Superconductivity in CeO1−xFxFeAs with upper critical field of 94 T

We have successfully synthesized Ce based oxypnictide superconductors with fluorine doping (CeO_1?xF_xFeAs) by a two step solid state reaction method. Detailed XRD and EDX confirm the crystal structure and chemical compositions. We observe that an extremely high H_c2(0) of 94 T can be achieved in the x = 0.1 composition. This increase in H_c2(0) is accompanied by a decrease in transition temperature (38.4 K in x = 0.1 composition) from 42.5 K for the x = 0.2 phase. The in-plane Ginzburg–Landau coherence length is estimated to be ～27 Å at x = 0.2 suggesting a moderate anisotropy in this class of superconductors. The Seebeck coefficient confirms the majority carrier to be electrons and strong dominance of electron–electron correlations in this multiband superconductor.     

Yttrium doped La1−xYxO0.9F0.1FeAs superconductors: Hall and thermopower studies

The effect of yttrium substitution at the lanthanum site on the superconducting properties of La_1?xY_xO_0.9F_0.1FeAs (‘x’ = 0, 0.10, 0.20, 0.30, 0.50 and 0.60) oxypnictides has been studied. Powder X-ray diffraction studies confirm single phases till x = 0.1 beyond which minor amount of Y₂O₃ is observed. The temperature dependence of resistivity measurements confirm the superconducting transition temperature (T_c) of 34.8 (±0.05) K and corresponding Meissner transition at 34.3 K in the ‘x’ = 0.3 composition which is higher than that reported for the parent phase (LaO_0.9F_0.1FeAs (T_c = 28 K)). Further increase in the concentration of yttrium leads to broadening and suppression of the superconducting transition. The value of H_c2 at zero temperature is estimated to be about 60.5 T. The Seebeck coefficient (S) shows a negative sign indicating that the major contribution to the conductivity is by electrons. The Hall coefficient (R_H) also remains negative throughout the temperature range supporting the thermopower results. The lattice parameters (a and c) decreases and the charge-carrier density increases with yttrium doping.     

The effect of chemical pressure in rutheno-cuprates

We have studied the effect of negative chemical pressure in the RuGd_1.5(Ce_0.5?xPr_x)Sr₂Cu₂O_10?δ with Pr content of 0.0 ? x ? 0.2. This is also investigated using the bond length results obtained from the Rietveld refinement analysis. The c parameter and cell volume increase with x for 0.0 ? x ? 0.15. The width of the resistivity transition also increases with Pr concentration, indicating higher inhomogeneity and oxygen deficiency. The difference in the ionic valences of Pr^3+,4+ and Ce⁴⁺ causing different hole doping, the difference in the ionic radii, and oxygen stoichiometry affect the superconducting transition. The magnetoresistance shows a cusp around 135 K which lies between the antiferromagnetic and ferromagnetic transition temperatures, which is probably due to the presence of a spin glass region. There exist two magnetic transition temperatures for 0.0 ? x ? 0.2 which respectively change from T_M = 155 K to 144 K and from T_irr = 115 K to 70 K. The magnetization versus applied magnetic field isotherms at 77 K and 300 K show that the remanent magnetization and coercivity are lower for samples with higher Pr content.     

Effect of fluorine doping on phase formation and properties of Bi(Pb)-2223 ceramics

Superconducting ceramics of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_yF_x (x = 0–0.6) are prepared in air by conventional solid state reaction and characterized. The study shows that the melting point of the samples decreases as fluorine content increases. As a consequence, the grain size increases with the doping level and for x = 0.6, the sample is completely deformed and presents a concave shape making impossible the measurements on it. The Vickers microhardness reaches its maximum for x = 0.2. The analysis of the X-ray diffraction results reveals that all the samples are composed of only Bi(Pb)-2212 and Bi(Pb)-2223 phases. The highest proportion of the high T_c phase (Bi(Pb)-2223) is also observed for x = 0.2 and is about 67.32%. The refinement of cell parameters is done by considering the structural modulation. The results show that the doping leads to a reduction of cell volume as well as the a axis component of modulation. From resistivity versus temperature measurements, it is shown that the doped phases exhibit higher onset critical transition temperatures than the undoped one. The residual resistivity increases with fluorine content suggesting that the doping introduces structural defects and disorder into the samples. The obtained critical current density at 77 K under zero magnetic field also increases with fluorine doping.     

