Zum 30. Todestag des Staatsrechtlers Rudolf Smend:Würdigung und Ausblick

Hinweis auf die politische Aktualität des Verfassungsverständnisses Rudolf Smends; Umriss eines Forschungsvorhaben

Terahertz photoresponse of a quantum Hall edge-channel diode

The Teraherz (THz) photoresponse of a two-dimensional electron gas in the quantum Hall regime is investigated. We use a sample structure which is topologically equivalent to a Corbino geometry combined with a cross-gate technique. This quasi-Corbino geometry allows us to directly investigate the THz-induced transport between adjacent edge-states, thus avoiding bulk effects. We find a pronounced photo voltage at zero applied bias, which rapidly decreases when an external current bias is applied. The photo voltage and its dependence on the bias current can be described using the model of an illuminated photodiode, resulting from the reconstruction of the Landau bands at the sample edge. Using the sample as a detector in a Fourier transform spectrometer setup, we find a resonant response from which we extract a reduced effective cyclotron mass. The findings support a non-bolometric mechanism of the induced photo voltage and the proposed edge-channel diode model.Comment: 5 pages, 5 eps-figures, accepted for Phys. Rev.

THz-Spektroskopie an zweidimensionalen Elektronengasen in Corbino-Geometrien

In dieser Arbeit wird die THz-induzierte Photo-Antwort eines zweidimensionalen elektronengases (2DEG) im Quanten-Hall-Regime untersucht. Dazu wird eine zweifach zusammenhängende Probengeometrie verwendet, die topologisch äquivalent zu einer ringförmigen Geometrie ist (Corbino-Geometrie). Diese Geometrie wird im Folgenden als Mäander-Corbino-Geometrie bezeichnet. Unter Quanten-Hall-Bedingungen wird der elektronische Transport, wie auch in einer Hall-bar-Probe, stark von den Randkanälen beeinflusst. Aufgrund der Ausbildung der Randkanäle entlang des Randes und einem semiisolierenden Volumenbereichs zwischen den Randkanälen ist es möglich, die inneren und äußeren Randkanäle getrennt von einander zu kontaktieren und die Photo-Antwort des semiisolierenden Volumenbereichs zu messen. Für Füllfaktoren ν < 2 wird in spektral aufgelösten Messungen eine Lorentz-förmige Resonanz bei der Zyklotronfrequenz beobachtet. Weiterhin zeigt die Analyse der Stromabhängigkeit der Photo-Antwort, dass sich das beobachtete Photosignal im Rahmen eines in der Literatur etablierten bolometrischen Modells beschreiben lässt. Die Situation ändert sich für Füllfaktoren oberhalb des ganzzahligen Füllfaktors (ν = 2). Hier eigen spektral aufgelöste Messungen eine asymmetrische Verbreiterung der Resonanz. Bei niedrigen Temperaturen (T = 300 mK) lassen sich zwei unabhängige Beiträge im Photosignal identifizieren. Ein Beitrag lässt sich dabei eindeutig auf das Heizen des Elektronengases durch die Zyklotronabsorption zurückführen (bolometrischer Beitrag), und der zweite Beitrag wird durch einen nicht-bolometrischen Mechanismus hervorgerufen. Die beobachtete Abhängigkeit der Amplitude der Photo-Antwort vom aufgeprägten Strom bei Füllfaktoren ν > 2 weist ebenfalls auf das Auftreten eines nicht-bolometrischen Beitrages zur Photo-Antwort hin. Außerdem werden Untersuchungen an einer so genannten Quasi-Corbino-Probe vorgestellt, die es erlaubt, zwei Randkanäle an einem Probenrand separat zu kontaktieren. Diese Quasi-Corbino-Probe ermöglicht es, die Photo-Antwort eines einzelnen inkompressiblen Streifens zwischen zwei Randkanälen (ohne den Einfluss des Volumens) zu untersuchen. Hier zeigen die Messungen, dass die Photo-Antwort aus der Erzeugung eines Photostroms innerhalb des inkompressiblen Randstreifens resultiert. Dementsprechend lässt sich der nicht-bolometrische Beitrag zur Photo-Antwort der Mäander-Corbino-Proben ebenfalls auf die Generation eines Photostroms innerhalb der inkompressiblen Streifen am Rand zurückführen. Darüber