Biomedical subjects

M Bichler

Publications and source records attributed to M Bichler.

11 recordsLinked to original sources

PubMed2002

Spin-sensitive bleaching and monopolar spin orientation in quantum wells.

Spin-sensitive bleaching of the absorption of far-infrared radiation has been observed in p-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 p-type material in the range of tens of ps were derived from the intensity dependence of the absorption.

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PubMed2001

How many-particle interactions develop after ultrafast excitation of an electron-hole plasma.

Electrostatic coupling between particles is important in many microscopic phenomena found in nature. The interaction between two isolated point charges is described by the bare Coulomb potential, but in many-body systems this interaction is modified as a result of the collective response of the screening cloud surrounding each charge carrier. One such system involves ultrafast interactions between quasi-free electrons in semiconductors-which are central to high-speed and future quantum electronic devices. The femtosecond kinetics of nonequilibrium Coulomb systems has been calculated using static and dynamical screening models that assume the instantaneous formation of interparticle correlations. However, some quantum kinetic theories suggest that a regime of unscreened bare Coulomb collisions might exist on ultrashort timescales. Here we monitor directly the temporal evolution of the charge-charge interactions after ultrafast excitation of an electron-hole plasma in GaAs. We show that the onset of collective behaviour such as Coulomb screening and plasmon scattering exhibits a distinct time delay of the order of the inverse plasma frequency, that is, several 10(-14) seconds.

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PubMed2001

Energy spectra of quantum rings.

Quantum mechanical experiments in ring geometries have long fascinated physicists. Open rings connected to leads, for example, allow the observation of the Aharonov-Bohm effect, one of the best examples of quantum mechanical phase coherence. The phase coherence of electrons travelling through a quantum dot embedded in one arm of an open ring has also been demonstrated. The energy spectra of closed rings have only recently been studied by optical spectroscopy. The prediction that they allow persistent current has been explored in various experiments. Here we report magnetotransport experiments on closed rings in the Coulomb blockade regime. Our experiments show that a microscopic understanding of energy levels, so far limited to few-electron quantum dots, can be extended to a many-electron system. A semiclassical interpretation of our results indicates that electron motion in the rings is governed by regular rather than chaotic motion, an unexplored regime in many-electron quantum dots. This opens a way to experiments where even more complex structures can be investigated at a quantum mechanical level.

Journal Article↗

PubMed2001

Signatures of spin pairing in chaotic quantum dots.

Coulomb blockade resonances are measured in a GaAs quantum dot in which both shape deformations and interactions are small. The parametric evolution of the Coulomb blockade peaks shows a pronounced pair correlation in both position and amplitude, which is interpreted as spin pairing. As a consequence, the nearest-neighbor distribution of peak spacings can be well approximated by a modified bimodal Wigner surmise, in which interactions are taken into account beyond the constant interaction model.

Journal Article↗

PubMed2001

Quantum interference in artificial band structures.

Magnetotransport experiments on two-dimensional electron systems with an atomically precise, one-dimensional potential modulation reveal striking quantum interference oscillations. Within a semiclassical framework, they are recognized either as self-interference along closed orbits, many of them rendered possible by magnetic breakdown between Fermi contour segments of the artificial band structure, or as interference-enhanced backscattering. The known commensurability oscillations appear as a special case of the latter mechanism.

Journal Article↗

PubMed2001

Bronchodilatory effects of salbutamol, ipratropium bromide, and their combination: double-blind, placebo-controlled crossover study in cystic fibrosis.