Effects of Ni and Co doping on the physical properties of tetragonal FeSe0.5Te0.5 superconductor

Isoelectronic Tellurium (Te) substitution for Selenium (Se) in the tetragonal phase of FeSe (β-FeSe) increases the superconducting transition temperature (T_c) by applying a negative pressure on the lattice. However, the normal state resistivity increases and shows semi-metallic behavior for samples with higher Te concentration. With increasing Te concentration, the T_c increases and reaches a maximum for FeSe_0.5Te_0.5 and then decreases with further increase of Te. We have investigated the effect of Cobalt (Co) and Nickel (Ni) doping in FeSe_0.5Te_0.5 in the nominal composition range Fe_1?xTM_xSe_0.5Te_0.5 (TM = Co (x = 0.05, 0.1, 0.15, 0.2) and Ni (x = 0.05, 0.1)). Both Co and Ni doping suppress T_c and drives the system to metal–insulator transition. The in-plane (‘a’) and out-of-plane (‘c’) lattice constants decrease with increasing dopant concentration.     

Sulfur substitution and pressure effect on superconductivity of α-FeSe

In this paper, sulfur substitution and pressure effect on superconductivity of α-FeSe has been investigated in Fe(Se_1 ? xS_x)_0.88 (x = 0.1, 0.2). For x = 0.1, the critical temperature T_c is slightly larger than that of non-substituted sample, in consistent with the pressure effect on the superconductivity of α-FeSe. However, with further increasing S content to x = 0.2, T_c decreases. Temperature dependent of specific heat showed that the structural transition seems to be suppressed. T_c for x = 0.2 can be further decreased by applying pressure of 5 kbar, in contrary to the pressure effect on α-FeSe. We suggest that, in addition to the suppression of structural transition, other factors like the increase of carrier concentration should be considered for understanding the pressure effect on the superconductivity of α-FeSe.     

Internal friction study on bilayer cuprates Pr(Ba1−xSrx)2Cu3O7−δ

The internal friction of partially Sr-substituted Pr(Ba_1?xSr_x)₂Cu₃O_7?δ (x = 0, 0.05, 0.1, 0.2, 0.3, and 0.4) ceramics was measured by the vibrating-reed method from liquid nitrogen temperature to room temperature at kilohertz frequencies. The intensity of the internal friction peak, which appears around 250 K, decreases upon Sr doping. The result is explained in terms of a possible combining structural freezing transition of CuO₅ square pyramids and oxygen atoms in CuO_x chains. Another internal friction peak around 134 K was observed in tetragonal Pr(Ba_0.6Sr_0.4)₂Cu₃O_7?δ which is temporarily explained by the hopping of holes in the CuO₂ layers.     

An oxygen permeable membrane La0.8Sr0.2(Ga0.8Mg0.2)1 − xCrxO3 − δ for a reduced atmosphere

Mixed electron hole and oxide ion conducting perovskite-type oxides, La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ (0 ≤ x ≤ 1.0)_, were prepared by solid state reaction. The phase stability and the oxygen permeation properties of the oxides were examined as a function of the content of Cr. La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ has a perovskite related tetragonal phase with x = 0.1 to 0.8. The total electrical conductivity of La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ increases with increasing x. The oxygen permeation flux across the La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ membranes at higher temperatures increases with x up to x = 04. The maximum oxygen permeation flux of 1.6 × 10^? 7 mol^? 1 cm^? 2 at 1100 °C in a oxygen activity gradient of air/10^? 2 Pa is observed in La_0.8Sr_0.2(Ga_0.8Mg_0.2)_0.6Cr_0.4O_3 ? δ. This perovskite-type oxide is stable under an oxygen partial pressure of 7 × 10^? 10 Pa at 1000 °C.     

Superconductivity of the platinum doped 122 iron arsenide SrFe2−xPtxAs2

Pt doped 122 iron arsenide SrFe_1?xPt_xAs₂ (0 ? x ? 0.4) was successfully synthesized. The tetragonal unit-cell volume and the lattice constant a increase with increasing the Pt content, while c decreases, suggesting that the Fe ions are indeed replaced by Pt ions. By the Pt doping, the magnetic order of the parent phase is suppressed, and superconductivity emerges at approximately x = 0.15. T_c reaches the maximum of 16 K at x = 0.2. The compounds series can be a suitable subject to investigate role of the doped 5d state in the superconducting 3d Fe–As layer.     

