J. Schäfer

Bismuthene on a SiC substrate: A candidate for a high-temperature quantum spin Hall material

Making a large-gap topological insulator Although of interest to basic research, topological insulators (TIs) have not yet lived up to their technological potential. This is partly because their protected surface-edge state usually lives within a narrow energy gap, with its exotic transport properties overwhelmed by the ordinary bulk material. Reis et al. show that a judicious choice of materials can make the gap wide enough for the topological properties to be apparent at room temperature. Numerical calculations indicate that a monolayer of Bismuth grown on SiC(0001) is a two-dimensional TI with a large energy gap. The researchers fabricated such a heterostructure and characterized it using scanning tunneling spectroscopy. The size of the experimentally measured gap was consistent with the calculations. Science, this issue p. 287 Scanning tunneling spectroscopy indicates a large energy gap and conducting edge states, consistent with calculations. Quantum spin Hall materials hold the promise of revolutionary devices with dissipationless spin currents but have required cryogenic temperatures owing to small energy gaps. Here we show theoretically that a room-temperature regime with a large energy gap may be achievable within a paradigm that exploits the atomic spin-orbit coupling. The concept is based on a substrate-supported monolayer of a high–atomic number element and is experimentally realized as a bismuth honeycomb lattice on top of the insulating silicon carbide substrate SiC(0001). Using scanning tunneling spectroscopy, we detect a gap of ~0.8 electron volt and conductive edge states consistent with theory. Our combined theoretical and experimental results demonstrate a concept for a quantum spin Hall wide-gap scenario, where the chemical potential resides in the global system gap, ensuring robust edge conductance.

Elemental topological insulator with tunable Fermi level: strained α-Sn on InSb(001).

We report on the epitaxial fabrication and electronic properties of a topological phase in strained α-Sn on InSb. The topological surface state forms in the presence of an unusual band order not based on direct spin-orbit coupling, as shown in density functional and GW slab-layer calculations. Angle-resolved photoemission including spin detection probes experimentally how the topological spin-polarized state emerges from the second bulk valence band. Moreover, we demonstrate the precise control of the Fermi level by dopants.

Experimental investigation and computational modeling of hot filament diamond chemical vapor deposition

A joint investigation has been undertaken of the gas-phase chemistry taking place in a hot-filament chemical vapor-deposition (HFCVD) process for diamond synthesis on silica surfaces by a detailed comparison of numerical modeling and experimental results. Molecular beam sampling using quadrupole mass spectroscopy and resonance-enhanced multiphoton ionization time of flight mass spectroscopy (REMPI-TOF-MS) has been used to determine absolute concentrations of stable hydrocarbons and radicals. Resulting species of a CH4/H2, a CH4/D2 (both 0.5%/99.5%) and a C2H2/H2 (0.25%/99.75%) feedgas mixture were investigated for varying filament and substrate temperatures. Spatially resolved temperature profiles at various substrate temperatures, obtained from coherent anti-Stokes Raman spectroscopy (CARS) of hydrogen, are used as input parameters for the numerical code to reproduce hydrogen atom, methyl radical, methane, acetylene, and ethylene concentration profiles in the boundary layer of the substrate. In addition,...

Performance of an atlas-based autosegmentation software for delineation of target volumes for radiotherapy of breast and anorectal cancer

