Thermal transport in the oxygenated magnetic superconductor, Eu1.5Ce0.5RuSr2Cu2O10+δ

The thermal conductivity and thermopower are reported for a hole doped Eu_1.5Ce_0.5RuSr₂Cu₂O_10+δ sample that has been annealed at 1100 K under an oxygen pressure of 54 atm. At T_c=45 K superconductivity and weak ferromagnetism coexist (T_m=180 K). Weak features in the thermopower, S(T), and thermal conductivity, κ(T), are observed both at T_m and at T^*=140 K. The thermopower begins to decrease sharply toward zero at T_c, and there is an extremely sharp increase of about 30% in the thermal conductivity at T_c. This “first order” transition may be related to the sudden appearance of a spontaneous vortex phase at T_c. A small shoulder is observed in κ(T) in the temperature range T=5–13 K.     

Preparation and spectroscopic characterization of γ-Al2O3 thin films

The electronic structure and surface properties of γ-Al₂O₃ thin films are studied. We have prepared the films by oxydizing Al foils under controlled conditions and we characterize the γ-Al₂O₃ samples by means of XPS, UPS, and TEM and found no charging. Pronounced effects in temperature-dependent changes of the work function are observed which result from changes in band bending and electron affinities by reorganisation and migration of defects. Thereby the ability of these systems for prototype studies in catalysis and analysis of defects is demonstrated.     

Origin of the metal-insulator transition in the superconducting series Bi2Sr2Ca1−xYxCu2O8+δ

The metal-insulator transition in the solid solution Bi₂Sr₂Ca_1−xY_xCu₂O_8+δ (0≤x≤1) has been investigated by TGA (oxygen content) and by X-ray absorption spectroscopy (Bi and Cu valence states). Resistivity and AC magnetic susceptibility measurements have shown that the superconducting properties and the metallic behavior vanish for x>0.55. The oxygen content δ is larger than x/2 for x≤0.3 and smaller than x/2 for x≥0.6. For x=0, the Cu K edge shows a shift towards high energy with respect to the Cu(II) oxide La₂CuO₄; this shift decreases with increasing x in agreement with the decrease of the doping hole density and the variations of the physical properties. For 0≤x≤0.3, the Bi L₃ edge shows a shift of 1 eV towards low energy with respect to the Bi(III) oxide Bi₂O₃ in agreement with the charge transfer between [CuO₂]_∞ and [BiO]_∞ planes. This shift also decreases with increasing x, but is still present for the x=0.6 composition for which δ is smaller than x/2. A model of the metal-insulator transition in this series is proposed based on the fact that the intercalation of excess oxygen raises the bottom of the Bi-O band with respect to the Fermi level and decreases the contribution of the Bi-O electron pocket to the hole density.     

Dynamic contribution to the fishtail effect in a twin-free DyBa2Cu3O7−δ single crystal

Superconducting current densities j_s and dynamic relaxation rates Q d ln j_s/d In(dB_e/dt), where dB_e/dt is the sweep rate of the external magnetic field B_e, were measured as a function of temperature (5 K < T < 65 K) in magnetic fields up to 7 T on a twin-free DyBa₂Cu₃O_7−δ single crystal by means of a high-sensitivity capacitance torque magnetometer. Above 15 K, we observe a “fishtail” effect, i.e. a pronounced minimum in the j_s(B_e) curve at fields around B_e = 1 T. The relaxation rate Q shows an anomalous increase at low fields which is correlated to the minimum in the j_s(B_e) curve. Both the j_s versus B_e and Q versus B_e data are used as input parameters into the generalized inversion scheme developed by Schnack et al. [Phys. Rev. B 48 (1993) 13178] to calculate the true critical current density j_c which is by definition independent of relaxation effects. Interestingly, the j_c(B_e, T) curves derived in this way do not show a minimum. This points clearly to a dynamic contribution to the fishtail effect. The true critical current density j_c(B_e, T) decreases weakly with increasing B_e over the entire measured temperature and field range, as expected for single-vortex pinning. This indicates that the observed fishtail effect is not caused by a crossover from single-vortex pinning to pinning of flux bundles. The temperature dependence of j_c is in good agreement with the predictions of a model based on single-vortex pinning caused by spatial fluctuations in the charge-carrier mean free path.     

