149 research outputs found
Phonons in SrTiO3 analyzed by difference bond-length spectrum
Phonons change remarkable the interatomic bond length in solids and this work
suggest a novel method how this behavior can be displayed and analyzed. The
bond-length spectrum is plotted for each of the different atomic bonding types.
When comparing the bond-length to an un-deformed crystal by the so-called
difference bond length spectrum, the effect of phonons is clearly visible. The
Perovskite lattice of SrTiO3 is used as an example and several lattice
vibration modes are applied in a frozen phonon calculation in a 2x2x2
supercell. Ab-initio DFT simulations using the Vasp software were performed to
calculate the density of states. The results show the important finding
reported here first, that certain phonon interactions with shorter Ti-O bonds
decrease the band gap, while changes in the Sr-Ti bond length enlarge the band
gap.Comment: 6 pages, 6 figure
Optimal Consensus Set for Digital Line Fitting
International audienceThis paper presents a new method for fitting a digital line to a given set of points in a 2D image in the presence of noise by maximizing the number of inliers, namely the consensus set. By using a digital line model instead of a continuous one, we show that we can generate all possible consensus sets for digital line fitting. We present a deterministic algorithm that efficiently searches the optimal solution with the time complexity O(N2 logN) and the space complexity O(N) where N is the number of points
BEAM DECOHERENCE DUE TO COMBINATION OF WAKE FORCE AND NONLINEARITY IN SP-RING-8 STORAGE RING
Abstract To understand particle behavior from a beam injection state to equilibrium state determined by radiation effects, we have performed a simple experiment to observe the beam decoherence, i.e., temporal variation of the damping of beam coherent motion generated by a single horizontal kicker. We found that the beam decoherence much depends on chromaticities, the sign of amplitudedependent tune shift and beam current. This suggests that short-range wake force and nonlinearity of ring parameters play important roles in the observed phenomena. Simulations with transverse wake fields show good agreements with the measurements
AMPLITUDE DEPENDENT BETATRON OSCILLATION CENTER SHIFT BY NON-LINEARITY AND BEAM INSTABILITY INTERLOCK
Abstract As a result of the even symmetry of the Sextupole field, it creates the horizontal shift of the averaged position of horizontal and vertical betatron oscillation and the amount of the shift depends on its oscillation amplitude. This shift can be observed with usual slow orbit beam position monitor. At the SPring-8 storage ring, this shift is used to detect the excitation of the betatron oscillation for the interlock system for the protection of the vacuum components from strong radiation of insertion devices. AMPLITUDE DEPENDENT BETATRON OSCILLATION CENTER SHIFT The transverse beam instability drives a horizontal or vertical betatron oscillation, and if this occurs in light sources, the strong synchrotron radiation from insertion devices also oscillates as the beam and cause heat damages on beam pipe components if the radiation continuously hit them. The even symmetry of the sextupole field produces the horizontal shift of the time averaged horizontal position if the horizontal and vertical betatron oscillation exists. Here we call it an amplitude dependent betatron oscillation center shift (ADCS). The ADCS on the sextupole strength can be derived by a canonical perturbation theory as the first order effect by sextupole field [1] as where the symbols with overline are the time averaged values, J z and φ z ( z = x, y ) are the action and the phase, respectively, and related to the position and beta function β z s The sextupole strength is expressed as for the magnetic field
Light emission from a scanning tunneling microscope: Fully retarded calculation
The light emission rate from a scanning tunneling microscope (STM) scanning a
noble metal surface is calculated taking retardation effects into account. As
in our previous, non-retarded theory [Johansson, Monreal, and Apell, Phys. Rev.
B 42, 9210 (1990)], the STM tip is modeled by a sphere, and the dielectric
properties of tip and sample are described by experimentally measured
dielectric functions. The calculations are based on exact diffraction theory
through the vector equivalent of the Kirchoff integral. The present results are
qualitatively similar to those of the non-retarded calculations. The light
emission spectra have pronounced resonance peaks due to the formation of a
tip-induced plasmon mode localized to the cavity between the tip and the
sample. At a quantitative level, the effects of retardation are rather small as
long as the sample material is Au or Cu, and the tip consists of W or Ir.
However, for Ag samples, in which the resistive losses are smaller, the
inclusion of retardation effects in the calculation leads to larger changes:
the resonance energy decreases by 0.2-0.3 eV, and the resonance broadens. These
changes improve the agreement with experiment. For a Ag sample and an Ir tip,
the quantum efficiency is 10 emitted photons in the visible
frequency range per tunneling electron. A study of the energy dissipation into
the tip and sample shows that in total about 1 % of the electrons undergo
inelastic processes while tunneling.Comment: 16 pages, 10 figures (1 ps, 9 tex, automatically included); To appear
in Phys. Rev. B (15 October 1998
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