Critical current density and flux pinning in La2−xPrxCa2x Ba2Cu4+2xOz (x=0.1-0.5) superconductors

Polycrystalline La_2−xPr_xCa_2xBa₂Cu_4+2xO_z (LPCaBCO) compounds with x=0.1-0.5 were synthesized by solid-state reaction method and studied by room temperature X-ray diffraction, dc resistivity, dc magnetization and iodometry. The superconducting transition temperatures in these tetragonal triple perovskite compounds increases from 32 to 62 K (T_c^onset values) with increasing dopant concentration. The mixing of rare earth La³⁺ and Pr^3+/4+ ions at rare earth site (La³⁺) along with substitution of divalent Ca²⁺ results in the shrinkage of unit cell volume. The contraction of unit cell volume due to larger ion being substituted by smaller ions, gives rise to creation of pinning centres in the unit cell leading to increase in critical current density and flux pinning.     

Soft x-ray absorption and high-resolution powder x-ray diffraction study of superconducting CaxLa1−xBa1.75−xLa0.25+xCu3Oy system

We have studied the electronic structure of unoccupied states measured by O K-edge and Cu L-edge x-ray absorption spectroscopy (XAS), combined with crystal structure studied by high resolution powder x-ray diffraction (HRPXRD), of charge-compensated layered superconducting Ca_xLa_1−xBa_1.75−xLa_0.25+xCu₃O_y (0≤x≤0.4 and 6.4≤y≤7.3) cuprate. A detailed analysis shows that, apart from hole doping, chemical pressure on the electronically active CuO₂ plane due to the lattice mismatch with the spacer layers greatly influences the superconducting properties of this system. The results suggest chemical pressure to be the most plausible parameter to control the maximum critical temperatures (T_c^max) in different cuprate families at optimum hole density.     

Superconductivity and magnetism of La2Cu1−xZnxO4+δ chemically oxidized by NaClO

The structural, superconducting and magnetic properties of La₂Cu_1−xZn_xO_4+δ (0≤x≤0.1) chemically oxidized by NaClO at room temperature were studied. All the samples before and after oxidation are single phase with orthorhombic structure, as indicated by their powder X-ray diffraction analysis. The iodometric titration results indicate that Zn-substituted La₂Cu_1−xZn_xO₄ is more favorable for the insertion of the excess oxygen, as compared to the Zn-free La₂CuO₄. The T_c suppression rate resulting from Zn substitution in La₂Cu_1−xZn_xO_4+δ is −12.4 K/%. The effective magnetic moment induced by the non-magnetic Zn ion is the order of one Bohr magneton, which decreases with increasing the Zn concentrations in the range examined. The latter two results are qualitatively well consistent with those obtained in La_2−xSr_xCu_1−yZn_yO₄ with the Sr optimal doping. This reveals that the non-magnetic Zn ions play the same role in both of the La₂Cu_1−xZn_xO_4+δ with the excess oxygen content of about 0.1 and the La_2−xSr_xCu_1−yZn_yO₄ with the Sr optimal doping.     

Structure and magnetic properties of Ca1−x−yMgxCu2+yO3 influenced by isoelectronic substitution x

Phases of the composition Ca_1−x−yMg_xCu_2+yO₃ have been prepared for the first time. The compounds are isostructural with the known end-members CaCu₂O₃ and MgCu₂O₃ showing a two-leg spin-ladder-like connection of copper and oxygen atoms within the Cu₂O₃-layer. Opposite the spin ladders this layer is folded, which results in a long-range antiferromagnetic ordering of these phases. The Néel temperature can be adapted by variation of x in Ca_1−x−yMg_xCu_2+yO₃ between 24 and 80 K. Several structural features, which influence the magnetic ordering, are discussed.     

