Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La2−x Ce x CuO4−y and Pr2−x Ce x CuO4−y

From measurements of the magnetic penetration depth, (T), from 1.6 K to T _c in films of electron-doped cuprates La_2–x Ce _x CuO_4–y and Pr_2–x Ce _x CuO_4–y we obtain the normalized density of states, N _s(E) at T=0 by using a simple model. In this framework, the flat behavior of ^–2(T) at low T implies N _s(E) is small, possibly gapped, at low energies. The upward curvature in ^–2(T) near T _c seen in overdoped films implies that superfluid comes from an anomalously small energy band within about 3k _B T _c of the Fermi surface.     

High temperature mechanical properties of La1−x Sr x Cr1−y Co y O3 for SOFC interconnect

Solid oxide fuel cells (SOFC) require an interconnect for fabrication into stacked cells. This is typically La(Sr, Ca)CrO₃, of which much data on the electrical and physical properties already exists. However, very little information exists on the high temperature mechanical properties of the material, which is a necessity for future design improvements. La_1–x Sr _x Cr_1–y Co _y O₃ samples were fabricated into green dry-pressed bars and pellets, and sintered under various heating and cooling regimes. The sinterability and high temperature mechanical properties of the material was then investigated as a function of the dopant concentration. It was observed, for example, that the modulus of rupture of the dry pressed La_0.7Sr_0.3Cr_1–yCo_yO₃ (y 0.3) gave a value of over 110 MPa at 1000 °C. This paper will provide data on the high temperature mechanical properties of the material and its application to the SOFC system.     

Growth of Ga x In1 − x As y P1 − y /GaAs quantum dot arrays by ion beam deposition

Experimental data are presented that demonstrate the possibility of producing GaInAsP quantum dots on GaAs by ion beam deposition. The morphology of the quantum dots and the effect of GaAs substrate temperature on parameters of GaInAsP quantum dot arrays have been studied by atomic force microscopy and scanning electron microscopy. We have determined the elemental composition of the quantum dots and obtained photoluminescence spectra of the GaInAsP/GaAs heterostructures.     

Transport Properties of Heavy Fermion Compounds: YbIn1−x Cu4+x and YbIn1−y Ag y Cu4

The intermetallic compounds of Yb with In and Cu (YbIn_1–x Cu_4+x ) and Yb with In, Ag, and Cu (YbIn_1–y Ag _y Cu₄) exhibit interesting magnetic and transport properties. Of the compounds of Yb with In and Cu the compound with x=0, namely YbInCu₄, has attracted particular attention, because—while being a Curie–Weiss paramagnet—it undergoes a first-order isostructural phase transition at T _v =approx. 40 to 80 K and atmospheric pressure. Below T _v the ytterbium in this compound is in a mixed-valence state and the compound as a whole is sometimes called a light heavy-fermion system. Above T _v , the compound is known as a Curie–Weiss paramagnet of localized magnetic moments and, below T _v , a Pauli paramagnet in a nonmagnetic Fermi-liquid state. In the present paper the results of measurements of the thermal conductivity of polycrystalline samples, YbIn_1–x Cu_4+x with x=0,0.015, 0.095, and 0.17 and YbIn_1–y Ag _y Cu₄ with y=0, 0.3, 0.7, and 1.0, are reported. The thermal conductivity is separated into the phonon thermal conductivity ( _ph ) (i.e., related to the heat carried by phonons) and into the electronic thermal conductivity ( _e ) (related to the heat carried by electrons). The electrical resistivity of the compounds was measured to determine the temperature dependence of the Lorenz number. The results show that in the temperature interval 4.2 to 300 K the latter quantity behavior follows the theoretical predictions for heavy fermion materials.     

Transport Properties and Electronic Specific Heat of La2−x Ba x Cu1−y Zn y O4+d Bulk

The effect of the partial substitution of Cu by Zn and La by Ba in La: 214 prepared by conventional solid state reaction method has been investigated by using Hall coefficient, Seebeck coefficient and electronic specific heat measurements, over the wide temperature range between 4.2 K and 300 K, in magnetic fields up to 5 T. The samples with y=0 and d=0, showed superconductivity for x between 0.055 and 0.30. The critical temperature, the Hall and Seebeck coefficients strongly depend on Zn content. The phonon specific heat C _ph and electronic specific heat C _el have been extracted from the total specific heat C. Zn-doping effect on the (T)=C _el/T behavior suggests that the large range stripe order and the fluctuating stripe suppress the singlet formation and pseudo gap.     

