One-dimensional multiband correlated conductors and Anderson impurity physics

A single Anderson impurity model recently predicted, through its unstable fixed point, the phase diagram of a two-band model correlated conductor, well confirmed by dynamical mean-field theory in infinite dimensions. We study here the one-dimensional version of the same model and extract its phase diagram in the opposite limit of reduced dimensionality. As expected for one dimension, the Mott metal-insulator transition at half filling is replaced by a dimerized insulator-undimerized Mott insulator transition, while away from half filling the strongly correlated superconductivity for inverted Hund's rule exchange in infinite dimensions is replaced by dominant pairing fluctuations. Many other aspects of the one-dimensional system, in particular, the field theories and their symmetries, are remarkably the same as those of the Anderson impurity, whose importance appears enhanced.     

Surface state excitons in semiconductors

We calculate binding energies and wave-functions of surface state excitons. Our approach is based on the effective mass approximation, and assumes a small but finite surface state penetration depth. Central cell corrections are included. We obtain binding energies in the tenth of eV range, in agreement with recent observations on GaAs (110) and Si (111) 7 × 7, for surface masses of order unity.     

Multipole and spin-flip interband transitions in the electron energy loss of ferromagnetic Fe

Electron energy loss measurements have been carried out in the reflection mode from polycrystalline iron. The observed structures are interpreted as surface and bulk collective losses by observing their dependence on the primary electron energy and surface oxidation. Interband transitions in the range 2–30 eV are pinpointed and found to be in excellent agreement with previous optical results, except for a new strong transition around 10 eV, which is not found in optical experiments. This transition is tentatively identified with a dipole-forbidden process, which becomes allowed because of finite momentum transfer by the electrons.     

Ultraviolet and soft X-ray photon-photon elastic scattering in an electron gas

We have considered the processes which lead to elastic scattering between two far ultraviolet or X-ray photons while they propagate inside a solid, modeled as a simple electron gas. The new ingredient, with respect to the standard theory of photon-photon scattering in vacuum, is the presence of low-energy, nonrelativistic electron-hole excitations. Owing to the existence of two-photon vertices, the scattering processes in the metal are predominantly of second order, as opposed to fourth order for the vacuum case. The main processes in second order are dominated by exchange of virtual plasmons between the two photons. For two photons of similar energy , this gives rise to a cross section rising like ² up to maximum of around 10^–32 cm², and then decreasing like ^–6. The maximal cross section is found for the photon wavevectorkk _F , the Fermi surface size, which typically means a photon energy in the keV range. Possible experiments aimed at checking the existence of these rare but seemingly measurable elastic photon-photon scattering processes are discussed, using in particular intense synchrotron sources.     

Retrieval of the vertical column of an atmospheric constituent from data fusion of remote sensing measurements

Techniques of data fusion are presently being considered with increasing interest for application to atmospheric observations from space because of their capability to optimally exploit the complementary information provided by different instruments operating aboard on-going and future satellite missions. The task of combining measurements of the same target, when carried out at the level of the retrieved state vectors, faces with two major problems: the need to interpolate the products represented on different retrieval grids which determines a loss of information and the presence of a priori information in the products that can determine a bias in the product of the data fusion. The measurement space solution method avoids these problems. Based on this method we present a novel approach to retrieve the vertical column of an atmospheric constituent from data fusion of remote sensing measurements. We apply the method to retrieve the ozone column from the fusion of simulated measurements of the IASI nadir-viewing spectrometer onboard the METOP-A platform and of the MIPAS limb sounder onboard the ENVISAT satellite. The performance of the method is evaluated in terms of retrieval errors and averaging kernels of the products. The results show the evidence of improved retrieval quality when comparing the outcome of data fusion with that of the inversion process applied to spectra from either of the two instruments.     

Chemoenzymatic synthesis of the enantiomerically pure 1,2,3,4-tetrahydroquinoline moiety of the antithrombotic (21R)- and (21S)-argatroban

The synthetic antithrombotic argatroban is a dipeptide between the nonproteogenic (2R,4R)-4-methyl-2-piperidine carboxylic acid and l-arginine, in turn bonded to a 3-methyltetrahydroquinoline sulfonyl group; the drug is usually prepared and administered as a mixture of C-21-diastereoisomers. By means of a biocatalytic transformation enantiomerically pure (R)- and (S)-synthons, suitable for the synthesis of separate (21R)- and (21S)- argatroban, were obtained.     

Some applications of metric currents to complex analysis

The aim of this paper is to extend the theory of metric currents, developed by Ambrosio and Kirchheim, to complex spaces. We define the bidimension of a metric current on a complex space and we discuss the Cauchy–Riemann equation on a particular class of singular spaces. As another application, we investigate the Cauchy–Riemann equation on complex Banach spaces, by means of a homotopy formula.     

Magnetism-induced ballistic conductance changes in palladium nanocontacts

We present first-principles calculations of the effects of magnetism on the ballistic conductance of a model Pd nanocontact, made of a short Pd monatomic stretched chain placed between two Pd leads, simulated by semi-infinite (100) slabs. The stretching makes the suspended Pd chain generally ferromagnetic. The spin-resolved ballistic conductance, calculated according to the Landauer-Büttiker formula is found to be 0.85G₀ for the spin-up and 1.15G₀ for the spin-down electrons (G₀ = 2e²/h is the conductance quantum). The total conductance ~2G₀ is lower, but still relatively close to that of the nonmagnetic Pd nanocontact with the same geometry, calculated to be 2.3G₀. To illustrate how magnetism and conductance depend on structural details, we change the three atom chain docking from the top to a hollow surface site, where at the same stress the Pd contact is nonmagnetic and the conductance decreases to 1.8G₀. Overall we find these calculated ballistic conductance values of very similar magnitude to the first histogram peak in the experimental data obtained for Pd at low temperature in mechanically controllable break junctions. We conclude that the 15% conductance changes caused by the onset or the demise of local magnetism, similar in magnitude to geometry-related conductance changes, are probably too small to be used as a diagnostic for the presence or absence of nanocontact magnetism.     

A new platinum vapor-derived highly efficient hydrosilylation catalyst: NMR structural investigation

The structural features of a highly efficient hydrosilylation catalyst generated by reaction of Pt vapor and a mixture of mesitylene and 1,3-divinyltetramethyldisiloxane (DVS) were investigated by mono- and bidimensional NMR analyses. The structure around the Pt atoms was highlighted and compared with a commercial sample (Karstedt catalyst) and previously reported Pt vapor-derived system.