Adsorption and diffusion of a single Pt atom on γ-Al2O3 surfaces

Motivated to better understand the interactions between Pt and γ-Al₂O₃ support, the adsorption and diffusion of a single Pt atom on γ-Al₂O₃ was studied using density functional theory. Two different surface models with atoms of various coordination (3–5) were used, one derived from a defected spinel structure, and another derived from the dehydration of boehmite (AlOOH). Adsorption energies are similar for the two surfaces, about −2 eV for the most stable sites, and involve Pt binding to surface O atoms. An unusually strong trapping geometry whereby Pt moves into the surface was identified over the boehmite-derived surface. In all cases the surface transfers 0.2–0.3 e⁻ to the Pt atom. The bonding is explained as being a combination of charge transfer between the surface and Pt atom, polarization of the metal atom, and some weak covalent bonding. The similarity of the two surfaces is attributed to the similar local environments of the surface atoms, as corroborated by geometry analysis, density of states, and Bader charge analysis. Calculated activation barriers (0.3–0.5 eV) for the defected spinel surface indicate fast diffusion and a kinetic Monte Carlo model incorporated these barriers to determine exact diffusion rates and behavior. The kinetic Monte Carlo results indicate that at low temperatures (<500 K) the Pt atom can become trapped at certain surface regions, which could explain why the sintering process is hindered at low temperature. Finally we modeled the adsorption of Pt on hydrated surfaces and found adsorption to be weaker due to steric repulsion and/or decreased electron-donating ability of the surface.     

Molecular dynamics analysis of impurity ions behavior in β″-Al2O3

The effects of foreign impurity ions in the conduction plane on the β″-Al₂O₃ lattice have been analyzed by molecular dynamics. As impurity ions, K and Ca ions were chosen and Na sites in the conduction plane were replaced with these ions. Results are summarized as follows: (1) The β″-Al₂O₃ lattice expands perpendicular to the conduction plane when K ions are doped; (2) Ca ions do not contribute to expansions of the β″-Al₂O₃ lattice; (3) both K and Ca ions reduce the mean square displacement of Na ions, which can be attributed to decreased Na ion diffusion.     

Phase separation in YBa2Cu3O7−δ single crystals near δ = 0

X-ray diffraction, resistivity, AC and DC magnetization data on high-quality single crystals of YBa₂Cu₃O_7−δ are presented. We demonstrate that for δ<0.08, the (0 0 l) diffraction lines are split into two, indicating that at these high oxygen concentrations the crystals are no longer single phase but actually consist of two (or more) different phases with slightly different c-axis parameters. In the two-phase region, the electrical resistivity and the AC and DC magnetic susceptibilities show a broadening of the superconducting transition. This broadening is thought to be due to the proximity effect or strains in two finely dispersed phases with slightly different transition temperatures.     

Low-temperature AC conductivity of Bi2Sr2CaCu2O8+δ

We report measurements of anamolously large dissipative conductivities, σ₁, in Bi₂Sr₂CaCu₂O_8+δ at low temperatures. We have measured the complex conductivity of Bi₂Sr₂CaCu₂O_8+δ thin films at 100–600 GHz as a function of doping from the underdoped to the overdoped state. At low temperatures there exists a residual σ₁ which scales with the T=0 superfluid density as the doping is varied. This residual σ₁ is larger than the possible contribution to σ₁ from a thermal population of quasiparticles (QP) at the d-wave gap nodes.     

Nb/Au/(1 1 0)YBa2Cu3O7−δ Josephson junctions: evidence against atomic scaled “corner junctions”

We report on I–V characteristics for in situ formed Nb/Au/(1 1 0)YBa₂Cu₃O_7−δ (YBCO) Josephson junction, where the homoepitaxial (1 1 0)YBCO film shows ultra-smooth surface morphology. The field dependence of critical supercurrent I_c shows anisotropic large junction behavior with normal Fraunhofer patterns expected from BCS model of d_x²−y² wave superconductors. This strongly suggests that the Nb/Au/(1 1 0)YBCO junctions cannot be regarded as atomic scaled corner junctions, in contrast with (0 0 1)/(1 1 0)YBCO grain boundary junctions to show “π-junction” with a pronounced dip near zero fields in field modulation of I_c.     

Peak effect and oxygen ordering in YBa2Cu3O7−δ single crystals

We have investigated the second magnetization peak in pure YBa₂Cu₃O_7−δ single crystals with various oxygen contents (6.91<7−δ<6.97) and degrees of oxygen vacancy ordering, as achieved by low (1 bar) and high (100 bar) oxygen pressure annealing. Although the position of the peak changes drastically with oxygen stoichiometry, no dependence on the distribution of oxygen vacancies has been found for temperatures below 70 K. For T>70 K, however, ordering effects become important as demonstrated by the disappearance of the peak for the high pressure annealed samples. These results suggest that while at low temperatures, pinning of the vortex system by clusters or a more homogeneous distribution of oxygen vacancies is similar, at elevated T, the former are much stronger pinning sites leading to larger hysteresis and the presence of the peak.     

