Imaging Localized States in Graphene Nanostructures

Probing techniques with spatial resolution have the potential to lead to a better understanding of the microscopic physical processes and to novel routes for manipulating nanostructures. We present scanning-gate images of a graphene quantum dot which is coupled to source and drain via two constrictions. We image and locate conductance resonances of the quantum dot in the Coulomb-blockade regime as well as resonances of localized states in the constrictions in real space.Comment: 18 pages, 7 figure

Joseph E. Johnson Author Of Frontier News, Promotion, And Progress

News editors in the nineteenth century have been viewed as paramount figures in establishing the community life of small towns. While opinions differ over exactly how much influence to ascribe to these newspersons, most historians agree that they held a prominent place. Political parties, as a primary source of funding for newspapers at the time, ensured that reporting provided a favorable spin on things. News editors of the frontier usually mixed together short items on travel, neighboring settlements, politics, editorial commentary, and advertisements, driven by patron political interests and the desire for community growth. In this process, editors functioned as open promoters, not expected to restrain their views with the lens of impartiality—nor expecting in return that readers would question the bright future that they saw as destiny. This brought newspapers into a kind of alliance with railroad companies as both sides campaigned to increase the population in support of their own economic and social interests, with news editors doing the promoting and railroads bringing people and trade prospects. But all of this depended on the cultural backdrop of “manifest destiny” at the time, a composite of American dreams for progress generally equated with the advancement of white settlement westward. Joseph Ellis (J. E.) Johnson fit this profile of editor-promoter, but he also expanded it, often placing greater stock in advertisements and community observations than in political issues. When he took the reins as editor of the Council Bluffs Bugle in 1853, he shifted the paper’s emphasis significantly toward local business, greatly increasing the advertising space. While the Bugle maintained a definite orientation toward the Democratic Party and its politics in the region, Johnson’s championing of party interests does not distinguish him from his contemporaries. Many other papers relied heavily on anonymous descriptions in praise of their town’s surrounding land, but Johnson more often filled this role himself, traveling regularly and crafting various romantic portrayals of the area

Joseph E. Johnson Author Of Frontier News, Promotion, And Progress

News editors in the nineteenth century have been viewed as paramount figures in establishing the community life of small towns. While opinions differ over exactly how much influence to ascribe to these newspersons, most historians agree that they held a prominent place. Political parties, as a primary source of funding for newspapers at the time, ensured that reporting provided a favorable spin on things. News editors of the frontier usually mixed together short items on travel, neighboring settlements, politics, editorial commentary, and advertisements, driven by patron political interests and the desire for community growth. In this process, editors functioned as open promoters, not expected to restrain their views with the lens of impartiality—nor expecting in return that readers would question the bright future that they saw as destiny. This brought newspapers into a kind of alliance with railroad companies as both sides campaigned to increase the population in support of their own economic and social interests, with news editors doing the promoting and railroads bringing people and trade prospects. But all of this depended on the cultural backdrop of “manifest destiny” at the time, a composite of American dreams for progress generally equated with the advancement of white settlement westward. Joseph Ellis (J. E.) Johnson fit this profile of editor-promoter, but he also expanded it, often placing greater stock in advertisements and community observations than in political issues. When he took the reins as editor of the Council Bluffs Bugle in 1853, he shifted the paper’s emphasis significantly toward local business, greatly increasing the advertising space. While the Bugle maintained a definite orientation toward the Democratic Party and its politics in the region, Johnson’s championing of party interests does not distinguish him from his contemporaries. Many other papers relied heavily on anonymous descriptions in praise of their town’s surrounding land, but Johnson more often filled this role himself, traveling regularly and crafting various romantic portrayals of the area

Electronic structure studies of BaFe2As2 by angle-resolved photoemission spectroscopy

We report high resolution angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure of BaFe$_2$As$_2$, which is one of the parent compounds of the Fe-pnictide superconductors. ARPES measurements have been performed at 20 K and 300 K, corresponding to the orthorhombic antiferromagnetic phase and the tetragonal paramagnetic phase, respectively. Photon energies between 30 and 175 eV and polarizations parallel and perpendicular to the scattering plane have been used. Measurements of the Fermi surface yield two hole pockets at the $\Gamma$-point and an electron pocket at each of the X-points. The topology of the pockets has been concluded from the dispersion of the spectral weight as a function of binding energy. Changes in the spectral weight at the Fermi level upon variation of the polarization of the incident photons yield important information on the orbital character of the states near the Fermi level. No differences in the electronic structure between 20 and 300 K could be resolved. The results are compared with density functional theory band structure calculations for the tetragonal paramagnetic phase.Comment: 11 pages, 5 figure

