Study on the superconducting composite material formation in the system Bi2Sr2CaCu2O8+x/Al-containing phases

Phase evolution in the Bi---Sr---Ca---Cu---Al---O system was studied. Two Al-containing phases BiSr_1.5Ca_0.5Al₂O_z and (Sr_1−xCa_x)₃Al₂O₆ (x = 0.4 − 0.45) were determined to be chemically compatible with Bi_2.18Sr₂CaCu₂O_8+x (Bi-2212) at temperatures of the samples processing. The phase equilibria in the title system were investigated above the solidus temperature. The BiSr_1.5Ca_0.5Al₂O_z was found to be in equilibrium only with the melt and the (Sr_1−xCa_x)₃Al₂O₆ phase. This latter aluminate equilibrated with Ca,Sr cuprates, CaO, the Cu-free phase, and the liquid. The melting and solidification in Bi-2212, doped with the aluminate, corresponded to the reversible reaction Bi-2212 + BiSr_1.5Ca_0.5Al₂O_z ↔ (Sr_1−xCa_x)₃Al₂O₆ + liquid. Two sets of superconducting composite materials with initial compositions Bi-2212 + nBiSr_1.5Ca_0.5Al₂O_z and Bi-2212 + m(Sr_1−xCa_x)₃Al₂O₆ were prepared by solidification from the partial melt. The former material was composed mostly of large Bi-2212 lamellas separated by the BiSr_1.5Ca_0.5Al₂O_z phase, which destroyed superconducting links between Bi-2212 grains. The latter material consisted of a Bi-2212 polycrystalline matrix with high concentration of small (ca. 3 μm) grains of (Sr_1−xCa_x)₃Al₂O₆ imbedded in Bi-2212 lamellas. The Bi-2212 + m(Sr_1−xCa_x)₃Al₂O₆ materials displayed a trend to enhance flux pinning at T = 60 K with the increase of aluminate phase content.     

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.     

Critical current density, magnetic relaxation and pinning distribution in Bi2Sr2CaCu2Ox superconductors

Critical current densities and magnetic relaxation have been measured for Bi₂Sr₂CaCu₂O_x (Bi-2212) single crystals and melt-processed ceramic samples. Strongly nonlogarithmic decay of the magnetization is observed down to 10 K in Bi-2212 single crystals over a time range of more than 3 decades. Different decay behaviours are found in Bi-2212 single crystals, bulk ceramics and powdered ceramics. The experimental results are discussed within the pinning distribution model of Hagen and Griessen. A pinning distribution function yielding the power law decay of magnetization as an analytical solution is found. A careful analysis of the pinning energy distribution reveals high energy tails which can hardly be associated with single vortex pinning mechanisms. These high pinning energies can, however, be explained by reinterpreting the Hagen-Griessen model in terms of collective pinning theory.     

Neutron scattering of fragile antiferromagnetic phases in heavy fermion compounds

Results of neutron scattering and specific heat measurements are reported for two kinds of weak antiferromagnetic compounds corresponding to (i) the situation at the quantum critical point and (ii) the double magnetic transition observed in CeRu₂Ge₂.     

Performance at 10–50 K of Bi-2212/Ag multilayer tape fabricated by using pre-annealing and intermediate rolling process

High-transport critical current density (J_c-oxide)>500 kA/cm² at 4.2 K, 10 T can be obtained for Bi-2212/Ag tapes fabricated by using pre-annealing and intermediate rolling (PAIR) and melt-solidification process. In this paper, we report high-temperature properties of PAIR-processed Bi-2212/Ag multilayer tape in order to show their potential for practical applications operated at cryocooling temperatures. Magnetic field dependence and angular dependence of critical current (I_c) are investigated at temperatures ranging 10–50 K by using helium gas cooling and liquid neon. Field-temperature curves for I_c=0.2 and 2.0 A are also determined in order to show the approximation of the irreversible field. High-temperature performance of the tape is attractive to consider future applications. For example, the best sample carries I_c=267 A (engineering-J_c=303 A/mm², J_c-oxide=151 kA/cm²) and 92 A (104 A/mm², 52 kA/cm²) at 27.1 K (in liquid neon), in magnetic fields (parallel to the tape surface) of 2 and 10 T, respectively. Engineering-J_c of 100 A/mm² is obtained even in the perpendicular field of 0.5 T at 27.1 K.     

