Ab initio study of semiconducting carbon nanotubes adsorbed on the Si(100) surface: diameter- and registration-dependent atomic configurations and electronic properties

We present a first-principles study of semiconducting carbon nanotubes adsorbed on the unpassivated Si(100) surface. We have found metallicity for the combined system caused by n-doping of the silicon slab representing the surface by the SWNT. We confirm this metallicity for nanotubes of different diameters and chiral angles, and find the effect to be independent of the orientation of the nanotubes on the surface. We also present adsorption energetics and configurations which show semiconducting SWNTs farther apart from the surface and transferring less charge, in comparison with metallic SWNTs of similar diameter.Comment: Replaces old (Jan 2006) version; more supporting material. 11 pages, 8 figures, 7 table

Carbon nanotubes on partially depassivated n-doped Si(100)-(2x1):H substrates

We present a study on the mechanical configuration and the electronic properties of semiconducting carbon nanotubes supported by partially depassivated silicon substrates, as inferred from topographic and spectroscopic data acquired with a room-temperature ultrahigh vacuum scanning tunneling microscope and density-functional theory calculations. A mechanical distortion and doping for semiconducting carbon nanotubes on Si(100)-(2x1):H with hydrogen-depassivated stripes up to 100 Angstrom wide are ascertained from both experiment and theory. The results presented here point towards novel and local functionalities of nanotube-semiconductor interfaces.Comment: 5 pages, 3 Figures, 1 Table. Accepted for publication in Physical Review

A high-reflectivity, ambient-stable graphene mirror for neutral atomic and molecular beams

The following article appeared in Applied Physics Letters Volume 99, Issue 21, 21 November 2011, Article number 211907 and may be found at http://scitation.aip.org/content/aip/journal/apl/99/21/10.1063/1.3663866We report a He and H2 diffraction study of graphene-terminated Ru(0001) thin films grown epitaxially on c-axis sapphire. Even for samples exposed for several weeks to ambient conditions, brief annealing in ultrahigh vacuum restored extraordinarily high specular reflectivities for He and H 2 beams (23 and 7 of the incident beam, respectively). The quality of the angular distributions recorded with both probes exceeds the one obtained from in-situ prepared graphene on Ru(0001) single crystals. Our results for graphene-terminated Ru thin films represent a significant step toward ambient tolerant, high-reflectivity curved surface mirrors for He-atom microscopyWork performed in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which issupported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Work supported by the Ministerio de Educacion y Ciencia through projects CONSOLIDER-INGENIO 2010 on Molecular Nanoscience (CSD 2007-00010), FIS2010-18847, and byComunidad de Madrid through the program NANOBIOMAGNET (S2009/MAT-1726)

Die Bedeutung einer Ausfallbedrohtheit von Versicherungskontrakten - ein Beitrag zur Behavioral Insurance

Kahneman/Tversky 1979 haben das theoretische Konstrukt der Probabilistic Insurance Kontrakte in die Literatur eingeführt. Hiermit werden Versicherungsverträge bezeichnet, deren Erfüllung im Leistungsfalle aufgrund einer möglichen Insolvenz des Versicherungsunternehmens nicht gewährleistet ist. In Ausweitung einer Studie von Wakker/Thaler/Tversky 1997 wird in der vorliegenden Arbeit eine experimentelle Untersuchung durchgeführt, wobei die Zahlungsbereitschaft potentieller Versicherungsnehmer in Abhängigkeit des Ratings des den Versicherungskontrakt anbietenden Unternehmens festgestellt wird. Dabei zeigt sich, daß diese ausfallbedrohte Versicherungsprodukte relativ zu ausfallfreien Verträgen mit erheblichen Prämienabschlägen sanktionieren. Der Preisabschlag nimmt dabei mit sinkender Unternehmensbonität (erhöhter Ausfallgefahr) zu. Die Befragungsergebnisse zeigen zudem das neuartige Phänomen, daß mit zunehmender Ausfallbedrohtheit immer weniger Personen bereit sind, ausfallbedrohte Versicherungsprodukte überhaupt zu akzeptieren. Schließlich werden Schlußfolgerungen für die Steuerung von Versicherungsunternehmen diskutiert

Ab initio Green's function formalism for band structures

Using the Green's function formalism, an ab initio theory for band structures of crystals is derived starting from the Hartree-Fock approximation. It is based on the algebraic diagrammatic construction scheme for the self-energy which is formulated for crystal orbitals (CO-ADC). In this approach, the poles of the Green's function are determined by solving a suitable Hermitian eigenvalue problem. The method is not only applicable to the outer valence and conduction bands, it is also stable for inner valence bands where strong electron correlations are effective. The key to the proposed scheme is to evaluate the self-energy in terms of Wannier orbitals before transforming it to a crystal momentum representation. Exploiting the fact that electron correlations are mainly local, one can truncate the lattice summations by an appropriate configuration selection scheme. This yields a flat configuration space; i.e., its size scales only linearly with the number of atoms per unit cell for large systems and, under certain conditions, the computational effort to determine band structures also scales linearly. As a first application of the new formalism, a lithium fluoride crystal has been chosen. A minimal basis set description is studied, and a satisfactory agreement with previous theoretical and experimental results for the fundamental band gap and the width of the F 2p valence band complex is obtained.Comment: 20 pages, 3 figures, 1 table, RevTeX4, new section on lithium fluorid

