The relaxation rate of the magnetic moment of a shallow acceptor center as a function of impurity concentration in silicon

A study is made into the temperature dependence of residual polarization of negative muons in crystalline silicon with the concentration of impurity of the n-and p-types ranging from 8.7×10¹³ to 4.1× 10¹⁸ cm^?3. The measurements are performed in a magnetic field of 1000 G transverse to the muon spin, in the temperature range from 4.2 to 300 K. The form of the temperature dependence of the relaxation rate v of the magnetic moment of the μAl⁰ acceptor in silicon is determined. For a nondegenerate semiconductor, the relaxation rate depends on temperature as v ∝ T ^q (q ≈ 3). A variation in the behavior of the temperature dependence and a multiple increase in the relaxation rate are observed in the range of impurity concentration in excess of 10¹⁸ cm^?3. The importance of phonon scattering and spin-exchange scattering of free charge carriers by an acceptor from the standpoint of relaxation of the acceptor magnetic moment is discussed. The constant of hyperfine interaction in an acceptor center formed by an atom of aluminum in silicon is estimated for the first time: |A _hf (Al)/2π| ～ 2.5×10⁶s^?1.     

Rheometry on magnetorheological (MR) fluids

Test fixtures of a commercial concentric cylinder rheometer (Physica Rheolab MC 20) were modified to enable measurements under magnetic inductions up to 0.5 Tesla in a shear rate range of 0.1 up to 1000 s ^–1 and temperatures 0° to 150°C. In the 2 x90°-cups only two 90° sectors of the stationary part of the double concentric cylinder arrangement are submitted to the magnetic field which is created outside the test tools by an electromagnet. A prototype of a 360°-cup contains the electromagnet within the cup and avoids the correction necessary for the sector geometry. Measurements are shown for a carbonyl iron MR fluid and two nano MR fluids. An encouraging comparison of the viscosity function and MR effect (shear stress changes due to the field) measured by using the various cups is presented. The detailed investigation of the magnetic field distribution in the tools yields a distinct radial field gradient and also stray fields that make the quantification of the effective field in the gap difficult. The change of the field when the gap is filled with MR fluid is addressed. MR effects up to 13 000 Pa have been found, the limited torque range of the rheometer making it necessary to use relatively small gap dimensions which introduce errors due to edge effects. Shear rates up to 40000 s^–1 as typical for the application in dampers were investigated by a piston-driven capillary rheometer making use of a thermostated rectangular slit with superimposed magnetic field. A satisfactory agreement of the magnetorheological data with the concentric cylinder results is found in the overlapping shear rate range.     

Accurate time propagation for the Schrodinger equation with an explicitly time-dependent Hamiltonian

Several different numerical propagation techniques for explicitly time-dependent Hamiltonians are discussed and compared, with the focus on models of pump-probe experiments. The quality of the rotating wave approximation is analyzed analytically, and we point out under which circumstances the modeling becomes inaccurate. For calculations with the fully time-dependent Hamiltonian, we show that for multistate systems, with either time or space dependence in the interstate coupling, the fourth order truncated Magnus expansion can be reformulated so that no commutators appear. Our results show that the split-operator method should only be used when low accuracy is acceptable. For accurate and efficient time stepping, the Magnus-Lanczos approach appears to be the best choice.     

Spectroscopic gas analyzers based on indium-phosphide,antimonide and lead-salt diode-lasers

Currently available semiconductor lasers for spectroscopy in the near- and mid-infrared region based on direct band-to-band transitions as gallium-arsenide, indium-phosphide, antimonides and lead-salt containing compounds will be discussed together with the main features of different tunable diode-laser absorption spectrometers for trace gas analysis. Measurements of atmospheric carbon dioxide with a room-temperature 2 microm indium-phosphide laser, applications of antimonide lasers for methane and formaldehyde sensing in the 3-4 microm range and a fast chemical sensor for methane flux measurements based on lead-salt diode-lasers operating near 7.8 microm will be presented.     

A consecutive Diels-Alder approach toward a Tet repressor directed combinatorial library

A combinatorial library of 180 tetracycline analogs was generated by solution phase parallel synthesis applying a consecutive Diels-Alder strategy. Chemical methodology suitable for three-dimensional solution phase parallel synthesis was developed that enabled us to generate a collection of potential TetR inducers. The synthesis was built on cross-conjugated trienes as central building blocks facilitating two consecutive cycloaddition processes with different dienophiles. Upon sequential exposure to naphthoquinone and maleimide derivatives, the generation of a carbocyclic skeleton of type 2 incorporating the diversity elements R¹-R⁵ was envisaged.     

A Fourier-Coefficient Based Solution of an Optimal Control Problem in Quantum Chemistry

We consider an optimal control problem for the time-dependent Schrödinger equation modeling molecular dynamics. The dynamics can be steered by interactions with a tuned laser field. The problem of designing an optimal field can be posed as an optimal control problem. We reformulate the optimization problem by using a Fourier transform of the electric field, and narrow the frequency band. The resulting problem is less memory intense, and can be solved with a superlinearly convergent quasi-Newton method. We show computational results for a Raman-transition example and give numerical evidence that our method can outperform the standard monotonically convergent algorithm.     

Application of a multi-laser tunable diode laser absorption spectrometer for atmospheric trace gas measurements at sub-ppbv levels

We describe the application of a three-laser tunable diode laser absorption spectrometer (TDLAS), called 'tracer in-situ TDLAS for atmospheric research' (TRISTAR), to measure nitrogen dioxide (NO2), formaldehyde (HCHO) and hydrogen peroxide (H2O2), during an intensive measurement campaign on Mt. Cimone (44 degrees 11'N, 10 degrees 42'E, 2165 m asl), Northern Appenines, Italy in June 2000 as part of the EU-project 'mineral dust and tropospheric chemistry' (MINATROC). The TRISTAR instrument was a major component of an instrument package, provided by the Max-Planck-Insitut für Chemie, to investigate free tropospheric gas-phase chemistry over the Appenines. Here we discuss the optical, electronic, gas flow, and calibration setup of the TDLAS used during the campaign. We characterized extensively the instrument's performance during a preparatory phase in the laboratory and compared the laboratory results to the in-field results. Consistency checks with additional trace gas measurements obtained during the campaign create high confidence in the measured concentrations. Correlations between different trace gas species, along with other evaluation tools, allow a full chemical characterization of air masses to meet the goals of the campaign.     

Perfectly matched layers for modelling seismic oceanography experiments

Seismic oceanography techniques are able to provide oceanographic properties of the water masses by processing seismic reflection data. These techniques have reported reflected waves due to the fine structure in the ocean, whose order of magnitude is as weak as −80 dB. Thus, if we focus our attention on numerical simulation of this kind of oceanography experiments, the numerical performance of the method should allow obtaining accurate results, where the spurious reflections from the artificial boundaries of the computational grid are, at least, one order of magnitude smaller than the physical phenomena. This can be achieved by introducing perfectly matched layers (PML), which simulate non-reflecting boundaries. The aim of this work is to propose a numerical underwater propagation method, which combines a second-order finite-difference scheme in the physical region of interest with a first-order pressure/velocity discretization in the PML domain. This numerical method provides a low-cost computational algorithm with an accuracy, which allows recovering the reflected phenomena from the ocean fine structure, and moreover, with a spurious error of order −100 dB from the PML domain.