Composition-dependent metal-insulator transition in perovskite CaXPb(1−X) TiO3 (CPT) ceramic

Ceramic compositions of a complex perovskite Ca_XPb(_1?X)TiO₃ (CPT) systems with x=0.6, 0.7 and 0.8 were prepared by mechanical mixing of their oxides (CaTiO₃ and PbTiO₃). The structure of the (CPT) ceramics was characterized by X-ray diffraction (XRD) The ceramics transform gradually from orthorhombic phase (pseudo cubic phase) to cubic phase by increasing pb content percent. The dc resistivity ρ(t) versus temperature (range 300–525 K) for x=0.6, 0.7 and 0.8. The ρ.T/curves reveal that samples exhibit a metallic behaviour at low temperature and undergo a metal-semiconductor transition with increasing temperature at T_p=373 K, 343 K and 333 K, for x=0.6, 0.7 and 0.8, respectively. The nature of conduction mechanism is studied in semiconductor region by studying the current–voltage temperature characteristics. The current–voltage characteristics were interpreted in terms of Poole–Frenkel type of conduction mechanism.     

Synthesis and critical current density promotion for Nb-doped 2212-BPSCCO HTc-superconducting materials

Although BPSCCO superconducting regime has very low stability under high oxygen pressures as reported in the literature, we managed to synthesize relatively pure 2212-BPSCCO and their Nb-doped samples having general formula Bi_1−xNb_xPbSr₂CaCu₂O₈, where x = 0.1, 0.2, 0.4 and 0.6 mole, respectively, at moderate oxygen pressure (∼30 bar). The superconducting measurements proved that the best recorded T_c ∼ 69 K was for the undoped 2212-BPSCCO, while the lowest T_c ∼ 58 K was recorded for the maximum doped sample x = 0.6 mole indicating that superconductive transition temperatures T_cs decrease regularly with increasing Nb-dopant concentration from x = 0.1 to 0.6, respectively. The lattice parameter c exhibited a slight length compression as Nb-dopant ratio increases from 0.1 to 0.6 mole, respectively. From SE-microscopic analysis, the average grain size was estimated and found in between 0.44 and 1.6 μm which is considered relatively high to that reported in the literature. The measured J_c’s values were found to be enhanced remarkably as Nb-dopant concentration increases.     

Effect of Ge4+ and Mg2+ doping on superconductivity,fluctuation induced conductivity and interplanar coupling of TlSr2CaCu2O7−δ superconductors

Substitution of Ge⁴⁺ in place of Cu in Tl_0.85Cr_0.15Sr₂CaCu_2?xGe_xO_7?δ (x = 0–0.6) showed initial increase in zero critical temperature value, T_{c zero} from 98 K (x = 0) to 100 K (x = 0.1) and in the range of 85–86 K for x = 0.2–0.3. The slow decrease in T_{c zero} is unexpected as tetravalent Ge⁴⁺ substitution is expected to strongly reduce hole concentration in the samples and suppress T_{c zero}. Excess conductivity analyses of resistance versus temperature data based on Asmalazov–Larkin (AL) theory revealed that the substitution induced 2D-to-3D transition of fluctuation induced conductivity with the highest transition temperature, T_2D–3D observed at x = 0.1. FTIR spectroscopy analysis indicates Ge⁴⁺ substitution cause reduction in CuO₂/GeO₂ interplanar distance while our calculation based on Lawrence–Doniach model revealed highest superconducting coherence length, ξ_c(0) and interplanar coupling, J at x = 0.3. On the other hand, substitution of divalent Mg²⁺ for Ca²⁺ in (Tl_0.5Pb_0.5)(Sr_1.8Yb_0.2)(Ca_1?yMg_y)Cu₂O₇ (y = 0–1.0), which is not expected to directly vary hole concentration, surprisingly caused T_{c zero} to increase from 89.6 K (y = 0) to an optimum value of 95.9 K (y = 0.6) before decreasing with further increase in y. Excess conductivity analyses showed 2D-to-3D transition of fluctuation induced conductivity for all samples where the highest T_2D–3D was at y = 0.4. Similar calculation revealed highest values of ξ_c(0) and J also at y = 0.4. FTIR analysis of the samples indicates inequivalent Cu(1)O(2)Pb/Tl lengths and possible tilting of CuO₂ plane as a result of Mg²⁺ substitution. The increased ξ_c(0) and J as a result of the Ge⁴⁺ and Mg²⁺ substitutions are suggested to contributed to sustenance of superconductivity above 80 K in the samples.     