Study of the ferroelectric–paraelectric phase transition of NaH3(SeO3)2 single crystals by H and Na NMR

The ¹H and ²³Na spin–lattice and spin–spin relaxation times of NaH₃(SeO₃)₂ single crystals grown using the slow-evaporation method were measured as functions of temperature and frequency in the ferroelectric and paraelectric phases. The changes in the symmetry of the (SeO₃)²⁻ dimers as a result of the ferroelectric–paraelectric phase transition are associated with large changes in the spin–lattice and spin–spin relaxation times, and in the number of resonance lines. The large changes in the relaxation times at 195 K indicate that the H and Na ions are significantly affected by this transition. The change in the number of resonance lines for the ¹H and ²³Na nuclei means that the orientations of the (SeO₃)²⁻ dimers and the environments of the Na ions change at T_C. Therefore, the orientations of the (SeO₃)²⁻ dimers and the environments of the Na ions play important roles in the phase transitions. In conclusion, the ferroelectric–paraelectric phase transition of NaH₃(SeO₃)₂ is accompanied by changes in hydrogen-bond structure and distortions of the (SeO₃)²⁻ and Na⁺ ion lattices, which form a slightly distorted octahedron.     

Superconducting and magnetic properties of La1-xGdxRu2 and La1−xPrxRu2 mixed crystals

LaRu₂ in contrast to CeRu₂ shows the normal T_c-depression by magnetic rare earth ions. This depression is much larger for praseodymium than for gadolinium.     

Electronic and magnetic properties of the rare earth intermetallic compounds RRu4Sn6 (R=Nd, Sm, Gd, Tb, Dy and Ho)

A number of compounds of structural formula RRu₄Sn₆ (R=rare-earth element) have previously been reported to form in the tetragonal crystal structure with space group I4¯2m. In this structure the R atoms are well isolated from each other. We embarked on this study to investigate the physical properties and to compare with earlier results obtained on the strongly correlated, low charge-carrier density compound CeRu₄Sn₆. Here we report our results of crystallographic, electrical resistivity, and magnetic studies on this family of compounds. In contrast to the behaviour in CeRu₄Sn₆, magnetic ordering is evident at low temperatures in the compounds with R=Sm, Gd, and Dy, as is evidenced by well-resolved anomalies in the temperature dependence of the electrical resistivity and static magnetic susceptibility.     

Phase transitions and electrical properties of CsH(SO4)0.76(SeO4)0.24 mixed crystals

The study by X-ray diffraction, calorimetry, vibrational and impedance spectroscopy of CsH(SO₄)_0.76(SeO₄)_0.24 new solid solution is presented. Crystals of this composition undergo two phase transitions at T = 333 and 408 K. The last one at 408 K is a superionic-protonic transition (SPT) related to a rapid [HS(Se)O₄⁻] reorientation and fast H⁺ diffusion. A sudden jump in the conductivity plot confirms the presence of this transition. Above 408 K, this high temperature phase is characterized by high electrical conductivity (7 × 10^t-3 Ω⁻ cm⁻¹) and low activation energy (E_a < 0.3 eV).     

Composite protonic solid electrolytes in the CsHSO4-SiO2 system

Transport, thermal and structural properties of the composite solid electrolytes (1 −x)CsHSO₄---xSiO₂ (where x = 0–0.8) were investigated. The composites were prepared by mechanical mixing of components followed by heating at temperatures near CsHSO₄ melting point (483 K). The dependence of low temperature phase conductivity on x has a maximum with a value 2.5 orders of magnitude higher than that of pure CsHSO₄ and conductivity is governed by protons. Heterogeneous doping is shown to change markedly the thermodynamic parameters of the ionic component. The phase transition temperature CsHSO₄ in the composites decreases from 414 to 350 K with the increase of the content of heterogeneous additive SiO₂ from 0 to 0.7. As x raises CsHSO₄ the amorphization takes place and the relative change of ionic conductivity at phase transition diminishes, the phase transition becomes diffusive and disappears for the 0.2CsHSO₄---0.8SiO₂ composite.     

Lithium insertion to ReO3-type metastable phase in the Nb2O5–WO3 system

Lithium insertion to distorted ReO₃-type metastable solid solution Nb_xW_1−xO_3−x/2 (0≤x<0.25) has been studied by chemical and electrochemical methods. In the course of lithium insertion into tetragonal compounds, transition to a cubic phase was found to occur in the region where values of y (in Li_yNb_xW_1−xO_3−x/2) fall between 0.2 and 0.3, and the phase transition was found to depend on the conditions of the reaction. Changes in OCV and lattice parameters in tetragonal region (y<0.2) were discussed from the viewpoint of the ordering of lithium ions. Also, the component diffusion coefficient of lithium in tetragonal compounds Li_0.1Nb_xW_1−xO_3−x/2 (0≤x≤0.23) was found to increase with niobium content when x≤0.10, and to saturate at 4×10⁻⁹ cm²/s.     

