High-Pressure Studies on Superconductivity in LaFeAsO1−x F x and SmFeAsO1−x F x

The pressure dependence on the superconducting transition temperature (T _c ) was investigated for the iron-based superconductors LaFeAsO_1−x F _x and SmFeAsO_1−x F _x . The T _c ’s increase largely for LaFeAsO_1−x F _x with a small increase of pressure, while a sharp decrease of T _c was observed for SmFeAsO_1−x F _x . The electrical resistivity measurements reveal pressure-induced superconductivity for undoped LaFeAsO and SmFeAsO. These pressure effects seem to be related to an anisotropic decrease of the lattice constants under high pressure from the x-ray diffraction measurements up to 10 GPa for the LaFeAsO_1−x F _x system.     

High Field ESR Spectroscopy on GdO1−x F x FeAs

In the present work we have studied polycrystalline samples of the GdO_1?x F _x FeAs superconductor by means of high field electron spin resonance (HF-ESR) spectroscopy. A set of the samples with different levels of fluorine doping was measured in the frequency range from 10 GHz to 400 GHz in magnetic fields up to 15 T. Surprising results have been obtained on the GdO_0.85F_0.15FeAs sample with the superconducting transition temperature T _c =20 K. Gd ESR gives clear indications of the enhancement of (quasi)-static magnetic correlations which set in below ～80 K and continue to develop even in the superconducting state. This suggests an occurrence of an intimate interplay between magnetism and superconductivity in the FeAs planes which evolves in GdO_1?x F _x FeAs compound upon the fluorine doping.     

Cu-NQR Study on Single Crystals Sr14−x Ca x Cu24O41 with x = 0–11.5

Cu-nuclear quadrupole resonance (NQR) measurements have been performed on single crystals Sr_14–x Ca _x Cu₂₄O₄₁ (Ca _x ) with x = 0–11.5, in order to clarify the relation between the magnetic order around x = 11.5 and local environments at the Cu sites upon x. Cu-NQR frequency _Q for the ladders at 4.2 K increases linearly with increasing x, whereas _Q for the nonmagnetic Zhang–Rice (ZR) sites in the chains does not. Detailed site assignment of the spectra, which are different from those in the previous paper, is presented.     

Carbon monoxide oxidation on (Mg6−x Al x )MnO8 (0⩽x⩽0.6)

Murdochite-type (Mg_6–x Al _x )MnO_m8 (0x0.6) was synthesized at low temperature (700 °C) with the sol-gel process. The specific surface area, crystallite size, catalytic activity for the CO oxidation, and adsorption isotherms of oxygen on (Mg_6–x Al _x )MnO₈ were measured. The increase of the specific surface area with increasing Al³⁺ ion content was explained by the decrease of the particle size. The catalytic activity increased with increasing Al³⁺ ion content, and this increase was caused by the valence deviation from the Mn⁴⁺ ion to the Mn^3.5+ ion.     

Theoretical Study on the Spin-State Transition of Electron-Doped Sr2−x La x CoO4 Compounds

The spin-state transition is an interesting and unresolved problem in layered perovskite Sr_2?x La _x CoO₄ under doping. The unrestricted Hartree-Fock approximation on a realistic multiband lattice model has been applied to study the various spin states of electron-doped Sr_2?x La _x CoO₄ compounds. In the doping range 0.0≤x≤0.9, various spin states in an enlarged double cell are investigated and four of them are the ground states at different doping concentration. The magnetic ground state of the doped system takes a low-spin–intermediate-spin ferromagnetically ordered state for x<0.26, a high-spin–intermediate-spin ferromagnetically ordered state for 0.26≤x<0.47, an intermediate-spin state for 0.47≤x<0.65, and followed by an high-spin antiferromagnetically ordered state for 0.65≤x≤0.9. The densities of states of all magnetic ground states are computed and their electronic and magnetic properties are discussed.     

