646 research outputs found
Electron-electron interaction in carbon nanostructures
The electron-electron interaction in carbon nanostructures was studied. A new
method which allows to determine the electron-electron interaction constant
from the analysis of quantum correction to the magnetic
susceptibility and the magnetoresistance was developed. Three types of carbon
materials: arc-produced multiwalled carbon nanotubes (arc-MWNTs), CVD-produced
catalytic multiwalled carbon nanotubes (c-MWNTs) and pyrolytic carbon were used
for investigation. We found that =0.2 for arc-MWNTs (before and
after bromination treatment); = 0.1 for pyrolytic graphite;
0 for c-MWNTs. We conclude that the curvature of graphene layers
in carbon nanostructures leads to the increase of the electron-electron
interaction constant .Comment: 12 pages, 18 figures, to be published in the Proceedings of the NATO
Advanced Research Workshop on Electron Correlation in New Materials and
Nanosystems, NATO Science Series II, Springer, 200
Pt(II) and Pd(II) Complexes with β-Alanine
A sequence of stages in the syntheses of isomeric bisamino acid complexes of Pt(II) with β-aminopropionic acid (β-alanine = β-AlaH) has been studied by the 195Pt NMR spectroscopy. The techniques have been developed of the synthesis of the cis- and trans-bischelates of Pt(II) and Pd(II) with β-alanine as well as of the halide complexes of trans-[M(β-AlaH)2Cl2] (M = Pt, Pd) and trans-K2[Pt(β-Ala)2I2] types. The NMR spectroscopy and IR spectroscopy (in the nuclei of 195Pt,13C,1H) and X-ray diffraction analysis have been used to examine the structures of the synthesized compounds
The High Eccentricity of the Planet Around 16 Cyg B
We consider the high eccentricity, 0.66, of the newly discovered planet
around 16 Cyg B, using the fact that the parent star is part of a wide binary.
We show that the high eccentricity of the planet could be the result of tidal
forces exerted on 16 Cyg B and its planet by 16 Cyg A, the distant companion in
the system. By following stellar triple systems with parameters similar to
those of 16 Cyg, we have established that the orbital eccentricity of the
planet could have gone through strong modulation, with an amplitude of 0.8 or
even larger, with typical timescale of tens of millions years. The amplitude of
the planet eccentricity strongly depends on the relative inclination between
the plane of motion of the planet and that of the wide binary 16 Cyg AB. To
account for the present eccentricity of the planet we have to assume that the
angle between the two planes of motion is large, at least 60 deg. We argue that
this assumption is reasonable for wide binaries like 16 Cyg AB.Comment: 2 Figures, Latex, submitted for publication to ApJ
Model for initiation of quality factor degradation at high accelerating fields in superconducting radio-frequency cavities
A model for the onset of the reduction in SRF cavity quality factor, the
so-called Q-drop, at high accelerating electric fields is presented. Breakdown
of the surface barrier against magnetic flux penetration at the cavity equator
is considered to be the critical event that determines the onset of Q-drop. The
worst case of triangular grooves with low field of first flux penetration Hp,
as analyzed previously by Buzdin and Daumens, [1998 Physica C 294: 257], was
adapted. This approach incorporates both the geometry of the groove and local
contamination via the Ginzburg-Landau parameter kappa, so the proposed model
allows new comparisons of one effect in relation to the other. The model
predicts equivalent reduction of Hp when either roughness or contamination were
varied alone, so smooth but dirty surfaces limit cavity performance about as
much as rough but clean surfaces do. When in combination, contamination
exacerbates the negative effects of roughness and vice-versa. To test the model
with actual data, coupons were prepared by buffered chemical polishing and
electropolishing, and stylus profilometry was used to obtain distributions of
angles. From these data, curves for surface resistance generated by simple flux
flow as a function of magnetic field were generated by integrating over the
distribution of angles for reasonable values of kappa. This showed that
combined effects of roughness and contamination indeed reduce the Q-drop onset
field by ~30%, and that that contamination contributes to Q-drop as much as
roughness. The latter point may be overlooked by SRF cavity research, since
access to the cavity interior by spectroscopy tools is very difficult, whereas
optical images have become commonplace. The model was extended to fit cavity
test data, which indicated that reduction of the superconducting gap by
contaminants may also play a role in Q-drop.Comment: 15 pages with 7 figure
Influence of the atomic-wall collision elasticity on the coherent population trapping resonance shape
We studied theoretically a coherent population trapping resonance formation
in cylindrical cell without buffer gas irradiated by a narrow laser beam. We
take into account non-zero probabilities of elastic ("specular") and inelastic
("sticking") collision between the atom and the cell wall. We have developed a
theoretical model based on averaging over the random Ramsey pulse sequences of
times that atom spent in and out of the beam. It is shown that the shape of
coherent population trapping resonance line depends on the probability of
elastic collision.Comment: 18 pages, 5 figure
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