Charge and spin density distributions around Zn impurities in cuprates

The effect of zinc substitution on the local electronic structure of several cuprates is investigated using first-principles cluster calculations. Clusters comprising 5, 9, and 13 copper atoms in the cuprate plane of La$_2$CuO$_4$, YBa$_2$Cu$_3$O$_7$, and YBa$_2$Cu$_4$O$_8$ are used. Spin polarized calculations with different multiplicities in the framework of density functional theory enable a detailed study of the changes in the charge and spin density distribution induced by Zn substitution. Furthermore, doping with charge carriers in the above materials is simulated and the resulting changes in the charge distribution are compared to the changes induced by Zn impurities. These differences are then discussed in terms of a phenomenological model related to properties expected from the generic phase diagram. The effects of zinc substitution are rather local and as expected the absolute values of the Mulliken charges at both nearest and next nearest neighbor oxygens to Zn are larger than in the unsubstituted clusters. The calculated electric field gradient at Cu sites that are nearest neighbor to Zn is found to be somewhat larger than in the unsubstituted cluster whereas that of next nearest neighbors is about 5% smaller. We conclude that the satellite peak in the Cu NQR spectrum occurring upon Zn substitution in YBa$_2$Cu$_3$O$_7$ and YBa$_2$Cu$_4$O$_8$ has its origin at Cu that are next nearest neighbors to Zn.Comment: 8 pages, 7 figure

First principles study of local electronic and magnetic properties in pure and electron-doped Nd2_2CuO4_4

The local electronic structure of Nd2CuO4 is determined from ab-initio cluster calculations in the framework of density functional theory. Spin-polarized calculations with different multiplicities enable a detailed study of the charge and spin density distributions, using clusters that comprise up to 13 copper atoms in the CuO2plane. Electron doping is simulated by two different approaches and the resulting changes in the local charge distribution are studied in detail and compared to the corresponding changes in hole doped La2CuO4. The electric field gradient (EFG) at the copper nucleus is investigated in detail and good agreement is found with experimental values. In particular the drastic reduction of the main component of the EFG in the electron-doped material with respect to LaCuO4 is explained by a reduction of the occupancy of the 3d3z^2-r^2 atomic orbital. Furthermore, the chemical shieldings at the copper nucleus are determined and are compared to results obtained from NMR measurements. The magnetic hyperfine coupling constants are determined from the spin density distribution

Forward-backward asymmetry of photoemission in C60_{60} excited by few-cycle laser pulses

We theoretically analyze angle-resolved photo-electron spectra (ARPES) generated by the interaction of C$_{60}$ with intense, short laser pulses. In particular, we focus on the impact of the carrier-envelope phase (CEP) onto the angular distribution. The electronic dynamics is described by time-dependent density functional theory, and the ionic background of \csixty is approximated by a particularly designed jellium model. Our results show a clear dependence of the angular distributions onto the CEP for very short pulses covering only very few laser cycles, which disappears for longer pulses. For the specific laser parameters used in a recent experiments, a very good agreement is obtained. Furthermore, the asymmetry is found to depend on the energy of the emitted photoelectrons. The strong influence of the angular asymmetry of electron emission onto the CEP and pulse duration suggests to use this sensitivity as a means to analyze the structure of few-cycle laser pulses.Comment: 8 pages, 6 figure

Strategies to valorise agrobiodiversity

The current food value chain is characterised by a highly standardised offer produced in an increasingly monotonous agricultural system. In parallel, there is a growing interest among consumers for traditional or regional crop varieties. The conservation of these crops is often done by civil society organisations like Arche Noah, Pro Specie Rara, Rete Semi Rurali, Réseau Semence Paysanne or Red Andaluza de Semillas. Some networks sell their food products in local niche markets, specialised stores and supermarkets. To increase their visibility, some producer groups in the networks are interested in developing or improving a label to valorise agrobiodiversity to the consumer. Based on a representative consumer survey in four European countries, we show that consumer awareness with respect to traditional, old varieties is low but the interest in "Diversifood", defined as more diverse, locally adapted, healthy and tasty produce, is substantial. Therefore, we state, that the availability of a label to valorise agrobiodiversity would respond to consumer needs and increase the visibility of the work done by the networks – but only if it is embedded in an appropriate communication strategy aimed at raising consumer awareness about agrobiodiversity

Evaluation matrix for products from underutilized crops

We developed a matrix to evaluate valorisation strategies for products from underutilized crops. It shall help networks involved in agrobiodiversity conservation and breeding to find and improve their valorisation strategies, such as a label

Nuclear magnetic resonance measurements reveal the origin of the Debye process in monohydroxy alcohols

Monohydroxy alcohols show a structural relaxation and at longer time scales a Debye-type dielectric peak. From spin-lattice relaxation experiments using different nuclear probes an intermediate, slower-than-structural dynamics is identified for n-butanol. Based on these findings and on diffusion measurements, a model of self-restructuring, transient chains is proposed. The model is demonstrated to explain consistently the so far puzzling observations made for this class of hydrogen-bonded glass forming liquids.Comment: 4 pages, 4 figure