Analyses of the Pinning Mechanism for the Superconductor/Normal-Metal and Superconductor/Superconductor Multilayers

The vortex pinning force of superconductor(S)/normal-metal(N) multilayers, Nb/Ti and Nb_100−x Ti _x /Ti, and superconductor(S)/superconductor(S′) multilayers, Nb_100−x Ti _x /Nb and Nb₂₈Ti₇₂/Nb₆₅Ti₃₅, has been studied as a function of the layer thickness d for various compositions of x. The transverse pinning force density F _{p ⊥} acting perpendicular to the layer planes makes a peak around d∼ 2ξ_GL (ξ_GL: the GL coherence length) for both the S/N and S/S′ multilayers, where they exhibit the quasi-two-dimensional superconductivity. In contrast, the longitudinal pinning force density F _p‖ acting parallel to the layer plane hardly depends on d. The pinning mechanism inherent in the layered structure has been analyzed based on the GL free energy, which clarified that the effective pinning force due to the multilayer structure originates from the difference in the potential energy for the S/N multilayer and from the difference in the kinetic energy for the S/S′ multilayer between the constituent layers. The d dependence of the F _{p ⊥}, however, has been found to be qualitatively similar for the S/N and S/S′ multilayers.     

Effect of Ni Doping at Pd Site in Nb<Subscript>2</Subscript>PdS<Subscript>5</Subscript> Superconductor

The superconducting properties of Nb₂PdS₅ superconductor have been investigated with Ni doping at Pd site All the bulk polycrystalline Nb₂Pd_1?xNi_x S ₅ (0 = x ≤ 0.10) samples are crystallized in singlephase monoclinic structure. The electrical resistivity and magnetic measurements of Nb₂Pd_1?xNi_x S ₅ (0 = x ≤ 0.15) were carried out to study the variation of superconducting critical parameters with Ni doping. Superconductivity in Nb₂PdS₅ sample completely disappears for x ≥ 0.15. We observed that the ratio of upper critical field to transition temperature decreases with increasing Ni concentration. Also, the magnetization study of Nb₂Pd_1?xNi_x S ₅ (0 = x ≤ 0.15) samples shows similar superconducting behaviour. In summary, the superconductivity in Nb₂PdS₅ sample is slightly varying with partial doping of Ni at Pd site in Nb₂PdS₅ superconductor.     

Novel transparent conducting oxide: Anatase Ti1−xNbxO2

Single-crystalline Ti_1−xNb_xO₂ (x = 0.2) films of 40 nm thickness were deposited on SrTiO₃ (100) substrates by the pulsed laser deposition (PLD) technique. X-ray diffraction measurement confirmed epitaxial growth of anatase (001) film. The resistivity of Ti_1−xNb_xO₂ films with x ≥ 0.03 is 2-3 × 10^− 4 Ω cm at room temperature. The carrier density of Ti_1−xNb_xO₂, which is almost proportional to the Nb concentration, can be controlled in a range of 1 × 10¹⁹ to 2 × 10²¹ cm^− 3. Optical measurements revealed that internal transmittance in the visible and near-infrared region for films with x = 0.03 was more than 97%. These results demonstrate that the presently developed anatase Ti_1−xNb_xO₂ is one of the promising candidates for the practical TCOs.     

Preparation of Nb<Subscript>3</Subscript>Al Superconducting Tapes by a Powder-in-Tube Method Combined with Hot-Pressed Sintering

The precursors of Nb-Al tape were fabricated by a powder-in-tube (PIT) technique. Supersaturated solid solutions of Nb-Al powder were obtained by high-energy ball milling, followed by a powder-in-tube process to prepare series of Nb₃Al precursor tapes. Compared with sintering under normal pressure, the hot-pressing sintering greatly increased the critical current density of the tapes. The results showed that the Nb₃Al tapes prepared via the powder-in-tube method and hot-pressed sintering could significantly improve the J _c performance, and the value of J _c (8 K, 0 T) was higher than 6 × 10⁵ A/cm². The Nb₃Al tapes after pressing under the pressure of 20 MPa and sintering at 950 ^°C for 3 h had the best magnetic flux pinning performance, and the sample reached the maximum flux pinning force (F _p,max) of 3.28109 N/m³ in the magnetic field of 2.2 T.     