Crystal structure and dielectric properties of (Pb1−x Ca x )(Zr1−y Sn y )O3 ceramics

There has been an increasing demand for dielectric resonator materials that operate in the microwave frequency range for applications in microwave communications. (Pb,Ca)ZrO₃ ceramics have a dielectric constant (_r), high quality factor (Q) and a small temperature coefficient of resonant frequency (_f). However, basic properties such as its crystal structure, temperature characteristics and the nature of its phase transformation are not yet fully understood. The temperature coefficient of resonant frequency can be controlled fairly well with the temperature coefficient of the dielectric constant. In this paper, we report the results of investigated crystal structure and the dielectric properties of (Pb_1–x Ca _x )(Zn₁– _y Sn _y )O₃ ceramics with the objective of elucidating the relationship between the crystal structure and the dielectric properties. The crystal structure refinement was performed by the Rietveld method. The dielectric properties were measured from-150–350 °C. The phase transformation was analysed from high and low temperature XRD data.     

Magnetization Fluctuation Analysis and Superconducting Parameters of La1.5−x Ba1.5+x−y Ca y Cu3O z Superconductor

In this work we report analysis of experimental data of the magnetization of La_1.5?x Ba_1.5+x?y Ca _y Cu₃O _z superconducting system. The data are analyzed in terms of thermal fluctuations on the magnetization excess M(T) for different values of temperature in each of the samples. We describe a procedure for extracting the penetration depth in the ab plane (1537–1650 Å) and the coherence length in the ab plane (21–23 Å) parameters from the magnetization, as a function of the applied magnetic field. This procedure was performed for polycrystalline samples using the theory of Bulaevskii, Ledvij and Kogan, which analyzes the vortex fluctuation in superconducting materials within the Lawrence-Doniach framework. These data allowed one to determine the characteristic temperature value T ^? (53–73 K) in the magnetization curves for several magnetic fields. We calculated the data of magnetization excess from the curves of the magnetization as a function of the logarithm of the applied field. We notice that the values for these superconducting parameters are in agreement with reports for high-temperature superconductors. The value obtained of the superconducting volumetric fraction is compared with the value obtained through the measure of the Meissner effect.     

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.     

Superconductivity of YBa2−x Nd x Cu3O y Solid Solution

The solubility of Nd at the Ba sites and the superconductivity of YBa_2?x Nd _x Cu₃O _y were investigated by X-ray powder diffraction and measurements of the electrical resistance and ac susceptibility. The single Re123 phase was obtained for x≤0.30. The onset transition temperature $T_{\text{c}}^{{\text{on}}}$ is insensitive to the Nd content x in the region of x≤0.40. All are higher than 95 K. The zero resistance transition temperatures $T_{\text{c}}^{{\text{zero}}}$ , however, exhibits two-step variation with the increase of x. For x≤0.25, $T_{\text{c}}^{{\text{zero}}}$ are all above 92 K. The highest $T_{\text{c}}^{{\text{zero}}}$ of 94 K was obtained for x=0.25. For x≥0.3 $T_{\text{c}}^{{\text{zero}}}$ drops sharply to about 84 K. Finally $T_{\text{c}}^{{\text{zero}}}$ falls to 30 K and $T_{\text{c}}^{{\text{zero}}}$ is below 10 K for x=0.5. The two-step variation of T _c might be an indication of the existence of two trap levels for holes.     

Normal and Anomalous Pseudogap of Superconducting Y0.9−x Pr x Ca0.1Ba2[Cu1−y Zn y ]3O7−δ

Electrical resistivity measurements on the superconducting oxides of the compositions Y_0.9−x Pr _x Ca_0.1Ba₂[Cu_1−y Zn _y ]₃O_7−δ (0≤x≤0.20 and 0.0≤y≤0.10) sintered in oxygen atmosphere were carried out to obtain the normal and anomalous pseudogaps in underdoped and overdoped samples. It is observed that pseudogap temperature T ^* decreases with increasing doping level p in the underdoped case. For the overdoped sample with y=0.06, T ^* shows no p dependence.      

