46 research outputs found
Modification of emission properties of ZnO layers due to plasmonic near-field coupling to Ag nanoislands
A simple fabrication method of Ag nanoislands on ZnO films is presented.
Continuous wave and time-resolved photoluminescence and transmission are
employed to investigate modifications of visible and UV emissions of ZnO
brought about by coupling to localized surface plasmons residing on Ag
nanoislands. The size of the nanoislands, determining their absorption and
scattering efficiencies, is found to be an important factor governing plasmonic
modification of optical response of ZnO films. The presence of the Ag
nanoislands of appropriate dimensions causes a strong (threefold) increase in
emission intensity and up to 1.5 times faster recombination. The experimental
results are successfully described by model calculations within the Mie theory.Comment: 14 pages, 5 figure
Single spin optical read-out in CdTe/ZnTe quantum dot studied by photon correlation spectroscopy
Spin dynamics of a single electron and an exciton confined in CdTe/ZnTe
quantum dot is investigated by polarization-resolved correlation spectroscopy.
Spin memory effects extending over at least a few tens of nanoseconds have been
directly observed in magnetic field and described quantitatively in terms of a
simple rate equation model. We demonstrate an effective (68%) all-optical
read-out of the single carrier spin state through probing the degree of
circular polarization of exciton emission after capture of an oppositely
charged carrier. The perturbation introduced by the pulsed optical excitation
serving to study the spin dynamics has been found to be the main source of the
polarization loss in the read-out process. In the limit of low laser power the
read-out efficiency extrapolates to a value close to 100%. The measurements
allowed us as well to determine neutral exciton spin relaxation time ranging
from 3.4 +/- 0.1 ns at B = 0 T to 16 +/- 3 ns at B = 5 T.Comment: to appear in Phys. Rev.
Possibility of long-range order in clean mesoscopic cylinders
A microscopic Hamiltonian of the magnetostatic interaction is discussed. This
long-range interaction can play an important role in mesoscopic systems leading
to an ordered ground state.
The self-consistent mean field approximation of the magnetostatic interaction
is performed to give an effective Hamiltonian from which the spontaneous,
self-sustaining currents can be obtained.
To go beyond the mean field approximation the mean square fluctuation of the
total momentum is calculated and its influence on self-sustaining currents in
mesoscopic cylinders with quasi-1D and quasi-2D conduction is considered. Then,
by the use of the microscopic Hamiltonian of the magnetostatic interaction for
a set of stacked rings, the problem of long-range order is discussed. The
temperature below which the system is in an ordered state is
determined.Comment: 14 pages, REVTeX, 5 figures, in print in Phys. Rev.
Coexistence of Anomalous Hall Effect and Weak Net Magnetization in Collinear Antiferromagnet MnTe
Anomalous Hall effect (AHE) plays important role in the rapidly developing
field of antiferromagnetic spintronics. It has been recently discussed that it
can be a feature of not only uncompensated magnetic systems but also in
altermagnetic materials. Hexagonal MnTe belongs to this appealing group of
compounds exhibiting AHE and is commonly perceived as magnetically compensated.
Here, we demonstrate that bulk form of MnTe exhibits small but detectable
magnetic moment correlating with hysteretic behaviour of the AHE. We formulate
a phenomenological model which explains how this feature allows to create a
disbalance between states with opposite N\'eel vector and prevent the AHE
signal from averaging out to zero. Moreover, we show how the dependence of AHE
on the N\'eel vector arises on microscopical level and highlight the
differences in Berry curvature between magnetically compensated and
uncompensated systems
Statistical Analysis of Sleep Spindle Occurrences
Spindles - a hallmark of stage II sleep - are a transient oscillatory phenomenon in the EEG believed to reflect thalamocortical activity contributing to unresponsiveness during sleep. Currently spindles are often classified into two classes: fast spindles, with a frequency of around 14 Hz, occurring in the centro-parietal region; and slow spindles, with a frequency of around 12 Hz, prevalent in the frontal region. Here we aim to establish whether the spindle generation process also exhibits spatial heterogeneity. Electroencephalographic recordings from 20 subjects were automatically scanned to detect spindles and the time occurrences of spindles were used for statistical analysis. Gamma distribution parameters were fit to each inter-spindle interval distribution, and a modified Wald-Wolfowitz lag-1 correlation test was applied. Results indicate that not all spindles are generated by the same statistical process, but this dissociation is not spindle-type specific. Although this dissociation is not topographically specific, a single generator for all spindle types appears unlikely
Characterization of K-Complexes and Slow Wave Activity in a Neural Mass Model
NREM sleep is characterized by two hallmarks, namely K-complexes (KCs) during sleep stage N2 and cortical slow oscillations (SOs) during sleep stage N3. While the underlying dynamics on the neuronal level is well known and can be easily measured, the resulting behavior on the macroscopic population level remains unclear. On the basis of an extended neural mass model of the cortex, we suggest a new interpretation of the mechanisms responsible for the generation of KCs and SOs. As the cortex transitions from wake to deep sleep, in our model it approaches an oscillatory regime via a Hopf bifurcation. Importantly, there is a canard phenomenon arising from a homoclinic bifurcation, whose orbit determines the shape of large amplitude SOs. A KC corresponds to a single excursion along the homoclinic orbit, while SOs are noise-driven oscillations around a stable focus. The model generates both time series and spectra that strikingly resemble real electroencephalogram data and points out possible differences between the different stages of natural sleep
A Thalamocortical Neural Mass Model of the EEG during NREM Sleep and Its Response to Auditory Stimulation
Few models exist that accurately reproduce the complex rhythms of the thalamocortical system that are apparent in measured scalp EEG and at the same time, are suitable for large-scale simulations of brain activity. Here, we present a neural mass model of the thalamocortical system during natural non-REM sleep, which is able to generate fast sleep spindles (12–15 Hz), slow oscillations (<1 Hz) and K-complexes, as well as their distinct temporal relations, and response to auditory stimuli. We show that with the inclusion of detailed calcium currents, the thalamic neural mass model is able to generate different firing modes, and validate the model with EEG-data from a recent sleep study in humans, where closed-loop auditory stimulation was applied. The model output relates directly to the EEG, which makes it a useful basis to develop new stimulation protocols