hinaus zeigt eine genauere Analyse der spektral aufgelösten Messungen an Mäander-Corbino-Proben einen dritten Beitrag zur Photo-Antwort. Dieser dritte Beitrag lässt sich dabei als eine neuartige Magnetoplasmon-Anregung am Probenrand interpretieren. Zum Abschluss dieser Arbeit wird die Anwendungsmöglichkeit der Mäander-Corbino-Proben als frequenzselektive, durchstimmbare und hochsensible THz-Detektoren vorgestellt.In this thesis, the THz-photoresponse between two separately connected edge-channels of a two-dimensional electron gas in the quantum Hall regime is investigated. We use a not-simply-connected sample geometry which is topologically equivalent to a ring shape (Corbino-geometry), in the following called meander-Corbino-geometry. Under quantizing magnetic fields, electronic transport is strongly influenced by the edge channels, as in Hall-bar samples, however with two disjoint boundaries. This allows us to electrically connect the edge-states of each boundary separately and measure the photoresponse across the insulating bulk between the edges. At filling factors ν < 2, spectrally resolved photoresponse measurements show a Lorentzian resonance, centered at the cyclotron-frequency. Furthermore, we find that the source-drain-current dependence of the photoresponse amplitude at filling factors ν < 2 is in a good agreement with a bolometric model, which is well established in literature. The situation changes above the integer filling factor. Spectrally resolved photoresponse measurements show an asymmetric broadening of the resonance. At low temperatures (T = 300 mK), two independent contributions to the photoresponse signal can be resolved. One contribution clearly results from bolometric heating inside the bulk and the other one is caused by a non-bolometric mechanism. The source-draincurrent dependence of the photoresponse amplitude at filling factors ν > 2 also indicates the occurrence of a non-bolometric contribution to the photoresponse. Additionally, we investigate the photoresponse of a so-called quasi-Corbino-sample, which allows us to separately contact to edge channels at the same boundary. This quasi-Corbino-sample enables us to study the photoresponse of a single incompressible strip between the edge channels, without any bulk effects. The experimental results show that the photoresponse of such a quasi-Corbino-sample is caused by a photocurrent, generated inside the incompressible strip. These measurements suggest that the non-bolometric contribution mentioned above is also caused by a photocurrent generated inside the incompressible strips at the sample edges. Furthermore, a closer examination of the spectrally resolved photoresponse measurements on meander-Corbino-samples reveal a third contribution to the photoresponse which can be interpreted as a new kind of a magnetoplasmon excitation on the sample edge. Finally, we demonstrate the use of the meander-Corbino-samples as frequency-selective, tunable and highly sensitive THz-detectors

Optical Levitation of a Droplet under Linear Increase of Gravitational Acceleration

Optical levitation of a liquid droplet in gas phase was investigated under timedependent change of the gravitational acceleration with specific flight pattern of an airplane. Through multiple trials under linear increase of effective gravitational acceleration, we performed the experiment of ptical trapping of a droplet from 0.3g_0 to 0.9g_0, where g_0 = 9.8 m/s^2. During such change of the effective gravitational acceleration, the trapping position on a droplet with the radius of 14 &#956;m was found to be lowered by ca. 100 &#956;m. The essential feature of the change of the trapping position is reproduced by a theoretical calculation under the framework of ray optics. As far as we know, the present study is the first report on optical levitation under time-dependent gravitational change