Evidence for electronically-driven ferroelectricity in the family of strongly correlated dimerized BEDT-TTF molecular conductors

By applying measurements of the dielectric constants and relative length changes to the dimerized molecular conductor $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl, we provide evidence for order-disorder type electronic ferroelectricity which is driven by charge order within the (BEDT-TTF)$_2$ dimers and stabilized by a coupling to the anions. According to our density functional theory calculations, this material is characterized by a moderate strength of dimerization. This system thus bridges the gap between strongly dimerized materials, often approximated as dimer-Mott systems at 1/2 filling, and non- or weakly dimerized systems at 1/4 filling exhibiting charge order. Our results indicate that intra-dimer charge degrees of freedom are of particular importance in correlated $\kappa$-(BEDT-TTF)$_2$X salts and can create novel states, such as electronically-driven multiferroicity or charge-order-induced quasi-1D spin liquids.Comment: 6 pages, 4 figures + Supplementary Information (8 pages, 8 figures

1S and MSbar Bottom Quark Masses from Upsilon Sum Rules

The bottom quark 1S mass, $M_b^{1S}$, is determined using sum rules which relate the masses and the electronic decay widths of the $\Upsilon$ mesons to moments of the vacuum polarization function. The 1S mass is defined as half the perturbative mass of a fictitious ${}^3S_1$ bottom-antibottom quark bound state, and is free of the ambiguity of order $\Lambda_{QCD}$ which plagues the pole mass definition. Compared to an earlier analysis by the same author, which had been carried out in the pole mass scheme, the 1S mass scheme leads to a much better behaved perturbative series of the moments, smaller uncertainties in the mass extraction and to a reduced correlation of the mass and the strong coupling. We arrive at $M_b^{1S}=4.71\pm 0.03$ GeV taking $\alpha_s(M_Z)=0.118\pm 0.004$ as an input. From that we determine the $\bar{MS}$ mass as $\bar m_b(\bar m_b) = 4.20 \pm 0.06$ GeV. The error in $\bar m_b(\bar m_b)$ can be reduced if the three-loop corrections to the relation of pole and $\bar{MS}$ mass are known and if the error in the strong coupling is decreased.Comment: 20 pages, latex; numbers in Tabs. 2,3,4 corrected, a reference and a comment on the fitting procedure added, typos in Eqs. 2 and 23 eliminate

Dilatonic current-carrying cosmic strings

We investigate the nature of ordinary cosmic vortices in some scalar-tensor extensions of gravity. We find solutions for which the dilaton field condenses inside the vortex core. These solutions can be interpreted as raising the degeneracy between the eigenvalues of the effective stress-energy tensor, namely the energy per unit length U and the tension T, by picking a privileged spacelike or timelike coordinate direction; in the latter case, a phase frequency threshold occurs that is similar to what is found in ordinary neutral current-carrying cosmic strings. We find that the dilaton contribution for the equation of state, once averaged along the string worldsheet, vanishes, leading to an effective Nambu-Goto behavior of such a string network in cosmology, i.e. on very large scales. It is found also that on small scales, the energy per unit length and tension depend on the string internal coordinates in such a way as to permit the existence of centrifugally supported equilibrium configuration, also known as vortons, whose stability, depending on the very short distance (unknown) physics, can lead to catastrophic consequences on the evolution of the Universe.Comment: 10 pages, ReVTeX, 2 figures, minor typos corrected. This version to appear in Phys. Rev.

Another Source of Baryons in BB Meson Decays

It is usually assumed that the production of baryons in $B$ meson decays is induced primarily by the quark level process $b\to c\bar ud$, where the charm quark hadronizes into a charmed baryon. With this assumption, the $\Lambda_c$ momentum spectrum would indicate that the transition $B\to\Lambda_c X$ is dominated by multi-body $B$ decays. However, a closer examination of the momentum spectrum reveals that the mass $m_X$ against which the $\Lambda_c$ is recoiling almost always satisfies m_X\agt m_{\Xi_c}. This fact leads us to examine the hypothesis that the production of charmed baryons in $B$ decays is in fact dominated by the underlying transition $b\to c\bar cs$, and is seen primarily in modes with two charmed baryons in the final state. We propose a number of tests of this hypothesis. If this mechanism is indeed important in baryon production, then there are interesting consequences and applications, including potentially important implications for the ``charm deficit'' in $B$ decays.Comment: 9 pages, 3 uuencoded figures included, uses REVTeX, preprint nos. CALT-68-1935, FERMILAB-PUB-94-132-T, JHU-TIPAC-940006, UCSD/PTH 94-0