Laboratory hard X-ray photoelectron spectroscopy of La1−x_{1-x}Srx_{x}MnO3_{3}

A laboratory hard X-ray photoelectron spectroscopy (HXPS) system equipped with a monochromatic Cr K$\alpha$ ($h\nu = 5414.7$ eV) X-ray source was applied to an investigation of the core-level electronic structure of La$_{1-x}$Sr$_x$MnO$_3$. No appreciable high binding-energy shoulder in the O $1s$ HXPS spectra were observed while an enhanced low binding-energy shoulder structure in the Mn $2p_{3/2}$ HXPS spectra were observed, both of which are manifestation of high bulk sensitivity. Such high bulk sensitivity enabled us to track the Mn $2p_{3/2}$ shoulder structure in the full range of $x$, giving us a new insight into the binding-energy shift of the Mn $2p_{3/2}$ core level. Comparisons with the results using the conventional laboratory XPS ($h\nu = 1486.6$ eV) as well as those using a synchrotron radiation source ($h\nu = 7939.9$ eV) demonstrate that HXPS is a powerful and convenient tool to analyze the bulk electronic structure of a host of different compounds.Comment: 5 pages, 6 figure

Homogeneous sets, clique-separators, critical graphs, and optimal χ\chi-binding functions

Given a set $\mathcal{H}$ of graphs, let $f_\mathcal{H}^\star\colon \mathbb{N}_{>0}\to \mathbb{N}_{>0}$ be the optimal $\chi$-binding function of the class of $\mathcal{H}$-free graphs, that is, $$f_\mathcal{H}^\star(\omega)=\max\{\chi(G): G\text{ is } \mathcal{H}\text{-free, } \omega(G)=\omega\}.$$ In this paper, we combine the two decomposition methods by homogeneous sets and clique-separators in order to determine optimal $\chi$-binding functions for subclasses of $P_5$-free graphs and of $(C_5,C_7,\ldots)$-free graphs. In particular, we prove the following for each $\omega\geq 1$: (i) $\ f_{\{P_5,banner\}}^\star(\omega)=f_{3K_1}^\star(\omega)\in \Theta(\omega^2/\log(\omega)),$ (ii) $\ f_{\{P_5,co-banner\}}^\star(\omega)=f^\star_{\{2K_2\}}(\omega)\in\mathcal{O}(\omega^2),$(iii)$\ f_{\{C_5,C_7,\ldots,banner\}}^\star(\omega)=f^\star_{\{C_5,3K_1\}}(\omega)\notin \mathcal{O}(\omega),$and (iv)$\ f_{\{P_5,C_4\}}^\star(\omega)=\lceil(5\omega-1)/4\rceil.$We also characterise, for each of our considered graph classes, all graphs$G$with$\chi(G)>\chi(G-u)$for each$u\in V(G)$. From these structural results, we can prove Reed's conjecture -- relating chromatic number, clique number, and maximum degree of a graph -- for$(P_5,banner)$-free graphs

Adsorption and diffusion of H2O molecule on the Be(0001) surface: A density-functional theory study

Using first-principles calculations, we systematically study the adsorption behavior of a single molecular H$_{2}$O on the Be(0001) surface. We find that the favored molecular adsorption site is the top site with an adsorption energy of about 0.3 eV, together with the detailed electronic structure analysis, suggesting a weak binding strength of the H$_{2}$O/Be(0001) surface. The adsorption interaction is mainly contributed by the overlapping between the $s$ and $p_{z}$ states of the top-layer Be atom and the molecular orbitals 1$b_{1}$ and 3$a_{1}$ of H$_{2}$O. The activation energy for H$_{2}$O diffusion on the surface is about 0.3 eV. Meanwhile, our study indicates that no dissociation state exists for the H$_{2}$O/Be(0001) surface.Comment: 10 pages, 4 figure

Molecular modeling of an antigenic complex between a viral peptide and a class I major histocompatibility glycoprotein

