2,311 research outputs found
Control of many electron states in semiconductor quantum dots by non-Abelian vector potentials
Adiabatic time evolution of degenerate eigenstates of a quantum system
provides a means for controlling electronic states since mixing between
degenerate levels generates a matrix Berry phase. In the presence of spin-orbit
coupling in n-type semiconductor quantum dots the electron Hamiltonian is
invariant under time reversal operation and the many body groundstate may be
doubly degenerate. This double degeneracy can generate non-Abelian vector
potentials when odd number of electrons are present. We find that the
antisymmetry of many electron wavefunction has no effect on the matrix Berry
phase. We have derived equations that allow one to investigate the effect of
electron correlations by expressing the non-Abelian vector potentials for many
electron system in terms of single electron non-Abelian vector potentials.Comment: minor changes included, accepted in Phys. Rev.
Far-infrared spectra of lateral quantum dot molecules
We study effects of electron-electron interactions and confinement potential
on the magneto-optical absorption spectrum in the far-infrared range of lateral
quantum dot molecules. We calculate far-infrared (FIR) spectra for three
different quantum dot molecule confinement potentials. We use accurate exact
diagonalization technique for two interacting electrons and calculate
dipole-transitions between two-body levels with perturbation theory. We
conclude that the two-electron FIR spectra directly reflect the symmetry of the
confinement potential and interactions cause only small shifts in the spectra.
These predictions could be tested in experiments with nonparabolic quantum dots
by changing the number of confined electrons. We also calculate FIR spectra for
up to six noninteracting electrons and observe some additional features in the
spectrum.Comment: For better quality Figs download manuscript from
http://www.fyslab.hut.fi/~mma/FIR/Helle_qdmfir.ps.g
Time-Resolved Studies of a Rolled-Up Semiconductor Microtube Laser
We report on lasing in rolled-up microtube resonators. Time-resolved studies
on these semiconductor lasers containing GaAs quantum wells as optical gain
material reveal particularly fast turn-on-times and short pulse emissions above
the threshold. We observe a strong red-shift of the laser mode during the pulse
emission which is compared to the time evolution of the charge-carrier density
calculated by rate equations
Experimental Investigation of Pervaporation Membranes for Biobutanol Separation
Biotechnological production of chemical building blocks is one important step towards a more sustainable production. Unfortunately, the products to be separated are often highly diluted. Pervaporation has received increasing attention for the separation of small amounts of organic compounds from aqueous solutions, especially in the separation of butanol from water or from fermentation broth. To evaluate the potential of pervaporation for biobutanol recovery a consistent database is required, describing the dependency of permeate fluxes and selectivities on process variables like temperature, permeate pressure as well as feed concentrations and compositions. Therefore, within this work we investigated the separation behaviour of a commercially available polydimethylsiloxane (PDMS) membrane and membranes based on poly(ether block amide) (PEBA) fabricated in our own laboratory. The membranes were tested under varying operating conditions. Fermentation by-products or impurities may affect the pervaporation separation performance. Therefore, in addition, the permeate fluxes and the influence of acetone, ethanol, acetic and butyric acid and 1,3-propanediol have been investigated in detail as well. Several differences in the permeability and selectivity of PDMS and PEBA were observed during the experimental study. Swelling experiments were applied to further analyse the separation behaviour of PDMS and PEBA more in detail. Finally the influence of the observed separation performances on the overall butanol pervaporation process is discussed. It was found that especially well permeating by-products like acetone can drastically influence the subsequent downstreaming process
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