5,013 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
¿El ciclo hidrológico o el ciclo de atención mediática? Estudio empírico de los encuadres noticiosos del AGUA en la prensa española
Este estudio se basa en los análisis de contenido de las noticias sobre el agua publicadas en la prensa española en el período de 1960-2004, con el fin de examinar la forma en que los temas hidrológicos con tradición más arraigada han incorporado temas modernos o han cedido a ellos durante los últimos 45 años de la historia. De este modo, se puede observar si se ha impulsado la agenda de nuevos temas del agua o, si se emplean encuadres peculiares para atribuir a los temas existentes ideas innovadoras
How extroversion affects student attitude toward the combined use of a wiki and video recording of group presentations
The aim of this paper is to analyze the effect of extroversion on students'' attitude toward the combined use of a wiki and the video recording of oral presentations to improve communication skills using a quantitative approach. The model includes stress because it is considered an important aspect in public speaking situations, especially so in a case like ours where the videos will be available to the class. The students’ enjoyment and the relative advantages of the learning activity are also included because they may have an influence on satisfaction and course recommendation. A survey was carried out among first-year undergraduate students. Using partial least squares methodology, the results suggest that extrovert individuals perceive less stress and more enjoyment when performing this activity, which results in them having a better attitude toward it. A total mediation effect of enjoyment between extroversion and attitude is found. Our findings also confirm that stress acts as a barrier to satisfaction, but it does not create a negative reaction toward the activity. Finally, the results show the mediation effect of attitude between the perceived relative advantages and satisfaction and between enjoyment and satisfaction
Critical fields for vortex expulsion from narrow superconducting strips
We calculate the critical magnetic fields for vortex expulsion for an
infinitely long superconducting strip, using the Ginzburg-Landau formalism. Two
critical fields can be defined associated with the disappearance of either the
energetic stability or metastability of vortices in the center of the strip for
decreasing magnetic fields. We compare the theoretical predictions for the
critical fields in the London formalism with ours and with recently published
experimental results. As expected, for narrow strips our results reproduce
better the experimental findings.Comment: 5 pages, 5 figure
Signature of Quantum Hall Effect Skyrmions in Tunneling: A Theoretical Study
We present a theoretical study of the tunneling characteristic between
two parallel two-dimensional electron gases in a perpendicular magnetic field
when both are near filling factor . Finite-size calculations of the
single-layer spectral functions in the spherical geometry and analytical
expressions for the disk geometry in the thermodynamic limit show that the
current in the presence of skyrmions reflects in a direct way their underlying
structure. It is also shown that fingerprints of the electron-electron
interaction pseudopotentials are present in such a current.Comment: 4 pages, 1 figur
Equilibrium susceptibilities of superparamagnets: longitudinal & transverse, quantum & classical
The equilibrium susceptibility of uniaxial paramagnets is studied in a
unified framework which permits to connect traditional results of the theory of
quantum paramagnets, \Sm=1/2, 1, 3/2, ..., with molecular magnetic clusters,
\Sm\sim5, 10, 20, all the way up, \Sm=30, 50, 100,... to the theory of
classical superparamagnets. This is done using standard tools of quantum
statistical mechanics and linear response theory (the Kubo correlator
formalism). Several features of the temperature dependence of the
susceptibility curves (crossovers, peaks, deviations from Curie law) are
studied and their scalings with \Sm identified and characterized. Both the
longitudinal and transverse susceptibilities are discussed, as well as the
response of the ensemble with anisotropy axes oriented at random. For the
latter case a simple approximate formula is derived too, and its range of
validity assessed, so it could be used in modelization of experiments.Comment: 32 pages, 5 figures. Submitted to J.Phys.Condens.Matte
Longitudinal relaxation and thermoactivation of quantum superparamagnets
The relaxation mechanisms of a quantum nanomagnet are discussed in the frame
of linear response theory. We use a spin Hamiltonian with a uniaxial potential
barrier plus a Zeeman term. The spin, having arbitrary , is coupled to a
bosonic environment. From the eigenstructure of the relaxation matrix, we
identify two main mechanisms, namely, thermal activation over the barrier, with
a time scale \eival_1^{-1}, and a faster dynamics inside the potential wells,
with characteristic time \eivalW^{-1}. This allows to introduce a simple
analytical formula for the response, which agrees well with the exact numerical
results, and cover experiments even under moderate to strong fields in the
superparamagnetic range. In passing, we generalize known classical results for
a number of quantities (e.g., integral relaxation times, initial decay time,
Kramers rate), results that are recovered in the limit .Comment: submitted to Phys. Rev.
Spin effects in a confined 2DEG: Enhancement of the g-factor, spin-inversion states and their far-infrared absorption
We investigate several spin-related phenomena in a confined two-dimensional
electron gas (2DEG) using the Hartree-Fock approximation for the mutual Coulomb
interaction of the electrons. The exchange term of the interaction causes a
large splitting of the spin levels whenever the chemical potential lies within
a Landau band (LB). This splitting can be reinterpreted as an enhancement of an
effective g-factor, g*. The increase of g* when a LB is half filled can be
accompanied by a spontaneous formation of a static spin-inversion state (SIS)
whose details depend on the system sision state (SIS) whose details depend on
the system size. The coupling of the states of higher LB's into the lowest band
by the Coulomb interaction of the 2DEG is essential for the SIS to occur. The
far-infrared absorption of the system, relatively insensitive to the spin
splitting, develops clear signs of the SIS.Comment: 7 figure
Vortex matter in superconducting mesoscopic disks: Structure, magnetization, and phase transitions
The dense vortex matter structure and associated magnetization are calculated
for type-II superconducting mesoscopic disks. The magnetization exhibits
generically first-order phase transitions as the number of vortices changes by
one and presents two well-defined regimes: A non-monotonous evolution of the
magnitude of the magnetization jumps signals the presence of a vortex glass
structure which is separated by a second-order phase transition at
from a condensed state of vortices (giant vortex) where the magnitude of the
jumps changes monotonously. We compare our results with Hall magnetometry
measurements by Geim et al. (Nature 390, 259 (1997)) and claim that the
magnetization exhibits clear traces of the presence of these vortex glass
states.Comment: 4 pages, 3 figure
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