2,950 research outputs found
MARGINAL AGRICULTURAL LAND CLASSIFICATION: A NEW APPROACH
Land Economics/Use,
Distribution of Soils by Natural Drainage Class and by Slope Class for Iowa Counties
Natural drainage class (namely, poor, imperfect, and well drained) and slope class are estimated for Iowa counties through a 2% ÂĽ-section sample soil survey. Estimate of soil drainage class is given also by slope class. Counties vary widely in the amount of poorly drained soils. Kossuth County has 56.4% of poorly drained soils, while Allamakee has less than 1%. Land use management and conservation considerations are discussed briefly
Distribution of Manganese in a Bio-Topo Sequence of Southeastern Iowa Soils
Manganese extractable by sodium hydrosulfite was determined for 9 soil profiles of a bio-topo (vegetation and drainage) sequence in southeastern Iowa. The distribution of manganese in the soils studied is influenced by vegetation, drainage, and pH. Under prairie vegetation the manganese is evenly distributed with depth in the well-drained soil, but with increasing wetness of the soil profile, manganese is apparently lost from the A1 horizon and accumulates in the lower part of the B horizon. Manganese accumulates in the A1 and A2 horizons of the well-drained soils developed under forest vegetation, but in profiles of increasing wetness the amount of manganese in the A1 and A2 horizons decreases and the amount of manganese in the lower B horizon increases. The distribution of manganese in the transition prairie forest soils was intermediate between soils developed under prairie vegetation and those developed under forest vegetation
Exciton-plasmon states in nanoscale materials: breakdown of the Tamm-Dancoff approximation
Within the Tamm-Dancoff approximation ab initio approaches describe excitons
as packets of electron-hole pairs propagating only forward in time. However, we
show that in nanoscale materials excitons and plasmons hybridize, creating
exciton--plasmon states where the electron-hole pairs oscillate back and forth
in time. Then, as exemplified by the trans-azobenzene molecule and carbon
nanotubes, the Tamm-Dancoff approximation yields errors as large as the
accuracy claimed in ab initio calculations. Instead, we propose a general and
efficient approach that avoids the Tamm--Dancoff approximation, and correctly
describes excitons, plasmons and exciton-plasmon states
Center-of-Mass Properties of the Exciton in Quantum Wells
We present high-quality numerical calculations of the exciton center-of-mass
dispersion for GaAs/AlGaAs quantum wells of widths in the range 2-20 nm. The
k.p-coupling of the heavy- and light-hole bands is fully taken into account. An
optimized center-of-mass transformation enhances numerical convergence. We
derive an easy-to-use semi-analytical expression for the exciton groundstate
mass from an ansatz for the exciton wavefunction at finite momentum. It is
checked against the numerical results and found to give very good results. We
also show multiband calculations of the exciton groundstate dispersion using a
finite-differences scheme in real space, which can be applied to rather general
heterostructures.Comment: 19 pages, 12 figures included, to be published in Phys. Rev.
Exact exchange-correlation potential for a time-dependent two electron system
We obtain an exact solution of the time-dependent Schroedinger equation for a
two-electron system confined to a plane by an isotropic parabolic potential
whose curvature is periodically modulated in time. From this solution we
compute the exact time-dependent exchange correlation potential v_xc which
enters the Kohn-Sham equation of time-dependent density functional theory. Our
exact result provides a benchmark against which various approximate forms for
v_xc can be compared. Finally v_xc is separated in an adiabatic and a pure
dynamical part and it is shown that, for the particular system studied, the
dynamical part is negligible.Comment: 23 pages, 6 figure
Phase separation at all interaction strengths in the t-J model
We investigate the phase diagram of the two-dimensional t-J model using a
recently developed Green's Function Monte Carlo method for lattice fermions. We
use the technique to calculate exact ground-state energies of the model on
large lattices. In contrast to many previous studies, we find the model phase
separates for all values of J/t. In particular, it is unstable at the hole
dopings and interaction strengths at which the model was thought to describe
the cuprate superconductors.Comment: Revtex, 4 pages, 3 figures. Some minor changes were made to the text
and figures, and some references were adde
Standardizing Type Ia Supernova Absolute Magnitudes Using Gaussian Process Data Regression
We present a novel class of models for Type Ia supernova time-evolving
spectral energy distributions (SED) and absolute magnitudes: they are each
modeled as stochastic functions described by Gaussian processes. The values of
the SED and absolute magnitudes are defined through well-defined regression
prescriptions, so that data directly inform the models. As a proof of concept,
we implement a model for synthetic photometry built from the spectrophotometric
time series from the Nearby Supernova Factory. Absolute magnitudes at peak
brightness are calibrated to 0.13 mag in the -band and to as low as 0.09 mag
in the blueshifted -band, where the dispersion includes
contributions from measurement uncertainties and peculiar velocities. The
methodology can be applied to spectrophotometric time series of supernovae that
span a range of redshifts to simultaneously standardize supernovae together
with fitting cosmological parameters.Comment: 47 pages, 15 figures, accepted for publication by Astrophysical
Journa
The Low-Energy Fixed Points of Random Quantum Spin Chains
The one-dimensional isotropic quantum Heisenberg spin systems with random
couplings and random spin sizes are investigated using a real-space
renormalization group scheme. It is demonstrated that these systems belong to a
universality class of disordered spin systems, characterized by weakly coupled
large effective spins. In this large-spin phase the uniform magnetic
susceptibility diverges as 1/T with a non-universal Curie constant at low
temperatures T, while the specific heat vanishes as T^delta |ln T| for T->0.
For broad range of initial distributions of couplings and spin sizes the
distribution functions approach a single fixed-point form, where delta \approx
0.44. For some singular initial distributions, however, fixed-point
distributions have non-universal values of delta, suggesting that there is a
line of fixed points.Comment: 19 pages, REVTeX, 13 figure
C in intense femtosecond laser pulses: nonlinear dipole response and ionization
We study the interaction of strong femtosecond laser pulses with the C
molecule employing time-dependent density functional theory with the ionic
background treated in a jellium approximation. The laser intensities considered
are below the threshold of strong fragmentation but too high for perturbative
treatments such as linear response. The nonlinear response of the model to
excitations by short pulses of frequencies up to 45eV is presented and analyzed
with the help of Kohn-Sham orbital resolved dipole spectra. In femtosecond
laser pulses of 800nm wavelength ionization is found to occur multiphoton-like
rather than via excitation of a ``giant'' resonance.Comment: 14 pages, including 1 table, 5 figure
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