320 research outputs found
Circular photogalvanic effect induced by monopolar spin orientation in p-GaAs/AlGaAs MQW
The circular photogalvanic effect (CPGE) has been observed in (100)-oriented
-GaAs/AlGaAs quantum wells at normal incidence of far-infrared radiation. It
is shown that monopolar optical spin orientation of free carriers causes an
electric current which reverses its direction upon changing from left to right
circularly polarized radiation. CPGE at normal incidence and the occurence of
the linear photogalvanic effect indicate a reduced point symmetry of studied
multi-layered heterostructures. As proposed, CPGE can be utilized to
investigate separately spin polarization of electrons and holes and the
symmetry of quantum wells.Comment: 4 pages, 3 figure
Laterally pumped GaAs/AlGaAs quantum wells as sources of broadband terahertz radiation
In this work we consider lateral current pumped GaAs/AlGaAs quantum wells as sources of incoherent terahertz radiation. The lateral field heats the electrons in a two-dimensional quantum layer and increases the population of higher subbands, hence also increasing the radiation power generated in spontaneous intersubband emission processes. Digitally graded quasi-parabolic and simple square quantum wells are considered, and the advantages of both types are discussed. Calculations at lattice temperatures of 77 K and 300 K, for electric fields up to 10 kV/cm, show that the optical output power of ~100â200 W/m2 may be achieved for the 7 THz source. The main peak of the spectrum, at 7 THz, of the quasi-parabolic quantum well exceeds the black body radiation at 300 K by approximately a factor of two and by two orders of magnitude at 77 K
Optical Spin Orientation under Inter- and Intra-Subband Transitions in QWs
It is shown that absorption of circularly polarized infrared radiation
achieved by inter-subband and intra-subband (Drude-like) transitions results in
a monopolar spin orientation of free carriers. The monopolar spin polarization
in zinc-blende-based quantum wells (QWs) is demonstrated by the observation of
the spin-galvanic and circular photogalvanic effects. It is shown that
monopolar spin orientation in n-type QWs becomes possible if an admixture of
valence band states to the conduction band wave function and the spin-orbit
splitting of the valence band are taken into account
Non-signalling energy use in the developing rat brain
Energy use in the brain constrains its information processing power, but only about half the brain's energy consumption is directly related to information processing. Evidence for which non-signalling processes consume the rest of the brain's energy has been scarce. For the first time, we investigated the energy use of the brain's main non-signalling tasks with a single method. After blocking each non-signalling process, we measured oxygen level changes in juvenile rat brain slices with an oxygen-sensing microelectrode and calculated changes in oxygen consumption throughout the slice using a modified diffusion equation. We found that the turnover of the actin and microtubule cytoskeleton, followed by lipid synthesis, are significant energy drains, contributing 25%, 22% and 18%, respectively, to the rate of oxygen consumption. In contrast, protein synthesis is energetically inexpensive. We assess how these estimates of energy expenditure relate to brain energy use in vivo, and how they might differ in the mature brain
HITRAP: A facility at GSI for highly charged ions
An overview and status report of the new trapping facility for highly charged
ions at the Gesellschaft fuer Schwerionenforschung is presented. The
construction of this facility started in 2005 and is expected to be completed
in 2008. Once operational, highly charged ions will be loaded from the
experimental storage ring ESR into the HITRAP facility, where they are
decelerated and cooled. The kinetic energy of the initially fast ions is
reduced by more than fourteen orders of magnitude and their thermal energy is
cooled to cryogenic temperatures. The cold ions are then delivered to a broad
range of atomic physics experiments.Comment: 8 pages, 11 figure
Monopolar Optical Orientation of Electronic Spins in Semiconductors
It is shown that absorption of circularly polarized infrared radiation due to
intraband (Drude-like) transitions in n-type bulk semiconductors and due to
intra-subband or inter-subband transitions in quantum well (QW) structures
results in a monopolar spin orientation of free electrons. Spin polarization in
zinc-blende-structure based QWs is demonstrated by the observation of the
spin-galvanic and the circular photogalvanic effects. The monopolar spin
orientation in n-type materials is shown to be possible if an admixture of
valence band states to the conduction band wave function and the spin-orbit
splitting of the valence band are taken into account
Mass measurements of very neutron-deficient Mo and Tc isotopes and their impact on rp process nucleosynthesis
The masses of ten proton-rich nuclides, including the N=Z+1 nuclides 85-Mo
and 87-Tc, were measured with the Penning trap mass spectrometer SHIPTRAP.
Compared to the Atomic Mass Evaluation 2003 a systematic shift of the mass
surface by up to 1.6 MeV is observed causing significant abundance changes of
the ashes of astrophysical X-ray bursts. Surprisingly low alpha-separation
energies for neutron-deficient Mo and Tc are found, making the formation of a
ZrNb cycle in the rp process possible. Such a cycle would impose an upper
temperature limit for the synthesis of elements beyond Nb in the rp process.Comment: Link to online abstract:
http://link.aps.org/doi/10.1103/PhysRevLett.106.12250
Direct mass measurements beyond the proton drip-line
First on-line mass measurements were performed at the SHIPTRAP Penning trap
mass spectrometer. The masses of 18 neutron-deficient isotopes in the
terbium-to-thulium region produced in fusion-evaporation reactions were
determined with relative uncertainties of about , nine of them
for the first time. Four nuclides (Ho and Tm) were
found to be proton-unbound. The implication of the results on the location of
the proton drip-line is discussed by analyzing the one-proton separation
energies
Spin sensitive bleaching and monopolar spin orientation in quantum wells
Spin sensitive bleaching of the absorption of far-infrared radiation has been
observed in -type GaAs/AlGaAs quantum well structures. The absorption of
circularly polarized radiation saturates at lower intensities than that of
linearly polarized light due to monopolar spin orientation in the first heavy
hole subband. Spin relaxation times of holes in -type material in the range
of tens of ps were derived from the intensity dependence of the absorption.Comment: Figures have been updated due to technical printing problems
(Postscript mismatch
Present and Future Experiments with Stored Exotic Nuclei at Relativistic Energies
Recent progress is presented from experiments on masses and lifetimes of bare
and few-electron exotic nuclei at GSI.Comment: Proceedings of International Conference on "Frontiers in Nuclear
Structure, Astrophysics and Reactions", Kos, Greece, September 12-17, 200
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