28 research outputs found

    Quantum lattice dynamical effects on the single-particle excitations in 1D Mott and Peierls insulators

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    As a generic model describing quasi-one-dimensional Mott and Peierls insulators, we investigate the Holstein-Hubbard model for half-filled bands using numerical techniques. Combining Lanczos diagonalization with Chebyshev moment expansion we calculate exactly the photoemission and inverse photoemission spectra and use these to establish the phase diagram of the model. While polaronic features emerge only at strong electron-phonon couplings, pronounced phonon signatures, such as multi-quanta band states, can be found in the Mott insulating regime as well. In order to corroborate the Mott to Peierls transition scenario, we determine the spin and charge excitation gaps by a finite-size scaling analysis based on density-matrix renormalization group calculations.Comment: 5 pages, 5 figure

    Hubbard model versus t-J model: The one-particle spectrum

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    The origin of the apparent discrepancies between the one-particle spectra of the Hubbard and t-J models is revealed: Wavefunction corrections, in addition to the three-site terms, should supplement the bare t-J. In this way a quantitative agreement between the two models is obtained, even for the intermediate-UU values appropriate for the high-Tc cuprate superconductors. Numerical results for clusters of up to 20 sites are presented. The momentum dependence of the observed intensities in the photoemission spectra of Sr2CuO2Cl2 are well described by this complete strong-coupling approach.Comment: 4 two-column RevTeX pages, including 4 Postscript figures. Uses epsf. Accepted for publication in Physical Review B, Rapid Communicatio

    Colossal dielectric constants in transition-metal oxides

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    Many transition-metal oxides show very large ("colossal") magnitudes of the dielectric constant and thus have immense potential for applications in modern microelectronics and for the development of new capacitance-based energy-storage devices. In the present work, we thoroughly discuss the mechanisms that can lead to colossal values of the dielectric constant, especially emphasising effects generated by external and internal interfaces, including electronic phase separation. In addition, we provide a detailed overview and discussion of the dielectric properties of CaCu3Ti4O12 and related systems, which is today's most investigated material with colossal dielectric constant. Also a variety of further transition-metal oxides with large dielectric constants are treated in detail, among them the system La2-xSrxNiO4 where electronic phase separation may play a role in the generation of a colossal dielectric constant.Comment: 31 pages, 18 figures, submitted to Eur. Phys. J. for publication in the Special Topics volume "Cooperative Phenomena in Solids: Metal-Insulator Transitions and Ordering of Microscopic Degrees of Freedom

    Thermal Infrared remote sensing and its usefulness in determining regional stream temperatures

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    Uncertainty associated with the Remote-sensing of stream temperatures at multiple spatial scales

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    Stream temperature is an important water quality indicator, particularly in the Pacific Northwest where endangered fish populations are sensitive to elevated water temperature. Regional assessment of stream temperatures from the ground is limited by sparse sampling of temperatures in both space and time. Remotely sensed thermal infrared (TIR) images can be used to derive spatially distributed estimates of the radiant skin temperature of streams. Even for fully resolved streams, however, our limited ability to compensate for atmospheric and emissivity effects constrains the accuracy of stream temperature measurements. When the stream is not fully resolved, it is the effect of mixed image pixels and thermal scattering from the near-bank environment that dominates both the accuracy and uncertainty of stream temperature measurements. Whereas airborne thermal sensors can resolve smaller streams than spaceborne sensors, their limited spatial coverage and cost makes satellite-derived TIR data an attractive option. We examine the accuracy and uncertainty associated with recovered stream temperatures for a range of stream sizes and pixel resolutions, using remotely sensed airborne (MASTER) and satellite (ASTER, LANDSAT-TM) TIR measurements. We find that when the stream width is resolved by fewer than 3 pixels, the accuracy of TIR measurements significantly decreases, and the uncertainty is greater, largely due to the reduction of mixed pixels (bank and stream). This limits the use of satellite TIR to temperature recovery for large rivers (currently, ~180 m stream widths with Landsat- TM)

    Accuracy of lake and stream temperatures estimated from thermal infrared images

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    Emitted thermal infrared radiation (TIR, λ= 8 to 14 μm) can be used to measure surface water temperatures (top approximately 100 μm). This study evaluates the accuracy of stream (50 to 500 m wide) and lake (300 to 5,000 m wide) radiant temperatures (15 to 22°C) derived from airborne (MASTER, 5 to 15 m) and satellite (ASTER 90 m, Landsat ETM+ 60 m) TIR images. Applied atmospheric compensations changed water temperatures by −0.2 to +2.0°C. Atmospheric compensation depended primarily on atmospheric water vapor and temperature, sensor viewing geometry, and water temperature. Agreement between multiple TIR bands (MASTER - 10 bands, ASTER - 5 bands) provided an independent check on recovered temperatures. Compensations improved agreement between image and in situ surface temperatures (from 2.0 to 1.1°C average deviation); however, compensations did not improve agreement between river image temperatures and loggers installed at the stream bed (from 0.6 to 1.6°C average deviation). Analysis of field temperatures suggests that vertical thermal stratification may have caused a systematic difference between instream gage temperatures and corrected image temperatures. As a result, agreement between image temperatures and instream temperatures did not imply that accurate TIR temperatures were recovered. Based on these analyses, practical accuracies for corrected TIR lake and stream surface temperatures are around 1°C
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