Nax/2Bi1−x/2MoxV1−xO4 and Bi1−x/3MoxV1−xO4 New Scheelite-related phases

New Scheelite-related solid solutions of the compositions Na_x/2Bi_1−x/2Mo_xV_1−xO₄ (0≤x≤1) and Bi_1−x/3 Mo_xV_1−xO₄(0≤x≤0.2) have been synthesised by the substitution of Na and Mo at the A and B sites respectively of the ABO₄ type ferroelastic BiVO₄. The phases were characterised using chemical analysis, powder X-ray diffraction, scanning electron microscopy, EDAX, and Raman spectroscopy. While almost a continuous solid solution is obtained for the series Na_x/2Bi_1−x/2Mo_xV_1−xO₄, the absence of Na at the A-site results only in a narrow stability region for the other series, Bi_1−x/3 Mo_xV_1−xO₄ where 0≤x≤0.2. Raman spectra of selected samples at room temperature also suggest that vanadium and molybdenum atoms are disordered at the tetrahedral sites.     

Ramans study of Y1−xPrxBa2Cu3O7−δ and YB2Cu3−xZnxO7−δ single crystals

Single crystals with known T_c values of Y_1−xPr_xBa₂Cu₃O_7−δ (Y---Pr1:2:3) and YBa₂Cu_3−xZn_3−xZn_xO_7−δ (Y---Zn1:2:3) systems are studied by Raman measurements. The Raman spectra for (Y---Pr1:2:3) single crystals show that the frequencies of Ba and O_z modes increase as the Pr content increases. The results are consistent with the hole-localization scheme proposed for the suppression of superconductivity in the polycrystalline Y---Pr1:2:3 systems. On the other hand, in the Y---Zn1:2:3 system, all the Raman modes do not change in frequencies. However, the FWHM of the Cu(2) mode increases with the decrease of T_c, indicating strong scattering of charge carriers by the substituted Zn ions in the CuO₂ planes. The induced disorder in the CuO₂ planes may be related with suppression of T_c in the Y---Zn1:2:3 system. Thus, the suppression mechanism in the Y---Zn1:2:3 systems seems to be different from that in the Y---Pr1:2:3 systems.     

Superconductivity and crystallographic properties of La2 − xMxCuO4 − δ (M = Na, K)

Superconductivity and crystallographic properties of La_{2 − x}M_xCuO_{4 − δ} (M = Na, K) are studied. In the La_{2 − x}M_xCuO_{4 − δ} system, superconductivity is detected for x 0.2. Oxygen content analysis shows that the system has more oxygen vacancies than the La_{2 − x}Sr_xCuO_{4 − δ} system. These oxygen vacancies may reduce the hole concentration, and high Na-doping is needed to produce superconductivity. In the La_{2 − x}K_xCuO_{4 − δ} system, superconductivity is observed for the first time. Resistivity and magnetic susceptibility measurements show that T_c(onset) is 40 K and the Meissner volume fraction is about 4% for x = 0.7. The system changes from orthorhombic to a tetragonal K₂NiF₄ structure at x ≈ 0.3 and only tetragonal samples show superconductivity.     

Phonon Raman scattering in Nd1+xBa2−xCu3O7−δ and Pr1+xBa2−xCu3O7−δ single crystals

The polarized Raman spectra of Nd_1+xBa_2−xCu₃O_7−δ (−0.023≤x≤0.107) and Pr_1+xBa_2−xCu₃O_7−δ (0.01≤x≤0.15) single crystals have been investigated. It was found that the Cu(2) A_g mode softens by 6 cm⁻¹ in Nd 1:2:3 and 4 cm⁻¹ in Pr 1:2:3 as x increases. These frequency shifts cannot be explained by the change in the relevant bond lengths due to Nd(Pr)-substitution for Ba. The variations with x of the two low frequency modes may be affected by change of their hybridization and/or change of their force constants. The linewidths of Ba mode in Pr 1:2:3 are broader than those in Y 1:2:3. This result suggests that the Pr substitution on Ba sites occurred even in a very small value of x. In x(yy) geometry the relative intensity of the Ba and O(4) modes in Nd 1:2:3 is greater than those in Pr 1:2:3. The difference between Nd 1:2:3 and Pr 1:2:3 in the relative intensity of the Ba and O(4) modes may be produced by the chains.     

The T-contribution to the electrical resistivity at low temperatures in NiAs-type Ni1−xS1−ySey

The y(1−y)-like maxima in Ni_0.98S_1−ySe_y are found beyond y_c in the y-dependences of the coefficient of the T² contribution to the resistivity and of the residual resistivity. Here, the critical concentration of the antiferromagnetic and less-conductive phase is determined to be y_c = 0.095.     

