Prediction of photoperiodic regulators from quantitative gene circuit models

Photoperiod sensors allow physiological adaptation to the changing seasons. The external coincidence hypothesis postulates that a light-responsive regulator is modulated by a circadian rhythm. Sufficient data are available to test this quantitatively in plants, though not yet in animals. In Arabidopsis, the clock-regulated genes CONSTANS (CO) and FLAVIN, KELCH, F-BOX (FKF1) and their lightsensitive proteins are thought to form an external coincidence sensor. We use 40 timeseries of molecular data to model the integration of light and timing information by CO, its target gene FLOWERING LOCUS T (FT), and the circadian clock. Among other predictions, the models show that FKF1 activates FT. We demonstrate experimentally that this effect is independent of the known activation of CO by FKF1, thus we locate a major, novel controller of photoperiodism. External coincidence is part of a complex photoperiod sensor: modelling makes this complexity explicit and may thus contribute to crop improvement

Three-to two-dimensional transition in electrostatic screening of point charges at semiconductor surfaces studied by scanning tunneling microscopy

The electrostatic screening of localized electric charges on semiconductor surfaces is investigated quantitatively by statistically analyzing the spatial distribution of thermally formed positively charged anion surface vacancies on GaAs and InP(110) surfaces. Two screening regimes are found: at low vacancy concentrations the vacancy charges are found to be three-dimensionally screened by bulk charge carriers. The corresponding screening length, which increases strongly with decreasing carrier concentration, is best described by the classical bulk screening length evaluated with a surface dielectric constant. With increasing vacancy concentration at given bulk carrier concentration, a three- to two-dimensional screening transition occurs. At high vacancy concentrations, the screening is found to be governed by charge carriers located in a two-dimensional surface vacancy defect band, which is partially filled due to the vacancy-induced surface band bending

Language Generation from Conceptual Structure

This paper idescribes the current state of the S~/~gYN project, whose goal is be develop a module for generation of German from a semantic representation. The first application of this module is within the framework of a Japanese/German machine translation project. The generation process is organized into three stages that use distinct knowledge sources. ~ne first stage is conceptually oriented and language independent, and exploits case and concept schemata. The second stage e~ploys realization schemata which specify choices to map from meaning structures into German linguistic constructs. The last stage constructs the surface string using knowledge about syntax, morphology, and style. This paper describes the first two stages. INTRO[X~TION ~'s generation module is developed within a German/Japanese MT project. FUjitsu Research Labs. provide semantic representations that are produced as an interim data structure of their Ja-panese/English MT system ATLAS/II (Uchida & Sugiyama, 1980). ~ne feasibility of the approach of using a semantic representation as an interlingua in a practical application will be investigated and demonstrated by translating titles of Japanese papers from the field of "Information Technology". This material comes from Japanese documentation data bases and contains in addition to titles also their respective abstracts. Our design of the generation component is not limited to titles, but takes extensibility to abstracts and full texts into account. The envisioned future application of a Japanese/German translation system is to provide natural language access to Japanese documentation data bases. OVERALL DESIGN CF Fig. 1 shows the stages of generation. The Japanese text is processed by the analysis part of FtUI"TS~'s ATLAS/II system. Its output is a semantic net which serves as the input for our system. 1 ~ is an acronym for semantic synthesis. The project is funded by the "Informationslinguistik&quot

Surface states and origin of the Fermi level pinning on nonpolar GaN(1100) surfaces

GaN1000 cleavage surfaces were investigated by cross-sectional scanning tunneling microscopy and spectroscopy. It is found that both the N and Ga derived intrinsic dangling bond surface states are outside of the fundamental band gap. Their band edges are both located at the Gamma point of the surface Brillouin zone. The observed Fermi level pinning at 1.0 eV below the conduction band edge is attributed to the high step and defect density at the surface but not to intrinsic surface states

Electronic properties of dislocations in GaN investigated by scanning tunneling microscopy

We investigated the type, spatial distribution, line direction, and electronic properties of dislocations in n-type GaN by scanning tunneling microscopy. We found uncharged perfect dislocations with a/3{1120} Burgers vectors and negatively charged Shockley partial dislocations with a/3{1100} Burgers vectors interconnected by a negatively charged stacking fault. The charges are traced to different charge transfer levels associated with the particular core structure

Measurement of the internal quantum efficiency of InGaN quantum wells

The internal quantum efficiency as a function of the internal electric field was studied in InGaN/GaN based quantumwell heterostructures. Most striking, we find the IQE to be independent of the electron hole overlap for a standard green-emitting single quantum-well LED structure. In standard c-plane grown InGaN quantum wells, internal piezo-fields are responsible for a reduced overlap of electron and hole wavefunction. Minimization of these fields, for example by growth on non-polar m- and a-planes, is generally considered a key to improve the performance of nitride-based light emitting devices. In our experiment, we manipulate the overlap by applying different bias voltages to the standard c-plane grown sample, thus superimposing a voltage induced band-bending to the internal fields. In contrast to the IQE measurement, the dependence of carrier lifetime and wavelength shift on bias voltage could be explained solely by the internal piezo-fields according to the quantum confined Stark effect. Measurements were performed using temperature and bias dependent resonant photoluminescence, measuring luminescence and photocurrent simultaneously. Furthermore, the doping profile in the immediate vicinity of the QWs was found to be a key parameter that strongly influences the IQE measurement. A doping induced intrinsic hole reservoir inside the QWs is suggested to enhance the radiative exciton recombination rate and thus to improve saturation of photoluminescence efficiency