Interface structure and electrical properties of PtSi/Si1−x Ge x diodes based on silicon

PtSi/strained Si_1–x Ge _x (x=0, 0.2, and 0.25) Schottky-barrier diodes (SBD) with extended cutoff wavelengths have been demonstrated by combining pulsed laser deposition (PLD) and molecular beam epitaxy (MBE). Pt was deposited by PLD on the Si_1–x Ge _x alloys with a thin Si sacrificial cap layer fabricated by MBE. By the reaction of deposited Pt film on Si, a sacrificial cap layer silicide SBD has been fabricated. Auger electron depth profiling was performed on the films before and after in vacuo annealing to study the redistribution of composition in the reactions. High-resolution transmission electron microscopy was used to investigate the interface structure. We have found that Pt reacts mainly with Si to form silicides at 350 °C, leaving some Ge to segregate at the surface. With annealing at 600 °C for 3 min the interface of PtSi/Si_1–x Ge _x is smooth. Since lowered-barrier-height silicide SBD are desirable for obtaining longer cutoff wavelength Si-based infrared detectors, the Schottky barrier heights of the PtSi/strained Si_1–x Ge _x SBDs with smooth interfaces were substantially lower than those of PtSi/Si SBDs, i.e., decreased with increasing Ge fraction, allowing for tuning of the SBDs cutoff wavelength. At 293 K, the ideality factor has been found to be 2.00 and 1.32 for PtSi/Si_0.80Ge_0.20 and PtSi/Si_0.75Ge_0.25 diodes, respectively. We have shown that high quantum efficiency and near-ideal dark current can be obtained in the film of PtSi/strained Si_1–x Ge _x with an excellent interface fabricated by MBE and PLD, after annealing at 600 °C for 3 min.     

Impurity scattering effect on the anisotropic resistivity for La2−x Sr x CuO4−δ with a wide range of x

We have investigated the effect of the impurity scattering due to a small amount of oxygen defect, ( < 0.01) on the anisotropic resistivity of La _2–x Sr _x CuO _4– in a wide x range up to x = 0.26. In the superconducting region with 0.06 x 0.22, we found that impurity scattering by the oxygen defect remarkably decreases the c-axis resistivity c over the whole temperature range. The finding is interpreted in terms of the nonmetallic conduction along the c axis which is enhanced by scattering the confined carriers in the CuO ₂ plane. On the other hand, the enhancement of the c-axis conduction diminishes in the nonsuperconducting region. This is consistent with the two-dimensional nature of the electronic state which stabilizes the superconducting ground state in the cuprates.     

About the Impact of Defect Phases on the Thermoelectric Properties of Cr3S4–x Se x

Phase relations in Cr₃S₄ and the substituted system Cr₃S_4–x Se _x are studied to determine the influence of chemical substitutions on the thermoelectric properties. In addition to the expected equilibrium phase crystallizing in the monoclinic space group I2/m, some samples exhibit a defect phase with Cr₂S₃-like structure. The defect phase can be observed in a few samples prior to sintering, with the majority being phase-pure Cr₃S₄. The defect phase can, however, be introduced in phase-pure samples through in situ heating. It can be proven that the defect phase has an influence on the thermoelectric properties, by lowering the electrical and thermal conductivity, while increasing the Seebeck coefficient. Substitution in the anion lattice of Cr₃S₄ with Se lowers the thermal conductivity. The improvement is mainly achieved through a reduction of the electronic contribution to the thermal conductivity, leading to total values as low as 1.6 Wm⁻¹ K⁻¹ for the substituted system in comparison to the pristine material 2.3 Wm⁻¹ K⁻¹.     

Hydrostatic pressure effects on the superconductivity of La1.2−x Eu x Mo6S8 pseudoternary compounds

The effect of hydrostatic pressure on the superconductivity of La_1.2–x Eu _x Mo₆S₈ pseudoternary compounds was studied by means of low-frequency ac and dc magnetic susceptibility measurements up to 20 kbar. The superconducting transition temperatureT _c at ambient pressure is a weak function of Eu concentrationx for 0x0.2, and then decreases smoothly withx forx>0.2 until superconductivity is completely suppressed forx0.95. Pressure was found to decreaseT _c in the La-rich compounds and to increaseT _c in the Eu-rich compounds. Pressure also induces superconductivity in compounds withx>0.95 that are not superconducting at ambient pressure. A La_0.2Eu_1.0Mo₆S₈ compound revealed a sizeable Meissner effect for pressures larger than 7 kbar.Research supported by the U.S. Department of Energy under contract DE-AT03-76ER70227.On leave from Instituto de Fisica Gleb Wataghin, UNICAMP, Brazil.Research supported by the National Science Foundation under grant DMR81-09950.Supported by the National Science Foundation.     