Sur de nouvelles phases oxyfluorees derivees du niobate de plomb: Etudes cristallographique et dielectrique

The solid solution Pb_2?xNa_xNb₂O_7?xF_x obtained by substitution of oxygen by fluorine in the ferroelectric oxide Pb₂Nb₂O₇ has the cubic pyrochlore structure for 0, 75 ? x ? 1, 85 and monoclinic deformations of this structure for other values of x. The temperature and frequency variations of the dielectric constant and of the dielectric losses have been studied. The high values of ε′_r have been attributed to a mechanism of ionic polarizability, and those of tan δ to the mobility of the F^? ions.     

Preparation of Ti1− zNbzN ceramics by spark plasma sintering Ti1− zNbzOxNy nanopowders

Ti_1−zNb_zN ceramics were fabricated by sintering nanocrystalline titanium-niobium oxynitride (Ti_1−zNb_zO_xN_y) powders using spark plasma sintering (SPS) technique at 1060 °C for 3 min in an N₂ atmosphere. The phase composition and microstructure were characterized by XRD, SEM, TEM and EDS. The results showed that Ti_1−zNb_zN ceramics remained the cubic structures of Ti_1−zNb_zO_xN_y powders. There were XRD peak shifts in the cubic phases between Ti_1−zNb_zN ceramics and corresponding Ti_1−zNb_zO_xN_y powders. During the sintering process, oxygen separated from Ti_1−zNb_zO_xN_y to form titanium-niobium oxides. Ti_1−zNb_zN (0 < z < 1) had a more compact structure than TiO_xN_y and NbO_xN_y. Ti_0.5Nb_0.5N ceramic had the biggest grain size in the series of Ti_1−zNb_zN.     

Preparation of new inorganic–organic complexes by using Fe x Ti1?2 x Nb1+ x O 5? ( x =?0–0.5) nanosheets

Novel inorganic–organic complex nanocomposites have been prepared by using exfoliated Fe _x Ti_1−2x Nb_1+x O₅⁻ nanosheets and poly (N-octadecyl-2-ethynyl pyridinium bromide) (PNOEtPyBr) by the method of exfoliation–reflocculation. The X-ray diffraction data of the nanocomposites indicate the formation of Fe _x Ti_1−2x Nb_1+x O₅/PNOEtPyBr nanostructure with the length of basal spacing of 4.6 nm along the stacking direction of Fe _x Ti_1−2x Nb_1+x O₅⁻ nanosheet layers. The a. c. electrical conductivity of nanocomposites decreased with the value of x. This trend was correlated with the hydration behavior of the interlayer for HFe _x Ti_1−2x Nb_1+x O₅·nH₂O (x = 0–0.5) which were starting compounds of nanosheets.     

Structural and microwave dielectric behavior of (Li1/4Nb3/4) substituted ZrxSnyTizO4 (x + y + z = 2) system

The phase structure, microwave dielectric properties, and their stability with different annealing conditions have been investigated in (Li_1/4Nb_3/4) substituted Zr_xSn_yTi_zO₄ system. The sintering temperature of Zr_xSn_yTi_zO₄ ceramic was lowered from 1500 to 1140 °C by (Li_1/4Nb_3/4) substitution. Both X-ray diffraction (XRD) analysis and electron diffraction (ED) analysis revealed that the (Li_1/4Nb_3/4) substituted Zr_xSn_yTi_zO₄ ceramic crystallized as the high-temperature disordered ZrTiO₄ phase. As the content of Sn increased from 0.10 to 0.30, the permittivity of the (Zr_1−xSn_x)(Li_1/4Nb_3/4)_0.4Ti_0.6O₄ ceramic decreased gradually from 35.5 to 31.5, the Q_f value increased from 37,800 to 58,300 GHz, and TCF value shifted slightly from −4.5 to −33.0 ppm °C⁻¹. Both the phase structure and microwave dielectric properties of (Zr_1−xSn_x)(Li_1/4Nb_3/4)_0.4Ti_0.6O₄ ceramics were stable with annealing conditions.     

