5,627 research outputs found
Strangeness in Compact Stars and Signal of Deconfinement
Phase transitions in compact stars are discussed including hyperonization,
deconfinement and crystalline phases. Reasons why kaon condensation is unlikely
is reviewed. Particular emphasis is placed on the evolution of internal
structure with spin-down of pulsars. A signature of a first order phase
transition in the timing structure of pulsars which is strong and easy to
measure, is identified.Comment: 17 pages, 15 figures, Latex. (Invited Talk at the International
Symposium on ``Strangeness In Quark Matter 1997'', Thera (Santorini), Hellas,
April 14-18, 1997, To be published in Journal of Physics G (Organizers: A
Panagiotou and J. Madsen
Adsorption of cobalt on graphene: Electron correlation effects from a quantum chemical perspective
In this work, we investigate the adsorption of a single cobalt atom (Co) on
graphene by means of the complete active space self-consistent field approach,
additionally corrected by the second-order perturbation theory. The local
structure of graphene is modeled by a planar hydrocarbon cluster
(CH). Systematic treatment of the electron correlations and the
possibility to study excited states allow us to reproduce the potential energy
curves for different electronic configurations of Co. We find that upon
approaching the surface, the ground-state configuration of Co undergoes several
transitions, giving rise to two stable states. The first corresponds to the
physisorption of the adatom in the high-spin ()
configuration, while the second results from the chemical bonding formed by
strong orbital hybridization, leading to the low-spin () state.
Due to the instability of the configuration, the adsorption energy of Co
is small in both cases and does not exceed 0.35 eV. We analyze the obtained
results in terms of a simple model Hamiltonian that involves Coulomb repulsion
() and exchange coupling () parameters for the 3 shell of Co, which we
estimate from first-principles calculations. We show that while the exchange
interaction remains constant upon adsorption ( eV), the Coulomb
repulsion significantly reduces for decreasing distances (from 5.3 to
2.60.2 eV). The screening of favors higher occupations of the 3
shell and thus is largely responsible for the interconfigurational transitions
of Co. Finally, we discuss the limitations of the approaches that are based on
density functional theory with respect to transition metal atoms on graphene,
and we conclude that a proper account of the electron correlations is crucial
for the description of adsorption in such systems.Comment: 12 pages, 6 figures, 2 table
Interfacial interactions between local defects in amorphous SiO and supported graphene
We present a density functional study of graphene adhesion on a realistic
SiO surface taking into account van der Waals (vdW) interactions. The
SiO substrate is modeled at the local scale by using two main types of
surface defects, typical for amorphous silica: the oxygen dangling bond and
three-coordinated silicon. The results show that the nature of adhesion between
graphene and its substrate is qualitatively dependent on the surface defect
type. In particular, the interaction between graphene and silicon-terminated
SiO originates exclusively from the vdW interaction, whereas the
oxygen-terminated surface provides additional ionic contribution to the binding
arising from interfacial charge transfer (-type doping of graphene). Strong
doping contrast for the different surface terminations provides a mechanism for
the charge inhomogeneity of graphene on amorphous SiO observed in
experiments. We found that independent of the considered surface morphologies,
the typical electronic structure of graphene in the vicinity of the Dirac point
remains unaltered in contact with the SiO substrate, which points to the
absence of the covalent interactions between graphene and amorphous silica. The
case of hydrogen-passivated SiO surfaces is also examined. In this
situation, the binding with graphene is practically independent of the type of
surface defects and arises, as expected, from the vdW interactions. Finally,
the interface distances obtained are shown to be in good agreement with recent
experimental studies.Comment: 10 pages, 4 figure
Graphene adhesion on mica: Role of surface morphology
We investigate theoretically the adhesion and electronic properties of
graphene on a muscovite mica surface using the density functional theory (DFT)
with van der Waals (vdW) interactions taken into account (the vdW-DF approach).
We found that irregularities in the local structure of cleaved mica surface
provide different mechanisms for the mica-graphene binding. By assuming
electroneutrality for both surfaces, the binding is mainly of vdW nature,
barely exceeding thermal energy per carbon atom at room temperature. In
contrast, if potassium atoms are non uniformly distributed on mica, the
different regions of the surface give rise to - or -type doping of
graphene. In turn, an additional interaction arises between the surfaces,
significantly increasing the adhesion. For each case the electronic states of
graphene remain unaltered by the adhesion. It is expected, however, that the
Fermi level of graphene supported on realistic mica could be shifted relative
to the Dirac point due to asymmetry in the charge doping. Obtained variations
of the distance between graphene and mica for different regions of the surface
are found to be consistent with recent atomic force microscopy experiments. A
relative flatness of mica and the absence of interlayer covalent bonding in the
mica-graphene system make this pair a promising candidate for practical use.Comment: 6 pages, 3 figure
Adsorption of diatomic halogen molecules on graphene: A van der Waals density functional study
The adsorption of fluorine, chlorine, bromine, and iodine diatomic molecules
on graphene has been investigated using density functional theory with taking
into account nonlocal correlation effects by means of vdW-DF approach. It is
shown that the van der Waals interaction plays a crucial role in the formation
of chemical bonding between graphene and halogen molecules, and is therefore
important for a proper description of adsorption in this system. In-plane
orientation of the molecules has been found to be more stable than the
orientation perpendicular to the graphene layer. In the cases of F, Br
and I we also found an ionic contribution to the binding energy, slowly
vanishing with distance. Analysis of the electronic structure shows that ionic
interaction arises due to the charge transfer from graphene to the molecules.
