Biomedical subjects

A Wixforth

Publications and source records attributed to A Wixforth.

15 recordsLinked to original sources

PubMed2002

Nonlinear charge spreading visualized in voltage-controlled lateral superlattices.

Voltage-controlled lateral potential superlattices are used to dramatically increase the lifetime of photogenerated carriers in a quantum well. These long lifetimes, together with the ability to deliberately trigger radiative recombination, enable us to directly visualize the spreading character of nonlinear Maxwell relaxation of 2D charges along narrow channels. Our system allows for temporal and spatial resolution of Maxwell kinetics, usually a very fast process and difficult to observe. The observed spreading dynamics of a 2D hole plasma is in perfect agreement with our nonlinear model.

Journal Article↗

PubMed2001

Self-induced acoustic transparency in semiconductor quantum films.

We develop a quantum theory of the nonlinear interaction between intense surface acoustic waves and electrons in a quantum well in the regime of moving quantum wires and dots. In the quantum nonlinear regime, the sound attenuation exhibits quantum oscillations and dramatically decreases with increasing quantization. In the case of dynamically created electron dots formed by two acoustic waves, the waves can propagate without any dissipation in the limit of high sound intensity and, hence, the electron quantum film acts as an acoustically quasitransparent material.

Journal Article↗

PubMed2000

Canted antiferromagnetic phase in a double quantum well in a tilted quantizing magnetic field.

We investigate the double-layer electron system in a parabolic quantum well at filling factor nu=2 in a tilted magnetic field using capacitance spectroscopy. The competition between two ground states is found at the Zeeman splitting appreciably smaller than the symmetric-antisymmetric splitting. Although at the transition point the system breaks up into domains of the two competing states, the activation energy turns out to be finite, signaling the occurrence of a new insulator-insulator quantum phase transition. We interpret the obtained results in terms of a predicted canted antiferromagnetic phase.

Journal Article↗

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

Determination of naphthodianthrones in plant extracts from Hypericum perforatum L. by liquid chromatography-electrospray mass spectrometry.

The determination of the naphthodianthrone constituents in extracts of dried blossoms of Hypericum perforatum L. by combined HPLC-electrospray mass spectrometry is described. Hypericin (1), pseudohypericin (2) and their precursor compounds produce intensive negative quasi-molecular ions by deprotonation provided a non-acidic eluent system is used in the HPLC separation. From the [M-H]- ions formed in the electrospray ionization process characteristic daughter ion spectra can be obtained by collisional activation which have been studied by tandem mass spectrometry.

Anthracenes↗

PubMed1997

Quantitation of urinary nucleosides by high-performance liquid chromatography.

It is known that some modified, especially methylated, nucleosides originating from RNA degradation are excreted in abnormal levels in the urine of patients with malignant tumours and they have been proposed as tumour markers. Their measurement could provide a non-invasive diagnostic method, be helpful in the identification of different cancers and in the monitoring of therapeutic effects. In this study, we developed and optimized an analytical procedure to isolate and quantify normal and modified ribonucleosides. The extraction of urinary nucleosides was performed by affinity chromatography on a phenylboronic acid column prior to separation. The reversed-phase high-performance liquid chromatography method allowed a complete separation of sixteen urinary ribonucleosides. The recoveries for the different nucleosides ranged from 83 to 100%, except for xanthosine (66%) and pseudouridine (74%). In normal 24 h urine, the mean levels of thirteen nucleosides (in nmol of nucleoside/mumol of creatinine) were found to be as follows: dihydrouridine (6.37), pseudouridine (25.52), cytidine (0.07), uridine (0.21), 1-methyladenosine (2.19), inosine (0.30), guanosine (0.06), xanthosine (0.59), 3-methyluridine (0.11), 1-methylinosine (1.13). 1-methylguanosine (0.74), adenosine (0.21) and 5'-deoxy-5'-methylthioadenosine (0.12). The first results concerning two kinds of tumours, i.e. breast and floor of mouth tumours, showed some abnormal levels of ribonucleosides. Further experiments are now in progress to measure the modified nucleosides in urine of patients with different forms of cancer.

Adenosine↗

PubMed1996