Trapping mechanism of superconductivity suppression induced by Pr substitution in the Ho1−xPrxBa2Cu3O7−δ system

Pr concentration dependence of the superconducting transition temperature T_c in the Ho_1−xPr_xBa₂Cu₃O_7−δ system is determined from measurements of DC electrical resistance. This dependence coincides with that for the parallely studied Y_1−xPr_xBa₂Cu₃O_7−δ reference system. Both systems have the same value of the critical concentration x_c=0.58, in accordance with nearly equal ionic radii of Ho³⁺ and Y³⁺ ions. It has been shown that the T_c(x) curve can be described with a single mechanism based on a decreasing number of sheet holes trapped by Pr^IV-ions, if one takes also into account that the number of these ions changes with x.     

Influence of Nd substitution on the superconducting properties of ceramics in the 2212 system Bi2−wPbwSr2−xCa1−yNdx+yCu2O8+z

In the system Bi_2−wPb_wSr_2−xCa_1−yNd_x+yCu₂O_8+z different fractions of Nd are substituted on either Sr of Ca sites in order to introduce intrinsic insulating pinning centres. It is shown that a Nd concentration around x or y = 0.2 is likewise favourable with an average Nd---Nd distance in the range of the coherence length in the a, b-plane. However, clear evidence of flux pinning is only present for charge compensation with Pb²⁺. A simultaneous substitution of the Bi-based 2212 superconductor with moderate amounts of Nd³⁺ and Pb²⁺ improves the superconducting properties by strengthening the flux pinning forces.     

NMR study of coexistence of ferro- and antiferromagnetism in (Zr1−xNbx)Fe2

The magnetic phase diagram has been investigated in the C14 type (Zr_1−xNb_x)Fe₂ with x0.7 from ⁹³Nb NMR and magnetization measurements. In the compound with x = 0.825 a first order-like transition has been found to occur around 25 K from a canted state with the ferromagnetic moment in the basal plane to a ferromagnetic state with decreasing temperature.     

Luminescence from Si-Si1−xGex/Si1−yCy-Si structures

Near band edge photoluminescence has been obtained from Si_1−yC_y quantum well (QW) and neighboring Si_1−xGe_x/Si_1−yC_y double QW (DQW) structures. Enhanced no-phonon recombination is observed from the DQW structures and it is attributed to a breaking of the k-selection rule in the presence of the heterointerface. The luminescence persists for measurement temperatures up to 30–50 K and the intensity exhibits a quenching behavior with an activation energy equal to 8–20 meV. In electroluminescence only recombination in the Si_1−xGe_x layer has been observed from neighboring Si_1−xGe_x and Si_1−yC_y DQW structures.     

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.     

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.     

Superconductivity in Ba2−xSrxYCu3Oz prepared at GPa

The compounds Ba_1.4Sr_0.6YCu₃O_z (7.0≤z<7.4) were synthesized at 2 GPa to clarify the effect of high temperature and pressure on the superconducting properties. The structure as observed by powder X-ray diffraction was tetragonal for z≥7.2, and orthorhombic or tetragonal for z<7.1. It has been revealed that high-pressure syntheses suppress the superconductivity in the compounds with z<7.1. Recovery of the superconducting properties occurs above z≥7.2. The effect of Sr substitution on the superconducting properties was also studied. The magnitude of diamagnetism at 5–30 K decreases monotonically from −0.003 to −0.0002 emu/g Oe with increasing x in Ba_2−xSr_xYCu₃O_z (x≥1.2).     

Magnetic properties of Gd2(Fe1−x−yCoyTix)17

Serial single-phase Gd₂(Fe_1−x−yCo_yTi_x)₁₇ compounds have been synthesized. These compounds have a crystal structure belonging to rhombohedral lattice with space group. The lattice parameters of compounds decrease with cobalt content and increase with titanium content, respectively. The saturation magnetization decreases with increasing cobalt and titanium contents. The anisotropy fields increase to maximum then decrease with cobalt concentration. The magnetocrystalline anisotropy constants increase with cobalt content from negative to positive maximum and then decrease with Co concentration. The saturation moment of the compounds decreases linearly with cobalt concentration and decreases nonlinearly with titanium concentration.     

A spectroscopic ellipsometric investigation of new critical points of Zn1−xMnxS epilayers

Zn_1−xMn_xS epilayers were grown on GaAs (1 0 0) substrates by hot-wall epitaxy. X-ray diffraction (XRD) patterns revealed that all the epilayers have a zincblende structure. The optical properties were investigated using spectroscopic ellipsometry at 300 K from 3.0 to 8.5 eV. The obtained data were analyzed for determining the critical points of pseudodielectric function spectra, (E) = ₁(E) + i₂(E), such as E₀, E₀ + Δ₀, and E₁, and three E₂ (Σ, Δ, Γ) structures at a lower Mn composition range. These critical points were determined by analytical line-shapes fitted to numerically calculated derivatives of their pseudodielectric functions. The observation of new peaks, as well as the shifting and broadening of the critical points of Zn_1−xMn_xS epilayers, were investigated as a function of Mn composition by ellipsometric measurements for the first time. The characteristics of the peaks changed with increasing Mn composition. In particular, four new peaks were observed between 4.0 and 8.0 eV for Zn_1−xMn_xS epilayers, and their characteristics were investigated in this study.     

