4,716 research outputs found
Hard thermal loops in static background fields
We discuss the high temperature behavior of retarded thermal loops in static
external fields. We employ an analytic continuation of the imaginary time
formalism and use a spectral representation of the thermal amplitudes. We show
that, to all orders, the leading contributions of static hard thermal loops can
be directly obtained by evaluating them at zero external energies and momenta.Comment: 5 pages, to be published in The European Physical Journal
Origin of the heavy elements in HD 140283. Measurement of europium abundance
HD 140283 is a nearby (V=7.7) subgiant metal-poor star, extensively analysed
in the literature. Although many spectra have been obtained for this star, none
showed a signal-to-noise (S/N) ratio high enough to enable a very accurate
derivation of abundances from weak lines. The detection of europium proves that
the neutron-capture elements in this star originate in the r-process, and not
in the s-process, as recently claimed in the literature. Based on the OSMARCS
1D LTE atmospheric model and with a consistent approach based on the spectrum
synthesis code Turbospectrum, we measured the europium lines at 4129 {\AA} and
4205 {\AA}, taking into account the hyperfine structure of the transitions. The
spectrum, obtained with a long exposure time of seven hours at the
Canada-France-Hawaii Telescope (CFHT), has a resolving power of 81000 and a S/N
ratio of 800 at 4100 {\AA}. We were able to determine the abundance A(Eu)=-2.35
dex, compatible with the value predicted for the europium from the r-process.
The abundance ratio [Eu/Ba]=+0.58 dex agrees with the trend observed in
metal-poor stars and is also compatible with a strong r-process contribution to
the origin of the neutron-capture elements in HD 140283.Comment: 10 pages, 7 figures. To be published in A\&
Resummation of infrared divergences in the free-energy of spin-two fields
We derive a closed form expression for the sum of all the infrared divergent
contributions to the free-energy of a gas of gravitons. An important ingredient
of our calculation is the use of a gauge fixing procedure such that the
graviton propagator becomes both traceless and transverse. This has been shown
to be possible, in a previous work, using a general gauge fixing procedure, in
the context of the lowest order expansion of the Einstein-Hilbert action,
describing non-interacting spin two fields. In order to encompass the problems
involving thermal loops, such as the resummation of the free-energy, in the
present work, we have extended this procedure to the situations when the
interactions are taken into account.Comment: 12 pages, 25 figure
High temperature limit in static backgrounds
We prove that the hard thermal loop contribution to static thermal amplitudes
can be obtained by setting all the external four-momenta to zero before
performing the Matsubara sums and loop integrals. At the one-loop order we do
an iterative procedure for all the 1PI one-loop diagrams and at the two-loop
order we consider the self-energy. Our approach is sufficiently general to the
extent that it includes theories with any kind of interaction vertices, such as
gravity in the weak field approximation, for space-time dimensions. This
result is valid whenever the external fields are all bosonic.Comment: 15 pages, 11 figures. To be published in Physical Review
Andreev tunneling through a double quantum-dot system coupled to a ferromagnet and a superconductor: effects of mean field electronic correlations
We study the transport properties of a hybrid nanostructure composed of a
ferromagnet, two quantum dots, and a superconductor connected in series. By
using the non-equilibrium Green's function approach, we have calculated the
electric current, the differential conductance and the transmittance for
energies within the superconductor gap. In this regime, the mechanism of charge
transmission is the Andreev reflection, which allows for a control of the
current through the ferromagnet polarization. We have also included interdot
and intradot interactions, and have analyzed their influence through a mean
field approximation. In the presence of interactions, Coulomb blockade tend to
localized the electrons at the double-dot system, leading to an asymmetric
pattern for the density of states at the dots, and thus reducing the
transmission probability through the device. In particular, for non-zero
polarization, the intradot interaction splits the spin degeneracy, reducing the
maximum value of the current due to different spin-up and spin-down densities
of states. Negative differential conductance (NDC) appears for some regions of
the voltage bias, as a result of the interplay of the Andreev scattering with
electronic correlations. By applying a gate voltage at the dots, one can tune
the effect, changing the voltage region where this novel phenomenon appears.
This mechanism to control the current may be of importance in technological
applications.Comment: 12 pages, 11 figure
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