Computer simulation of the conformations of short antigenic peptides (&lo residues) either free or bound to their receptor, the major histocompatibility complex (MHC)- encoded glycoprotein H-2 Ld, was employed to explain experimentally determined differences in the antigenic activities within a set of related peptides. Starting for each sequence from the most probable conformations disclosed by a pattern-recognition technique, several energyminimized structures were subjected to molecular dynamics simulations (MD) either in vacuo or solvated by water molecules. Notably, antigenic potencies were found to correlate to the peptides propensity to form and maintain an overall a-helical conformation through regular i,i + 4 hydrogen bonds. Accordingly, less active or inactive peptides showed a strong tendency to form i,i+3 hydrogen bonds at their Nterminal end. Experimental data documented that the C-terminal residue is critical for interaction of the peptide with H-2 Ld. This finding could be satisfactorily explained by a 3-D Q.S.A.R. analysis postulating interactions between ligand and receptor by hydrophobic forces. A 3-D model is proposed for the complex between a high-affinity nonapeptide and the H- 2 Ld receptor. First, the H-2 Ld molecule was built from X-ray coordinates of two homologous proteins: HLA-A2 and HLA-Aw68, energyminimized and studied by MD simulations. With HLA-A2 as template, the only realistic simulation was achieved for a solvated model with minor deviations of the MD mean structure from the X-ray conformation. Water simulation of the H-2 Ld protein in complex with the antigenic nonapeptide was then achieved with the template- derived optimal parameters. The bound peptide retains mainly its a-helical conformation and binds to hydrophobic residues of H-2 Ld that correspond to highly polymorphic positions of MHC proteins. The orientation of the nonapeptide in the binding cleft is in accordance with the experimentally determined distribution of its MHC receptor-binding residues (agretope residues). Thus, computer simulation was successfully employed to explain functional data and predicts a-helical conformation for the bound peptid

The Okamoto-Nolen-Schiffer anomaly without rho-omega mixing

We examine the effect of isospin-violating meson-nucleon coupling constants and of $\pi$-$\eta$ mixing on the binding-energy differences of mirror nuclei in a model that possesses no contribution from $\rho$-$\omega$ mixing. The ${}^{3}$He-${}^{3}$H binding-energy difference is computed in a nonrelativistic approach using a realistic wave function. We find the ${}^{3}$He-${}^{3}$H binding-energy difference very sensitive to the short-distance behavior of the nucleon-nucleon potential. We conclude that for the typically hard Bonn form factors such models can not account for the observed binding-energy difference in the three-nucleon system. For the medium-mass region (A=15--41) the binding-energy differences of mirror nuclei are computed using a relativistic mean-field approximation to the Walecka model. We obtain large binding-energy differences---of the order of several hundred keV---arising from the pseudoscalar sector. Two effects are primarily responsible for this new finding: a) the inclusion of isospin breaking in the pion-nucleon coupling constant, and b) the in-medium enhancement of the small components of the bound-state wave functions. We look for off-shell ambiguities in these results and find them to be large.Comment: 19 LaTeX pages and 2 postscript figures. Revisions/additions: Manuscript now includes a treatment of the binding-energy difference in the three-nucleon system as well as a study of possible off-shell ambiguities in the binding-energy differences of (A=15-41) mirror nucle

Phenomenological Lambda-Nuclear Interactions

Variational Monte Carlo calculations for ${_{\Lambda}^4}H$ (ground and excited states) and ${_{\Lambda}^5}He$ are performed to decipher information on ${\Lambda}$-nuclear interactions. Appropriate operatorial nuclear and ${\Lambda}$-nuclear correlations have been incorporated to minimize the expectation values of the energies. We use the Argonne $\upsilon_{18}$ two-body NN along with the Urbana IX three-body NNN interactions. The study demonstrates that a large part of the splitting energy in ${_{\Lambda}^4}H$ ($0^+-1^+$) is due to the three-body ${\Lambda}$ NN forces. $_{\Lambda}^{17}O$ hypernucleus is analyzed using the {\it s}-shell results. $\Lambda$ binding to nuclear matter is calculated within the variational framework using the Fermi-Hypernetted-Chain technique. There is a need to correctly incorporate the three-body ${\Lambda}$ NN correlations for $\Lambda$ binding to nuclear matter.Comment: 18 pages (TeX), 2 figure