The Effect of Zn-Substitution on the Anomalous Resistance Behavior of La1−x Ca x Mn1−y Zn y O3 Compounds

We have studied the effect of Zn substitution on the temperature dependence of the resistivity and the Curie temperature T _p of La_1–x Ca _x Mn_1–y Zn _y O₃ system (with x=0, y=0, and x=0.33, 0.00y0.15) during their pass to the paramagnetic conducting phase. All the specimens show a clear metal-insulator transition passing through the Curie temperature T _p . With increasing Zn concentration, T _p decreases for La/Ca substitution and also for Zn/Mn substitution with fixed 33 at.% of Ca. T _p is found to be slightly decreases for low concentrations of Zn (up to y=0.075) followed by a sharp decrease up to y=0.15. A possible explanation is given. Moreover, the activation energy E _a is calculated in the three different temperature regions characterized in the (-T) curve. Decreasing the temperature below T _p can increase the parallel alignment of the Mn spins and, thus decreases the resistivity of the samples. E _a increases with Zn content (up to y=0.075) followed by a sharp decrease. A strong correlation in the behavior of the normal state and residual resistivity as well as T _p and E _a with Zn content is reported.     

Static bismuth disorder in Bi2−x(CrTa)O7−y

The crystal structure of the pyrochlore Bi_2−x(CrTa)O_7−y has been refined by the Rietveld method from powder neutron and synchrotron X-ray diffraction data. Using the ideal pyrochlore structural model, anomalously high atomic displacement parameters (APD), which could not be satisfactorily explained with anisotropic ADP, of both Bi and O′ sites were observed. The structure was successfully refined with a model that incorporated static displacive disorder of both the Bi and O′ atoms. Using this model physically reasonable ADP parameters for both the Bi and O′ atoms, and improvement in the various measure of fit were obtained. The underbonding of the Bi atoms, observed in ideal pyrochlore model, but removed in the disordered structure, is believed to be the main driving force for this disorder.     

Two distinct structural phases in Sr3−x Ca x Ru2O y

Non-cuprate layered perovskite Sr₂RuO₄, which is an analogue of (La,Sr)₂CuO₄, exhibits superconductivity below T_c1K. We have synthesized a new system Sr_3–xCa_xRu₂O_y and found the solubility of Ca to be at least as much as x = 2. These samples are not superconductive down to 2K but magnetically ordered. The x dependence of the lattice parameters and that of T_mag clearly indicate that there are at least two distinct structural phases in Sr_3–xCa_xRu₂O_y.On leave from Hiroshima University.     

The nonstoichiometry and physical properties of the Nd1−x Ca1+x FeO4−y system

The solid solutions of the Nd_1–x Ca_1+x FeO_4–y system for compositions ofx=0.000,0.125, 0.250,0.375, and 0.500 are prepared by drip pyrolysis. XRD analysis shows all the solid solutions are tetragonal I_4/mmm. The Fe⁴⁺ ratio to the total Fe ions or value has a maximum for the compositionx=0.375. From the X-ray powder diffraction analysis and the Mössbauer spectroscopy, the distortion and symmetry change of oxygen octahedra of Fe ions are observed. The structural change of oxygen octahedra of Fe ions strongly affects the physical properties. The solid solution whenx=0.000 shows a weak ferromagnetic behaviour due to the spin canting of the distorted octahedra. The other solid solutions withx=0.125, 0.250, 0.375, and 0.500 show a paramagnetic behaviour over room temperature. The decrease of the magnetic transition temperature is due to the distortion of oxygen octahedra of Fe ions and the existence of the Fe⁴⁺ ion. The formation site of oxygen vacancies plays an important role in the conductivity of the Nd_1–x Ca_1+x FeO_4–y system. Although the oxygen vacancies in [Nd, Ca]-O layer have little effect on conductivity, the oxygen vacancies in the FeO₂ plane of the perovskite layer act as electron trapping sites and thus increase the activation energy.     

X-ray absorption spectroscopy of Pr x Y1−x Ba2Cu3O7−y single crystals

Substituting Y in orthorhombic (Y,RE)Ba₂Cu₃O₇ by any rare-earth element RE has generally little effect on the superconducting properties. For RE = Pr, however, superconductivity is completely suppressed. To elucidate this effect we have studied the unoccupied electronic structure of Pr_xY_1–xBa₂Cu₃O_7–y (x = 0.0,0.4, 0.8) by polarization-dependent O1s x-ray absorption spectroscopy on detwinned single crystals. Along with the comparison of undoped (y 0.9) to the doped materials (y 0.1), this allows a test of the current theoretical explanations for the suppression of superconductivity. While we can rule out models involving hole filling or charge transfer from planes to chains our data is consistent with approaches based on Pr4f–02p hybridization.     