Photoinduced Br Desorption from CsBr Thin Films Grown on Cu(100)

Thin films of CsBr deposited onto metals such as copper are potential photocathode materials for light sources and other applications. We investigate desorption dynamics of Br atoms from CsBr films grown on insulator (KBr, LiF) and metal (Cu) substrates induced by sub-bandgap 6.4 eV laser pulses. The experimental results demonstrate that the peak kinetic energy of Br atoms desorbed from CsBr/Cu films is much lower than that for the hyperthermal desorption from CsBr/LiF films. Kelvin probe measurements indicate negative charge at the surface following Br desorption from CsBr/Cu films. Our ab initio calculations of excitons at CsBr surfaces demonstrate that this behavior can be explained by an exciton model of desorption including electron trapping at the CsBr surface. Trapped negative charges reduce the energy of surface excitons available for Br desorption. We examine the electron-trapping characteristics of low-coordinated sites at the surface, in particular, divacancies and kink sites. We also provide a model of cation desorption caused by Franck-Hertz excitation of F centers at the surface in the course of irradiation of CsBr/Cu films. These results provide new insights into the mechanisms of photoinduced structural evolution of alkali halide films on metal substrates and activation of metal photocathodes coated with CsBr

Dirac Spectrum in Piecewise Constant One-Dimensional Potentials

We study the electronic states of graphene in piecewise constant potentials using the continuum Dirac equation appropriate at low energies, and a transfer matrix method. For superlattice potentials, we identify patterns of induced Dirac points which are present throughout the band structure, and verify for the special case of a particle-hole symmetric potential their presence at zero energy. We also consider the cases of a single trench and a p-n junction embedded in neutral graphene, which are shown to support confined states. An analysis of conductance across these structures demonstrates that these confined states create quantum interference effects which evidence their presence.Comment: 10 pages, 12 figures, additional references adde

J/Psi Suppression in Heavy Ion Collisions at the CERN SPS

We reexamine the production of J/Psi and other charmonium states for a variety of target-projectile choices at the SPS. For this study we use a newly constructed cascade code LUCIFER II, which yields acceptable descriptions of both hard and soft processes, specifically Drell-Yan and hidden charm production, and soft energy loss and meson production, at the SPS. Glauber calculations of other authors are redone, and compared directly to the cascade results. The modeling of the charmonium states differs from that of earlier workers in its unified treatment of the hidden charm meson spectrum, which is introduced from the outset as a set of coupled states. The result is a description of the NA38 and NA50 data in terms of a conventional hadronic picture. The apparently anomalous suppression found in the most massive Pb+Pb system arises from three sources: destruction in the initial nucleon-nucleon cascade, use of coupled channels to exploit the larger breakup in the less bound Chi and Psi' states, and comover interaction in the final low energy phase.Comment: 36 pages (15 figures

Progress in the determination of the J/ψ−πJ/\psi-\pi cross section

Improving previous calculations, we compute the $J/\psi \pi\to {charmed mesons}$ cross section using QCD sum rules. Our sum rules for the $J/\psi \pi\to \bar{D} D^*$, $D \bar{D}^*$, ${\bar D}^* D^*$ and ${\bar D} D$ hadronic matrix elements are constructed by using vaccum-pion correlation functions, and we work up to twist-4 in the soft-pion limit. Our results suggest that, using meson exchange models is perfectly acceptable, provided that they include form factors and that they respect chiral symmetry. After doing a thermal average we get $\sim 0.3$ mb at T=150\MeV.Comment: 22 pages, RevTeX4 including 7 figures in ps file

On the determination of the Fermi surface in high-Tc superconductors by angle-resolved photoemission spectroscopy

We study the normal state electronic excitations probed by angle resolved photoemission spectroscopy (ARPES) in Bi2201 and Bi2212. Our main goal is to establish explicit criteria for determining the Fermi surface from ARPES data on strongly interacting systems where sharply defined quasiparticles do not exist and the dispersion is very weak in parts of the Brillouin zone. Additional complications arise from strong matrix element variations within the zone. We present detailed results as a function of incident photon energy, and show simple experimental tests to distinguish between an intensity drop due to matrix element effects and spectral weight loss due to a Fermi crossing. We reiterate the use of polarization selection rules in disentangling the effect of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite all the complications, the Fermi surface can be determined unambiguously: it is a single large hole barrel centered about (pi,pi) in both materials.Comment: Expanded discussion of symmetrization method in Section 5, figures remain the sam