226 research outputs found
Linear polarization of the photoluminescence of quantum wells
The degree and orientation of the magnetic-field induced linear polarization
of the photoluminescence from a wide range of heterostructures containing
(Cd,Mn)Te quantum wells between (Cd,Mn,Mg)Te barriers has been studied as a
function of detection photon energy, applied magnetic field strength and
orientation in the quantum well plane. A theoretical description of this effect
in terms of an in-plane deformation acting on the valence band states is
presented and is verified by comparison with the experimental data. We
attempted to identify clues to the microscopic origin of the valence band spin
anisotropy and to the mechanisms which actually determine the linear
polarization of the PL in the quantum wells subject to the in-plane magnetic
field. The conclusions of the present paper apply in full measure to
non-magnetic QWs as well as ensembles of disk-like QDs with shape and/or strain
anisotropy.Comment: 21 pages, 10 figure
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The 1/N Expansion in Noncommutative Quantum Mechanics
We study the 1/N expansion in noncommutative quantum mechanics for the
anharmonic and Coulombian potentials. The expansion for the anharmonic
oscillator presented good convergence properties, but for the Coulombian
potential, we found a divergent large N expansion when using the usual
noncommutative generalization of the potential. We proposed a modified version
of the noncommutative Coulombian potential which provides a well-behaved 1/N
expansion.Comment: v2: resided version, to appear in PRD, 18 pages, 4 figure
Suris tetrons: possible spectroscopic evidence for four-particle optical excitations of the 2D electron gas
The excitations of a two-dimensional electron gas in quantum wells with
intermediate carrier density (~10^{11} cm^{-2}), i.e., between the
exciton-trion- and the Fermi-Sea range, are so far poorly understood. We report
on an approach to bridge this gap by a magneto-photoluminescence study of
modulation-doped (Cd,Mn)Te quantum well structures. Employing their enhanced
spin splitting, we analyzed the characteristic magnetic-field behavior of the
individual photoluminescence features. Based on these results and earlier
findings by other authors, we present a new approach for understanding the
optical transitions at intermediate densities in terms of four-particle
excitations, the Suris tetrons, which were up to now only predicted
theoretically. All characteristic photoluminescence features are attributed to
emission from these quasi-particles when attaining different final states.Comment: 12 pages, 3 figure
Exciton spin decay modified by strong electron-hole exchange interaction
We study exciton spin decay in the regime of strong electron-hole exchange
interaction. In this regime the electron spin precession is restricted within a
sector formed by the external magnetic field and the effective exchange fields
triggered by random spin flips of the hole. Using Hanle effect measurements, we
demonstrate that this mechanism dominates our experiments in CdTe/(Cd,Mg)Te
quantum wells. The calculations provide a consistent description of the
experimental results, which is supported by independent measurements of the
parameters entering the model.Comment: 5 pages, 3 figure
Survival of adult neurons lacking cholesterol synthesis in vivo
BACKGROUND: Cholesterol, an essential component of all mammalian plasma membranes, is highly enriched in the brain. Both during development and in the adult, brain cholesterol is derived from local cholesterol synthesis and not taken up from the circulation. However, the contribution of neurons and glial cells to total brain cholesterol metabolism is unknown. RESULTS: Using conditional gene inactivation in the mouse, we disrupted the squalene synthase gene (fdft1), which is critical for cholesterol synthesis, in cerebellar granule cells and some precerebellar nuclei. Mutant mice showed no histological signs of neuronal degeneration, displayed ultrastructurally normal synapses, and exhibited normal motor coordination. This revealed that these adult neurons do not require cell-autonomous cholesterol synthesis for survival or function. CONCLUSION: We conclude that at least some adult neurons no longer require endogenous cholesterol synthesis and can fully meet their cholesterol needs by uptake from their surrounding. Glia are a likely source of cholesterol in the central nervous system
Guanosine effect on cholesterol efflux and apolipoprotein E expression in astrocytes
The main source of cholesterol in the central nervous system (CNS) is represented by glial cells, mainly astrocytes, which also synthesise and secrete apolipoproteins, in particular apolipoprotein E (ApoE), the major apolipoprotein in the brain, thus generating cholesterol-rich high density lipoproteins (HDLs). This cholesterol trafficking, even though still poorly known, is considered to play a key role in different aspects of neuronal plasticity and in the stabilisation of synaptic transmission. Moreover, cell cholesterol depletion has recently been linked to a reduction in amyloid beta formation. Here we demonstrate that guanosine, which we previously reported to exert several neuroprotective effects, was able to increase cholesterol efflux from astrocytes and C6 rat glioma cells in the absence of exogenously added acceptors. In this effect the phosphoinositide 3 kinase/extracellular signal-regulated kinase 1/2 (PI3K/ERK1/2) pathway seems to play a pivotal role. Guanosine was also able to increase the expression of ApoE in astrocytes, whereas it did not modify the levels of ATP-binding cassette protein A1 (ABCA1), considered the main cholesterol transporter in the CNS. Given the emerging role of cholesterol balance in neuronal repair, these effects provide evidence for a role of guanosine as a potential pharmacological tool in the modulation of cholesterol homeostasis in the brain
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