5,690 research outputs found
Linear Chains of Styrene and Methyl-Styrene Molecules and their Heterojunctions on Silicon: Theory and Experiment
We report on the synthesis, STM imaging and theoretical studies of the
structure, electronic structure and transport properties of linear chains of
styrene and methyl-styrene molecules and their heterojunctions on
hydrogen-terminated dimerized silicon (001) surfaces. The theory presented here
accounts for the essential features of the experimental STM data including the
nature of the corrugation observed along the molecular chains and the
pronounced changes in the contrast between the styrene and methyl-styrene parts
of the molecular chains that are observed as the applied bias is varied. The
observed evolution with applied bias of the STM profiles near the ends of the
molecular chains is also explained. Calculations are also presented of electron
transport along styrene linear chains adsorbed on the silicon surface at
energies in the vicinity of the molecular HOMO and LUMO levels. For short
styrene chains this lateral transport is found to be due primarily to direct
electron transmission from molecule to molecule rather than through the silicon
substrate, especially in the molecular LUMO band. Differences between the
calculated position-dependences of the STM current around a junction of styrene
and methyl-styrene molecular chains under positive and negative tip bias are
related to the nature of lateral electron transmission along the molecular
chains and to the formation in the LUMO band of an electronic state localized
around the heterojunction.Comment: 17 pages plus 11 figures. To appear in Physical Review
Interference Effects, Time Reversal Violation and Search for New Physics in Hadronic Weak Decays
We propose some methods for studying hadronic sequential two-body decays
involving more spinning particles. It relies on the analysis of T-odd and
T-even asymmetries, which are related to interference terms. The latter
asymmetries turn out to be as useful as the former ones in inferring time
reversal violating observables; these in turn may be sensitive, under some
particular conditions, to possible contributions beyond the standard model. Our
main result is that one can extract such observables even after integrating the
differential decay width over almost all of the available angles. Moreover we
find that the correlations based exclusively on momenta are quite general,
since they provide as much information as those involving one or more spins. We
generalize some methods already proposed in the literature for particular decay
channels, but we also pick out a new kind of time reversal violating
observables. Our analysis could be applied, for example, to data of LHCb
experiment.Comment: 35 page
Origin of atomic clusters during ion sputtering
Previous studies have shown that the size distributions of small clusters ( n<=40 n = number of atoms/cluster) generated by sputtering obey an inverse power law with an exponent between -8 and -4. Here we report electron microscopy studies of the size distributions of larger clusters ( n>=500) sputtered by high-energy ion impacts. These new measurements also yield an inverse power law, but one with an exponent of -2 and one independent of sputtering yield, indicating that the large clusters are produced when shock waves, generated by subsurface displacement cascades, ablate the surface
Performance Characteristics of a Cluster of 5-kW Laboratory Hall Thrusters
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76117/1/AIAA-19752-159.pd
Bound-to-bound and bound-to-continuum optical transitions in combined quantum dot - superlattice systems
By combining band gap engineering with the self-organized growth of quantum
dots, we present a scheme of adjusting the mid-infrared absorption properties
to desired energy transitions in quantum dot based photodetectors. Embedding
the self organized InAs quantum dots into an AlAs/GaAs superlattice enables us
to tune the optical transition energy by changing the superlattice period as
well as by changing the growth conditions of the dots. Using a one band
envelope function framework we are able, in a fully three dimensional
calculation, to predict the photocurrent spectra of these devices as well as
their polarization properties. The calculations further predict a strong impact
of the dots on the superlattices minibands. The impact of vertical dot
alignment or misalignment on the absorption properties of this dot/superlattice
structure is investigated. The observed photocurrent spectra of vertically
coupled quantum dot stacks show very good agreement with the calculations.In
these experiments, vertically coupled quantum dot stacks show the best
performance in the desired photodetector application.Comment: 8 pages, 10 figures, submitted to PR
Strongly Non-Equilibrium Bose-Einstein Condensation in a Trapped Gas
We present a qualitative (and quantitative, at the level of estimates)
analysis of the ordering kinetics in a strongly non-equilibrium state of a
weakly interacting Bose gas, trapped with an external potential. At certain
conditions, the ordering process is predicted to be even more rich than in the
homogeneous case. Like in the homogeneous case, the most characteristic feature
of the full-scale non-equilibrium process is the formation of superfluid
turbulence.Comment: 4 pages, revtex, no figures. Submitted to PR
Self-organized Vortex State in Two-dimensional Dictyostelium Dynamics
We present results of experiments on the dynamics of Dictyostelium discoideum
in a novel set-up which constraints cell motion to a plane. After aggregation,
the amoebae collect into round ''pancake" structures in which the cells rotate
around the center of the pancake. This vortex state persists for many hours and
we have explicitly verified that the motion is not due to rotating waves of
cAMP. To provide an alternative mechanism for the self-organization of the
Dictyostelium cells, we have developed a new model of the dynamics of
self-propelled deformable objects. In this model, we show that cohesive energy
between the cells, together with a coupling between the self-generated
propulsive force and the cell's configuration produces a self-organized vortex
state. The angular velocity profiles of the experiment and of the model are
qualitatively similar. The mechanism for self-organization reported here can
possibly explain similar vortex states in other biological systems.Comment: submitted to PRL; revised version dated 3/8/9
Galactic Abundances: Report of Working Group 3
We summarize the various methods and their limitations and strengths to derive galactic abundances from in-situ and remote-sensing measurements, both from ground-based observations and from instruments in space. Because galactic abundances evolve in time and space it is important to obtain information with a variety of different methods covering different regions from the Very Local Insterstellar Medium (VLISM) to the distant galaxy, and different times throughout the evolution of the galaxy. We discuss the study of the present-day VLISM with neutral gas, pickup ions, and Anomalous Cosmic Rays, the study of the local interstellar medium (ISM) at distances <1.5 kpc utilizing absorption line measurements in H I clouds, and the study of galactic cosmic rays, sampling contemporary (~15 Myr) sources in the local ISM within a few kiloparsec of the solar system. Solar system abundances, derived from solar abundances and meteorite studies are discussed in several other chapters of this volume. They provide samples of matter from the ISM from the time of solar system format ion, about 4.5 Gyr ago. The evolution of galactic abundances on longer time scales is discussed in the context of nuclear synthesis in the various contributing stellar objects
Torque magnetometry on single-crystal high temperature superconductors near the critical temperature: a scaling approach
Angular-dependent magnetic torque measurements performed near the critical
temperature on single crystals of HgBa_{2}CuO_{4+y}, La_{2-x}Sr{x}CuO_{4}, and
YBa_{2}Cu_{3}O_{6.93} are scaled, following the 3D XY model, in order to
determine the scaling function dG^{\pm}(z)/dz which describes the universal
critical properties near T_{c}. A systematic shift of the scaling function with
increasing effective mass anisotropy \gamma = (m_{ab}*/m_{c}*)^{1/2} is
observed, which may be understood in terms of a 3D-2D crossover. Further
evidence for a 3D-2D crossover is found from temperature-dependent torque
measurements carried out in different magnetic fields at different field
orientations \delta, which show a quasi 2D "crossing region'' (M*,T*). The
occurrence of this "crossing phenomenon'' is explained in a phenomenological
way from the weak z dependence of the scaling function around a value z = z*.
The "crossing'' temperature T* is found to be angular-dependent. Torque
measurements above T_{c} reveal that fluctuations are strongly enhanced in the
underdoped regime where the anisotropy is large, whereas they are less
important in the overdoped regime.Comment: 9 pages, 10 figures, submitted to PR
- …