First-principles study of light-element doping effects on iron-based superconductors

First-principles calculations are performed for hydrogen-doped iron-based superconductors, LaFeAsOH_x, which exhibits higher transition temperature than hydrogen-free LaFeAsO. We find that hydrogen atoms favor the positions near FeAs layers and induce FeAs₄ tetrahedrons to regular ones, which are considered to bring about higher transition temperature. However, hydrogen doping more than x ～ 0.25 breaks typical Fermi surface and therefore we estimate the optimal doping as x ～ 0.2.     

Single crystals of LnFeAsO1−xFx (Ln = La,Pr, Nd,Sm, Gd) and Ba1−xRbxFe2As2: Growth,structure and superconducting properties

A review of our investigations on single crystals of LnFeAsO_1?xF_x (Ln = La, Pr, Nd, Sm, Gd) and Ba_1?xRb_xFe₂As₂ is presented. A high-pressure technique has been applied for the growth of LnFeAsO_1?xF_x crystals, while Ba_1?xRb_xFe₂As₂ crystals were grown using a quartz ampoule method. Single crystals were used for electrical transport, structure, magnetic torque and spectroscopic studies. Investigations of the crystal structure confirmed high structural perfection and show incomplete occupation of the (O, F) position in superconducting LnFeAsO_1?xF_x crystals. Resistivity measurements on LnFeAsO_1?xF_x crystals show a significant broadening of the transition in high magnetic fields, whereas the resistive transition in Ba_1?xRb_xFe₂As₂ simply shifts to lower temperature. The critical current density for both compounds is relatively high and exceeds 2 × 10⁹ A/m² at 15 K in 7 T. The anisotropy of magnetic penetration depth, measured on LnFeAsO_1?xF_x crystals by torque magnetometry is temperature dependent and apparently larger than the anisotropy of the upper critical field. Ba_1?xRb_xFe₂As₂ crystals are electronically significantly less anisotropic. Point-Contact Andreev-Reflection spectroscopy indicates the existence of two energy gaps in LnFeAsO_1?xF_x. Scanning Tunneling Spectroscopy reveals in addition to a superconducting gap, also some feature at high energy (～20 meV).     

Effects of excessive Zn2+ ions on intrinsic magnetic and structural properties of Ni0.2Zn0.6Cu0.2Fe2O4 powder prepared by chemical coprecipitation method

Zn(OH)₂ is a kind of amphoteric compound. Therefore, for chemical coprecipitation method, the precipitation of Zn²⁺ ions may be incomplete if using NaOH as precipitator. In this study, single-phase powder specimens with a nominal composition Ni_0.2Zn_0.6Cu_0.2Fe₂O₄ were prepared with chemical coprecipitation method, and the effects of excessive Zn²⁺ content (x, x = 3%, 5%, 7%, 9%) in working solution on intrinsic magnetic and structural properties were studied by vibrating sample magnetometer and X-ray diffractometer, respectively. It was found that the magnetization when H_m = 398 kA/m (5000 Oe) reached a maximum when x = 5%, and then decreased with the increase of x, which was attributed to the effect of different amount of Zn²⁺ in A sites on the A–B and B–B exchange interaction. Moreover, it was found that the lattice parameter was affected by the Zn²⁺ and Fe³⁺ ions due to their different ion radius to a certain extent.     

Effect of Eu3+ concentration on microstructure and photoluminescence of Sr2SiO4:Eu3+ phosphors prepared by microwave assisted sintering

In this paper, effect of Eu³⁺ doping concentrations on microstructure and photoluminescence of Sr₂SiO₄ phosphors was investigated. The Sr_2?xSiO₄:xEu³⁺ phosphors with x=0.05, 0.1, 0.2, 0.3 were synthesized by microwave assisted sintering at 1200 °C for 60 min in air. X-ray powder diffraction analysis confirmed the formation of pure Sr₂SiO₄ phase without second phase or phases of starting materials irrespective of the adding amount of Eu³⁺. From scanning electron microscopy image, it is found that with more Eu³⁺ ions introduced to Sr₂SiO₄, the shape of the particles is not much different from each other, but the particle size decreases significantly from 1 to 2 μm (when x=0.05) to less than 500 nm (when x=0.3). The emission spectrum was located obviously at 617 nm as the excitation spectrum at λ_ex=395 nm, and it had best emission intensity when x=0.1.     

