47,070 research outputs found
Electron-phonon interaction for adiabatic anharmonic phonons
A model with Holstein-like electron-phonon coupling is studied in the limit
of adiabatic phonons. The phonon distribution is anharmonic with two degenerate
maxima. This model can be related to fermions in a correlated binary alloy and
describes microscopic phase separation. We discuss the weak and strong
electron-phonon coupling limit and present a qualitative phase diagram. In
terms of the phononic displacements it consists of a homogeneous, an
alternating, and a disordered phase. There is a first order phase transition
between the homogeneous and the alternating phase, and second order phase
transition between the alternating and the disordered phase. The opening of a
gap inside the disordered phase is treated by a dynamical mean-field theory.Comment: 11 pages, 3 figures, revised and published versio
A Note on Asymptotic Freedom at High Temperatures
This short note considers, within the external field approach outlined in
hep-ph/0202026, the role of the lowest lying gluon Landau mode in QCD in the
high temperature limit. Its influence on a temperature- and field-dependent
running coupling constant is examined. The thermal imaginary part of the mode
is temperature-independent in our approach and exactly cancels the well-known
zero temperature imaginary part, thus rendering the Savvidy vacuum stable.
Combining the real part of the mode with the contributions from the higher
lying Landau modes and the vacuum contribution, a field-independent coupling
alpha_s(T) is obtained. It can be interpreted as the ordinary zero temperature
running coupling constant with average thermal momenta \approx 2pi T for
gluons and \approx pi T for quarks.Comment: 4 pages; minor changes, version to appear in Phys. Rev.
Diffusion of Neon in White Dwarf Stars
Sedimentation of the neutron rich isotope Ne may be an important
source of gravitational energy during the cooling of white dwarf stars. This
depends on the diffusion constant for Ne in strongly coupled plasma
mixtures. We calculate self-diffusion constants from molecular dynamics
simulations of carbon, oxygen, and neon mixtures. We find that in a
mixture does not differ greatly from earlier one component plasma results. For
strong coupling (coulomb parameter few), has a modest
dependence on the charge of the ion species, .
However depends more strongly on for weak coupling (smaller
). We conclude that the self-diffusion constant for
Ne in carbon, oxygen, and neon plasma mixtures is accurately known so
that uncertainties in should be unimportant for simulations of
white dwarf cooling.Comment: 6 pages, 5 figures, minor changes, Phys. Rev. E in pres
Seventy-One New L and T Dwarfs from the Sloan Digital Sky Survey
We present near-infrared observations of 71 newly discovered L and T dwarfs,
selected from imaging data of the Sloan Digital Sky Survey (SDSS) using the
i-dropout technique. Sixty-five of these dwarfs have been classified
spectroscopically according to the near-infrared L dwarf classification scheme
of Geballe et al. and the unified T dwarf classification scheme of Burgasser et
al. The spectral types of these dwarfs range from L3 to T7, and include the
latest types yet found in the SDSS. Six of the newly identified dwarfs are
classified as early- to mid-L dwarfs according to their photometric
near-infrared colors, and two others are classified photometrically as M
dwarfs. We also present new near-infrared spectra for five previously published
SDSS L and T dwarfs, and one L dwarf and one T dwarf discovered by Burgasser et
al. from the Two Micron All Sky Survey. The new SDSS sample includes 27 T
dwarfs and 30 dwarfs with spectral types spanning the complex L-T transition
(L7-T3). We continue to see a large (~0.5 mag) spread in J-H for L3 to T1
types, and a similar spread in H-K for all dwarfs later than L3. This color
dispersion is probably due to a range of grain sedimentation properties,
metallicity, and gravity. We also find L and T dwarfs with unusual colors and
spectral properties that may eventually help to disentangle these effects.Comment: accepted by AJ, 18 pages, 10 figures, 5 tables, emulateapj layou
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