Increase of Tc in the 1:2:3 superconductors YBCO and (CaxLa1−x)(LauBa1−u)2Cu3Oy after slow cooling or low temperature annealing

The well known phenomenon of the increase of T_c of YBCO after slow cooling or low temperature annealing without change of the oxygen content, was found also for the YBCO like tetragonal superconductors of (Ca_xLa_1−x)(La_uBa_1−u)₂Cu₃O_y (this compound has been previously denoted as CLBLCO, CLBCO or CaLaBaCuO). It has been observed at 150 and 100 °C for oxygen underdoped, optimally- and overdoped ceramics. The products retain their tetragonal unit cells. The possible reasons of this phenomenon are discussed.     

Structural studies and T c dependence in La2−x Dy x Ca y Ba2Cu4+y O z type mixed oxide superconductors

A new series of mixed oxide superconductors with the stoichiometric composition La_2−x Dy _x Ca _y Ba₂Cu_4+y O _z (x=0.0 − 0.5, y=2x) has been studied for structural and superconductiong properties. Our earlier studies on La_2−x (Y/Er) _x Ca _y Ba₂Cu_4+y O _z series, show a strong dependence of T _c on hole concentration (p _sh). In the present work, the results of the analysis of the neutron diffraction measurements at room temprerature on x=0.3 and 0.5 samples are reported. It is interesting to know that Ca substitutes for both La and Ba site with concomitant displacement of La onto Ba site. Superconductivity studies show that maximum T _c is obtained for x=0.5, y=1.0 sample (T _c ∼ 75 K), for La_1.5Dy_0.5Ca₁Ba₂Cu₅O _z (La-2125).     

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.     

Structure and superconductivity in FexCu1−xBa2YCu2O7+y superconductors synthesized by high pressure

Comprehensive studies of X-ray diffraction, oxygen content, superconductivity and Mössbauer effect have been made on Fe_xCu_1−xBa₂YCu₂O_7+y superconductors (0.00≤x≤0.70) synthesized by ambient (AM) and high pressure (HP). Results indicate that all the HP-samples have tetragonal structure, smaller lattice parameter c and unit-cell volume than the AM-samples. The studies of oxygen content, and Mössbauer spectroscopy indicate that the HP-samples have higher oxygen content, carrier concentration and average valence of Fe than the AM-samples. Moreover, for the HP-samples more Fe atoms located in CuO_x chains have fivefold-oxygen coordination. These are important reasons for the enhancement of T_c in the HP-samples.     

Effects of excess oxygen content on the hole-carrying CuO2-layers in Tl2(Ba1−xSrx)2Ca2Cu3Oy superconducting oxides

The effects of oxygen doping on the hole-carrying CuO₂-layers in Tl₂(Ba_1−xSr_x)₂Ca₂Cu₃O_y were studied by combined chemical and valence analysis, T_c measurements and neutron diffraction. The highest T_c is characterized by an optimal excess oxygen content, Δy, dichotomizing the under- and over-doped regions for each Sr concentration. While the average Tl valence is close to 3.0 and independent of Δy, the average Cu valence shows a linear dependence with Δy. An increase of the flatness of the CuO₂ plane, characterized by the O(2)-Cu(2)-O(2) bond angle of ∼176°, was observed at the optimal Δy.     

Phase separation, ferromagnetism and magnetic irreversibility in La1−xSrxMn1−yFeyO3