Consequences for nuclear physics of a NNγ and A NNγ time reversal violating vertex

This paper studies the effects in low energy nuclear physics of a possible time reversal invariance (TRI) violation in the electromagnetic interaction. It is shown that this effect appears as a two body short range TRI violating transition operator or as a two and three body TRI violating potential. Two cases are studied. Firstly a TRI violating NNγ vertex is considered and found to have very little effect. Secondly the TRI violation is assumed to occur in the NN^*γ vertex and it is shown that if the violation is “maximal” the contribution to the imaginary part of the “mixing ratio” δ is Im δ ≈ ¦δ¦ × 10⁻³. This should be measurable.     

Structural and transport properties of the triple-layer compounds Ba4(Pb1−xBix)3O10 (0≤ x<0.3)

A series of new compounds Ba₄(Pb_1−xBi_x)₃O₁₀ with 0≤x<0.3 has been prepared and characterized by X-ray and electron diffraction. The d.c. resistivity vs. temperature data for different x values are presented and structural features are compared to those of the superconducting phase Ba(Pb_1−xBi_x)O₃. The “mother-compound” Ba₄Pb₃O₁₀ is tetragonal, with lattice constants A=0.4280(1) and C=3.017(1) nm. Its structure can be regarded as a stacking of Ba(PbBi)O₃ triple-layers, separated by single BaO layers. Electron diffraction reveals the presence of a weak superstructure with the a-axis related to that of the perovskite by a=√2a_p. Frequent intergrowth of this phase with the perovskite structure is observed. The conductivity of the samples Ba₄Pb₃O₁₀ is nearly independent of temperature, while Bi-doped samples exhibit semiconductor-like behaviour at low temperature. No superconducting transition is observed down to 2K.     

Determination of different oxygen transport mechanisms and measurement of the oxygen ion conductivity of Y1Ba2Cu3O7−x

By undertaking AC electrochemical impedance experiments on yttria stabilised zirconia electrolytes with polished Y₁Ba₂Cu₃O_7−x electrodes, the activation energy for oxygen ion transport within the bulk of Y₁Ba₂Cu₃O_7−x, in air, over the temperature range 823 K–1043 K, was determined to be 1.50 ± 0.05 eV. At 1000 K the oxygen ionic conductivity was calculated to be around one order of magnitude lower than that in yttria stabilised zirconia. Typical calculated values were σ=5×10⁻⁵ (ω cm)⁻¹ and 6×10⁻³ (ω cm)⁻¹ at the respective temperatures 823 K and 1043 K. By employing a similar cell but with Y₁Ba₂Cu₃O_7−x paste electrodes, oxygen transfer between the Y₁Ba₂Cu₃O_7−x and the electrolyte was found to occur via a surface diffusional processes. Over the temperature range 873 K–1098 K, in air, the activation energy for in-diffusion at the surface was found to be 1.4±0.1 eV and that for out-diffusion at the surface to be 1.76±0.05 eV.     

Structural refractometry of β- and β″-Al2O3-type aluminates and gallates

The ionicity of oxide and β- and β″-Al₂O₃ crystals is identified via Lorentz–Lorenz molar refractivity. The investigation was extended to sodium gallates, the refractivity and average refractive index of which are estimated.     

Yb (Ba, Sr)2Cu4O8: a “124” phase superconductor with a reduced unit cell volume

Yb(Ba_1−xSr_x)₂Cu₄O₈ (0.1x0.3) superconductors of the YBa₂Cu₄O₈(“124”) structure were successfully synthesized using an O₂-hot-isostaticc-pressing (HIP) technique. The samples were characterized with respect to the crystal structure and superconducting properties. The lattice parameters of the samples decreased as the substitution of Sr for Ba proceeded. The superconducting transition temperatures of all the Yb-“124” samples were more or less the same, being around 80 K.     

