Resistivity,thermopower and single-particle tunneling studies on some zinc-doped yttrium barium copper oxide superconductors

Abstract

Resistance and thermopower measurements have been made on a series of compounds, YBa₂Cu₃- _x Zn _x O_7-y, with x = 0.025, 0.05, 0.1, 0.15 and 0.2. The superconducting transition temperature decreases as the zinc concentration increases. In a range of temperatures below T_M , the mid point of the transition, the resistance shows an exponential temperature dependence fitting the phenomenological formula proposed by Ausloos et al. From the plot of logarithm of resistivity vs. (T_M ? T) ^1/2/T, one deduces a value of the average dimension of the Josephson junction to be a few tens of Å, suggesting the microtwin boundaries to be the location of the junctions. The thermopower shows a peak always just above T_c . This conclusively shows that phonon drag is not the cause of the peak. The temperature dependence of the thermopower appears to resemble closely the earlier observations of Srinivasan et al. on yttrium barium copper oxide. Single-particle tunneling measurements carried out for two concentrations, x = 0 and 0.05, appear to indicate that the energy gap parameter scales with T_c , and 2Δ/kT_c has an approximate value of 5.5.     

Photochromic and fluorescence properties of coumarin fulgimides

Four new fulgimides possessing a fluorescent coumarin unit were synthesized from the corresponding fulgides, and their photochromic as well as fluorescence properties were investigated. The open-ring forms of coumarin fulgimides were found to exhibit fluorescence in the visible region. Upon exposure to UV light, the fulgimides were transformed into the nonfluorescent closed-ring forms, which can be reverted to the initial fluorescent open-ring forms on exposure to visible light. The efficiency of quenching of fluorescence was as high as 95% at the photostationary state of UV irradiation.     

Thermodynamic and kinetic properties of Ho1−xTixCo2-hydrogen system

The hydrogen absorption behavior of Laves phase Ho_1−xTi_xCo₂ (x=0.1-0.6) alloys has been investigated by pressure-concentration (PC) isotherms and cyclic-, temperature- and pressure-dependent absorption kinetics. The PC isotherms and kinetics of hydrogen absorption have been studied in the pressure range 0.01-1 bar and temperature range 50-200 °C using Sievert's-type apparatus. The drastic changes in the induction period and particle size during the activation process have been discussed based on the kinetics of repeated hydrogenation cycles and scanning electron microscopy (SEM) images of the hydrides at different hydriding cycles, respectively. The experimental results of kinetic curves are interpreted using the Johnson-Mehl-Avrami (JMA) model, and the reaction order and reaction rate have been determined. The α-, (α+β)- and β-phase regions in Ho_1−xTi_xCo₂-H have been identified from the different slope regions of the first-order-type kinetic plots. The dependence of the reaction rate parameter on hydriding pressure and temperature in the (α+β)-phase region has been discussed.     

Yield surfaces for non-homogeneous anisotropic shells of revolution

The state of stress in a rotationally symmetric shell is characterized by the direct stresses and moments in the circumferential and longitudinal directions. It is assumed that the material of the shell is rigid perfectly plastic and that the yield stress of the material varies over the thickness of the shell. The material has different yield stresses in tension and compression and the yield stresses in the principal directions have different values. The yield condition for the shell is obtained in terms of the stress resultants assuming that the material of the shell obeys the maximum shear stress criterion.     

Electron transport in Chevrel phase superconductors,Cu1.8Mo6S8−y Se y , 0⩽y⩽8 and Cu1.8Mo6S8−y Te y , 0⩽y⩽4—Fit to Cote-Meisel theory

Normal state electrical resistivity of the Chevrel phase compounds of the type Cu_1.8Mo₆S_8−y Se _y , 0⩽y⩽8 and Cu_1.8Mo₆S_8−y Te _y , 0⩽y⩽4 is analysed on the basis of the generalized diffraction model which incorporates a postulate on electron-phonon interaction,viz phonons with wavelength exceeding the electron mean-free path are ineffective electron scatterers. Fit obtained by this model was found to be superior to other models based on the interbands-d scattering of electrons.     

Electronic Structure Insights into the Solvation of Magnesium Ions with Cyclic and Acyclic Carbonates

A computational framework to rank the solvation behavior of Mg²⁺ in carbonates by using molecular dynamics simulations and density functional theory is reported. Based on the binding energies and enthalpies of solvation calculated at the M06‐2X/6‐311++G(d,p) level of theory and the free energies of solvation from ABF‐MD simulations, we find that ethylene carbonate (EC) and the ethylene carbonate:propylene carbonate (EC:PC) binary mixture are the best carbonate solvents for interacting with Mg²⁺. Natural bond orbital and quantum theory of atoms in molecules analyses support the thermochemistry calculations with the highest values of charge transfer, perturbative stabilization energies, electron densities, and Wiberg bond indices being observed in the Mg²⁺(EC) and Mg²⁺(EC:PC) complexes. The plots of the noncovalent interactions indicate that those responsible for the formation of Mg²⁺ carbonate complexes are strong‐to‐weak attractive interactions, depending on the regions that are interacting. Finally, density of state calculations indicate that the interactions between Mg²⁺ and the carbonate solvents affects the HOMO and LUMO states of all carbonate solvents and moves them to more negative energy values.     