Annealing of swift heavy ion tracks in amorphous silicon dioxide

The annealing kinetics of the high energy ion damage in amorphous silicon dioxide (a-SiO2) are still not well understood, despite the material's widespread application in material science, physics, geology, and biology. This study investigates how annealing temperature, duration, and ambient environment affect the recovery of irradiation damage produced along the trajectory of swift heavy ions in a-SiO2. The track-annealing kinetics and the changing ion track morphology were investigated using synchrotron-based small-angle X-ray scattering (SAXS) and etching methods. We found that track annealing proceeds quicker near the sample surface demonstrated by a changing track etch rate as a function of depth. Measurements of ion tracks using SAXS show only small changes in the radial density distribution profile of the ion tracks. Activation energy of the annealing process at different sample depths was determined and the effect of the capping layer during the annealing process was also studied. Combination of oxygen diffusion and stress relaxation may contribute to the observed behaviour of preferential and anisotropic healing of the ion track. The results add to the fundamental understanding of ion track damage recovery and may have direct implications for materials for radioactive waste storage and solid state nanopores

Probing the band structure of InAs/GaAs quantum dots by capacitance-voltage and photoluminescence spectroscopy

The band structure of self-assembled InAs quantum dots, embedded in a GaAs matrix, is probed with capacitance-voltage spectroscopy and photoluminescence(PL)spectroscopy. The electron energy levels in the quantum dots with respect to the electron ground state of the wetting layer (WL) are determined from the capacitance-voltage measurements with a linear lever arm approximation. In the region where the linear lever arm approximation is not valid anymore (after the charging of the WL), the energetic distance from the electron ground state of the WL to the GaAs conduction band edge can be indirectly inferred from a numerical simulation of the conduction band under different gate voltages. In combination with PL measurements, the complete energy band diagram of the quantum dot sample is extracted

Swift heavy ion irradiation of GaSb: from ion tracks to nano-porous networks

Ion track formation, amorphisation, and the formation of porosity in crystalline GaSb induced by 185 MeV $^{197}$Au swift heavy ion irradiation is investigated as a function of fluence and irradiation angle relative to the surface normal. RBS/C and SAXS reveal an ion track radius between 3 nm and 5 nm. The observed pore morphology and saturation swelling of GaSb films shows a strong irradiation angle dependence. Raman spectroscopy and scanning electron microscopy show that the ion tracks act as a source of strain in the material leading to macroscopic plastic flow at high fluences and off normal irradiation. The results are consistent with the ion hammering model for glasses. Furthermore, wide angle X-ray scattering reveals the formation of nano crystallites inside otherwise amorphous GaSb after the onset of porosity

Adverse Life Events: Do Home Care Clients Have Resources for Mastering Them?

Objectives: Research on life stressors and adverse life events has a long tradition. Few studies have addressed this topic in connection to very old people. Life stressors, especially major life stressors (MLSs) experienced by clients of home care services in the community have rarely been the subject of studies. Considering this gap, we investigated the prevalence of MLSs in home care clients. We examined the effects that MLSs have on their mood and health status as well as the impact of clients’ social resources on MLSs and their outcomes. Method: We used assessment data from 2,884 home care clients in six European countries. The methodological basis was the comprehensive and standardized interRAI Home Care Assessment (interRAI HC). Results: Fifteen point four percent of the sample—that consisted of women and men with an average age of 82.89 years–experienced an MLS in the last 6 months before the assessment. They were more depressed than persons without these experiences, and their health status indicated a higher level of instability and deterioration. At reassessment after 6 months, the situation changed. Despite the fact that both outcomes of the MLSs, depression and health status became worse in the reassessment-sample, home care clients without MLS were more affected by the worsening, especially that of depression. The expected buffering impact of social resources was low. Discussion: Although this study worked with limited information on MLSs, it could contribute to closing various knowledge gaps. The study shows that the MLSs represent a prevalent problem in a population of home care clients and that this problem has negative consequences for their mood and the stability of their health status. Furthermore, this research took up the situation of very old and vulnerable adults, who have previously rarely been considered in studies on major critical life events and stressors. Conclusion and Research Perspective: Future research on MLSs has to take up the issue of the time passage between the MLS and the impact on health and well-being of individuals dependent on care. It has to determine immediate as well as later consequences and identify those factors that are appropriate to reduce the MLS-effects on very old people dependent on care