Magnetic susceptibility, χ(T), is investigated in ceramic La_1−xSr_xMn_1−yFe_yO₃ (LSMFO) samples with x=0.3 and y=0.15−0.25. A ferromagnetic (FM) transition observed in LSMFO is accompanied with an appreciable decrease of the transition temperature with increasing y, which is connected to breaking of the FM double-exchange interaction by doping with Fe. Strong magnetic irreversibility, observed in low (B=10 G) field, gives evidence for frustration of the magnetic state of LSMFO. The FM transition, which is expanded with increasing B, is more pronounced in the samples with y=0.15-0.20 and broadens considerably at y=0.25, where the irreversibility is increased. Well above the transition, χ(T) exhibits a Curie-Weiss asymptotic behavior, yielding very large values of the effective Bohr magneton number per magnetic ion, incompatible with those of Mn or Fe single ions. At y=0.15 and 0.20 a critical behavior of χ⁻¹(T)∼(T/T_C−1)^γ in the region of the FM transition is characterized by influence of two different magnetic systems, a 3D percolative one with γ₌γ_p≈1.8 and T_C=T_C^(p), and a non-percolative 3D Heisenberg spin system, with γ=γ_H≈1.4 and T_C=T_C^(H), where T_C^(p)<T_C^(H). At y=0.25 the percolative contribution to the critical behavior of χ(T) is not observed. The dependence of χ on T and y gives evidence for phase separation, with onset already near the room temperature, leading to generation of nanosize FM particles in the paramagnetic host matrix of LSMFO. The ferromagnetism of LSMFO is attributable to percolation over the system of such particles and generation of large FM clusters, whereas the frustration is governed presumably by a system of smaller weakly-correlated magnetic units, which do not enter the percolative FM clusters.     

Contribution of polaron hopping to the electron paramagnetic resonance linewidth in La1−xCaxMnO3 and related materials

We analyze the contribution of polaron hopping to the electron paramagnetic resonance linewidth in La_1−xCa_xMnO₃ and related materials. The material is assumed to be in the paramagnetic phase and the conductivity is associated with the activated polaron hopping. It is also assumed that the adiabatic, small polaron picture is appropriate so that the conductivity varies as exp[−E_a/T]/T, where E_a denotes the polaron activation energy. The polaron contribution to the linewidth is given by the expression C[χ₀(T)/χ(T)]exp[−E_a/T] where χ₀(T) is the Curie susceptibility (∼1/T), χ(T) is the measured susceptibility and C is a material-dependent parameter. Various experimental studies reporting polaron contributions to the linewidth are discussed. It is pointed out that fitting the linewidth to the functional form ΔH₀+(A/T)exp[−E_a/T] is not physically justified. In the high temperature–mean field regime, the exchange narrowed width, (1−Θ/T)k(∞), where Θ is the paramagnetic Curie temperature, replicates the exponential functional form with reasonable values for the activation energy. From previous measurements of the conductivity that showed activated polaron hopping as the leading transport mechanism, we concluded that the linewidth in La_0.7Ca_0.3MnO₃ is a sum of exchange narrowing and one-phonon spin–lattice terms with no evidence of a contribution from polaron hopping or band transport as had been previously proposed. A similar conclusion is reached for La_0.8Ca_0.2MnO₃, nanometer-sized La_0.9Ca_0.1MnO₃, and La_0.9Te_0.1MnO₃.     

Effects of compositions on characteristics and microstructures for melt-spun ribbons and die-upset magnets of Nd12.8+xFe81.2−x−y−zCoyGazB6

The effects of varying Nd-contents on the magnetic properties of Nd_12.8+xFe_{81.2−x−y−z}Co_yGa_zB₆ ribbons and the resulting die-upset magnets have been investigated. It was found that the Nd content has a significant effect on both magnetic properties and texture of the die-upset magnets. An optimum Nd content exhibits good texture, while both Nd-lean and excessively Nd-rich Nd-Fe-B alloys lead to unsatisfactory textures. The magnetic properties and texture of the die-upset magnets strongly deteriorated with a further increase in the Nd-content. The thermo-mechanical characteristics and the formation mechanism of two kinds of cracks in backward-extruded ring magnets have also been investigated.     