Effect of different oxidizing gases on the in-situ growth of YBa2Cu3O7−δ films by pulsed laser deposition

The effectiveness of oxygen (O₂), nitrous oxide (N₂O), and nitrogen dioxide (NO₂) as oxidizing agents during in-situ growth of YBa₂Cu₃O_7−δ (YBCO) films on (100) SrTiO₃ substrates by pulsed laser deposition has been studied as a function of deposition temperature (700–800°C), and laser wavelength (193,248 and 355 nm), for a wide range of oxidizer gas pressure (0.1–200 mTorr). In general, the superconducting transition temperature of the films has been found to increase with increasing oxidant pressure, with zero-resistance temperature ≈90 K only obtained in films prepared in a relatively high pressure (150–200 mTorr) of oxidizer gas. At lower pressures, the transition temperature while being depressed is quite sensitive to the nature of the oxidant, the laser wavelength and the deposition temperature. Nevertheless, independent of the oxygen source or other growth parameters, an almost linear decrease in transition temperature with a corresponding increase in the c-axis lattice parameter has been observed for all the film. YBCO films have also been deposited in a low pressure background (≤ 1 mTorr) using a combination of atomic oxygen and pulsed molecular oxygen. The results are discussed in terms of the oxygen requirement for kinetic and thermodynamic stability of YBCO during growth of the film by pulsed laser deposition.     

Synthesis and investigation of the complex copper oxides Bi2Sr2R2−xCexCu2O10+δ (R=rare earth)

Bi-2222 phases were prepared by solid state reactions and were studied using X-ray diffraction, chemical and EDS-analysis. Compounds with composition Bi₂Sr₂R_1.33Ce_0.67Cu₂O_10+δ were obtained for R=Pr, Nd, Sm-Er. The phases Bi₂Sr₂Cu₂O_10+δ with similar structure but without Ce were prepared for R = Pr, Sm-Dy. For Bi₂Sr₂Eu_2−xCe_xCu₂O_10+δ the solubility limit was determined and the effect of treatment under different oxygen pressures was investigated. Weak diamagnetic signals (<3%) were detected for some of these samples.     

Intercalation thermodynamics and chemical diffusion of oxygen in the solid solution YBa2Cu3−xCoxO6+δ

The equilibrium oxygen content as a function of the temperature and oxygen pressure was measured for the solid solution YBa₂Cu_3−xCo_xO_6+δ, where x=0, 0.2, 0.4, 0.6, 0.8, by using coulometric titration in the temperature range 600–850°C and oxygen pressures between 10⁻⁵ and 1.0 atm. The change in the partial molar enthalpy and entropy of the intercalated oxygen was determined at different oxygen and cobalt contents. The oxygen chemical diffusion was studied by thermogravimetric relaxation in the oxygen-controlled atmosphere. The thermodynamic data were employed to determine how the chemical diffusion coefficient, the thermodynamic factor and the random-diffusion coefficient depend on oxygen content in specimens with different cobalt concentration. The oxygen intercalation thermodynamics and diffusivity results provide evidence of ordering phenomena on a microscopic scale in the basal plane of the tetragonal solid solution YBa₂Cu_3−xCo_xO_6+δ. A model for the oxygen diffusion is suggested to explain the large difference between the random and tracer diffusion coefficients in YBa₂Cu₃O_6+δ     

Photo-excitation of single crystals of La2CuO4+δ near the metal-insulator transition

Transient photoconductivity (σ_ph) experiments have been carried out in single crystals of insulating La₂CuO_4+δ near the metal-insulator transition (δ≈ 1–2%). The time evolution of the σ_ph changes dramatically with light intensity (I_L). At low I_L, σ_ph is characterized by power law time decay, t⁻, in the nanosecond time regime, and the exponent, , decreases significantly with increasing I_L. As I_L increases to 5 × 10¹⁵ photons/cm₂, σ_ph reaches 15 S/cm, and the lifetime of the conducting state is enhanced by more than two orders of magnitude: σ_ph exhibits a delayed peak centered at approximately 30 ns. followed by exponential decay with time constant of 360 ns. The data reveal a continuous distribution of localized electronic states above the (hole) mobility edge; the M-I transition is achieved when the Fermi level is shifted away from the localized states and across the mobility edge either by chemical dopiing or by photo-excitation. The time of σ_ph at high I_L implies the formation of metastable metallic droplets after photo-excitation.     

Preparation and characterization of carbonate terminated polycrystalline Al2O3/Al films

X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface reactivity of polycrystalline Al films in contact with a gas mixture of carbon dioxide and oxygen at room temperature. Based on the characterization of interactions between these substrates and the individual gases at selected exposures, various surface functionalities were identified. Simultaneously dosing both carbon dioxide and oxygen is shown to create surface-terminating carbonate species, which contribute to inhibiting the formation of an Al₂O₃ layer. Finally, a reaction scheme is suggested to account for the observed dependence of surface group formation on the dosing conditions.