Influence of Nd Doping on the Magnetoresistance of La0.7−x Nd x Pb0.3MnO3

This paper presents an investigation on the magnetoresistance properties of the polycrystalline manganite oxides, La_0.7?x Nd _x Pb_0.3MnO₃ (x=0.0, 0.1, 0.5 and 0.7). We demonstrate that the magnetoresistance ratio is enhanced with the increase of Nd content due to the suppression of spin scattering in the presence of an applied magnetic field. We find that the magnetoresistance shows an increase with decreasing temperatures at low temperatures. These features have been conventionally attributed to spin-polarized tunneling at grain boundaries. The temperature dependence of the magnetoresistance can be described quite well by an expression of the type a+b/(T+c). At high temperatures the magnetoresistance ratio increases with increasing temperature and the resistivity shows the existence of the variable range hopping with Coulomb effects for x=0.5 and 0.7.     

Gd Substitution Effects on the Magnetic Properties of the Pr1−x Gd x Co4Si Compounds

The crystal structure and magnetic properties of single phase Pr_1?x Gd _x Co₄Si compounds with x=0,0.2,0.4,0.6,0.8, and 1.0 have been investigated. X-ray analysis reveals that the compounds crystallize as a single phase having the hexagonal CaCu₅-type structure with the space group P6/mmm. The substitution of Gd for Pr causes a linear decrease of the unit-cell parameters a and c, and the unit-cell volume V. Magnetic measurements indicate that all samples are ordered magnetically below the Curie temperature. The saturation magnetization at 4.2 K decreases upon the Gd substitution up to x=0.6, and then increases.     

A study on martensitic transformation in Ti50−x/2Ni50−x/2Cu x alloys with X≤10 at%

The effect of Cu additions on the martensitic transformation sequence and temperature in Ti_50–x/2Ni_50–x/2Cu _x alloys with x: 1–10 at% are investigated by ER, DSC, X-ray and IF measurements. Experimental results show that the transformation sequence of Ti_50–x/2Ni_50–x/2Cu _x alloys with x: 1–4 at% proceeding as two-stage B2RB19 transformation on cooling and Ti_50–x/2Ni_50–x/2Cu _x alloys with x=5, 10 at% have no martensitic transformation. The addition of Cu in Ti_50–x/2Ni_50–x/2Cu _x alloys assists the formation of R-phase, a behaviour which is quite different from that in Ti₅₀Ni_50–x Cu _x alloys. Both the Ms and T _R temperatures decrease rapidly with increasing Cu addition in Ti_50–x/2Ni_50–x/2Cu _x alloys with x: 1–4 at%. It is proposed that the Cu+Ni effects on the Ms temperature in Ti_50–x/2Ni_50–x/2Cu _x alloys is similar as Cu +Ni effects in Ti₅₀Ni_50–x Cu _x alloys and as Ni effects in as-quenched Ni-rich TiNi alloys.     

Influence of the cation sublattice defectness on the electronic thermoelectric power of Li x Cu(2−x)−δ (x ≤ 0.25)

This paper presents an experimental study of the electronic thermoelectric power as a function of copper content for Li _x Cu_(2?x)?δS (x ≤ 0.25) electronic-ionic mixed conductors. Using a vacancy model for lattice defects, we calculate their electronic thermoelectric power as a function of x. By adjusting the hole effective mass, we reach satisfactory agreement with experimental data.     

