6,087 research outputs found
Single exciton spectroscopy of semimagnetic quantum dots
A photo-excited II-VI semiconductor nanocrystal doped with a few Mn spins is
considered. The effects of spin-exciton interactions and the resulting
multi-spin correlations on the photoluminescence are calculated by numerical
diagonalization of the Hamiltonian, including exchange interaction between
electrons, holes and Mn spins, as well as spin-orbit interaction. The results
provide a unified description of recent experiments of photoluminesnce of dots
with one and many Mn atoms as well as optically induced ferromagnetism in
semimagnetic nanocrystals.Comment: 5 pages, 3 figure
Redefining the boundaries of interplanetary coronal mass ejections from observations at the ecliptic plane
On 2015 January 6-7, an interplanetary coronal mass ejection (ICME) was
observed at L1. This event, which can be associated with a weak and slow
coronal mass ejection, allows us to discuss on the differences between the
boundaries of the magnetic cloud and the compositional boundaries. A fast
stream from a solar coronal hole surrounding this ICME offers a unique
opportunity to check the boundaries' process definition and to explain
differences between them. Using Wind and ACE data, we perform a complementary
analysis involving compositional, magnetic, and kinematic observations
providing relevant information regarding the evolution of the ICME as
travelling away from the Sun. We propose erosion, at least at the front
boundary of the ICME, as the main reason for the difference between the
boundaries, and compositional signatures as the most precise diagnostic tool
for the boundaries of ICMEs.Comment: 9 pages and 7 figures in the original forma
Supergranular-scale magnetic flux emergence beneath an unstable filament
Here we report evidence of a large solar filament eruption on 2013, September
29. This smooth eruption, which passed without any previous flare, formed after
a two-ribbon flare and a coronal mass ejection towards Earth. The coronal mass
ejection generated a moderate geomagnetic storm on 2013, October 2 with very
serious localized effects. The whole event passed unnoticed to flare-warning
systems.
We have conducted multi-wavelength analyses of the Solar Dynamics Observatory
through Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager
(HMI) data. The AIA data on 304, 193, 211, and 94 \AA sample the transition
region and the corona, respectively, while HMI provides photospheric
magnetograms, continuum, and linear polarization data, in addition to the fully
inverted data provided by HMI.
[...]
We have observed a supergranular-sized emergence close to a large filament in
the boundary of the active region NOAA11850. Filament dynamics and magnetogram
results suggest that the magnetic flux emergence takes place in the
photospheric level below the filament. Reconnection occurs underneath the
filament between the dipped lines that support the filament and the
supergranular emergence. The very smooth ascent is probably caused by this
emergence and torus instability may play a fundamental role, which is helped by
the emergence.Comment: 9 pages, 6 figures, online material at Journa
Electronic transport and vibrational modes in the smallest molecular bridge: H2 in Pt nanocontacts
We present a state-of-the-art first-principles analysis of electronic
transport in a Pt nanocontact in the presence of H2 which has been recently
reported by Smit et al. in Nature 419, 906 (2002). Our results indicate that at
the last stages of the breaking of the Pt nanocontact two basic forms of bridge
involving H can appear. Our claim is, in contrast to Smit et al.'s, that the
main conductance histogram peak at G approx 2e^2/h is not due to molecular H2,
but to a complex Pt2H2 where the H2 molecule dissociates. A first-principles
vibrational analysis that compares favorably with the experimental one also
supports our claim .Comment: 5 pages, 3 figure
A Carrington-like geomagnetic storm observed in the 21st century
In September 1859 the Colaba observatory measured the most extreme
geomagnetic disturbance ever recorded at low latitudes related to solar
activity: the Carrington storm. This paper describes a geomagnetic disturbance
case with a profile extraordinarily similar to the disturbance of the
Carrington event at Colaba: the event on 29 October 2003 at Tihany magnetic
observatory in Hungary. The analysis of the H-field at different locations
during the "Carrington-like" event leads to a re-interpretation of the 1859
event. The major conclusions of the paper are the following: (a) the global Dst
or SYM-H, as indices based on averaging, missed the largest geomagnetic
disturbance in the 29 October 2003 event and might have missed the 1859
disturbance, since the large spike in the horizontal component (H) of
terrestrial magnetic field depends strongly on magnetic local time (MLT); (b)
the main cause of the large drop in H recorded at Colaba during the Carrington
storm was not the ring current but field-aligned currents (FACs), and (c) the
very local signatures of the H-spike imply that a Carrington-like event can
occur more often than expected.Comment: 18 pages, 2 figures, accepted for publication in SWS
An effective lowest Landau level treatment of demagnetization in superconducting mesoscopic disks
Demagnetization, which is inherently present in the magnetic response of
small finite-size superconductors, can be accounted for by an effective
within a two-dimensional lowest Landau level approximation of the
Ginzburg-Landau functional. We show this by comparing the equilibrium
magnetization of superconducting mesoscopic disks obtained from the numerical
solution of the three-dimensional Ginzburg-Landau equations with that obtained
in the ``effective'' LLL approximation.Comment: 5 pages, 4 figures, submitted to Phys. Rev.
Localized basis sets for unbound electrons in nanoelectronics
It is shown how unbound electron wave functions can be expanded in a suitably
chosen localized basis sets for any desired range of energies. In particular,
we focus on the use of gaussian basis sets, commonly used in first-principles
codes. The possible usefulness of these basis sets in a first-principles
description of field emission or scanning tunneling microscopy at large bias is
illustrated by studying a simpler related phenomenon: The lifetime of an
electron in a H atom subjected to a strong electric field.Comment: 6 pages, 5 figures, accepted by J. Chem. Phys. (http://jcp.aip.org/
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