The efficacy of inhaled sympathomimetic and anticholinergic agents on airway obstruction in cystic fibrosis (CF) has been proven in several studies. However, studies comparing combined therapy with monotherapy led to divergent results, probably due to different study designs, different dosages, and the small numbers of patients investigated. Therefore, we wanted to answer the question which inhalation has the best short term effect: a sympathomimetic or an anticholinergic agent, or the combination of both. We investigated 17 patients with CF on 4 successive days in the morning, using pulmonary function testing before and 30 min after inhalation. Each patient received aerosolized salbutamol (SB, maximum dose (max.) 2.5 mg), ipratropium bromide (IB, max. 0.5 mg), the combination of both, or placebo (normal saline) in a randomized, double-blind crossover design. The mean forced expiratory volume in the first second improved significantly (adjusted P-value < 0.017) after each treatment compared to placebo. Analysis of variance showed that SB and combination therapy with SB and IB were superior to IB alone, without significant difference between SB and combination therapy. Response of a patient to combined therapy was usually associated with response to SB. Long-term efficacy and side effects of treatment with bronchodilators still remain to be investigated after this short term study. We conclude that in CF patients bronchodilator therapy with sympathomimetic agents is usually sufficient. Only in cases with proven additional benefit from inhalation by anticholinergics should combination therapy be recommended.

Administration, Inhalation↗

PubMed2000

Quantum hall ferromagnetism in a two-dimensional electron system

Experiments on a nearly spin degenerate two-dimensional electron system reveals unusual hysteretic and relaxational transport in the fractional quantum Hall effect regime. The transition between the spin-polarized (with fill fraction nu = 1/3) and spin-unpolarized (nu = 2/5) states is accompanied by a complicated series of hysteresis loops reminiscent of a classical ferromagnet. In correlation with the hysteresis, magnetoresistance can either grow or decay logarithmically in time with remarkable persistence and does not saturate. In contrast to the established models of relaxation, the relaxation rate exhibits an anomalous divergence as temperature is reduced. These results indicate the presence of novel two-dimensional ferromagnetism with a complicated magnetic domain dynamic.

Journal Article↗

PubMed2000

Ultrafast optical spectroscopy of large-momentum excitons in GaAs.

Highly energetic excitons with wave vectors much larger than that of an absorbed photon are excited in thin GaAs films. We observe propagation beats between three polariton modes up to 300 meV above the absorption edge employing femtosecond transmission spectroscopy. The dispersion relations of the coherent excitations are measured. Ultrafast exciton damping via scattering with nonequilibrium carriers and with phonons is investigated. The dynamics is found to deviate strongly from the relaxation of free carriers. Theoretical simulations are in quantitative agreement with the data.

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PubMed1999

A Semiconductor-Based Photonic Memory Cell.

Photonic signals were efficiently stored in a semiconductor-based memory cell. The incident photons were converted to electron-hole pairs that were locally stored in a quantum well that was laterally modulated by a field-effect tunable electrostatic superlattice. At large superlattice potential amplitudes, these pairs were stored for a time that was at least five orders of magnitude longer than their natural lifetime. At an arbitrarily chosen time, they were released in a short and intense flash of incoherent light, which was triggered by flattening the superlattice amplitude.

Journal Article↗

PubMed1997

Coupled quantum dots fabricated by cleaved edge overgrowth: from artificial atoms to molecules

Atomically precise quantum dots of mesoscopic size have been fabricated in the gallium arsenide-aluminum gallium arsenide material system by cleaved edge overgrowth, with a high degree of control over shape, composition, and position. The formation of bonding and antibonding states between two such "artificial atoms" was studied as a function of quantum dot separation by microscopic photoluminescence (PL) spectroscopy. The coupling strength within these "artificial molecules" is characterized by a systematic dependence of the separation of the bonding and antibonding levels, and of the PL linewidth, on the "interatomic" distance. This model system opens new insights into the physics of coupled quantum objects.

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PubMed1994

Application of short-lived radionuclides in neutron activation analysis of biological and environmental samples.

The application of short-lived nuclides, especially in connection with the 6LiD-converter, in biological and environmental samples is demonstrated on I and Br determination in human urine, on I in pet food, and on the analysis of all the halogens in volcanic gases in a single activation. Trace element determination in lichens indicates polluted and unpolluted areas. The use of the .74-s 38mCl enables the rapid screening of great number of samples.

Animal Feed↗