Effect of Co substitution for Mn on Y1−x Sr x MnO3 properties for SOFC cathode material

Y_0.6Sr_0.4Mn_1–y Co _y O₃ (0 y 0.4) perovskite oxides were prepared by the coprecipitation method. The effect of Co substitution for Mn on the crystal structure, electrical conductivity and thermal expansion properties were investigated. By X-ray powder diffraction, the crystal structure was found to change from hexagonal symmetry of Y_0.8Sr_0.2MnO₃ to orthorhombic of Y_0.6Sr_0.4Mn_1–y Co _y O₃. The differences in the structure of the unsubstituted Y_1–x Sr _x MnO₃ (0.2 x 0.4) are attributed to the average ionic radii of the cations and the amounts of Mn⁴⁺ present. The results of electrical conductivity analysis can be described by the small polaron hopping conductivity model. With Co substitution, the activation energy increases, possibly due to an increase of Jahn–Teller distortion, at an extent higher than the increase of the concentration of charge carriers; thus, the electrical conductivity decreases. In addition, the relative densities of the materials reached 94% with sintering at 1350°C for 12 h and had higher concentration of the available lattice sites, thus showing higher conductivity, than that with sintering at 1300°C for 6 h, which achieved 70% relative density. It is also found that the thermal expansion coefficient (TEC) increases as the Sr and Co content of Y_1–x Sr _x Mn_1–y Co _y O₃ increases and those with Co content of y = 0.2 exhibit TEC compatibility with YSZ.     

Dependence of Superconductivity and Its Weakly Linked Behavior in Bulk LaO1?x F x FeAs on F Doping

Samples of oxypnictide compound LaO_1?x F _x FeAs, with x=0.15 and 0.2 corresponding to over- and highly over-doped compositions, respectively, were prepared by solid-state reaction. We present their characterization by XRD and HRTEM, as well as resistivity ??(T), magnetization M(B) and microwave modulated absorption (MMA) response between 4.2?C300?K and applied fields B=0?C8?T. With change in?x, both the superconducting and magnetic behavior of the samples shows an interesting pattern. The ??magnetic anomaly?? at T??130?K, observed in M(T) for x=0, instead of getting totally suppressed shows a tendency to reappear in x=0.2 sample. Both samples typically show ??(300?K)>2.8×10^?3????cm and critical current density J _c(5?K, 1?T)<2×10⁷?A/m². The superconducting transitions as measured by ??(T) at B=0 are found broad for both x=0.15 and 0.2 samples with transition widths ??2.5 and 6?K, respectively. The slope |dB _c2/dT| (where B _c2 is upper critical field) determined by resistive onsets, for the x=0.15 and 0.2 samples, has values ??7.5 and 3.5?T/K, respectively. The superconducting state characteristics as reflected by ??(T,B), M(T), magnetic J _c(B) and MMA response are typical of the presence of weakly linked inter-grain regions in both the samples. Our HRTEM images of the x=0.15 sample show the presence of high angle (??43^°) grain boundaries, which are well known to limit the J _c in cuprate-based high T _c bulk materials.     

Growth,chemical composition,and structure of thin La x Hf1 − x O y films on Si

The chemical structure, phase composition, and crystal structure of La _x Hf_{1 ? x} O _y films grown on Si using volatile metalorganic compounds as Hf and La precursors have been studied by X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive X-ray microanalysis, and atomic force microscopy. By varying the lanthanum and hafnium source temperatures, we were able to grow films with 2 at % < C_La < 30 at %. The Hf 4f and La 3d peak positions in the XPS spectra of the films correspond to hafnium and lanthanum in the Hf⁴⁺ and La³⁺ states. With increasing La concentration, the reflections in the X-ray diffraction patterns of the films shift to smaller 2θ angles, indicating the formation of solid solutions. At 18 at % La, we observed a transition from a fluorite-like structure to the pyrochlore structure (La₂Hf₂O₇). The film containing 30 at % La consisted of a mixture of c-La₂O₃ and La₂Hf₂O₇. The surface roughness of the films was shown to increase with increasing La concentration. Capacitance-voltage (C-V) measurements were used to assess the relative permittivity (k) of the films as a function of La concentration. The minimum k value was obtained at the La concentration corresponding to the transition from the fluorite structure to an ordered pyrochlore structure (second-order phase transition).     

