a whole is sometimes called a light heavy-fermion system. Above T _v , the compound is known as a Curie–Weiss paramagnet of localized magnetic moments and, below T _v , a Pauli paramagnet in a nonmagnetic Fermi-liquid state. In the present paper the results of measurements of the thermal conductivity of polycrystalline samples, YbIn_1–x Cu_4+x with x=0,0.015, 0.095, and 0.17 and YbIn_1–y Ag _y Cu₄ with y=0, 0.3, 0.7, and 1.0, are reported. The thermal conductivity is separated into the phonon thermal conductivity ( _ph ) (i.e., related to the heat carried by phonons) and into the electronic thermal conductivity ( _e ) (related to the heat carried by electrons). The electrical resistivity of the compounds was measured to determine the temperature dependence of the Lorenz number. The results show that in the temperature interval 4.2 to 300 K the latter quantity behavior follows the theoretical predictions for heavy fermion materials.     

Magnetoresistance of Sr1−x K x BiO3: a second-family of bismuth-oxide-based superconductors

The superconducting Sr _1–x K _x BiO ₃ samples with x = 0.45 – 0.6 were synthesized by the high-pressure-high-temperature technique in a belt type apparatus (2 GPa, 700 °C, 1 h, Pt capsules) with stoichiometric mixtures of Sr ₂ Bi ₂ O ₅ , Bi ₂ O ₃ and KO ₂ as described earlier. ¹ The X-ray diffraction results appear as a single perovskite-like phase. The superconductivity occurs at T _c 12 K in the A. C. susceptibility measurement. The onset (zero resistivity) temperature of superconductivity in resistivity measurement of the investigated sample was . The transition temperature region was a little bit broad and a shoulder was present about 11.3 K indicating probably the existence of crystallites of different K content. The particularly interesting point is that the resistance begins to reappear at T < 6 K at zero magnetic field. As the external magnetic field is applied, the reentrant resistance disappears and superconductivity is recovered until the applied magnetic field becomes higher than 0.65 Tesla. The superconductivity for T < 6 K is destroyed for the higher magnetic field. The T_c onset decreases as the magnetic field increases like in the BCS type superconductors. The transition region becomes broader under the magnetic field, which indicate a kind of vortex transition as in the case of high T _c cuprates.     

Transport Properties and Electronic Specific Heat of La2−x Ba x Cu1−y Zn y O4+d Bulk

The effect of the partial substitution of Cu by Zn and La by Ba in La: 214 prepared by conventional solid state reaction method has been investigated by using Hall coefficient, Seebeck coefficient and electronic specific heat measurements, over the wide temperature range between 4.2 K and 300 K, in magnetic fields up to 5 T. The samples with y=0 and d=0, showed superconductivity for x between 0.055 and 0.30. The critical temperature, the Hall and Seebeck coefficients strongly depend on Zn content. The phonon specific heat C _ph and electronic specific heat C _el have been extracted from the total specific heat C. Zn-doping effect on the (T)=C _el/T behavior suggests that the large range stripe order and the fluctuating stripe suppress the singlet formation and pseudo gap.     

Substitution Effect of Sn for Cu in the Bi2Sr2CaCu2−xSnxO8+δ System

The Bi₂Sr₂CaCu₂O_8+δ system samples doped with Sn on Cu sites were synthesized. Resistivity temperature dependence, X-ray powder diffraction and photoemission experiments have been performed. Suppression of superconductivity in the Sn-doped Bi₂Sr₂CaCu₂O_8+δ system has been observed. X-ray photoemission measurements show that Sn ion enters the lattice. Our results reveal that Sn ions are mostly in bivalent states.     

Structural and mechanical characterization of Bi1.75Pb0.25Sr2Ca2Cu3−xSnxO10+y superconductor ceramics using Vickers microhardness test

In this work we investigated the role of Sn doping on mechanical and structural properties of Bi_1.75Pb_0.25Sr₂Ca₂Cu_3?xSn_xO_10+y (x = 0.0, 0.1, 0.3 and 0.5) superconducting ceramic material. All samples were fabricated with glass ceramic method. The prepared samples were characterized by using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray powder diffractometer (XRD) and static microhardness indenter. Surface morphology, orientation of grains, and elemental composition analysis of the samples were made by SEM and EDS measurements, respectively. Texturing and lattice parameters a and c were determined by XRD measurements. Load dependent and load independent microhardness, elastic modulus, yield strength and fracture toughness were obtained by hardness measurements. In this work we focused on Vickers microhardness measurements in order to characterize the mechanical properties of our samples. Experimental results of Vickers microhardness measurements were analyzed by using Meyer’s law, the elastic/plastic deformation model, proportional sample resistance model (PSR), modified PSR model and Hays-Kendall (HK) approach. From these analyses, HK approach was determined as the most successful model describing the mechanical properties of our samples. Finally in this study, the changes on the mechanical and microstructural properties of Sn doped Bi-2223 superconductors and their possible reasons were also discussed in detail.     

High-pressure synthesis of a new homologous series of superconductors, (Hg1−xTIx)2Ba2Can−1CunOy (0.3 ≤ x ≤ 0.6,n=3,4)

We have recently synthesized new superconducting cuprates containing a mercury-based bilayer, i.e. (Hg,Tl)₂Ba₂(Ca,Y)Cu₂O_y and (Hg_1–x Tl_x)₂Ba₂ Ca_n–1 Cu_nO_y (x 0.4, n = 3,4) by a high-pressure technique. These compounds are the n=2, 3 and 4 members of a new homologous series, (Hg, Tl)-22(n–1)n. The effects of Tl substitution for Hg on the formation and superconductivity of the (Hg, Tl)-2223 and -2234 phases are investigated. Both the phases are stabilized for the Tl content x of 0.3–0.6. T_c of the as-synthesized (Hg,Tl)-2223 compound increases from 45 K to 81 K with an increase in x. In contrast, T_c of the (Hg,Tl)-2234 compound is almost independent of x (114 K).     

μSR Study in La2−x Sr x Cu1−y Zn y O4 with x≃l/8

In order to investigate the magnetic state around hole density 1/8 in LaSr-214, SR experiments were carried out for the samples of Zn-free La _2–x Sr _x CuO ₄ and Zn-doped La _2–x Sr _x Cu _1–y Zn _y O ₄. For Zn-free samples, -spin rotation was observed in x=0.115 below about T _r =10K, implying magnetic order, but not observed in x=0.125. On the other hand, Zn-doping brought about -spin rotation in x=0.125 sample below T _r =7K, implying growth of magnetic order but decreased T _r in x=0.115, meaning destruction of magnetic order. This indicates that Zn-doping pins dynamical spin modulation, resulting in static magnetic order in x=0.125 sample.