972 research outputs found

    Discrete Dirac system: rectangular Weyl functions, direct and inverse problems

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    A transfer matrix function representation of the fundamental solution of the general-type discrete Dirac system, corresponding to rectangular Schur coefficients and Weyl functions, is obtained. Connections with Szeg\"o recurrence, Schur coefficients and structured matrices are treated. Borg-Marchenko-type uniqueness theorem is derived. Inverse problems on the interval and semiaxis are solved.Comment: Section 2 is improved in the second version: some new results on Halmos extension are added and arguments are simplifie

    A Study of the Efficacy of Unit Contingency Contracting Training

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    This research is a follow-on effort to Capt Pete Lasch\u27s thesis, which examined the Air Force training needs of contingency contracting officers (CCOs). Through this study, Lasch (2002) identified a list of critical training tasks for CCO training. This study investigates how units currently conduct training and to what extent the recommended tasks are being addressed in training. Interviews, surveys, and archival training logs were used to capture data for the study. CCO demographic information and recommendations were collected through these interviews and surveys. Interviews were used as a pre-test and later were combined with the electronic survey responses. Surveys were sent in two phases to CCOs who deployed in the period of interest from September 11th of 2001 to December 15th of 2002. Phase one described the purpose of the study and phase two involved collection of surveys from population of interest. Training logs or plans were also requested from the units survey participants deployed from. These logs provided insight into the content of training and allowed the researcher to investigate which of the recommended tasks from Lasch (2002) were included in unit training programs across the Air Force. Descriptive statistics and pattern matching were used to analyze the data. This analysis resulted in recommendations to both improve training programs and to provide relevant training material for future unit level CCO training programs

    Inhomogeneous nuclear spin polarization induced by helicity-modulated optical excitation of fluorine-bound electron spins in ZnSe

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    Optically-induced nuclear spin polarization in a fluorine-doped ZnSe epilayer is studied by time-resolved Kerr rotation using resonant excitation of donor-bound excitons. Excitation with helicity-modulated laser pulses results in a transverse nuclear spin polarization, which is detected as a change of the Larmor precession frequency of the donor-bound electron spins. The frequency shift in dependence on the transverse magnetic field exhibits a pronounced dispersion-like shape with resonances at the fields of nuclear magnetic resonance of the constituent zinc and selenium isotopes. It is studied as a function of external parameters, particularly of constant and radio frequency external magnetic fields. The width of the resonance and its shape indicate a strong spatial inhomogeneity of the nuclear spin polarization in the vicinity of a fluorine donor. A mechanism of optically-induced nuclear spin polarization is suggested based on the concept of resonant nuclear spin cooling driven by the inhomogeneous Knight field of the donor-bound electron.Comment: 12 pages, 11 figure
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