35,931 research outputs found
Spectroscopic study of early-type multiple stellar systems II. New binary subsystems
Context. This work is part of a long-term spectroscopic study of a sample of
30 multiple stars with early-type components. In this second paper we present
the results of six multiple systems in which new stellar components have been
detected.
Aims. The main aim is to increase the knowledge of stellar properties and
dynamical structure of early-type multiple stellar systems.
Methods. Using spectroscopic observations taken over a time baseline of more
than 5 years we measured RVs by cross-correlations and applied a spectral
disentangling method to double-lined systems. Besides the discovery of objects
with double-lined spectra, the existence of new spectroscopic subsystems have
been inferred from the radial velocity variations of single-lined components
and through the variation of the barycentric velocity of double-lined
subsystems. Orbital elements have been calculated when possible.
Results. Seven new stellar components and two members that we expect to
confirm with new observations have been discovered in the six studied
multiples. We present orbital parameters for two double-lined binaries and
preliminary orbits for three single-lined spectroscopic binaries. Five of the
six analysed systems are quadruples, while the remaining has five components
distributed in four hierarchical levels. These multiplicity orders are in fact
lower limits, since these systems lack high-resolution visual observations and
additional hierarchical level might exist in that separation range.
Conclusions. The six analysed systems have greater multiplicity degree and a
more complex hierarchical structure than previously known, which suggests that
high-order multiple systems are significantly more frequent that it is
currently estimated. The long term spectroscopic monitoring of multiple systems
has shown to be useful for the detection of companions in intermediate
hierarchical levels.Comment: 13 pages, 9 figures. Accepted by Astronomy and Astrophysic
Room temperature ferromagnetic-like behavior in Mn-implanted and post-annealed InAs layers deposited by Molecular Beam Epitaxy
We report on the magnetic and structural properties of Ar and Mn implanted
InAs epitaxial films grown on GaAs (100) by Molecular Beam Epitaxy (MBE) and
the effect of Rapid Thermal Annealing (RTA) for 30 seconds at 750C. Channeling
Particle Induced X- ray Emission (PIXE) experiments reveal that after Mn
implantation almost all Mn atoms are subsbtitutional in the In-site of the InAs
lattice, like in a diluted magnetic semiconductor (DMS). All of these samples
show diamagnetic behavior. But, after RTA treatment the Mn-InAs films exhibit
room-temperature magnetism. According to PIXE measurements the Mn atoms are no
longer substitutional. When the same set of experiments were performed with As
as implantation ion all of the layers present diamagnetism without exception.
This indicates that the appearance of room-temperature ferromagnetic-like
behavior in the Mn-InAs-RTA layer is not related to lattice disorder produce
during implantation, but to a Mn reaction produced after a short thermal
treatment. X-ray diffraction patterns (XRD) and Rutherford Back Scattering
(RBS) measurements evidence the segregation of an oxygen deficient-MnO2 phase
(nominally MnO1.94) in the Mn-InAs-RTA epitaxial layers which might be on the
origin of room temperature ferromagnetic-like response observed.Comment: 16 pages, 5 figures. Acepted in J. Appl. Phy
Electron-induced rippling in graphene
We show that the interaction between flexural phonons, when corrected by the
exchange of electron-hole excitations, may place the graphene sheet very close
to a quantum critical point characterized by the strong suppression of the
bending rigidity of the membrane. Ripples arise then due to spontaneous
symmetry breaking, following a mechanism similar to that responsible for the
condensation of the Higgs field in relativistic field theories. In the presence
of membrane tensions, ripple condensation may be reinforced or suppressed
depending on the sign of the tension, following a zero-temperature buckling
transition in which the order parameter is given essentially by the square of
the gradient of the flexural phonon field.Comment: 4 pages, 3 figure
Many-body effects in doped graphene on a piezoelectric substrate
We investigate the many-body properties of graphene on top of a piezoelectric
substrate, focusing on the interaction between the graphene electrons and the
piezoelectric acoustic phonons. We calculate the electron and phonon
self-energies as well as the electron mobility limited by the substrate
phonons. We emphasize the importance of the proper screening of the
electron-phonon vertex and discuss the various limiting behaviors as a function
of electron energy, temperature, and doping level. The effect on the graphene
electrons of the piezoelectric acoustic phonons is compared with that of the
intrinsic deformation acoustic phonons of graphene. Substrate phonons tend to
dominate over intrinsic ones for low doping levels at high and low
temperatures.Comment: 13 pages, 8 figure
Charge distribution and screening in layered graphene systems
The charge distribution induced by external fields in finite stacks of
graphene planes, or in semiinfinite graphite is considered. The interlayer
electronic hybridization is described by a nearest neighbor hopping term, and
the charge induced by the self consistent electrostatic potential is calculated
within linear response (RPA). The screening properties are determined by
contributions from inter- and intraband electronic transitions. In neutral
systems, only interband transitions contribute to the charge polarizability,
leading to insulating-like screening properties, and to oscillations in the
induced charge, with a period equal to the interlayer spacing. In doped
systems, we find a screening length equivalent to 2-3 graphene layers,
superimposed to significant charge oscillations.Comment: 8 page
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