First principle computation of stripes in cuprates

We present a first principle computation of vertical stripes in $La_{15/8}Sr_{1/8}CuO_4$ within the LDA+U method. We find that Cu centered stripes are unstable toward O centered stripes. The metallic core of the stripe is quite wide and shows reduced magnetic moments with suppressed antiferromagnetic (AF) interactions. The system can be pictured as alternating metallic and AF two-leg ladders the latter with strong AF interaction and a large spin gap. The Fermi surface shows warping due to interstripe hybridization. The periodicity and amplitude of the warping is in good agreement with angle resolved photoemission experiment. We discuss the connection with low-energy theories of the cuprates.Comment: 5 pages,4 figure

Role of c-axis pairs in V2O3 from the band-structure point of view

The common interpretation of the LDA band structure of V$_{2}$O$_{3}$ is that the apparent splitting of the $a_{1g}$ band into a low intensity structure deep below the Fermi energy and a high intensity feature above it, is due to the bonding-antibonding coupling of the vertical V-V pair. Using tight-binding fitting to --as well as first-principles NMTO downfolding of-- the spin-up LDA+U $a_{1g}$ band, we show that there are other hopping integrals which are equally important for the band shape as the integral for hopping between the partners of the pair

On the correlation of elemental abundances with kinematics among galactic disk stars

We have performed the detailed analysis of 174 high-resolution spectra of FGK dwarfs obtained with the ELODIE echelle spectrograph at the Observatoire de Haute-Provence. Abundances of Fe, Si and Ni have been determined from equivalent widths under LTE approximation, whereas abundances of Mg have been determined under NLTE approximation using equivalent widths of 4 lines and profiles of 5 lines. Spatial velocities with an accuracy better than 1 km/s, as well as orbits, have been computed for all stars. They have been used to define 2 subsamples kinematically representative of the thin disk and the thick disk in order to highlight their respective properties. A transition occurs at [Fe/H]=-0.3. Stars more metal-rich than this value have a flat distribution with Zmax<1 kpc and sigma_W<20 km/s, and a narrow distribution of [alpha/Fe]. There exist stars in this metallicity regime which cannot belong to the thin disk because of their excentric orbits, neither to the thick disk because of their low scale height. Several thin disk stars are identified down to [Fe/H]=-0.80. Their Mg enrichment is lower than thick disk stars with the same metallicity. We confirm from a larger sample the results of Feltzing et al (2003) and Bensby et al (2003) showing a decrease of [alpha/Fe] with [Fe/H] in the thick disk interpreted as the signature of the SNIa which have progressively enriched the ISM with iron. However our data suggest that the star formation in the thick disk stopped when the enrichment was [Fe/H]=-0.30, [Mg/Fe]=+0.20, [Si/Fe]=+0.17. A vertical gradient in [alpha/Fe] may exist in the thick disk but should be confirmed with a larger sample. Finally we have identified 2 new candidates of the HR1614 moving group.Comment: Accepted in A&A, 16 pages, 14 figure

Barium and Yttrium abundance in intermediate-age and old open clusters

Barium is a neutron capture element, that, in open clusters, is frequently over-abundant with respect to the Iron. A clear explanation for this is still missing. Additionally, its gradient across the Galactic disk is poorly constrained. We measure the abundance of yttrium and barium using the synthetic spectrum method from UVES high-resolution spectra of eight distant open clusters, namely Ruprecht 4, Ruprecht 7, Berkeley 25, Berkeley 73, Berkeley 75, NGC 6192, NGC 6404, and NGC 6583. The barium abundance was estimated using NLTE approximation. We confirm that Barium is indeed over-abundant in most clusters, especially young clusters. Finally, we investigated the trend of yttrium and barium abundances as a function of distance in the Galaxy and ages. Several scenarios for the barium over-abundance are then discussed.Comment: 9 pages, 10 figure

On the subject of the Ba overabundance in the open clusters stars

For eight distant open clusters, namely Ruprecht 4, Ruprecht 7, Berkeley 25, Berkeley 73, Berkeley 75, NGC 6192, NGC 6404, and NGC 6583, we determined the yttrium and barium abundances using the UVES, VLT spectra (ESO, Chile). The stars of one young cluster (Ruprecht 7) demonstrate significant barium overabundance( 3c0.55 dex) that can not be due to the determination error. We have considered the Ba abundance determination errors due to LTE approach, saturation of the lines, synthetic and observed barium line fitting, and the causes of the Ba overabundance associated with the Galactic disc enrichment or the origin of open clusters. Possible explanation for this overabundance can be the origin of n-capture elements enrichment of the clusters (galactic or extragalactic) or additional sources of the Ba production

First Principle Electronic Model for High-Temperature Superconductivity

Using the structural data of the La2CuO4 compound both in the low temperature tetragonal phase and in the isotropic phase we have derived an effective t-J model with hoppings t and superexchange interactions J extended up to fourth and second neareast neighbors respectively. By numerically studying this hamiltonian we have then reproduced the main experimental features of this HTc compound: d-wave superconductivity is stabilized at small but finite doping delta>6% away from the antiferromagnetic region and some evidence of dynamical stripes is found at commensurate filling 1/8.Comment: 4 pages including 4 figures and 2 table