Effect of aliovalent ion substitution on the oxide ion conductivity in rare-earth pyrohafnates RE2−xSrxHf2O7−δ and RE2Hf2−x AlxO7−δ (RE=Gd and Nd; x=0, 0.1 and 0.2)

Oxide ion conductivity of the pure and aliovalent ion substituted rare-earth pyrohafnates in the series RE_2−xSr_xHf₂O₇ and RE₂Hf_2−xAl_xO₇ (RE=Gd and Nd; x=0–0.2) has been explored in the temperature range 400°C–700°C for the first time. It is seen that, conductivity is enhanced by doping 5 atom% Sr at the rare–earth site in these systems. Well defined impedance plots due to grain interior and grain boundary resistances were obtained in the Gd pyrohafnate with Sr substitution. The results of the conductivity variation for the pure, Sr and Al doped phases are explained on the basis of pyrochlore structure.     

Effect of Pr and Ca substitution on superconductivity in Y(1−x−yPrxCay)Ba2Cu3O7−δ

Pr substituted at constant Ca concentration for Y in (Y_1−x−yPr_xCa_y)Ba₂Cu₃O_7−δ superconductors have been prepared under identical conditions and the temperature dependence of the electrical resistivity of these samples are measured. The resistively determined values of T_c decrease linearly with increasing x (0 ≤ x ≤ 0.2) for constant y = 0.10 and 0.15 which provides convincing evidence that the suppression of superconductivity by Pr is mainly due to magnetic pair breaking. The suppression of superconductivity can also be correlated to the observed changes in oxygen content determined by iodometric analysis and to the average Cu-valences. However, it is found that the observed suppression of T_c cannot be compensated by appropriate hole doping with Ca.     

Localized canting of spins structure in the spinel oxide system: ZnzTizFe2−x−zCrx−zCoO4

The spinel oxide system Zn_zTi_zFe_2−x−zCr_x−zCoO₄; z=x²; x=0.60, 0.65, 0.70 and 0.80, was studied using neutron diffraction technique, low field DC magnetization measurements (ZFC–FC measurements), magnetic hysterisis, Mössbauer spectroscopy and low field AC susceptibility measurements. All the compositions show significantly less B-site magnetic moments at 10 K temperature derived from neutron diffraction data than the free ions site moments deduced assuming collinear arrangement of spins. This combined with some other features seen in the low temperature neutron diffraction patterns suggest localized canting of spins (LCS) type of magnetic ordering in the present system where a long range order of longitudinal component of moments co-exists along with totally disordered transverse component of moments. The conclusion is also supported by the features seen in the other measurements. The magnetic moments derived from 10 K neutron diffraction data are explained using the LCS approach for different exchange integrals ratios.     

Non-Fermi liquid regimes in U1−xMxPd2Al3(M=Y,Th)

The temperature-composition (T–x) phase diagram and NFL characteristics in the electrical resistivity ρ(T), specific heat C(T), and magnetic susceptibility χ(T) at low temperatures for the systems U_1−xM_xPd₂Al₃ (M=Y,Th) are described. The T–x phase diagram, the NFL characteristics, and the underlying mechanism for the NFL behavior are distinctly different for M=Y³⁺ and Th⁴⁺, apparently reflecting the difference in valence of the M atom substituents, and suggesting that U is tetravalent in these two systems.     

Positive linear magnetoresistance in Fex–C1−x composites

A large positive magnetoresistance (MR) has been found in micro-sized Fe_x–C_1−x composites. At a magnetic field of 5 T, the Fe_0.2–C_0.8 composite has the largest MR, 53.8% and 190% at room temperature and at 5 K, respectively. The magnetic field dependence of the MR can be described approximately as MR∝Bⁿ, and the value of exponent n is determined by the Fe weight concentration and temperature, ranging from 1/4 to 6/4. It appears that Fe_x–C_1−x has a linear field dependence of the positive MR at different temperatures. The possible mechanism for the positive MR is discussed.     

Short-range order and F ion diffusion inside the Pb1 − xAlxF2 + x solid solution Part I: F-NMR investigation

A ¹⁹F-NMR investigation of the Pb_{1 − x}Al_xF_{2 + x} (0 ≤ x ≤ 0.12) solid solution and of the ordered Pb₉Al₂F₂₄ phase is undertaken as a function of temperature. Several F⁻ ion groups become progressively mobile at the NMR time scale at increasing temperature, the first, (F_i)_pb and (F_n)_pb, are located in the surroundings of Pb²⁺ cations, the second ones, (F_rd)_AI and (F_ra)_Al, in that of Al³⁺ cations. Four types of fluoride ions are distinguished in the ¹⁹F-NMR spectra of rigid lattice at low temperature (T = 175 K) and the distribution of fluoride ions among the four sites is determined by deconvolution of the spectra registered. The sites occupied by the (F_n)_pb anions are identified with the normal anionic sites of the fluorite network, the others with different interstitial sites.