Effect of the Crystallographic Orientation of GaAs Substrates on the Composition of Ga x In1 – x P Layers

The effect of the crystallographic orientation of GaAs substrates on the composition of Ga _x In_{1 – x} P solid solutions grown by liquid-phase heteroepitaxy is studied. It is shown that GaAs-lattice-matched Ga _x In_{1 – x} P solid solutions can be grown, under identical conditions, on (100), (111), and (111) GaAs substrates using different melt compositions. The results are interpreted in terms of the contribution from the energy of the crystal–melt interface to the total energy of mixing of the solid solution.     

Effects of Non-Magnetic Impurities on Cu-Spin Dynamics and Superconductivity in La2−x Sr x Cu1−y Zn y O4 Around x = 0.115

Muon-spin-relaxation (SR) and magnetic susceptibility measurements have been carried out in La_2–x Sr _x Cu_1–y Zn _y O₄ with x=0.13 changing y finely up to 0.10, with the aim at clarifying effects of the non-magnetic impurity Zn on the Cu-spin dynamics and superconductivity. It has been found that the y dependence of the volume fraction of the superconducting region estimated from susceptibility measurements highly correlates with that of the magnetically ordered region estimated from the SR results. The rapid decrease in the superconducting region and the rapid increase in the magnetically ordered region by the doping of a small amount of Zn can be interpreted as follows; Zn pins the dynamical spin correlation or the dynamical stripe correlations and hence the superconductivity is destroyed around itself.     

Structural,thermal and transport studies on Ag1 − x Cuxl (0.05 ⩽ x ⩽ 0.25) solid electrolyte

Preparation and characterization studies on polycrystalline samples of Ag_{1 – x}Cu_xl wherex=0.05, 0.1, 0.15, 0.2 and 0.25, respectively, have been reported. Samples were analysed using powder X-ray diffraction (XRD) and differential scanning calorimetric (DSC) techniques in order to identify the compositions and phase transition temperatures. A.c. electrical conductivity studies were carried out on pelleted specimens of various compositions in the frequency range 65.5 kHz to 1 Hz and over the temperature range 293–412 K. DSC results obtained in the temperature range 373–473 K have shown that the ß- to -phase transition temperature is enhanced from 426 K to 438 K whenx is increased from 0.05 to 0.25. XRD results have indicated that there is a shift ind-spacing when the Cul content is increased, suggesting changes in the crystal structure. Typical XRD patterns recorded for the composition Ag_0.95Cu_0.05l at three different temperatures (room temperature, 373 and 473 K, respectively) have confirmed that both face-centred cubic and hexagonal phases would be present at room temperature and at 373 K as well, whereas at 473 K the structure would be purely body-centred cubic in nature. A.c. impedance analysis of the above samples appears to suggest that their electrical conductivity, predominantly due to the migration of Ag⁺ ions, lies in the order of 10^–4S cm^–1 at room temperature.     

Effects of Zn Doping on Superconductivity in Tl0.4K0.4Fe2−δ−x Zn x Se2

We have studied the transport and magnetic properties of the Tl_0.4K_0.4Fe_2?δ?x Zn _x Se₂ (nominal ratio 0.01≤x≤0.15) single crystal samples. It is found that although Zn doping leads to a decrease of charge carrier density, it does not affect the superconductivity significantly when x≤0.1. The doped superconductors exhibit properties similar to the parent compound (Tl_0.4K_0.4Fe_2?δ Se₂), e.g., the almost unchanged superconducting transition temperature (T _c) and the “hump”-like behavior in the resistivity curves, which denotes the transition from insulating phase to metallic phase. As the doping density rises, the hump obviously shifts to low temperature region, implying the quick enlargement of the ratio of insulating phase. The most plausible explanation for the above phenomena is that the Zn impurities prefer entering into the insulating phase only, therefore, merely hindering the formation of superconducting phase without directly changing its properties. The ESR measurement gives low signal-to-noise ratio and just weak paramagnetic signals, revealing strong itinerancy of the Zn doped superconductor.