6,049 research outputs found
Interactions and star formation activity in Wolf-Rayet galaxies
We present the main results of the PhD Thesis carried out by
L\'opez-S\'anchez (2006), in which a detailed morphological, photometrical and
spectroscopical analysis of a sample of 20 Wolf-Rayet (WR) galaxies was
realized. The main aims are the study of the star formation and O and WR
stellar populations in these galaxies and the role that interactions between
low surface companion objects have in the triggering of the bursts. We analyze
the morphology, stellar populations, physical conditions, chemical abundances
and kinematics of the ionized gas, as well as the star-formation activity of
each system.Comment: 16 pages, 15 figure
Coherent spin rotations in open driven double quantum dots
We analyze the charge and spin dynamics in a DC biased double quantum dot
driven by crossed DC and AC magnetic fields. In this configuration, spatial
delocalization due to inter-dot tunnel competes with intra-dot spin rotations
induced by the time dependent magnetic field, giving rise to a complicated time
dependent behavior of the tunnelling current. When the Zeeman splitting has the
same value in both dots and spin flip is negligible, the electrons remain in
the triplet subspace (dark subspace) performing coherent spin rotations and the
current does not flow. This electronic trapping is removed either by finite
spin relaxation or when the Zeeman splitting is different in each quantum dot.
In the first case, our results show that measuring the current will allow to
get information on the spin relaxation time. In the last case, we will show
that applying a resonant bichromatic magnetic field, the electrons become
trapped in a coherent superposition of states and electronic transport is
blocked. Then, manipulating AC magnetic fields, electrons are driven to perform
coherent spin rotations which can be unambiguously detected by direct
measurement of the tunneling current.Comment: 7 pages, 6 figures. Extended published versio
Strength distribution of solar magnetic fields in photospheric quiet Sun regions
The magnetic topology of the solar photosphere in its quietest regions is
hidden by the difficulties to disentangle magnetic flux through the resolution
element from the field strength of unresolved structures. The observation of
spectral lines with strong coupling with hyperfine structure, like the observed
MnI line at 553.7 nm, allows such differentiation.
The main aim is to analyse the distribution of field strengths in the network
and intranetwork of the solar photosphere through inversion of the MnI line at
553.7 nm.
An inversion code for the magnetic field using the Principal Component
Analysis (PCA) has been developed. Statistical tests are run on the code to
validate it. The code has to draw information from the small-amplitude spectral
feature oppearing in the core of the Stokes V profile of the observed line for
field strengths below a certain threshold, coinciding with lower limit of the
Paschen-Back effect in the fine structure of the involved atomic levels.
The inversion of the observed profiles, using the circular polarization (V)
and the intensity (I), shows the presence of magnetic fields strengths in a
range from 0 to 2 kG, with predominant weak strength values. Mixed regions with
mean strength field values of 1130 and 435 Gauss are found associated with the
network and intranetwork respectively.
The MnI line at 553 nm probes the field strength distribution in the quiet
sun and shows the predominance of weak, hectoGauss fields in the intranetwork,
and strong, kiloGauss fields in the network. It also shows that both network
and intranetwork are to be understood at our present spatial resolutions as
field distributions of which we hint the mean properties.Comment: 10 pages, 6 figure
Exciting polaritons with quantum light
We discuss the excitation of polaritons---strongly-coupled states of light
and matter---by quantum light, instead of the usual laser or thermal
excitation. As one illustration of the new horizons thus opened, we introduce
Mollow spectroscopy, a theoretical concept for a spectroscopic technique that
consists in scanning the output of resonance fluorescence onto an optical
target, from which weak nonlinearities can be read with high precision even in
strongly dissipative environments.Comment: 5 pages, 3 figure
Kramers polarization in strongly correlated carbon nanotube quantum dots
Ferromagnetic contacts put in proximity with carbon nanotubes induce spin and
orbital polarizations. These polarizations affect dramatically the Kondo
correlations occurring in quantum dots formed in a carbon nanotube, inducing
effective fields in both spin and orbital sectors. As a consequence, the carbon
nanotube quantum dot spectral density shows a four-fold split SU(4) Kondo
resonance. Furthermore, the presence of spin-orbit interactions leads to the
occurrence of an additional polarization among time-reversal electronic states
(polarization in the time-reversal symmetry or Kramers sector). Here, we
estimate the magnitude for the Kramer polarization in realistic carbon nanotube
samples and find that its contribution is comparable to the spin and orbital
polarizations. The Kramers polarization generates a new type of effective field
that affects only the time-reversal electronic states. We report new splittings
of the Kondo resonance in the dot spectral density which can be understood only
if Kramers polarization is taken into account. Importantly, we predict that the
existence of Kramers polarization can be experimentally detected by performing
nonlinear differential conductance measurements. We also find that, due to the
high symmetry required to build SU(4) Kondo correlations, its restoration by
applying an external field is not possible in contrast to the compensated SU(2)
Kondo state observed in conventional quantum dots.Comment: 8 pages, 4figure
The intriguing HI gas in NGC 5253: an infall of a diffuse, low-metallicity HI cloud?
(Abridged) We present new, deep HI line and 20-cm radio continuum data of the
very puzzling blue compact dwarf galaxy NGC 5253, obtained with the ATCA as
part of the `Local Volume HI Survey' (LVHIS). Our low-resolution HI maps show
the disturbed HI morphology that NGC 5253 possesses, including tails, plumes
and detached HI clouds. The high-resolution map reveals an HI plume at the SE
and an HI structure at the NW that surrounds an Ha shell. We confirm that the
kinematics of the neutral gas are highly perturbed and do not follow a rotation
pattern. We discuss the outflow and infall scenarios to explain such disturbed
kinematics, analyze the environment in which it resides, and compare it
properties with those observed in similar star-forming dwarf galaxies. The
radio-continuum emission of NGC 5253 is resolved and associated with the
intense star-forming region at the center of the galaxy. We complete the
analysis using multiwavelength data extracted from the literature. We estimate
the SFR using this multiwavelength approach. NGC 5253 does not satisfy the
Schmidt-Kennicutt law of star-formation, has a very low HI mass-to-light ratio
when comparing with its stellar mass, and seems to be slightly metal-deficient
in comparison with starbursts of similar baryonic mass. Taking into account all
available multiwavelength data, we conclude that NGC 5253 is probably
experiencing the infall of a diffuse, low-metallicity HI cloud along the minor
axis of the galaxy, which is comprising the ISM and triggering the powerful
starburst. The tidally disturbed material observed at the east and north of the
galaxy is a consequence of this interaction, which probably started more than
100 Myr ago. The origin of this HI cloud may be related with a strong
interaction between NGC 5253 and the late-type spiral galaxy M 83 in the past.Comment: 19 pages, 12 figures, accepted for publication in MNRA
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