Furthermore, we found that the increase of impurity concentration leads to the
conduction band formation in graphene due to interaction between halogen
molecules. In addition, graphite intercalation by halogen molecules has been
investigated. In the presence of halogen molecules the binding between graphite
layers becomes significantly weaker, which is in accordance with the results of
recent experiments on sonochemical exfoliation of intercalated graphite.Comment: Submitted to PR
Deconfinement transition in protoneutron stars: analysis within the Nambu-Jona-Lasinio model
We study the effect of color superconductivity and neutrino trapping on the
deconfinement transition of hadronic matter into quark matter in a protoneutron
star. To describe the strongly interacting matter a two-phase picture is
adopted. For the hadronic phase we use different parameterizations of a
non-linear Walecka model which includes the whole baryon octet. For the quark
matter phase we use an Nambu-Jona-Lasinio effective model which
includes color superconductivity. We impose color and flavor conservation
during the transition in such a way that just deconfined quark matter is
transitorily out of equilibrium with respect to weak interactions. We find that
deconfinement is more difficult for small neutrino content and it is easier for
lower temperatures although these effects are not too large. In addition they
will tend to cancel each other as the protoneutron star cools and deleptonizes,
resulting a transition density that is roughly constant along the evolution of
the protoneutron star. According to these results the deconfinement transition
is favored after substantial cooling and contraction of the protoneutron star
XMM-Newton observation of the ULIRG NGC 6240: The physical nature of the complex Fe K line emission
We report on an XMM-Newton observation of the ultraluminous infrared galaxy
NGC 6240. The 0.3-10 keV spectrum can be successfully modelled with: (i) three
collisionally ionized plasma components with temperatures of about 0.7, 1.4,
and 5.5 keV; (ii) a highly absorbed direct power-law component; and (iii) a
neutral Fe K_alpha and K_beta line. We detect a significant neutral column
density gradient which is correlated with the temperature of the three plasma
components. Combining the XMM-Newton spectral model with the high spatial
resolution Chandra image we find that the temperatures and the column densities
increase towards the center.
With high significance, the Fe K line complex is resolved into three distinct
narrow lines: (i) the neutral Fe K_alpha line at 6.4 keV; (ii) an ionized line
at about 6.7 keV; and (iii) a higher ionized line at 7.0 keV (a blend of the Fe
XXVI and the Fe K_beta line). While the neutral Fe K line is most probably due
to reflection from optically thick material, the Fe XXV and Fe XXVI emission
arises from the highest temperature ionized plasma component.
We have compared the plasma parameters of the ultraluminous infrared galaxy
NGC 6240 with those found in the local starburst galaxy NGC 253. We find a
striking similarity in the plasma temperatures and column density gradients,
suggesting a similar underlying physical process at work in both galaxies.Comment: 8 pages including 9 figures. Accepted for publication in A&
Characterization and Modeling of Non-Uniform Charge Collection in CVD Diamond Pixel Detectors
A pixel detector with a CVD diamond sensor has been studied in a 180 GeV/c
pion beam. The charge collection properties of the diamond sensor were studied
as a function of the track position, which was measured with a silicon
microstrip telescope. Non-uniformities were observed on a length scale
comparable to the diamond crystallites size. In some regions of the sensor, the
charge drift appears to have a component parallel to the sensor surface (i.e.,
normal to the applied electric field) resulting in systematic residuals between
the track position and the hits position as large as 40 m. A numerical
simulation of the charge drift in polycrystalline diamond was developed to
compute the signal induced on the electrodes by the electrons and holes
released by the passing particles. The simulation takes into account the
crystallite structure, non-uniform trapping across the sensor, diffusion and
polarization effects. It is in qualitative agreement with the data. Additional
lateral electric field components result from the non-uniform trapping of
charges in the bulk. These provide a good explanation for the large residuals
observed.Comment: Accepted by Nucl. Instr. and Met
Direct Urca neutrino rate in colour superconducting quark matter
If deconfined quark matter exists inside compact stars, the primary cooling
mechanism is neutrino radiation via the direct Urca processes d->u+e+antinu_e
and u+e->d+nu_e. Below a critical temperature, T_c, quark matter forms a colour
superconductor, one possible manifestation of which is a condensate of
quark Cooper pairs in an electric-charge neutralising background of electrons.
We compute the neutrino emission rate from such a phase, including charged
pair-breaking and recombination effects, and find that on a material
temperature domain below T_c the pairing-induced suppression of the neutrino
emission rate is not uniformly exponential. If gapless modes are present in the
condensed phase, the emissivity at low temperatures is moderately enhanced
above that of completely unpaired matter. The importance of charged current
pair-breaking processes for neutrino emission both in the fully gapped and
partially gapped regimes is emphasised.Comment: 5 pages, 2 figures; to appear in Phys. Rev. C (Rapid Comm.
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