33 research outputs found
Polar surface engineering in ultra-thin MgO(111)/Ag(111) -- possibility of metal-insulator transition and magnetism
A recent report [Kiguchi {\it et al.}, Phys. Rev. B {\bf 68}, 115402 (2003)]
that the (111) surface of 5 MgO layers grown epitaxially on Ag(111) becomes
metallic to reduce the electric dipole moment raises a question of what will
happen when we have fewer MgO layers. Here we have revealed, first
experimentally with electron energy-loss spectroscopy, that MgO(111) remains
metallic even when one-layer thick, and theoretically with the density
functional theory that the metallization should depend on the nature of the
substrate. We further show, with a spin-density functional calculation, that a
ferromagnetic instability may be expected for thicker films.Comment: 5 pages, 7 figure
Metal-Induced Gap States at Well Defined Alkali-Halide/Metal Interfaces
In order to search for states specific to insulator/metal interfaces, we have
studied epitaxially grown interfaces with element-selective near edge X-ray
absorption fine structure (NEXAFS). An extra peak is observed below the bulk
edge onset for LiCl films on Cu and Ag substrates. The nature of chemical bonds
as probed by X-ray photoemission spectroscopy and Auger electron spectroscopy
remains unchanged, so we regard this as evidence for metal-induced gap
states(MIGS) formed by the proximity to a metal, rather than local bonds at the
interface. The dependence on the film thickness shows that the MIGS are as thin
as one monolayer. An ab initio electronic structure calculation supports the
existence of the MIGS that are strongly localized at the interface.Comment: 4 pages, 5 figures, to be published in Phys. Rev. Let
Calculation of absolute free energy of binding for theophylline and its analogs to RNA aptamer using nonequilibrium work values
The massively parallel computation of absolute binding free energy with a
well-equilibrated system (MP-CAFEE) has been developed [H. Fujitani, Y. Tanida,
M. Ito, G. Jayachandran, C. D. Snow, M. R. Shirts, E. J. Sorin, and V. S.
Pande, J. Chem. Phys. , 084108 (2005)]. As an application, we
perform the binding affinity calculations of six theophylline-related ligands
with RNA aptamer. Basically, our method is applicable when using many compute
nodes to accelerate simulations, thus a parallel computing system is also
developed. To further reduce the computational cost, the adequate non-uniform
intervals of coupling constant , connecting two equilibrium states,
namely bound and unbound, are determined. The absolute binding energies thus obtained have effective linear relation between the computed and
experimental values. If the results of two other different methods are
compared, thermodynamic integration (TI) and molecular mechanics
Poisson-Boltzmann surface area (MM-PBSA) by the paper of Gouda [H.
Gouda, I. D. Kuntz, D. A. Case, and P. A. Kollman, Biopolymers , 16
(2003)], the predictive accuracy of the relative values is
almost comparable to that of TI: the correlation coefficients (R) obtained are
0.99 (this work), 0.97 (TI), and 0.78 (MM-PBSA). On absolute binding energies
meanwhile, a constant energy shift of -7 kcal/mol against the
experimental values is evident. To solve this problem, several presumable
reasons are investigated.Comment: 23 pages including 6 figure
Image-potential band-gap narrowing at a metal/semiconductor interface
GW approximation is used to systematically revisit the image-potential
band-gap narrowing at metal/semiconductor interfaces proposed by Inkson in the
1970's. Here we have questioned how the narrowing as calculated from
quasi-particle energy spectra for the jellium/Si interface depends on of
the jellium. The gap narrowing is found to only weakly depend on (i.e.,
narrowing eV even for a large . Hence we can turn to
smaller polarizability in the semiconductor side as an important factor in
looking for larger narrowing.Comment: 6 pages, 7 figure
Electronic properties of metal induced gap states at insulator/metal interfaces -- dependence on the alkali halide and the possibility of excitonic mechanism of superconductivity
Motivated from the experimental observation of metal induced gap states
(MIGS) at insulator/metal interfaces by Kiguchi {\it et al.} [Phys. Rev. Lett.
{\bf 90}, 196803 (2003)], we have theoretically investigated the electronic
properties of MIGS at interfaces between various alkali halides and a metal
represented by a jellium with the first-principles density functional method.
We have found that, on top of the usual evanescent state, MIGS generally have a
long tail on halogen sites with a -like character, whose penetration depth
() is as large as half the lattice constant of bulk alkali halides.
This implies that , while little dependent on the carrier density in
the jellium, is dominated by the lattice constant (hence by energy gap) of the
alkali halide, where . We also propose a possibility of the MIGS working favorably for the
exciton-mediated superconductivity.Comment: 7 pages, 9 figure
Metal-induced gap states in epitaxial organic-insulator/metal interfaces
We have shown, both experimentally and theoretically, that the metal-induced gap states (MIGS) can exist in epitaxially grown organic insulator/metal interfaces. The experiment is done for alkane/Cu(001) with an element-selective near edge x-ray absorption fine structure (NEXAFS), which exhibits a prepeak indicative of MIGS. An ab initio electronic structure calculation supports the existence of the MIGS. When the Cu substrate is replaced with Ni, an interface magnetism may be possible with a carrier doping