Possible relevance of lattice effects in high-T c superconductors

The relevance of the lattice-mediated superconducting pairing in a system of quasilocalized polarons dressed by local lattice deformations is considered. The spin correlations are taken into account using thet-J model expressed in terms of holon and spin operators. The Holstein-like Hamiltonian for holons with the transport term depending on spin correlations is transformed by the generalized Lang-Firsov transformation which implies the spread of the charge and the deformations to the nearest neighbours. The analytical formula for the superconducting transition temperatureT _c is deduced using the assumption of an extremely narrow polaron band. The hole-concentration dependence ofT _c and the isotope exponent are discussed using the classical approximation for incommensurate spiral spin correlations. This work was supported by the Grant Agency of the Czech Republic, project No. 202/96/0864.     

Phonon dispersion in small-polaron conductivity problem: A solvable model

The stochastic properties of harmonically coupled oscillators studied by Ford, Kac and Mazur are used to investigate the role of the phonon dispersion in the phonon-assisted small-polaron hopping. It is shown that a particular choice of the vibrational frequency spectrum allows one to express the effect of a weak phonon dispersion as an effective damping of independent local oscillators. Therefore, this particular choice makes it possible to employ literally the theory developed in a previous paper. The dependence on the phonon-dispersion width of the small polaron hopping conductivity determined in this way is illustrated by numerical calculations.     

Linear HThThH: a candidate for a Th-Th triple bond

The acetylene-like linear HThThH structure is calculated to lie almost degenerate to its rhombic alternative. Its unsupported Th-Th triple bond would be the first one of its kind.     

Can weakly coordinating anions stabilize mercury in its oxidation state +IV?

While the thermochemical stability of gas-phase HgF4 against F2 elimination was predicted by accurate quantum chemical calculations more than a decade ago, experimental verification of "truly transition-metal" mercury(IV) chemistry is still lacking. This work uses detailed density functional calculations to explore alternative species that might provide access to condensed-phase Hg(IV) chemistry. The structures and thermochemical stabilities of complexes Hg(IV)X4 and Hg(IV)F2X2 (X- = AlF4-, Al2F7-, AsF6-, SbF6-, As2F11-, Sb2F11-, OSeF5-, OTeF5-) have been assessed and are compared with each other, with smaller gas-phase HgX4 complexes, and with known related noble gas compounds. Most species eliminate F2 exothermically, with energies ranging from only about -60 kJ mol(-1) to appreciable -180 kJ mol(-1). The lower stability of these species compared to gas-phase HgF4 is due to relatively high coordination numbers of six in the resulting Hg(II) complexes that stabilize the elimination products. Complexes with AsF6 ligands appear more promising than their SbF6 analogues, due to differential aggregation effects in the Hg(II) and Hg(IV) states. HgF2X2 complexes with X- = OSeF5- or OTeF5- exhibit endothermic fluorine elimination and relatively weak interactions in the Hg(II) products. However, elimination of the peroxidic (OEF5)2 coupling products of these ligands provides an alternative exothermic elimination pathway with energies between -120 and -130 kJ mol(-1). While all of the complexes investigated here thus have one exothermic decomposition channel, there is indirect evidence that the reactions should exhibit nonnegligible activation barriers. A number of possible synthetic pathways towards the most interesting condensed-phase Hg(IV) target complexes are proposed.     

Can Weakly Coordinating Anions Stabilize Mercury in Its Oxidation State + IV?

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access the actual ChemInform Abstract, please click on HTML or PDF.     

Proteotypic peptides of hairs for the identification of common European domestic and wild animal species revealed by in-sample protein digestion and mass spectrometry analysis

The aim of this work is to offer an alternative or complementary analytical tool to the time-consuming and expensive methods commonly used for the recognition of animal species according to their hair. The paper introduces a simple and fast way for species differentiation of animal hairs called in-sample digestion. A total of 10 European animal species, including cat, cow, common degu, dog, fallow deer, goat, horse, sika deer, rabbit, roe deer, and 17 different breeds of dogs were examined using specific tryptic cleavage directly in hair followed by matrix-assisted laser desorption/ionization–time of flight mass spectrometry and liquid chromatography-electrospray ionization quadrupole time of flight. Principal component analysis was used for the subsequent mass spectrometric data evaluation. This novel approach demonstrates the ability to distinguish among individual animal species, which is supported by finding characteristic m/z values obtained by the mass spectrometry for each animal species. The approach was successfully tested on two “blind” samples. On the other hand, the attempt to distinguish among hairs of different dog breeds has not been successful due to the very similar protein composition and their amino acid sequences.     

Uranium recovery from LiF–CaF2–UF4–GdF3 system on Ni electrode

Electrochemistry of gadolinium and uranium in LiF–CaF₂ (79–21 mol%) melt was studied using reactive Ni electrode and alloying reactions were observed. Deposits of gadolinium and uranium in the form of Gd–Ni and U–Ni intermetallic alloys were obtained after electrolysis by modulated current. Electrolysis of the same parameters was used also in the complex system of LiF–CaF₂–UF₄–GdF₃ to demonstrate feasibility of selective deposition of uranium and therefore its separation from the system. Compact deposit of U–Ni alloy containing only traces of gadolinium was obtained.     

Image restoration in a highly non-steady-state regime of stimulated Brillouin scattering in a photodissociation iodine laser

For what we believe to be the first time, mask restoration in a backscattered signal in a highly non-steady-state regime of stimulated Brillouin scattering was attained. A special two-lens system of laser beam insertion into a scattering medium was used. Results are compared with those of ordinary one-lens focusing.     

Fractional Dynamics at Multiple Times

A continuous time random walk (CTRW) imposes a random waiting time between random particle jumps. CTRW limit densities solve a fractional Fokker-Planck equation, but since the CTRW limit is not Markovian, this is not sufficient to characterize the process. This paper applies continuum renewal theory to restore the Markov property on an expanded state space, and compute the joint CTRW limit density at multiple times.