13,836 research outputs found
Chemical evolution models with a new stellar nucleosynthesis
Numerical models for the chemical evolution of the Galaxy have been computed
with the new stellar yields published by Maeder (1992). These metallicity
dependent yields represent an important improvement in the chemical evolution
of galaxies but there are still uncertainties in the stellar evolution which
prevent completely satisfactory results. {}From the comparison of the model
predictions with the corresponding observational constraints we find that
Maeder's nucleosynthesis reproduces the oxygen and carbon abundances and
provides consistent Y/(O/H) ratios if a significant amount of
gas is accreted by the galactic disc during its whole lifetime. The lower mass
limit for the black hole formation (M) must be larger than 22.5
M to avoid oxygen underproduction.Comment: 6 pages, MNRAS LaTex style, figures available on request from
[email protected]
Nonequilibrium transport through magnetic vibrating molecules
We calculate the nonequilibrium conductance through a molecule or a quantum
dot in which the occupation of the relevant electronic level is coupled with
intensity to a phonon mode, and also to two conducting leads. The
system is described by the Anderson-Holstein Hamiltonian. We solve the problem
using the Keldysh formalism and the non-crossing approximation (NCA) for both,
the electron-electron and the electron-phonon interactions. We obtain a
moderate decrease of the Kondo temperature with for fixed
renormalized energy of the localized level . The meaning and value
of are discussed. The spectral density of localized electrons
shows in addition to the Kondo peak of width , satellites of this peak
shifted by multiples of the phonon frequency . The nonequilibrium
conductance as a function of bias voltage at small temperatures, also
displays peaks at multiples of in addition to the central dominant
Kondo peak near .Comment: 11 pages, 13 figures, accepted in Phys. Rev.
Replicas of the Kondo peak due to electron-vibration interaction in molecular transport properties
The low temperature properties of single level molecular quantum dots
including both, electron-electron and electron-vibration interactions, are
theoretically investigated. The calculated differential conductance in the
Kondo regime exhibits not only the zero bias anomaly but also side peaks
located at bias voltages which coincide with multiples of the energy of
vibronic mode . We obtain that the evolution with
temperature of the two main satellite conductance peaks follows the
corresponding one of the Kondo peak when , being the Kondo temperature, in agreement with recent transport measurements in
molecular junctions. However, we find that this is no longer valid when is of the order of a few times .Comment: 6 pages, 4 figures. Accepted for publication in Physical Review
Orbital Kondo spectroscopy in a double quantum dot system
We calculate the nonequilibrium conductance of a system of two capacitively
coupled quantum dots, each one connected to its own pair of conducting leads.
The system has been used recently to perform pseudospin spectroscopy by
controlling independently the voltages of the four leads. The pseudospin is
defined by the orbital occupation of one or the other dot. Starting from the
SU(4) symmetric point of spin and pseudospin degeneracy in the Kondo regime,
for an odd number of electrons in the system, we show how the conductance
through each dot varies as the symmetry is reduced to SU(2) by a pseudo-Zeeman
splitting, and as bias voltages are applied to any of the dots. We analize the
expected behavior of the system in general, and predict characteristic
fingerprint features of the SU(4) to SU(2) crossover that have not been
observed so far.Comment: 5 pages, 6 figures, submitted to Phys. Rev.
Restoring the SU(4) Kondo regime in a double quantum dot system
We calculate the spectral density and occupations of a system of two
capacitively coupled quantum dots, each one connected to its own pair of
conducting leads, in a regime of parameters in which the total coupling to the
leads for each dot are different. The system has been used recently
to perform pseudospin spectroscopy by controlling independently the voltages of
the four leads. For an odd number of electrons in the system,
, equal dot levels and sufficiently large interdot
repulsion the system lies in the SU(4) symmetric point of spin and
pseudospin degeneracy in the Kondo regime. In the more realistic case , pseudospin degeneracy is broken and the symmetry is reduced to
SU(2). Nevertheless we find that the essential features of the SU(4) symmetric
case are recovered by appropriately tuning the level difference
. The system behaves as an SU(4) Kondo one at low energies. Our
results are relevant for experiments which look for signatures of SU(4)
symmetry in the Kondo regime of similar systems.Comment: 9 pages, 10 figure
The intermediate age open cluster NGC 2660
We present CCD UBVI photometry of the intermediate old open cluster NGC2660,
covering from the red giants region to about seven magnitudes below the main
sequence turn-off. Using the synthetic Colour - Magnitude Diagram method, we
estimate in a self-consistent way values for distance modulus ((m-M)0 ~= 12.2),
reddening (E(B-V) ~= 0.40), metallicity ([Fe/H] about solar), and age (age ~ 1
Gyr). A 30% population of binary stars turns out to be probably present.Comment: 12 pages, 8 (encapsulated) figures, to be published on MNRA
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