Conductivity and oxygen permeability of a novel oxide Pr2Ni0.8 − x Cu0.2FexO4 and its application to partial oxidation of CH4

Iron-doped Pr₂Ni_0.8Cu_0.2O₄ was studied as a new mixed electronic and oxide-ionic conductor for use as an oxygen-permeating membrane. An X-ray diffraction analysis suggested that a single phase K₂NiF₄-type structure was obtained in the composition range from x = 0 to 0.05 in Pr₂Ni_0.8 − xCu_0.2Fe_xO₄. It is considered that the doped Fe is partially substituted at the Ni position in Pr₂NiO₄. The prepared Pr₂NiO₄-based oxide exhibited a dominant hole conduction in the P_O2 range from 1 to 10^− 21 atm. The electrical conductivity of Pr₂Ni_0.8−xCu_0.2Fe_xO₄ is as high as 10² S cm^− 1 in the temperature range of 873–1223 K and it gradually decreased with the increasing amount of Fe substituted for Ni. The oxygen permeation rate was significantly enhanced by the Fe doping and it was found that the highest oxygen permeation rate (60 μmol min^− 1 cm^− 2) from air to He was achieved for x = 0.05 in Pr₂Ni_0.8 − xCu_0.2Fe_xO₄. Since the chemical stability of the Pr₂NiO₄-based oxide is high, Pr₂Ni_0.75Cu_0.2Fe_0.05O₄ can be used as the oxygen-separating membrane for the partial oxidation of CH₄. It was observed that the oxygen permeation rate was significantly improved by changing from He to CH₄ and the observed permeation rate reached a value of 225 μmol min^− 1 cm^− 2 at 1273 K for the CH₄ partial oxidation.     

A ceria layer as diffusion barrier between LAMOX and lanthanum strontium cobalt ferrite along with the impedance analysis

A thin interlayer of samarium doped ceria (SDC) is applied as diffusion barrier between La_1 ? xSr_xCo_yFe_1 ? yO₃ x = 0.1–0.4, y = 0.2–0.8 (LSCF) cathode and La_1.8Dy_0.2Mo_1.6W_0.4O₉ (LDMW82) electrolyte to obstruct Mo–Sr diffusion and solid state reaction in the intermediate temperature range of SOFC. We demonstrate the effectiveness of the diffusion barrier through contrasting the clearly defined interfaces of LSCF/SDC/LDMW82 against a rugged growing product layer of LSCF/LDMW82 in 800 °C thermal annealing, and analyze the product composition and the probable new phase. In addition, the measured polarization resistance is considerably lower for the half-cell with a diffusion barrier. Therefore, the electrochemical performance of the LSCF cathode is investigated on the SDC-protected LDMW82. The cell with LSCF (x = 0.4) persistently outperforms the one with x = 0.2 in polarization resistance because of its small low-frequency contribution. The activation energy of polarization resistance is also lower for La_0.6Sr_0.4Co_yFe_1 ? yO₃ (112–135 kJ/mol), than that for La_0.8Sr_0.2Co_yFe_1 ? yO₃ (156–164 kJ/mol). La_0.6Sr_0.4Co_yFe_1 ? yO₃ y = 0.4–0.8 is the proper composition for the cathode interfaced to SDC/LDMW82.     

Superconductivity in bulk T′-(La,Sm)2CuO4 prepared via a molten alkaline hydroxide route

We have synthesized La_2?xSm_xCuO₄ (0 ? x ? 2.0) with the Nd₂CuO₄ structure via a molten alkaline hydroxide route at temperatures as low as 400–480 °C. After reduction heat treatment in vacuum at 600–750 °C for removal of excess oxygen atoms at the interstitial apical site, superconductivity with T_c = 20–24 K was observed in the samples with x = 0.05–1.0. The superconducting volume fraction is nearly 100% for x = 0.3–0.7. Our results demonstrate that La_2?xSm_xCuO₄ with no nominal carrier doping is a bulk superconductor.     

Large magnetoresistance in Ni1–xVxS

Large magnetoresistance (MR) was observed in Ni_1−x V_xS(x=0, 0.02, 0.04, 0.06 and 0.08), MR=1530% at 268 K for x=0, MR=1180% at 255 K for x=0.02, MR=980% at 248 K for x=0.04, MR=810% at 224 K for x=0.06 and MR=490% at 198 K for x=0.08 in magnetic field 4 T. The large MR is due to magnetic field-induced magnetic and electrical transition from antiferromagnetic (AFM) nonmetal phase to paramagnetic (PM) metal phase.