H. Karl

Ultrafast helicity control of surface currents in topological insulators with near-unity fidelity

In recent years, a class of solid-state materials, called three-dimensional topological insulators, has emerged. In the bulk, a topological insulator behaves like an ordinary insulator with a band gap. At the surface, conducting gapless states exist showing remarkable properties such as helical Dirac dispersion and suppression of backscattering of spin-polarized charge carriers. The characterization and control of the surface states via transport experiments is often hindered by residual bulk contributions. Here we show that surface currents in Bi2Se3 can be controlled by circularly polarized light on a picosecond timescale with a fidelity near unity even at room temperature. We reveal the temporal separation of such ultrafast helicity-dependent surface currents from photo-induced thermoelectric and drift currents in the bulk. Our results uncover the functionality of ultrafast optoelectronic devices based on surface currents in topological insulators.

Ultrafast electronic readout of diamond nitrogen–vacancy centres coupled to graphene

Non-radiative transfer processes are often regarded as loss channels for an optical emitter because they are inherently difficult to access experimentally. Recently, it has been shown that emitters, such as fluorophores and nitrogen-vacancy centres in diamond, can exhibit a strong non-radiative energy transfer to graphene. So far, the energy of the transferred electronic excitations has been considered to be lost within the electron bath of the graphene. Here we demonstrate that the transferred excitations can be read out by detecting corresponding currents with a picosecond time resolution. We detect electronically the spin of nitrogen-vacancy centres in diamond and control the non-radiative transfer to graphene by electron spin resonance. Our results open the avenue for incorporating nitrogen-vacancy centres into ultrafast electronic circuits and for harvesting non-radiative transfer processes electronically.

Ketoconazole inhibits cortisol secretion of an adrenal adenoma in vivo and in vitro

SummaryKetoconazole (Nizoral), an oral broad spectrum antifungal agent, inhibits ergosterol synthesis in fungi and cholesterol synthesis in mammalian cells by inhibition of the 14-demethylation of lanosterol. After a blunted cortisol response to ACTH in normal men after ketoconazole has been shown by others we studied the influence of the antifungal agent on the cortisol secretion in a patient with a cortisol producing adrenal adenoma in vivo and in vitro. Repeated oral doses of ketoconazole (200 mg every 5 h over a period of 48 h) induced a reproducible clear cutt fall of serum cortisol levels under 2.5 µg/dl. The inhibitory effect on the cortisol secretion could be detected first 5 h after the first dose, 9 h after the last dose cortisol levels recovered. In addition the inhibitory effect of ketoconazole on cortisol secretion could be reproduced in vitro by incubating tissue slices of the excised adrenal tumor together with the antifungal agent in concentrations equivalent to therapeutic serum levels. These findings emphasize that patients with an autonomous cortisol production caused by an adrenal tumor are proned to dangerous hypoadrenalism if treated with ketoconazole.

Time-Resolved Photoinduced Thermoelectric and Transport Currents in GaAs Nanowires

In order to clarify the temporal interplay of the different photocurrent mechanisms occurring in single GaAs nanowire based circuits, we introduce an on-chip photocurrent pump-probe spectroscopy with a picosecond time resolution. We identify photoinduced thermoelectric, displacement, and carrier lifetime limited currents as well as the transport of photogenerated holes to the electrodes. Moreover, we show that the time-resolved photocurrent spectroscopy can be used to investigate the drift velocity of photogenerated carriers in semiconducting nanowires. Hereby, our results are relevant for nanowire-based optoelectronic and photovoltaic applications.

RHEED studies of epitaxial growth of YBCO-films prepared by thermal co-evaporation

Abstract We prepare YBCO films of high quality by reactive co-evaporation of the metals. The rates are controlled by individual quartz crystal monitors, or else by a quadrupole mass spectrometer. On the standard substrates MgO, LaAlO 3 and SrTiO 3 we obtain T c ( R = 0) = 92 K, j c = 3.10 6 A/cm 2 at 77 K and surface resistances of 30 mΩ at 87 GHz and 77 K. These results are comparable to those of the best alternative techniques. Moreover we achieve good quality even on bare silicon substrates, probably because our substrate temperature required for the in-situ-formation of the perovskite structure is only 600–650 ° C. The further improvement of the films requires detailed information on the epitaxial growth conditions of the films. This can be obtained by in-situ-RHEED even with the oxygen present. Our RHEED studies demonstrate that the crystallinity of the substrate is the relevant parameter which decides upon the successful epitaxy. On good MgO-substrates which have Kikuchi lines in their RHEED patterns, we observe the perovskite structure of the YBCO films already when they are as thin as 0.6 nm. These films are superconductors of high quality. In contrast, we do not observe epitaxial growth on more disordered MgO substrates without Kikuchi lines, and the resulting films are poor superconductors. The growth properties on LaAlO 3 and on bare Si are also discussed.

