4,182 research outputs found
Bipolaron Density-Wave Driven By Antiferromagnetic Correlations and Frustration in Organic Superconductors
We describe the Paired Electron Crystal (PEC) which occurs in the interacting
frustrated two-dimensional 1/4-filled band. The PEC is a charge-ordered state
with nearest-neighbor spin singlets separated by pairs of vacant sites, and can
be thought of as a bipolaron density wave. The PEC has been experimentally
observed in the insulating state proximate to superconductivity in the organic
charge-transfer solids. Increased frustration drives a PEC-to-superconductor
transition in these systems.Comment: submitted to Physica B special issue for ISCOM 200
Absence of superconductivity in the half-filled band Hubbard model on the anisotropic triangular lattice
We report exact calculations of magnetic and superconducting pair-pair
correlations for the half-filled band Hubbard model on an anisotropic
triangular lattice. Our results for the magnetic phases are similar to those
obtained with other techniques. The superconducting pair-pair correlations at
distances beyond nearest neighbor decrease monotonically with increasing
Hubbard interaction U for all anisotropy, indicating the absence of
frustration-driven superconductivity within the model.Comment: 4 pages, 4 EPS figure
Cooperative orbital ordering and Peierls instability in the checkerboard lattice with doubly degenerate orbitals
It has been suggested that the metal-insulator transitions in a number of
spinel materials with partially-filled t_2g d-orbitals can be explained as
orbitally-driven Peierls instabilities. Motivated by these suggestions, we
examine theoretically the possibility of formation of such orbitally-driven
states within a simplified theoretical model, a two-dimensional checkerboard
lattice with two directional metal orbitals per atomic site. We include orbital
ordering and inter-atom electron-phonon interactions self-consistently within a
semi-classical approximation, and onsite intra- and inter-orbital
electron-electron interactions at the Hartree-Fock level. We find a stable,
orbitally-induced Peierls bond-dimerized state for carrier concentration of one
electron per atom. The Peierls bond distortion pattern continues to be period 2
bond-dimerization even when the charge density in the orbitals forming the
one-dimensional band is significantly smaller than 1. In contrast, for carrier
density of half an electron per atom the Peierls instability is absent within
one-electron theory as well as mean-field theory of electron-electron
interactions, even for nearly complete orbital ordering. We discuss the
implications of our results in relation to complex charge, bond, and
orbital-ordering found in spinels.Comment: 8 pages, 5 figures; revised versio
Comment on "Origin of Giant Optical Nonlinearity in Charge-Transfer--Mott Insulators: A New Paradigm for Nonlinear Optics"
Comment on Phys. Rev. Lett. 86, 2086 (2001)Comment: 1 page, 1 eps figur
Morphology of Three Imported Aphthona Flea Beetles Used as Biological Control Agents of Leafy Spurge
The following morphological study of the three imported Aphtona flea beetles supplies a detailed description of selected structures that will distinguish each of the three imported species and will separate them from a common native fle beetle (Glyptina atriventris) found at South Dakota release sites
Local singlets, frustration, and unconventional superconductivity in the organic charge-transfer solids
We suggest that superconductivity (SC) in the organic charge transfer solids
(CTS) is reached from a Bond-Charge Density Wave (BCDW). We discuss an
effective model for the BCDW to SC transition, an attractive U extended Hubbard
Hamiltonian with repulsive nearest neighbor interaction V. We discuss
experimental consequences of the theory for different classes of CTS
superconductors as well as related inorganic materials.Comment: 4 pages, 2 figures, ECRYS 200
Average echoes from randomly oriented random-length finite cylinders : zooplankton models
Author Posting. © Acoustical Society of America, 1993. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 94 (1993): 3463-3472, doi:10.1121/1.407200.By heuristically extending the previously developed ray solution [Stanton et al. J. Acoust. Soc. Am. 94, 3454–3462 (1993)] to predict the scattering by cylinders over all angles of incidence, approximate expressions are derived which describe the echo energy due to sound scattered by finite cylinders averaged over orientation and length. Both straight and bent finite length cylinders of high aspect ratio are considered over the full range of frequencies (Rayleigh through geometric scattering). The results show that for a sufficiently broad range of orientation, the average echo is largely independent of the degree of bend—that is, the results are essentially the same for both the straight and bent cylinders of various radii of curvature (provided the bend is not too great). Also, in the limit of high frequency (i.e., the acoustic wavelength is much smaller than the cross-sectional radius of the object), the averages are independent of frequency. The resultant formulas derived herein are useful in describing the scattering by elongated zooplankton whose shape may not necessarily be known in the natural ocean environment. The average echo is shown to depend directly upon standard deviation (s.d.) of the angle of orientation as well as size. If independent measurements of size are made (such as from trawling samples), then the properties of the angle distribution and hence behavior may be inferred from the data. Averages over both angle and a narrow distribution of size are shown to only partially smooth out deep nulls in the scatter versus frequency curves. The formulas compare favorably with laboratory data involving aggregations of animals and a broad range of frequencies (38 kHz to 1.2 MHz).This research was supported by the Oceanic
Biology and Ocean Acoustics Programs of the U.S. Office
of Naval Research Grant No. N00014-89-J-1729 and the
Office of Naval Technology (through the U.S. Naval Undersea
Warfare Center, Newport, Contract No. N66604-
91-C-5401)
- …