TEM analysis of planar defects induced by Ti doping in Bi-2212 single crystals

An electron microscopy study is performed on Ti doped Bi-2212 single crystals. High resolution electron microscopy reveals that the Ti doping leads to incorporation of (010) planar defects in the Bi-2212 lattice. These defects consist of pairs of antiphase boundaries, which are separated along the b-axis by one modulation distance. The atomic shift at the boundary is such that one of the two CuO₂ planes is continuous throughout the defect. Energy dispersive X-ray analysis clearly reveals the presence of Ti in the planar defects, whereas no Ti could be detected in defect free areas. This evidences the fact that these defects are induced by the Ti doping.     

Double peak behavior of resistivity curves in Cd doped LaMnO3 perovskite systems

The effect of Cd doping on transport, magnetotransport, and magnetic properties has been investigated in the perovskite La_1−xCd_xMnO₃ (0x0.5) systems. The ρ(T) curves exhibit a sharp metal insulator transition (T_p1), which is close to paramagnetic to ferromagnetic transition (T_c) obtained from M−T curves for all samples. In addition, ρ(T) curves for Cd doped samples exhibit another broad transition (T_P2) below T_p1. This transition becomes more prominent and the transition temperature (T_p2) shifts to lower temperature with increasing Cd content. Such double peak behavior in the ρ(T) curve is attributed to the phase separation between the ferromagnetic metallic phases and the ferromagnetic insulating phases induced by the electronic inhomogeneity in the samples.     

Structural and transport properties of the triple-layer compounds Ba4(Pb1−xBix)3O10 (0≤ x<0.3)

A series of new compounds Ba₄(Pb_1−xBi_x)₃O₁₀ with 0≤x<0.3 has been prepared and characterized by X-ray and electron diffraction. The d.c. resistivity vs. temperature data for different x values are presented and structural features are compared to those of the superconducting phase Ba(Pb_1−xBi_x)O₃. The “mother-compound” Ba₄Pb₃O₁₀ is tetragonal, with lattice constants A=0.4280(1) and C=3.017(1) nm. Its structure can be regarded as a stacking of Ba(PbBi)O₃ triple-layers, separated by single BaO layers. Electron diffraction reveals the presence of a weak superstructure with the a-axis related to that of the perovskite by a=√2a_p. Frequent intergrowth of this phase with the perovskite structure is observed. The conductivity of the samples Ba₄Pb₃O₁₀ is nearly independent of temperature, while Bi-doped samples exhibit semiconductor-like behaviour at low temperature. No superconducting transition is observed down to 2K.     

Itinerant f-electron systems of cerium and uranium compounds

The f-electron systems of the cerium and uranium compounds have attracted attention in the context of heavy fermion and anisotropic superconductivity. We clarified an itinerant f-electron nature for UPt₃ by the de Haas–van Alphen experiments, detecting heavy conduction electrons with 100m₀. The dHvA oscillation was also observed clearly in both the normal and superconducting mixed states in CeRu₂,URu₂Si₂ and UPd₂Al₃. An anisotropic energy gap with a line node for URu₂Si₂ was discussed from the angular dependence of the dHvA amplitude in the mixed state.     

Monte Carlo and coarse graining renormalization studies for the XY model with cubic anisotropy

The XY model with cubic anisotropy is studied using a Monte Carlo coarse graining renormalization calculation. Particular emphasis is placed on the accuracy of phase diagrams and the finite-size effects on the phase transition. Non-universal behavior of the correlation-length critical exponent v with respect to variation of the cubic anisotropic field h₄ is analysed and an empirical formula v = [1 − exp(− ch₄)]⁻¹ is proposed. The results are discussed with reference to the surface reconstruction phase transition of the W(001) surface.     

Bi-2212超导线材研究进展

Bi-2212高温超导材料在高磁场中具有优异的磁场载流能力,可制备成各向同性圆线,可以绞制成大电流的电缆导体,是大型强磁场磁体的首选材料之一。经过30多年的发展,Bi-2212超导线材的临界工程电流密度(J_E)也已达到工程应用要求。综述了国内外近年来Bi-2212超导材料在载流性能、绝缘、内插线圈和热处理等方面的研究进展。     

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.     

Itinerant metamagnetic transition in Y(Co1−xFex)2

High field magnetization measurements have been performed to examine the existence of itinerant metamagnetism in exchange-enhanced systems related to YCo₂ together with Fe_1−x Co_xSi. In the Y(Co_−xFe_x)₂ system, the meta magnetism inherent in YCo₂ has been observed in 0.04 x0.07. The transition is not as sharp as in the Y(Co_1−xAl_x)₂ system. Other exchange-enhanced paramagnets Y(Co_−xCu_x)₂ and Y_1−xLa_xCo_x2 and weakly itinerant ferromagnet Fe_1−xCo_xSi exhibit no metamagnetic transition up to 430 kOe.