Simultaneous enhancements of Curie temperature and magnetocaloric effects in the La1−xCexFe11.5Si1.5Cy compounds

The effects of introducing Ce and C atoms on the Curie temperature (T_C), the magnetic entropy change (ΔS_M) and the hysteresis loss have been investigated in the NaZn₁₃-type LaFe_11.5Si_1.5 compound. Partial replacement of La with Ce leads to a decrease in T_C and an increase in ΔS_M; however, the introduction of interstitial C atoms can adjust T_C to high temperature. The itinerant-electron metamagnetic transition is weakened after carbonization, which results in a reduction of both the hysteresis loss and magnetocaloric effect (MCE). The maximum value of ΔS_M for La_0.8Ce_0.2Fe_11.5Si_1.5C_0.2 is found to be −28 J/kg K at T_C=207 K with an effective refrigeration capacity of 420 J/kg for a field change from 0 to 5 T. Our study reveals that the enhancements of both T_C and MCEs can be achieved simultaneously in the La_1−xCe_xFe_11.5Si_1.5C_y compounds by adjusting the concentrations of Ce and C atoms appropriately.     

Thermoluminescence studies of bismuth doped BaxCa1−xS nanostructures

Bismuth doped Ba_1−xCa_xS:Bi (x=0-1) nanocrystallities have been prepared by the solid state reaction method and characterized by XRD and TEM. X-ray diffraction analysis shows the formation of the compounds in cubic structure at room temperature. Only partial replacement of Ba is possible and we found that Ba_0.5Ca_0.5S:Bi could not be prepared due to the difference between ionic radii of barium and calcium. Thermoluminescence studies of these samples after exposure to UV radiation have been carried out. The TL glow curve of Ba_xCa_1−xS:Bi has been found to be a simple structure with a single peak at 405, 428 and 503 K for x=1, 0.8 and 0, respectively. The kinetic parameters at various heating rates namely activation energy (E), order of kinetics (b) and frequency factor (s) of the Ba_1−xCa_xS:Bi (x=0.2) (0.4 mol%) sample have been determined using Chen’s method. The deconvolution of curve was done using the GCD function suggested by Kitis. The effect of different heating rates and different amount of dose has also been discussed.     

Electrical conductivity of (Er,U)O2+x and (Ce,U)O2+x

The electrical conductivity (σ) of (Er_yU_1−y)O_2+x (y=0.06, 0.20) and (Ce_yU_1−y)O_2+x (y=0.05, 0.15, 0.25) has been measured as a function of oxygen partial pressure in the temperature range of 1100≤T/°C≤1300 by a d.c. 4-probe method. Both of the oxides exhibited Po₂-regions where the electrical conductivity is independent of oxygen partial pressure, which indicates that doped Er and Ce exist as trivalent cations on uranium sites and fix the hole concentration by acting as electron acceptors, i.e. [h]=[Er′_U] and [h]=[Ce′_U], respectively. It is considered that strong oxidization tendency of uranium and reduction tendency of cerium simultaneously render the cerium ions exist exclusively as Ce³⁺ in the uranium dioxide. The electron-hole mobility of (Er_yU_1−y)O_2+x and (Ce_yU_1−y)O_2+x in the Po₂ region where σ is constant has been calculated by the combination of the electrical conductivity and charge carrier concentration; the activation energy (E_H) of each oxide has been obtained from the temperature dependence of the mobility. Small polaron hopping conduction mechanism was confirmed by small magnitude of the mobility (0.018-0.052 cm² V⁻¹ s⁻¹) and the activation energy (0.12-0.22 eV).     

Electric quadrupole interaction of Cd in (Ce1−xLax)Ru2 and (Ce1−yCay)Ru2 synthesized at high pressure

Quasibinary Laves systems (Ce_1−xLa_x)Ru₂ and (Ce_1−yCa_y)Ru₂ doped with ¹¹¹In were synthesized at a pressure of 8 GPa, and variations of the electric quadrupole interaction of ¹¹¹Cd at the Ru sites have been studied by the method of time-differential perturbed angular correlation in a wide range of Ce-La and Ce-Ca relative concentrations. In the first case two sites with quadrupole frequencies ν_Q≅220 and 150 MHz persist at x≤0.2, while at x≥0.3 only the higher frequency component remains in the spectra, which are similar to that of pure LaRu₂. In the series (Ce_1−yCa_y)Ru₂, at y≥0.03 the lower frequency component was washed out except in samples with y=0.1 and 0.2, where it was restored.     

