9,797 research outputs found
Activated escape of periodically modulated systems
The rate of noise-induced escape from a metastable state of a periodically
modulated overdamped system is found for an arbitrary modulation amplitude .
The instantaneous escape rate displays peaks that vary with the modulation from
Gaussian to strongly asymmetric. The prefactor in the period-averaged
escape rate depends on nonmonotonically. Near the bifurcation amplitude
it scales as . We identify three scaling
regimes, with , and 1/2
Accounting for Convective Blue-Shifts in the Determination of Absolute Stellar Radial Velocities
For late-type non-active stars, gravitational redshifts and convective
blueshifts are the main source of biases in the determination of radial
velocities. If ignored, these effects can introduce systematic errors of the
order of ~ 0.5 km/s. We demonstrate that three-dimensional hydrodynamical
simulations of solar surface convection can be used to predict the convective
blue-shifts of weak spectral lines in solar-like stars to ~ 0.070 km/s. Using
accurate trigonometric parallaxes and stellar evolution models, the
gravitational redshifts can be constrained with a similar uncertainty, leading
to absolute radial velocities accurate to better than ~ 0.1 km/s.Comment: To appear in the proceedings of the Joint Discussion 10, IAU General
Assembly, Rio de Janeiro, August 10-11, 200
Definition of socioeconomic scenarios for land surface hydrology simulations of the 21st century
Multifractality and Conformal Invariance at 2D Metal-Insulator Transition in the Spin-Orbit Symmetry Class
We study the multifractality (MF) of critical wave functions at boundaries
and corners at the metal-insulator transition (MIT) for noninteracting
electrons in the two-dimensional (2D) spin-orbit (symplectic) universality
class. We find that the MF exponents near a boundary are different from those
in the bulk. The exponents at a corner are found to be directly related to
those at a straight boundary through a relation arising from conformal
invariance. This provides direct numerical evidence for conformal invariance at
the 2D spin-orbit MIT. The presence of boundaries modifies the MF of the whole
sample even in the thermodynamic limit.Comment: 5 pages, 4 figure
Minimal conductivity in bilayer graphene
Using the Landauer formula approach, it is proven that minimal conductivity
of order of found experimentally in bilayer graphene is its intrinsic
property. For the case of ideal crystals, the conductivity turns our to be
equal to per valley per spin. A zero-temperature shot noise in
bilayer graphene is considered and the Fano factor is calculated. Its value
is close to the value 1/3 found earlier for the single-layer
graphene.Comment: 3 pages, 1 figur
Isotope Spectroscopy
The measurement of isotopic ratios provides a privileged insight both into
nucleosynthesis and into the mechanisms operating in stellar envelopes, such as
gravitational settling. In this article, we give a few examples of how isotopic
ratios can be determined from high-resolution, high-quality stellar spectra. We
consider examples of the lightest elements, H and He, for which the isotopic
shifts are very large and easily measurable, and examples of heavier elements
for which the determination of isotopic ratios is more difficult. The presence
of 6Li in the stellar atmospheres causes a subtle extra depression in the red
wing of the 7Li 670.7 nm doublet which can only be detected in spectra of the
highest quality. But even with the best spectra, the derived Li abundance
can only be as good as the synthetic spectra used for their interpretation. It
is now known that 3D non-LTE modelling of the lithium spectral line profiles is
necessary to account properly for the intrinsic line asymmetry, which is
produced by convective flows in the atmospheres of cool stars, and can mimic
the presence of 6Li. We also discuss briefly the case of the carbon isotopic
ratio in metal-poor stars, and provide a new determination of the nickel
isotopic ratios in the solar atmosphere.Comment: AIP Thinkshop 10 "High resolution optical spectroscopy", invited
talk, AN in pres
Non-Fermi liquid behavior in Kondo models
Despite the fact that the low energy behavior of the basic Kondo model cannot
be studied perturbatively it was eventually shown by Wilson, Anderson, Nozieres
and others to have a simple "local Fermi liquid theory" description. That is,
electronic degrees of freedom become effectively non-interacting in the zero
energy limit. However, generalized versions of the Kondo model involving more
than one channel or impurity may exhibit low energy behavior of a less trivial
sort which can, nonetheless, be solved exactly using either Bethe ansatz or
conformal field theory and bosonization techniques. Now the low energy limit
exhibits interacting many body behavior. For example, processes in which a
single electron scatters off the impurity into a multi electron-hole state have
a non-vanishing (and sometimes large) amplitude at zero energy. This
corresponds to a rare solveable example of non-Fermi liquid behavior. Essential
features of these phenomena are reviewed.Comment: A brief review submitted to the special issue of J. Phys. Soc. of
Japan, "Kondo effect -- 40 years after the discovery
Abelian bosonization approach to quantum impurity problems
Using Abelian Bosonization, we develop a simple and powerful method to
calculate the correlation functions of the two channel Kondo model and its
variants. The method can also be used to identify all the possible boundary
fixed points and their maximum symmetry, to calculate straightforwardly the
finite size spectra, to demonstrate the physical picture at the boundary
explicitly. Comparisons with Non-Abelian Bosonization method are made. Some
fixed points corresponding to 4 pieces of bulk fermions coupled to s=1/2
impurity are listed.Comment: 12 pages, REVTEX, 1 Table, no figures. To appear in Phys. Rev. Letts.
July 21, 199
3D molecular line formation in dwarf carbon-enhanced metal-poor stars
We present a detailed analysis of the carbon and nitrogen abundances of two
dwarf carbon-enhanced metal-poor (CEMP) stars: SDSS J1349-0229 and SDSS
J0912+0216. We also report the oxygen abundance of SDSS J1349-0229. These stars
are metal-poor, with [Fe/H] < -2.5, and were selected from our ongoing survey
of extremely metal-poor dwarf candidates from the Sloan Digital SkySurvey
(SDSS). The carbon, nitrogen and oxygen abundances rely on molecular lines
which form in the outer layers of the stellar atmosphere. It is known that
convection in metal-poor stars induces very low temperatures which are not
predicted by `classical' 1D stellar atmospheres. To obtain the correct
temperature structure, one needs full 3D hydrodynamical models. Using CO5BOLD
3D hydrodynamical model atmospheres and the Linfor3D line formation code,
molecular lines of CH, NH, OH and C2 were computed, and 3D carbon, nitrogen and
oxygen abundances were determined. The resulting carbon abundances were
compared to abundances derived using atomic CI lines in 1D LTE and NLTE. There
is not a good agreement between the carbon abundances determined from C2 bands
and from the CH band, and molecular lines do not agree with the atomic CI
lines. Although this may be partly due to uncertainties in the transition
probabilities of the molecular bands it certainly has to do with the
temperature structure of the outer layers of the adopted model atmosphere. We
explore the influence of the 3D model properties on the molecular abundance
determination. In particular, the choice of the number of opacity bins used in
the model calculations and its subsequent effects on the temperature structure
and molecular line formation is discussed. (Abridged)Comment: Poster presented at IAU JD 10, Rio de Janeiro, 10-11 August 2009,
published in Memorie della Societa' Astronomica Italiana, Vol. 80 n.3 P.735.
One reference corrected, matches the published versio
- …