Indentation fracture in the In1−x Ga x As y P1−y /InP system and its effect on microhardness anisotropy characteristics

Knoop microhardness anisotropy measurements on the {100}-orientated In_1–x Ga _x As _y P_1–y /InP system have disclosed an appreciable variation in hardness behaviour across the composition range of the alloy. This paper relates these variations to changes in the directional fracture characteristics of the system. The qualitative investigation of both Vickers and Knoop indentation fracture has established the emergence of a secondary 100 cleavage direction at high values of the composition parameter, abovey 0.6. Furthermore, the number and extent of cracks emanating from indentations were seen to increase appreciably asy increases from 0 to 1. A quantitative analysis of Vickers indentation fracture in the quaternary system has indicated a marked dependence of fracture-related parameters on both the indentor orientation and composition. Fracture toughness values from 3.10⁵ to 1.10⁶ N m^/12 have been derived using expressions from the literature. The marked change in Knoop microhardness anisotropy characteristics with increasingy in the In_1–x Ga _x As _y P_1–y /InP system has been partially correlated to the emergence of 100 cleavage and the general increase in indentation fracture, highlighted by the quantitative fracture measurements. Thus, it is concluded that even at very low indentation loads, the effect of fracture on the measured hardness of crystalline materials cannot be ignored.     

Amorphous chalcogenide Se1−x−y TexPy semiconducting alloys: thermal and mechanical properties

Thermal and mechanical properties of ternary Se rich Se_1–x–yTe_xP_y semiconducting glasses (Te < 20 at % and P < 10 at %) in vitreous bulk and film form have been studied by differential scanning calorimetry (DSC) and microhardness measurements. Bulk vitreous samples were prepared by conventional melt quenching techniques and the amorphous photoreceptor films were prepared by vacuum deposition onto oxidized aluminum substrates whose electrophotographic properties were reported previously. We measured the glass transition temperature T_g starting from a well defined thermal history and using both heating and cooling scans as a function of composition. T_g increases monotonically with both Te and P content. Both bulk and film samples evince similar compositional T_g dependence. The increase in T_g with the P content in the glasses follows the Tanaka rule, that is, P addition has a networking effect due to the trivalent nature of the P atom and increases the mean coordination number. Both Te and P additions initially inhibit crystallization but at high Te contents (20 at %) the crystallization behavior is comparable to the pure a-Se case. Glasses with 10 at % Te seem to have the greatest resistance to crystallization. The crystallization behavior does not correlate with the T_g behavior over the whole composition range. The Vickers microhardness H_V increases with both Te and P content. H_V vs. Te and P behavior is similar to that of T_g vs. Te and P content . The compositional dependence of both H_V and T_g can be explained by the same factors that reduce Se chain mobility.     

Hydrothermal synthesis and magnetic properties of CoxFe1−x/CoyLazFe3−y−zO4 composites

A series of iron–cobalt alloy and cobalt–ferrite composites doped with La³⁺ (Co_xFe_1−x/Co_yLa_zFe_3−y−zO₄) in which the Fe–Co alloy has either a bcc or a fcc structure and the oxide is a spinel phase, have been synthesized by using the disproportionation of Fe (OH)₂ and the reduction of Co (II) by Fe⁰ in a concentrated and hot KOH solution. when x ≤ 0.1, the structures of the Fe_xCo_1−x alloy and cobalt–ferrite are fcc structure; and when x ≥ 0.25, the structures of the Fe_xCo_1−x alloy is bcc structure. The fcc structure of alloy is favored for [KOH] close to 9 N, Co(II)/Fe(II) ratios between 0.5 and 0.9 and short reaction time of synthesis. And the bcc structure of the alloy is favored for [KOH] close to 1 N, Co(II)/Fe(II) ratios between 0.1 and 0.5 and long reaction time of synthesis. A low [KOH] favors nucleation leading to octahedral of 1 μm. And [KOH] of 9–12 N favors particle growth. The metal occurs in square particles of 100–150 nm included within the spinel. Powder X-ray diffraction (XRD), thermal gravity analysis (TGA) and different thermal analysis (DTA), scanning electron microscope (SEM), transmission electron micrograph (TEM) and vibrating sample magnetometer (VSM) were employed characterize the crystallite sizes, structure, morphology and magnetic properties of the composites. And the effect of the Co(II)/Fe (II) ratio (0 ≤ Co/Fe ≤ 1), concentration of KOH, reaction time and substitution Fe³⁺ ions by La³⁺ ions on structure, magnetic properties of the composites were investigated.