Long-Range Spin-Triplet Superconductivity Induced by Magnetic Field in d Wave Superconductor/Ferromagnet Hybrid System

We theoretically study the long-range spin- triplet superconductivity in d wave superconductor/ ferromagnet/ferromagnet (S/F₁/F₂) trilayer junction, in which the magnetization of F₁ layer could be rotated in the y–z plane by an external magnetic field. The four-component Eilenberger equations were constructed to calculate the superconducting order parameters and density of states (DOS). Near the clean limit, the p wave equal-spin triplet component could be induced when the magnetization directions of F₁/F₂ layers are non-collinear, and the DOS exhibits a split zero-bias conductance peak. The various parameters such as ferromagnetic exchange energy, thickness of ferromagnetic layers, and angles between F₁/F₂ magnetization directions are studied for the effect on inducing triplet superconductivity. By magnetic field controlling the emergence of equal-spin triplet pairings or not, such a tunable S/F₁/F₂ trilayer junction based on long-range spin-triplet superconductivity could be used as a superconducting switch device, which would open up a new view of spintronics.     

Synthesis and characterizations of A-site deficient perovskite Sr0.9Ti0.8−xGaxNb0.2O3

The effects of reduction and Ga-doping on the physicochemical properties of A-site deficient perovskites Sr_0.9Ti_0.8−xGa_xNb_0.2O₃ (x = 0, 0.05, 0.1, 0.15 and 0.2) are reported. With 10% Ga doping, the sample sintered in air and treated at 1400 °C in H₂ atmosphere exhibits the highest electrical conductivity. It is found that the Ga-doping lowers the sinterability but promotes the reduction of Sr_0.9Ti_0.8−xGa_xNb_0.2O₃. The XRD analysis on the reduced samples suggests that some cations are reduced during the treatment. However, without high temperature pre-reduction, the improvement of Ga-doping is limited and the overall cell performance using Sr_0.9Ti_0.8−xGa_xNb_0.2O₃ as an anode without catalysts is still relatively low.     

Influence of the Extra Layer on the Transport Properties of NdFeAsO1?0.14F0.14 and FeSe0.88 Superconductors from Magneto Dynamic Analysis

In this letter, the electromagnetic response of the NdFeAsO_1?0.14F_0.14 (T _c =49?K) superconductor system, characterized by FeAs and NdO alternating layers, has been compared with that of FeSe_0.88. We have studied the flux dynamics of these two systems by means of ac multi-harmonic magnetic susceptibility. The analysis shows that although characterized by larger thermal fluctuations due to its higher T _c , NdFeAsO_1?0.14F_0.14 exhibits a stronger pinning force relative to FeSe_0.88. The further Irreversibility Line (IL) analysis also points out that both superconductors have a 3D flux pinning behavior. We associate the stronger pinning force in the NdFeAsO_1?0.14F_0.14 structure to the presence of the extra NdO layer. Different pinning contributions can be associated to the structural stress associated to FeAs superconducting layers and/or to the Nd³⁺ ions magnetic moment (????3.6???_B) contribution on the flux cores. We will also show that these pinning are over imposed to a weak collective contribution due to the dopant F atoms.     

New substitutions and novel derivatives of the Aurivillius phases, Bi5TiNbWO15 and Bi4Ti3O12

We investigated isomorphous substitution of several metal atoms in the Aurivillius structures, Bi₅TiNbWO₁₅ and Bi₄Ti₃O₁₂, in an effort to understand structure-property correlations. Our investigations have led to the synthesis of new derivatives, Bi₄LnTiMWO₁₅ (Ln = La, Pr; M = Nb, Ta), as well as Bi₄PbNb₂WO₁₅ and Bi₃LaPbNb₂WO₁₅, that largely retain the Aurivillius (n = 1) + (n = 2) intergrowth structure of the parent oxide Bi₅TiNbWO₁₅, but characteristically tend toward a centrosymmetric/tetragonal structure for the Ln-substituted derivatives. On the other hand, coupled substitution, 2Ti^IV → M^V + Fe^III in Bi₄Ti₃O₁₂, yields new Aurivillius phases, Bi₄Ti_3−2xNb_xFe_xO₁₂ (x = 0.25, 0.50) and Bi₄Ti_3−2xTa_xFe_xO₁₂ (x = 0.25) that retain the orthorhombic noncentrosymmetric structure of the parent Bi₄Ti₃O₁₂. Two new members of this family, Bi₂Sr₂Nb₂RuO₁₂ and Bi₂SrNaNb₂RuO₁₂ that are analogous to Bi₂Sr₂Nb₂TiO₁₂, possessing tetragonal (I4/mmm) Aurivillius structure have also been synthesized.     