Advanced apparatus for combinatorial synthesis of buried II-VI nanocrystals by ion implantation

Abstract The understanding, discovery and optimization of new complex functional materials requires combinatorial synthesis techniques and suitable fast screening and analysis methods. In this contribution the synthesis of buried II–VI compound semiconductor nanocrystals by combinatorial ion-implantation in SiO 2 on silicon will be presented. To this end we constructed a computer controlled implanter target station, in which a 4-in. wafer can be implanted with a lateral pattern of distinct dose or energy combinations. The chemical reaction of the implanted components is initiated either during the implantation process or in a second step, with the advantage that also a reactive atmosphere can be applied, during annealing. The resulting optical photoluminescence properties of the individual fields of the pattern can then be screened in rapid succession in an optical cryostat into which the whole wafer is mounted and cooled down. In this way complex interdependences of the physical parameters will be studied on one wafer and the technically relevant properties optimized.

Ferrimagnetic Tb–Fe Alloy Thin Films: Composition and Thickness Dependence of Magnetic Properties and All-Optical Switching

Ferrimagnetic rare earth - transition metal Tb-Fe alloy thin films exhibit a variety of different magnetic properties, which depends strongly on composition and temperature. In this study, first the influence of the film thickness (5 - 85 nm) on the sample magnetic properties was investigated in a wide composition range between 15 at.% and 38 at.% of Tb. From our results, we find that the compensation point, remanent magnetization, and magnetic anisotropy of the Tb-Fe films depend not only on the composition but also on the thickness of the magnetic film up to a critical thickness of about 20-30 nm. Beyond this critical thickness, only slight changes in magnetic properties are observed. This behavior can be attributed to a growth-induced modification of the microstructure of the amorphous films, which affects the short range order. As a result, a more collinear alignment of the distributed magnetic moments of Tb along the out-of-plane direction with film thickness is obtained. This increasing contribution of the Tb sublattice magnetization to the total sample magnetization is equivalent to a sample becoming richer in Tb and can be referred to as an “effective” composition. Furthermore, the possibility of all-optical switching, where the magnetization orientation of Tb-Fe can be reversed solely by circularly polarized laser pulses, was analyzed for a broad range of compositions and film thicknesses and correlated to the underlying magnetic properties.

Quantum confinement in CdSe-nanocrystallites synthesized by ion implantation

Abstract Nanocrystalline semiconductors offer a wide variety of possible applications in optical and optoelectronic devices. Embedding nanocrystals in compatible and stable host materials is of fundamental technical and scientific interest. In this work, buried nanocrystals of the direct band gap II–VI compound semiconductor CdSe were synthesized by sequential high dose ion beam implantation into thin thermally grown silicon dioxide layers and subsequent rapid thermal processing. Bulk CdSe has a bandgap energy of 1.75 eV that can be shifted to larger values by reducing the crystal size to dimensions smaller than the Bohr radius of the exciton. The size of the CdSe precipitates was controlled by the implantation and annealing parameters. The photoluminescence (PL) emission was exited by an Ar ion laser and measured in a temperature range between 80 and 295 K. The energy shift of the PL emission of the embedded CdSe nanocrystals is in quantitative agreement with the shift due to quantum confinement calculated from the mean size of the nanocrystals determined from thin film X-ray diffraction spectra (XRD) and transmission electron microscopy (TEM) images.

Ultrafast Photodetection in the Quantum Wells of Single AlGaAs/GaAs-Based Nanowires

We investigate the ultrafast optoelectronic properties of single Al0.3Ga0.7As/GaAs core-shell nanowires. The nanowires contain GaAs-based quantum wells. For a resonant excitation of the quantum wells, we find a picosecond photocurrent which is consistent with an ultrafast lateral expansion of the photogenerated charge carriers. This Dember-effect does not occur for an excitation of the GaAs-based core of the nanowires. Instead, the core exhibits an ultrafast displacement current and a photothermoelectric current at the metal Schottky contacts. Our results uncover the optoelectronic dynamics in semiconductor core-shell nanowires comprising quantum wells, and they demonstrate the possibility to use the low-dimensional quantum well states therein for ultrafast photoswitches and photodetectors.

Field induced photoluminescence quenching and enhancement of CdSe nanocrystals embedded in SiO2

The authors describe the operation of an electro-optical photoluminescence quenching device based on CdSe nanoclusters formed using sequential ion implantation of Cd+ and Se+ in thermally grown SiO2 on silicon. A sample geometry consisting of a semitransparent gold electrode on top and an ohmic contact to the p-doped Si on the reverse side of the device was chosen to apply high electric fields. Under high field strength an effective photoluminescence quenching and enhancement with a dynamical range of more than one order in magnitude have been observed. Field induced Stark shift has not been seen most probably due to polarization effects.