49 research outputs found
Effect of Coulomb correlation on electron transport through concentric quantum ring-quantum dot structure
We theoretically study the single electron transfer through two-terminal
quantum ring capacitively coupled to charged dot placed in its center. For this
purpose we solve time-dependent Schrodinger equation for fully correlated
two-particle system constituted by the transferred electron and the second
particle confined in the dot. Analysis of transmission probability dependence
on magnetic field in Ahronov-Bohm effect indicates that the maxima of
transmission probability may be enhanced as well as reduced for attractive or
repulsive Coulomb interaction respectively. The existence of Coulomb
correlation in the system may also lead to inelastic scattering of the
transferred electron. In such case, transmission of electron thorugh the ring
is not completely blocked for (n+1/2) magnetic flux quanta
Few-electron artificial molecules formed by laterally coupled quantum rings
We study the artificial molecular states formed in laterally coupled double
semiconductor nanorings by systems containing one, two and three electrons. An
interplay of the interring tunneling and the electron-electron interaction is
described and its consequences for the magnetization and charging properties of
the system are determined. It is shown that both the magnetic dipole moment
generated by the double ring structure and the chemical potential of the system
as function of the external magnetic field strongly depend on the number of
electrons and the interring barrier thickness. Both the magnetization and
chemical potentials exhibit cusps at the magnetic fields inducing ground-state
parity and / or spin transformations. The symmetry transformations are
discussed for various tunnel coupling strengths: from rings coupled only
electrostatically to the limit of coalesced rings. We find that in the
ground-states for rings of different radii the magnetic field transfers the
electron charge from one ring to the other. The calculations are performed with
the configuration interaction method based on an approach of Gaussian functions
centered on a rectangular array of points covering the studied structure.
Electron-electron correlation is also discussed
Stability of negative and positive trions in quantum wires
Binding energies of negative () and positive trions () in quantum
wires are studied for strong quantum confinement of carriers which results in a
numerical exactly solvable model. The relative electron and hole localization
has a strong effect on the stability of trions. For equal hole and electron
confinement, is more stable but a small imbalance of the particle
localization towards a stronger hole localization e.g. due to its larger
effective mass, leads to the interchange of and recombination lines
in the photoluminescent spectrum as was recently observed experimentally. In
case of larger stability, a magnetic field oriented parallel to the wire
axis leads to a stronger increase of the binding energy resulting in a
crossing of the and lines
Exciton and negative trion dissociation by an external electric field in vertically coupled quantum dots
We study the Stark effect for an exciton confined in a pair of vertically
coupled quantum dots. A single-band approximation for the hole and a parabolic
lateral confinement potential are adopted which allows for the separation of
the lateral center-of-mass motion and consequently for an exact numerical
solution of the Schr\"odinger equation. We show that for intermediate tunnel
coupling the external electric field leads to the dissociation of the exciton
via an avoided crossing of bright and dark exciton energy levels which results
in an atypical form of the Stark shift. The electric-field-induced dissociation
of the negative trion is studied using the approximation of frozen lateral
degrees of freedom. It is shown that in a symmetric system of coupled dots the
trion is more stable against dissociation than the exciton. For an asymmetric
system of coupled dots the trion dissociation is accompanied by a positive
curvature of the recombination energy line as a function of the electric field.Comment: PRB - in prin
In situ biospectroscopic investigation of rapid ischemic and postmortem induced biochemical alterations in the rat brain
© 2014 American Chemical Society. Rapid advances in imaging technologies have pushed novel spectroscopic modalities such as Fourier transform infrared spectroscopy (FTIR) and X-ray absorption spectroscopy (XAS) at the sulfur K-edge to the forefront of direct in situ investigation of brain biochemistry. However, few studies have examined the extent to which sample preparation artifacts confound results. Previous investigations using traditional analyses, such as tissue dissection, homogenization, and biochemical assay, conducted extensive research to identify biochemical alterations that occur ex vivo during sample preparation. In particular, altered metabolism and oxidative stress may be caused by animal death. These processes were a concern for studies using biochemical assays, and protocols were developed to minimize their occurrence. In this investigation, a similar approach was taken to identify the biochemical alterations that are detectable by two in situ spectroscopic methods (FTIR, XAS) that occur as a consequence of ischemic conditions created during humane animal killing. FTIR and XAS are well suited to study markers of altered metabolism such as lactate and creatine (FTIR) and markers of oxidative stress such as aggregated proteins (FTIR) and altered thiol redox (XAS). The results are in accordance with previous investigations using biochemical assays and demonstrate that the time between animal death and tissue dissection results in ischemic conditions that alter brain metabolism and initiate oxidative stress. Therefore, future in situ biospectroscopic investigations utilizing FTIR and XAS must take into consideration that brain tissue dissected from a healthy animal does not truly reflect the in vivo condition, but rather reflects a state of mild ischemia. If studies require the levels of metabolites (lactate, creatine) and markers of oxidative stress (thiol redox) to be preserved as close as possible to the in vivo condition, then rapid freezing of brain tissue via decapitation into liquid nitrogen, followed by chiseling the brain out at dry ice temperatures is required
Synchrotron Radiation X-Ray Microfluorescence Reveals Polarized Distribution of Atomic Elements during Differentiation of Pluripotent Stem Cells
The mechanisms underlying pluripotency and differentiation in embryonic and reprogrammed stem cells are unclear. In this work, we characterized the pluripotent state towards neural differentiated state through analysis of trace elements distribution using the Synchrotron Radiation X-ray Fluorescence Spectroscopy. Naive and neural-stimulated embryoid bodies (EB) derived from embryonic and induced pluripotent stem (ES and iPS) cells were irradiated with a spatial resolution of 20 µm to make elemental maps and qualitative chemical analyses. Results show that these embryo-like aggregates exhibit self-organization at the atomic level. Metallic elements content rises and consistent elemental polarization pattern of P and S in both mouse and human pluripotent stem cells were observed, indicating that neural differentiation and elemental polarization are strongly correlated