Hochdrucksynthesen und transformationen von alkali-oxofluoro-metallaten des typs Cs2NaMOxF6−x und Rb2LiMOxF6−x

Compounds and mixtures of compositions Cs₂NaMO_xF_6?x and Rb₂LiMOF₅ were transformed under high pressure to metastable modifications crystallizing in the hexagonal HT-K₂LiAlF₆ type structure and/or the cubic K₂NaCrF₆ elpasolithe type structure.Mixtures of composition Rb₂LiMO_xF_6?x (M = Nb⁵⁺or Mo⁶⁺) yielded under high pressure besides LiF metastable compounds A₂MO_xF_5?x crystallizing orthorhombic in the K₂VO₂F₃ type structure.     

Synthesis and characterization of bulk amorphous/amorphous composite alloys through powder metallurgy route

In the present study, amorphous Ni₆₀Nb₂₀Zr₂₀ and Ti₅₀Cu₂₈Ni₁₅Sn₇ alloy powders were synthesized separately using a mechanical alloying (MA) technique. The dual-amorphous-phased (Ti₅₀Cu₂₈Ni₁₅Sn₇)_100−x (Ni₆₀Nb₂₀Zr₂₀) _x (x = 0, 10, 20, and 30 vol%) powders were prepared by mixing the corresponding amorphous powders. The dual-amorphous-phased powders were then consolidated into bulk amorphous/amorphous composite (BA/AC) alloy discs. The amorphization status of as-prepared powders and bulk BA/AC composite discs was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The microstructure of the BA/AC discs showed that the Ni₆₀Nb₂₀Zr₂₀ phase is distributed homogeneously within the Ti₅₀Cu₂₈Ni₁₅Sn₇ matrix. The (Ti₅₀Cu₂₈Ni₁₅Sn₇)₇₀(Ni₆₀Nb₂₀Zr₂₀)₃₀ BA/AC disc exhibited a relative density of 96.6% and its Vickers microhardness was 726 kg/mm².     

Nb掺杂Bi4Ti3O12层状结构铁电陶瓷的电行为特性研究

采用固相烧结工艺制备了Nb5+掺杂的Bi4Ti3O12层状结构铁电陶瓷.运用XRD和AFM对Bi4Ti3-xNbxO12+x/2材料的微观结构进行表征,发现所制备的陶瓷均具有单一的正交相结构,抛光热腐蚀表面晶粒的显微形貌表现为随机排列的棒状结构.通过对材料直流电导率与温度关系的Arrhenius拟合,分析了Bi4Ti3-xNbxO12+x/2的导电机理.Nb5+掺杂提高了材料的介电常数,但居里温度随掺杂含量的增加呈线性下降趋势.DSC结果显示Bi4Ti3-xNbxO12+x/2材料在居里温度处经历了一级铁电相变.样品的铁电性能测试结果表明,Nb5+掺杂Bi4Ti3O12提高了材料的剩余极化Pr,这主要是由于Nb5+取代Ti4+大大降低了材料中氧空位的浓度,使得氧空位对畴的钉扎作用减弱的缘故.     

