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991.
Mathias Steiner Marcus Freitag James C. Tsang Vasili Perebeinos Ageeth A. Bol Antonio V. Failla Phaedon Avouris 《Applied Physics A: Materials Science & Processing》2009,96(2):271-282
We study the optical properties of a single, semiconducting single-walled carbon nanotube (CNT) that is partially suspended
across a trench and partially supported by a SiO2-substrate. By tuning the laser excitation energy across the E
33 excitonic resonance of the suspended CNT segment, the scattering intensities of the principal Raman transitions, the radial
breathing mode (RBM), the D mode and the G mode show strong resonance enhancement of up to three orders of magnitude. In the
supported part of the CNT, despite a loss of Raman scattering intensity of up to two orders of magnitude, we recover the E
33 excitonic resonance suffering a substrate-induced red shift of 50 meV. The peak intensity ratio between G band and D band
is highly sensitive to the presence of the substrate and varies by one order of magnitude, demonstrating the much higher defect
density in the supported CNT segments. By comparing the E
33 resonance spectra measured by Raman excitation spectroscopy and photoluminescence (PL) excitation spectroscopy in the suspended
CNT segment, we observe that the peak energy in the PL excitation spectrum is red-shifted by 40 meV. This shift is associated
with the energy difference between the localized exciton dominating the PL excitation spectrum and the free exciton giving
rise to the Raman excitation spectrum. High-resolution Raman spectra reveal substrate-induced symmetry breaking, as evidenced
by the appearance of additional peaks in the strongly broadened Raman G band. Laser-induced line shifts of RBM and G band
measured on the suspended CNT segment are both linear as a function of the laser excitation power. Stokes/anti-Stokes measurements,
however, reveal an increase of the G phonon population while the RBM phonon population is rather independent of the laser
excitation power. 相似文献
992.
Furkan Bulut Wolfgang Rosellen Mathias Getzlaff 《Applied Physics A: Materials Science & Processing》2009,97(1):185-189
Size-selected iron and iron–cobalt alloy clusters have been studied with high resolution transmission electron microscopy
(HRTEM) and scanning tunneling microscopy (STM). The clusters were produced by a continuously working arc cluster ion source
and subsequently size-selected by an electrostatic quadrupole deflector. The crystalline structure of pure clusters has been
investigated with HRTEM to ensure a reliable determination of the lattice parameter for the alloy clusters. The composition
of the alloy clusters was checked with energy dispersive X-ray spectroscopy (EDX). The height of the deposited FeCo clusters
on the (110) surface of tungsten was determined via STM. These results were compared with the lateral size distribution being
investigated by TEM and allow a conclusion on the shape of the deposited alloy clusters. Furthermore, the behavior of the
alloy clusters on the W(110) surface at elevated temperatures has been examined, at which the clusters show anisotropic spreading. 相似文献
993.
994.
The results of a muonic X-ray study of the charge radii of stable barium isotopes are presented and compared with optical isotope shifts. The isotope shifts Δ〈r2〉 of a wide range of barium isotopes are found to be in good agreement with the droplet model and with IBA calculations. 相似文献
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996.
We present experimental results on the reversibility of ultrasound in a multiple scattering medium. An ultrasonic pulsed wave is transmitted from a point source to a 128-element receiving array through 2D samples with various thickness. The samples consist of random collections of parallel steel rods immersed in water. The scattered waves are recorded, time reversed and sent back into the medium. The time-reversed waves are converging back to their source and the quality of spatial and temporal focusing on the source is related to the second-order moments of the scattered wave (correlation) in time and in space. Experimental results show that it is possible to obtain a robust estimation of the correlations on a single realisation of disorder, taking advantage of the wide frequency bandwidth. The spatial resolution of the system is only limited by the correlation length of the scattered field, and no longer by diffraction. Moreover, successful time-reversal focusing using a single element instead of an array is possible, whereas a one-channel monochromatic phase conjugation fails. The efficiency of broad-band time reversal compared to monochromatic phase conjugation lies in the number of 'information grains' in the frequency bandwidth. 相似文献
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