Urbach rule and disordering processes in Cu6P(S1−xSex)5Br1−yIy superionic conductors

Compositional behavior of Urbach absorption edge is studied as well as the effect of compositional disordering on the parameters of exciton-phonon interaction, phase transition temperatures and electric conductivity in Cu₆P(S_1−xSe_x)₅Br_1−yI_y superionic solid solutions. The effect of different types of disordering on the optical absorption processes and specific features of compositional changes in the absorption edge spectra under S→Se and Br→I anion substitution in the mixed crystals are investigated. (x, T) phase diagrams for Cu₆P(S_1−xSe_x)₅X (X=I, Br) solid solutions are studied.     

Raman spectra of Ba6−3xSm8+2xTi18O54 solid solution

Raman spectra of Ba_6−3xSm_8+2xTi₁₈O₅₄ solid solution were investigated as the function of x and sintering time. Reasonable explanations were provided about the Raman shifts and their intensities at 1013, 590, 751, 280, 232 cm⁻¹. 1013 cm⁻¹ demonstrates the existence of BaCO₃ phase in solid solution, 590 cm⁻¹ is the symmetric stretching mode of the basal oxygens of the octahedral; 280 and 232 cm⁻¹ are the symmetric stretching modes resulted from the tilt of octahedral when large cation sites are Sm³⁺ and Ba²⁺. The shoulder peak appearing around 302 cm⁻¹ is related to the vacancy produced by the unequal valence of Sm³⁺ and Ba²⁺.     

Superconductivity in CoSr2(Y1−xCax)Cu2O7+δ

The roles of aliovalent Ca^II-for-Y^III substitution and high-pressure-oxygen annealing in the process of ‘superconducterizing’ the Co-based layered copper oxide, CoSr₂(Y_1−xCa_x)Cu₂O_7+δ (Co-1212), were investigated. The as-air-synthesized samples up to x=0.4 were found essentially oxygen stoichiometric (−0.03≤δ≤0.00). These samples, however, were not superconducting, suggesting that the holes created by the divalent-for-trivalent cation substitution are trapped on Co in the charge reservoir. Ultra-high-pressure heat treatment carried out at 5 GPa and 500 °C for 30 min in the presence of Ag₂O₂ as an excess oxygen source induced bulk superconductivity in these samples. The highest T_c was obtained for the high-oxygen-pressure treated x=0.3 sample at ∼40 K.     

On the coexistence of spin and lattice polarons in the La0.67−xEuxCa0.33MnO3 CMR system

The electrical and magnetic transport properties of the La_0.67−xEu_xCa_0.33MnO₃ system exhibit lowering of insulator to metal and paramagnetic to ferromagnetic transition temperature (T_C) with the increase of Eu concentration in addition to possessing CMR property. The temperature variation of electrical resistivity and magnetic susceptibility for x=0.21 is found to have two distinct regions in the paramagnetic state for T>T_P; one with the localization of lattice polaron in the high-temperature region (T>1.5T_P) satisfying the dynamics of variable range hopping (VRH) model and the other being the combination of the spin and lattice polarons in the region T_P<T<1.5T_P. The resistivity variation with temperature and magnetic field, the cusp in the resistivity peak and CMR phenomenon are interpreted in terms of coexistence of spin and lattice small polarons in the intermediate region (T_P<T<1.5T_P). The spin polaron energy in the La_0.46Eu_0.21Ca_0.33MnO₃ system is estimated to be 106.73±0.90 meV and this energy decreases with the increase of external magnetic field. The MR ratio is maximal with a value of 99.99% around the transition temperature and this maximum persists till T→0 K, at the field of 8 T.