Photoinduced C-N bond cleavage in 2-azido-1,3-diphenyl-propan-1-one derivatives: photorelease of hydrazoic acid

Photolysis of 3-azido-1,3-diphenyl-propan-1-one (1a) in toluene yields 1,3-diphenyl-propen-1-one (2), whereas irradiation of 3-azido-2,2-dimethyl-1,3-diphenyl-propan-1-one (1b) results in the formation of mainly 2,2-dimethyl-1,3-diphenyl-propan-1-one. Laser flash photolysis (308 nm) of 1a,b in acetonitrile reveals a transient absorption (lambda max = approximately 310 nm) due to the formation of radicals 4a and 4b, respectively, which have lifetimes of approximately 14 micros at ambient temperature. TD-DFT calculations (B3LYP/6-31+G(d)) reveal that the first and second excited states of the triplet ketone (T1K (n,pi*) and T2K (pi,pi*)) in azide 1a are almost degenerate, at approximately 74 and 76 kcal/mol above the ground state (S0), respectively. Similarly, azide 1b has T1K and T2K 75 and 82 kcal/mol above S0, respectively. The calculated transition state for cleaving the C-N bond is located 71 and 74 kcal/mol above S0 in azides 1a and 1b, respectively. The calculated bond dissociation energies for breaking the C-N bond are 55 and 58 kcal/mol for azides 1a and 1b, respectively, making C-N bond breakage accessible from T1K in azides 1 at ambient temperature. In comparison, the irradiation of azides 1 in argon matrices at 14 K lead to the formation of the corresponding triplet alkyl nitrenes (1-n), via intramolecular energy transfer from T2K. The characterization of 1-n was supported by isotope labeling, IR spectroscopy, and molecular modeling.     

Design, synthesis, and validation of a branched flexible linker for bioactive peptides

A branched flexible linker that incorporates a fluorescent dansyl moiety was synthesized and used to connect two high affinity NDP-alpha-MSH ligands or two low affinity MSH(4) ligands. The linker was incorporated into the conjugate by solid-phase synthesis. In vitro biological evaluations showed that potency of binding to the human melanocortin 4 receptor was not diminished for linker-ligand combinations relative to the corresponding ligand alone.     

Synthesis and pharmacological evaluation of some 3-(4-methoxyphenyl)-2-substitutedamino-quinazolin-4(3H)-ones as analgesic and anti-inflammatory agents

A variety of novel 3-(4-methoxyphenyl)-2-substitutedamino-quinazolin-4(3H)-ones were synthesized by reacting the amino group of 2-hydrazino-3-(4-methoxyphenyl)-quinazolin-4(3H)-one with a variety of alkyl and aryl ketones. The starting material 2-hydrazino-3-(4-methoxyphenyl)-quinazolin-4(3H)-one was synthesized from 4-methoxyaniline. The title compounds were investigated for analgesic, anti-inflammatory and ulcerogenic index activities. While the test compounds exhibited significant activity, compounds 2-(1-methylpropylidene)-hydrazino-3-(4-methoxyphenyl)-quinazolin-4(3H)-one (A1), 2-(1-ethylpropylidene)-hydrazino-3-(4-methoxyphenyl)-quinazolin-4(3H)-one (A2) and 2-(1-methylbutylidene)-hydrazino-3-(4-methoxyphenyl)-quinazolin-4(3H)-one (A3) showed moderately more potent analgesic activity and the compound 2-(1-methylbutylidene)-hydrazino-3-(4-methoxyphenyl)-quinazolin-4(3H)-one (A3) showed moderately more potent anti-inflammatory activity when compared to the reference standard diclofenac sodium. Interestingly the test compounds showed only mild ulcerogenic potential when compared to aspirin.     

Low loaded Pt-Co catalyst surfaces optimized by magnetron sputtering sequential deposition technique for PEM fuel cell applications: physical and electrochemical analysis on carbon paper support

A series of thin Pt-Co films with different metal ratios were deposited by using the sequential cosputtering directly on a commercial hydrophobic carbon paper substrate at room temperature and in ultra-high vacuum (UHV) conditions. Their electrocatalytic properties toward the oxygen reduction reaction were investigated in 0.5 M H₂SO₄ solution by means of cyclic voltammetry (CV) and linear sweep voltammetry (LSV) on rotating disc electrode (RDE). The results showed that Pt particles, deposited by dc-magnetron gun, surround the large Co-clusters deposited by rf-magnetron gun. In addition, the increase of Co content led to an increase in the electrochemical active surface area (EASA) from 23.75 m²/gPt to 47.54 m²/gPt for pure Pt and Pt:Co (1:3), respectively, which corresponded the improvement of the utilization of Pt by a factor of 1.91. This improvement indicated that the sequential magnetron cosputtering was one of the essential technique to deposit homogeneous metal clusters with desirable size on the gas diffusion layer by adjustment plasma parameters.