2,879 research outputs found

    Quantum confinement effects on the ordering of the lowest-lying excited states in conjugated chains

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    The symmetrized density matrix renormalization group approach is applied within the extended Hubbard-Peierls model (with parameters U/t, V/t, and bond alternation \delta) to study the ordering of the lowest one-photon (1^{1}B^{-}_u) and two-photon (2^{1}A^{+}_g) states in one- dimensional conjugated systems with chain lengths, N, up to N=80 sites. Three different types of crossovers are studied, as a function of U/t, \delta, and N. The U-crossover emphasizes the larger ionic character of the 2A_g state compared to the lowest triplet excitation. The \delta crossover shows strong dependence on both N and U/t. The N-crossover illustrates the more localized nature of the 2A_g excitation relative to the 1B_u excitation at intermediate correlation strengths.Comment: Latex file; figures available upon request. Submitted to PR

    Low-Lying Electronic Excitations and Nonlinear Optic Properties of Polymers via Symmetrized Density Matrix Renormalization Group Method

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    A symmetrized Density Matrix Renormalization Group procedure together with the correction vector approach is shown to be highly accurate for obtaining dynamic linear and third order polarizabilities of one-dimensional Hubbard and UVU-V models. The UVU-V model is seen to show characteristically different third harmonic generation response in the CDW and SDW phases. This can be rationalized from the excitation spectrum of the systems.Comment: 4 pages Latex; 3 eps figures available upon request; Proceedings of ICSM '96, to appear in Synth. Metals, 199

    Femtosecond Pump-Probe Studies of Reduced Graphene Oxide Thin Films

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    The dynamics of photocarriers in reduced graphene oxide thin films is studied by using ultrafast pump-probe spectroscopy. Time dependent differential transmissions are measured with sample temperatures ranging from 9 to 300 K. At each sample temperature and probe delay, the sign of differential transmission remains positive. A fast energy relaxation of hot carriers is observed, and is found to be independent of sample temperature. Our experiments show that the carrier dynamics in reduced graphene oxide is similar to other types of graphene, and that the differential transmission is caused by phase-state filling of carriers.Comment: 3 pages, 3 figure

    Who Benefits from Job Creation at County Level? An Analysis of Leakage and Spillover of New Employment Opportunities in Virginia

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    Using an econometric model system built on county level labor market data, this study allocates new employments in Virginia from 1990 to 2000 into various demographic segments: commuters, residents, and new immigrants. The study finds significant leakage of new employment opportunities in Virginia. 52% of new jobs created in the 1990s in a locality were taken by outside commuters. However, Virginia’s localities also benefit from spillover benefits from job creation elsewhere. Economists need to account for employment leakage and spillover to accurately evaluate the fiscal impacts of potential economic development projects