BACKGROUND AND PURPOSE To validate atlas-based autosegmentation for contouring breast/anorectal targets. METHODS AND MATERIALS ABAS uses atlases with defined CTVs as template cases to automatically delineate target volumes in other patient CT-datasets. Results are compared with manually contoured CTVs of breast/anorectal cancer according to RTOG-guidelines. The impact of using specific atlases matched to individual patient geometry was evaluated. Results were quantified by analyzing Dice Similarity Coefficient (DSC), logit(DSC) and Percent Overlap (PO). DSC >0.700 and logit(DSC) >0.847 are acceptable. In addition a new algorithm (STAPLE) was evaluated. RESULTS ABAS produced good results for the CTV of breast/anorectal cancer targets. Delineation of inguinal lymphatic drainage, however, was insufficient. Results for breast CTV were (DSC: 0.86-0.91 ([0,1]), logit(DSC): 1.82-2.36 ([-∞,∞]), PO: 75.5-82.89%) and for anorectal CTVA (DSC: 0.79-0.85, logit(DSC): 1.40-1.77, PO: 68-73.67%). CONCLUSIONS ABAS produced satisfactory results for these clinical target volumes that are defined by more complex tissue interface geometry, thus streamlining and facilitating the radiotherapy workflow which is essential to face increasing demand and limited resources. STAPLE improved contouring outcome. Small target volumes not clearly defined are still to be delineated manually. Based on these results, ABAS has been clinically introduced for precontouring of CTVs/OARs.

Suppression of Apoptosis and Clonogenic Survival in Irradiated Human Lymphoblasts with Different TP53 Status

Abstract Schäfer, J., Bachtler, J., Engling, A., Little, J. B., Weber, K.-J. and Wenz, F. Suppression of Apoptosis and Clonogenic Survival in Irradiated Human Lymphoblasts with Different TP53 Status. Radiat. Res. 158, 699–706 (2002). The influence of radiation-induced apoptosis on radiosensitivity was studied in a set of closely related human lymphoblastoid cell lines differing in TP53 status. The clonogenic survival of irradiated TK6 cells (expressing wild-type TP53), WTK1 cells (overexpressing mutant TP53), and TK6E6 cells (negative for TP53 owing to transfection with HPV16 E6) was assessed in relation to the induction of apoptosis and its suppression by caspase inhibition or treatment with PMA as well as after treatment with caffeine. Measurements using the alkaline comet assay and pulsed-field electrophoresis of the induction and repair of DNA strand breaks showed similar kinetics of the processing of early DNA damage in these cell lines. The cytochalasin B micronucleus assay revealed identical levels of residual damage in the first postirradiation mitosis of these cells. Abrogation of TP53-dependent apoptosis in TK6E6 cells resulted in a distinct increase in radioresistance. Further suppression of apoptosis as observed in WTK1 cells overexpressing mutant TP53 apparently was not responsible for the high radioresistance of WTK1 cells, since other means of highly efficient suppression of apoptosis (caspase inhibition or PMA treatment) increased the clonogenic survival of irradiated TK6 cells only to levels similar to those of TK6E6 cells with abrogated TP53-dependent apoptosis. Considering the similar levels of residual chromosomal damage in TK6E6 cells and WTK1 cells, a hitherto unknown mechanism of tolerance needs to be inferred for these TP53 mutant cells. This residual damage tolerance, however, appears to require an intact G2/M-phase checkpoint function since the relative radioresistance of the WTK1 cells was completely lost upon caffeine treatment, which also resulted in a failure of the TK6 and TK6E6 cells to execute apoptosis. In this situation, the cellular response seems to be dominated entirely by TP53-independent mitotic failure.

Self-organized atomic nanowires of noble metals on Ge(001): atomic structure and electronic properties

Atomic structures of quasi-one-dimensional (1D) character can be grown on semiconductor substrates by metal adsorption. Significant progress concerning study of their 1D character has been achieved recently by condensing noble metal atoms on the Ge(001) surface. In particular, Pt and Au yield high quality reconstructions with low defect densities. We report on the self-organized growth and the long-range order achieved, and present data from scanning tunneling microscopy (STM) on the structural components. For Pt/Ge(001), we find hot substrate growth is the preferred method for self-organization. Despite various dimerized bonds, these atomic wires exhibit metallic conduction at room temperature, as documented by low-bias STM. For the recently discovered Au/Ge(001) nanowires, we have developed a deposition technique that allows complete substrate coverage. The Au nanowires are extremely well separated spatially, exhibit a continuous 1D charge density, and are of solid metallic conductance. In this review, we present structural details for both types of nanowires, and discuss similarities and differences. A perspective is given for their potential to host a 1D electron system. The ability to condense different noble metal nanowires demonstrates how atomic control of the structure affects the electronic properties.

