Magneto-transport properties of La0.7Ca0.3MnO3/SrTiO3/La0.7Ce0.3MnO3 tunnel junction

Hole-doped rare-earth manganite La_0.7Ca_0.3MnO₃ and the electron-doped manganite La_0.7Ce_0.3MnO₃ both show a metal-insulator transition around 250 K associated with a ferromagnetic transition and colossal magnetoresistance. In an earlier publication we have reported the rectifying characteristic of La_0.7Ca_0.3MnO₃/SrTiO₃/La_0.7Ce_0.3MnO₃ tunnel junction at room temperature, showing that it is possible to fabricate a diode out of the polaronic insulator regime of doped manganites. Here we report the magneto-transport properties of such a tunnel junction above and below the metal-insulator transition. We show, from the large positive magnetoresistance of the tunnel junction at low temperature, that La_0.7Ce_0.3MnO₃ could be a minority spin carrier ferromagnet. The implication of this observation is discussed.     

Structure and reactivity of bimetallic electrochemical interfaces: infrared spectroscopic studies of carbon monoxide adsorption and formic acid electrooxidation on antimony-modified Pt(100) and Pt(111)

The influence of predosed antimony on the adlayer structures of carbon monoxide and on the electro-oxidation kinetics of formic acid on Pt(100) and Pt(111) in 0.1M HClO₄ is examined by means of in-situ infrared spectroscopy in conjunction with cyclic voltammetry. Preadsorbed antimony inhibits the adsorption of CO on these surfaces, the attenuation in CO coverage being accompanied by a selective removal of the two-fold bridging geometry as deduced from the relative ν_CO band intensities. At saturation antimony coverages, the CO binding is exclusively terminal on Pt(100) and Pt(111). These findings are consistent with the adsorption of antimony at multi-fold sites, yielding microscopically intermixed adlayers with CO. The electro-oxidation rates of formic acid are enhanced substantially by preadsorbed antimony on Pt(100) and Pt(111). The real-time infrared spectra in the C-O stretching region and the CO coverages thereby deduced in the presence of predosed antimony under reactive voltammetric conditions suggest that the metal adatoms are actively involved in the dissociation of formic acid. The origins of the enhanced electrocatalytic activity of the bimetallic Sb/Pt surfaces are discussed in terms of geometric and chemical effects.     

Molecular solvent-dynamical effects on activated electron-transfer kinetics: How important is solvent friction?

The likely extent of retardation exerted by solvent friction upon the rates of activated electron-transfer (ET) processes is explored and evaluated with reference to some extant solvent-dependent data for metallocene ET self exchanges. Barrier-crossing frequencies extracted from the experimental kinetic and barrier data for suitably adiabatic reactions are compared with solvent inertial frequencies (i.e. the zero-friction limit) as estimated from currently available analytic expressions. The additional extent of rate retardation seen in passing from low-friction to ostensibly strongly overdamped solvents, as deduced in this manner, is seen to be muted substantially in comparison with the predictions of conventional Debye-continuum approaches, although following a solvent functionality that is nevertheless roughly in accord with the latter. The likely importance of more rapid dynamics associated with short-range (molecular) solvation is addressed in the light of these findings.     

Entrainment and stimulated emission of ultrasonic piezoelectric auto-oscillators

Theoretical modeling and laboratory tests are conducted for nonlinear auto-oscillating piezoelectric ultrasonic devices coupled to reverberant elastic bodies. The devices are shown to exhibit behavior familiar from the theory of coupled auto-oscillators. In particular, these spontaneously emitting devices adjust their limit-cycle frequency to the spectrum of the body. It is further shown that the auto-oscillations can be entrained by an applied field; an incident wave at a frequency close to the frequency of the natural limit cycle entrains the oscillator. Special attention is paid to the phase of entrainment. Depending on details, the phase is such that the oscillator can be in a state of stimulated emission: the incident field amplifies the ultrasonic power emitted by the oscillator. These behaviors are essential to eventual design of an ultrasonic system that would consist of a number of such devices all synchronized to their mutual field, a system that would be an analog to a laser. A prototype uaser is constructed.     

Propane σ‐Complexes on PdO(101): Spectroscopic Evidence of the Selective Coordination and Activation of Primary C?H Bonds

Achieving selective C H bond cleavage is critical for developing catalytic processes that transform small alkanes to value‐added products. The present study clarifies the molecular‐level origin for an exceptionally strong preference for propane to dissociate on the crystalline PdO(101) surface via primary C H bond cleavage. Using reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations, we show that adsorbed propane σ‐complexes preferentially adopt geometries on PdO(101) in which only primary C H bonds datively interact with the surface Pd atoms at low propane coverages and are thus activated under typical catalytic reaction conditions. We show that a propane molecule achieves maximum stability on PdO(101) by adopting a bidentate geometry in which a H Pd dative bond forms at each CH₃ group. These results demonstrate that structural registry between the molecule and surface can strongly influence the selectivity of a metal oxide surface in activating alkane C H bonds.     

Diliberto–Straus algorithm for the uniform approximation by a sum of two algebras

In 1951, Diliberto and Straus [5] proposed a levelling algorithm for the uniform approximation of a bivariate function, defined on a rectangle with sides parallel to the coordinate axes, by sums of univariate functions. In the current paper, we consider the problem of approximation of a continuous function defined on a compact Hausdorff space by a sum of two closed algebras containing constants. Under reasonable assumptions, we show the convergence of the Diliberto–Straus algorithm. For the approximation by sums of univariate functions, it follows that Diliberto–Straus’s original result holds for a large class of compact convex sets.     

A parametric study of laser induced thin film spallation

We report parametric studies of elastic wave generation by a pulsed laser and associated spalling of thin surface films by the corresponding high stresses. Two different substrate materials, single crystal Si (100) and fused silica, are considered. Spallation behavior of Al thin films is investigated as a function of substrate thickness, film thickness, laser energy, and various parameters governing the source. Surface displacement due to the stress wave is measured by Michaelson interferometry and used to infer the stresses on the film interface. Consistent with previous studies, the maximum stress in the substrate and at the film/substrate interface increases with increasing laser fluence. For many of the conditions tested, the substrate stress is large enough to damage the Si. Moreover, the maximum interface stress is found to increase with increasing film thickness, but decrease with increasing substrate thickness due to geometric attenuation. Of particular significance is the development of a decompression shock in the fused sillica substrates, which results in very high tensile stresses at the interface. This shock enhances the failure of thin film interfaces, especially in thicker samples.