New rutile solid solutions,Ti1−4xLixM3xO2: M = Nb,Ta, Sb

Three extensive new rutile solid solution series have been prepared in which Ti⁴⁺ is replaced by a combination of Li⁺ and a pentavalent cation: Nb⁵⁺, Ta⁵⁺, Sb⁵⁺. The formulae are Ti_1?4xLi_xM_3xO₂: 0 < x ? 0.15, M = Ta; 0 < x ? 0.17, M = Nb; 0 < x ? 0.12, M = Sb. The solid solutions were characterised by X-ray powder diffraction and density measurements. In addition to the rutile solid solutions, LiNb₃O₈ forms a limited range of solid solutions, Li_1?yNb_3?3yTi_4yO₈: 0 < y ? 0.06.     

The Effect of the Pinning Center Size on the Vortex Pinning by?Embedded ZrO2 Nano-particles

The influence of pinning centers size on the superconducting properties was investigated. Through the addition of three batches of ZrO₂ nano-particles with mean size of D ₁=13 nm, D ₂=21 nm, and D ₃=85 nm, we have succeeded in incorporating effective artificial pinning centers within the YBCO matrix of the bulk superconductor. An enhancement in the flux pinning and an improvement in the critical current densities (transport critical current density J _ct and magnetic critical current density J _cm) were achieved. The results indicate that slight inclusions of ZrO₂ can greatly enhance the flux pinning capability of samples. Comparative analyses of the critical current densities and the resulting pinning force F _p for the three diameters have shown that pinning centers with finer size are much more efficient than those with a size larger than the coherence length ??.     

Effect of Different Nanosized NiO Addition on Ag-Sheathed (Bi1.6Pb0.4)Sr2Ca2Cu3O10 Superconductor Tapes

(Bi_1.6Pb_0.4)Sr₂Ca₂Cu₃O₁₀–(NiO) _x superconductor with x=0, 0.01, 0.02, 0.03, 0.04, and 0.05 wt.% was prepared using the coprecipitation technique. The average size of NiO used was 8 nm and 16 nm. The highest value of the transport critical current density, J _c in the bulk form was observed in the x=0.01 wt.% samples for both sizes. Based on this result, Ag sheathed superconductor tapes with starting composition (Bi_1.6Pb_0.4)Sr₂Ca₂Cu₃O₁₀–(NiO)_0.01 (with NiO particle size 8 and 16 nm) were fabricated using the powder-in-tube method. The effect of these nanoparticles addition on the microstructure, phase formation, and transport critical current density were studied. The J _c of the tape at 77 K in zero fields with NiO (8 nm) addition (4500 A/cm²) was slightly higher than that of NiO (16 nm) added tape (4120 A/cm²). The nonadded tape showed a lower J _c (1080 A/cm² at 77 K). These results indicated that magnetic nanoparticles such as NiO could act as an effective flux pinning centers leading to the enhancement of J _c in the bulk and tape form. Moreover, NiO with size closer to the coherence length, ξ was more effective in enhancing J _c.     

Optical and structural properties of TiO2 films as a function of Nb doping concentration

Ti_1?Cx Nb _x O₂ (x?=?0?C0.06) films were prepared on quartz substrates by sol?Cgel spin coating and characterized by a variety techniques. The effect of Nb doping on the structural and optical properties of Ti_1?Cx Nb _x O₂ films were mainly investigated by X-Ray diffraction (XRD), Raman spectroscopy, Field-emission scanning electron microscopy and UV?Cvis transmittance spectroscopy. XRD and Raman study showed that the Nb doping inhibited the grain growth. The grain size decreased from 39.4 to 23.4?nm as the doping concentration increased from 0 to 0.06 in atomic percent. The UV?Cvis transmittance spectroscopy analysis revealed that the films exhibited 55?C65% transmittance in the visible region and the band gap of films became wider with increasing Nb doping concentration.     

Scaling Behavior in Aluminum Doped MgB2 Superconductor

Magnetization measurements have been performed on Mg_1?x Al _x B₂ (0≤x≤0.1) at various temperatures below T _c and in fields up to 9 T. The reversible magnetization has been used to calculate the thermodynamic critical field (H _c ) which is then used to obtain a new universal behavior for both the critical current density (J _c ) and the pinning force (P _f ). This approach is based on vortex core energy ( ${\sim}H^{2}_{c}$ ) and clearly exhibits the deviation from single vortex pinning with increasing temperature and higher concentration of Al-doping.