Symmetry-breaking phase transition without a Peierls instability in conducting monoatomic chains.

The one-dimensional (1D) model system Au/Ge(001), consisting of linear chains of single atoms on a surface, is scrutinized for lattice instabilities predicted in the Peierls paradigm. By scanning tunneling microscopy and electron diffraction we reveal a second-order phase transition at 585 K. It leads to charge ordering with transversal and vertical displacements and complex interchain correlations. However, the structural phase transition is not accompanied by the electronic signatures of a charge density wave, thus precluding a Peierls instability as origin. Instead, this symmetry-breaking transition exhibits three-dimensional critical behavior. This reflects a dichotomy between the decoupled 1D electron system and the structural elements that interact via the substrate. Such substrate-mediated coupling between the wires thus appears to have been underestimated also in related chain systems.

Long-Term Health-Related Quality-of-Life Outcomes after Permanent Prostate Brachytherapy

Background: We evaluated the long-term health-related quality of life (HRQoL) after modern prostate brachytherapy (PB). Patients and Methods: The European Organization for Research and Treatment of Cancer (EORTC) prostate cancer quality-of-life questionnaire, QLQ-PR25, and the modified International Continence Society (ICS) male questionnaire were sent to 296 men treated with PB at the University Medical Center Mannheim. To evaluate the influence of age, we subclassified two groups: group 1 <65 years, group 2 ≧65 years. 258 of the 296 patients were alive at the time of assessment. Of 238 questionnaires (92% response rate), 231 were suitable for analysis. Median follow-up was 51 months. Results: Of group 1 77.8% and of group 2 73.4% of the men were in good, very good or excellent health. There was a low rate of moderate (10.4%) or strong (1.0%) symptoms regarding urinary functioning. Stress incontinence was uncommon. Only 28.2% reported moderate or strong (up to 17.6%) symptoms regarding sexual functioning. In group 1 48.6% and in group 2 25.0% of patients reported no or minor erectile dysfunction (p < 0.007). There was no severe overall rectal dysfunction (<2%). Conclusion: Our data substantiate the favorable long-term HRQoL outcomes associated with modern PB techniques. Significant age differences were observed in sexual symptoms and less pronounced age differences in urinary symptoms. We found a low rate of urinary symptoms and no evidence of severe rectal dysfunction.

Monitoring of HyGaNx and Ga in OMCVD of GaN, using molecular beam quadrupole and REMPI-TOF mass spectrometry

Abstract A new route in organometallic chemical vapour deposition (OMCVD) of GaN layers is the single-source precursor method. Molecular beam sampling using quadrupole mass spectrometry and resonance-enhanced multiphoton ionisation time-of-flight mass spectrometry (REMPI-TOF-MS) has been used to show that gallium atoms and gallium–nitrogen compounds, like HGaNx (x=2–6) and GaNx (x=2–6), appear in the boundary layer of a sapphire substrate during thermal decomposition of (N3)2Ga[CH2CH2CH2N(CH3)2] in the temperature range 400–1000 K. The temperature dependence of the species is shown to be directly correlated with the growth rate of GaN layers.

High purity chemical etching and thermal passivation process for Ge(001) as nanostructure template

An advanced two-step cleaning process of the Ge(001) surface for nanoscience requirements is presented. First, wet-chemical etching with a variant of the Piranha solution (H(2)SO(4), H(2)O(2), H(2)O) is used to remove contaminants as well as the native oxide layer. Second, passivation of the surface is achieved by a rapid thermal oxidation step, leading to a homogeneous protective oxide layer. The thickness of the oxide layer is tuned to be thick enough to protect the surface, yet thin enough to be completely removed by thermal treatment in ultra-high vacuum. The application of this recipe results in an outstandingly clean and atomically flat surface, with carbon contamination at the detection limit of x-ray photoelectron spectroscopy. Scanning tunneling microscopy and electron diffraction reveal a long range ordered surface with typical terrace diameters of ~100 nm, suitable for the growth of atomic-scale nanostructures.