2,270 research outputs found
Gravitational potential of a homogeneous circular torus: new approach
The integral expression for gravitational potential of a homogeneous circular
torus composed of infinitely thin rings is obtained. Approximate expressions
for torus potential in the outer and inner regions are found. In the outer
region a torus potential is shown to be approximately equal to that of an
infinitely thin ring of the same mass; it is valid up to the surface of the
torus. It is shown in a first approximation, that the inner potential of the
torus (inside a torus body) is a quadratic function of coordinates. The method
of sewing together the inner and outer potentials is proposed. This method
provided a continuous approximate solution for the potential and its
derivatives, working throughout the region.Comment: 10 pages, 9 figures, 1 table; some misprints in formulae were
correcte
Influence of Collision Cascade Statistics on Pattern Formation of Ion-Sputtered Surfaces
Theoretical continuum models that describe the formation of patterns on
surfaces of targets undergoing ion-beam sputtering, are based on Sigmund's
formula, which describes the spatial distribution of the energy deposited by
the ion. For small angles of incidence and amorphous or polycrystalline
materials, this description seems to be suitable, and leads to the classic BH
morphological theory [R.M. Bradley and J.M.E. Harper, J. Vac. Sci. Technol. A
6, 2390 (1988)]. Here we study the sputtering of Cu crystals by means of
numerical simulations under the binary-collision approximation. We observe
significant deviations from Sigmund's energy distribution. In particular, the
distribution that best fits our simulations has a minimum near the position
where the ion penetrates the surface, and the decay of energy deposition with
distance to ion trajectory is exponential rather than Gaussian. We provide a
modified continuum theory which takes these effects into account and explores
the implications of the modified energy distribution for the surface
morphology. In marked contrast with BH's theory, the dependence of the
sputtering yield with the angle of incidence is non-monotonous, with a maximum
for non-grazing incidence angles.Comment: 12 pages, 13 figures, RevTe
Independent determination of the two gaps by directional point-contact spectroscopy in MgB_2 single crystals
Directional point-contact spectroscopy measurements were performed for the
first time in state-of-the-art MgB_2 single crystals. The selective suppression
of the superconductivity in the "pi" band by means of a suitable magnetic field
allowed separating the partial contribution of each band to the total
point-contact conductance. By fitting the partial conductance curves
sigma_sigma(V) and sigma_pi(V), we got an independent determination of the two
gaps, Delta_sigma and Delta_pi, with a strong reduction of the experimental
uncertainty. Their temperature dependence was found to agree well with the
predictions of the two-band models for MgB_2.Comment: 6 pages, 4 eps figures. References added, abstract rewritten, text
slightly changed. Proceedings of the BOROMAG Conference, June 17-19, Genoa,
Ital
Quasiparticle Density of States of Clean and Dirty s-Wave Superconductors in the Vortex State
The quasiparticle density of states (DOS) in the vortex state has been probed
by specific heat measurements under magnetic fields (H) for clean and dirty
s-wave superconductors, Y(Ni1-xPtx)2B2C and Nb1-xTaxSe2. We find that the
quasiparticle DOS per vortex is appreciably H-dependent in the clean-limit
superconductors, while it is H-independent in the dirty superconductors as
expected from a conventional rigid normal electron core picture. We discuss
possible origins for our observations in terms of the shrinking of the vortex
core radius with increasing H.Comment: 5 pages, 4 figures, to appear in J. Phys. Soc. Jpn. Vol. 68 No.
Infrared and optical properties of pure and cobalt-doped LuNi_2B_2C
We present optical conductivity data for Lu(NiCo)BC over
a wide range of frequencies and temperatures for x=0 and x=0.09. Both materials
show evidence of being good Drude metals with the infrared data in reasonable
agreement with dc resistivity measurements at low frequencies. An absorption
threshold is seen at approximately 700 cm-1. In the cobalt-doped material we
see a superconducting gap in the conductivity spectrum with an absorption onset
at 24 +/- 2 cm-1 = 3.9$ +/- 0.4 k_BT_c suggestive of weak to moderately strong
coupling. The pure material is in the clean limit and no gap can be seen. We
discuss the data in terms of the electron-phonon interaction and find that it
can be fit below 600 cm-1 with a plasma frequency of 3.3 eV and an
electron-phonon coupling constant lambda_{tr}=0.33 using an alpha^{2}F(omega)
spectrum fit to the resistivity.Comment: 10 pages with 10 embedded figures, submitted to PR
Equilibrium basal-plane magnetization of superconductive YNi(2)B(2)C - the influence of non-local electrodynamics
For a single crystal of YNi(2)B(2)C superconductor, the equilibrium
magnetization M in the square basal plane has been studied experimentally as a
function of temperature and magnetic field. While the magnetization M(H)
deviates from conventional London predictions, a recent extension of London
theory (to include effects of non-local electrodynamics) describes the
experiments accurately. The resulting superconductive parameters are well
behaved. These results are compared with corresponding findings for the case
with M perpendicular to the basal plane.Comment: 7 pages, 5 Postscript Figures, 2 table
Theory of the Optical Conductivity in the Cuprate Superconductors
We present a study of the normal state optical conductivity in the cuprate
superconductors using the nearly antiferromagnetic Fermi liquid (NAFL)
description of the magnetic interaction between their planar quasiparticles. We
find that the highly anisotropic scattering rate in different regions of the
Brillouin zone, both as a function of frequency and temperature, a benchmark of
NAFL theory, leads to an average relaxation rate of the Marginal Fermi Liquid
form for overdoped and optimally doped systems, as well as for underdoped
systems at high temperatures. We carry out numerical calculations of the
optical conductivity for several compounds for which the input spin fluctuation
parameters are known. Our results, which are in agreement with experiment on
both overdoped and optimally doped systems, show that NAFL theory explains the
anomalous optical behavior found in these cuprate superconductors.Comment: REVTEX file, 8 PostScript figure
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