1,759 research outputs found
Plowed Under: Food Policy Protests and Performance in New Deal America
Review of: Plowed Under: Food Policy Protests and Performance in New Deal America, by Ann Folino
Umklapp scattering of pairs in BCS superconductivity theory
The BCS theory of superconductivity is extended to recognize pairing of
electrons by both normal and umklapp scattering. Application of the variational
approach shows that coexistence of normal and umklapp scattering frustrates
superconductivity.Comment: 9 pages, 5 figures. to be published in Journal of Physics: Condensed
Matte
Book Reviews
Review of the following books: Nearby History: Exploring the Past Around You by David E. Kyvig and Myron A. Marty; The Forerunners: The Tragic Story of 156 Down-East Americans Led to Jaffa in 1866 by Charismatic G.J. Adams to Plant the Seeds of Modern Israel by Reed M. Holmes; Islands of Maine: Where America Really Began by Bill Caldwell; Foundations of Northeast Archaeology edited by Dean R. Snow; A Short History of the American Locomotive Builders in the Steam Era by John H. Whit
Using ensemble-mean climate scenarios for future crop yield projections: a stochastic weather generator
Using climate scenarios from only 1 or a small number of global climate models (GCMs) in climate change impact studies may lead to biased assessment due to large uncertainty
in climate projections. Ensemble means in impact projections derived from a multi-GCM ensemble are often used as best estimates to reduce bias. However, it is often time consuming to run process-based models (e.g. hydrological and crop models) in climate change impact studies using numerous climate scenarios. It would be interesting to investigate if using a reduced number of climate scenarios could lead to a reasonable estimate of the ensemble mean. In this study, we generated a single ensemble-mean climate scenario (En-WG scenario) using ensemble means of the change factors derived from 20 GCMs included in CMIP5 to perturb the parameters in a weather generator, LARS-WG, for selected locations across Canada. We used En-WG scenarios to drive crop growth models in DSSAT ver. 4.7 to simulate crop yields for canola and spring wheat under RCP4.5 and RCP8.5 emission scenarios. We evaluated the potential of using the En-WG scenario to simulate crop yields by comparing them with crop yields simulated with the LARS-WG generated climate scenarios based on each of the 20 GCMs (WG scenarios). Our results showed that simulated crop yields using the En-WG scenarios were often close to the ensemble means of simulated crop yields using the 20 WG scenarios with a high probability of outperforming simulations based on a randomly selected GCM. Further studies are required, as the results of the proposed approach may be influenced by selected crop types, crop models, weather generators, and GCM ensembles
Spin tunnelling in mesoscopic systems
We study spin tunnelling in molecular magnets as an instance of a mesoscopic
phenomenon, with special emphasis on the molecule Fe8. We show that the tunnel
splitting between various pairs of Zeeman levels in this molecule oscillates as
a function of applied magnetic field, vanishing completely at special points in
the space of magnetic fields, known as diabolical points. This phenomena is
explained in terms of two approaches, one based on spin-coherent-state path
integrals, and the other on a generalization of the phase integral (or WKB)
method to difference equations. Explicit formulas for the diabolical points are
obtained for a model Hamiltonian.Comment: 13 pages, 5 figures, uses Pramana style files; conference proceedings
articl
Effectiveness of using representative subsets of global climate models in future crop yield projections.
Representative subsets of global climate models (GCMs) are often used in climate change impact studies to account for uncertainty in ensemble climate projections. However, the effectiveness of such subsets has seldom been assessed for the estimations of either the mean or the spread of the full ensembles. We assessed two different approaches that were employed to select 5 GCMs from a 20-member ensemble of GCMs from the CMIP5 ensemble for projecting canola and spring wheat yields across Canada under RCP 4.5 and 8.5 emission scenarios in the periods 2040-2069 and 2070-2099, based on crop simulation models. Averages and spreads of the simulated crop yields using the 5-GCM subsets selected by T&P and KKZ approaches were compared with the full 20-GCM ensemble. Our results showed that the 5-GCM subsets selected by the two approaches could produce full-ensemble means with a relative absolute error of 2.9 – 4.7% for canola and 1.5 – 2.2% for spring wheat, and covers 61.8 – 91.1% and 66.1 – 80.8% of the full-ensemble spread for canola and spring wheat, respectively. Our results also demonstrated that both approaches were very likely to outperform a subset of randomly selected 5 GCMs in terms of a smaller error and a larger range
Tricritical Behavior in the Extended Hubbard Chains
Phase diagrams of the one-dimensional extended Hubbard model (including
nearest-neighbor interaction ) at half- and quarter-filling are studied by
observing level crossings of excitation spectra using the exact
diagonalization. This method is based on the Tomonaga-Luttinger liquid theory
including logarithmic corrections which stem from the renormalization of the
Umklapp- and the backward-scattering effects.
Using this approach, the phase boundaries are determined with high accuracy,
and then the structure of the phase diagram is clarified. At half-filling, the
phase diagram consists of two
Berezinskii-Kosterlitz-Thouless (BKT) transition lines and one Gaussian
transition line in the charge sector, and one spin-gap transition line.
This structure reflects the U(1) SU(2) symmetry of the electron
system. Near the line, the Gaussian and the spin-gap transitions take
place independently from the weak- to the intermediate-coupling region, but
these two transition lines are coupled in the strong-coupling region. This
result demonstrates existence of a tricritical point and a
bond-charge-density-wave (BCDW) phase between charge- and spin-density-wave
(CDW, SDW) phases. To clarify this mechanism of the transition, we also
investigate effect of a correlated hopping term which plays a role to enlarge
BCDW and bond-spin-density-wave (BSDW) phases. At quarter-filling, a similar
crossover phenomenon also takes place in the large- region involving
spin-gap and BKT-type metal-insulator transitions.Comment: 18 pages(REVTeX), 17 figures(EPS(color)), 3 tables, Detailed paper of
JPSJ 68 (1999) 3123 (cond-mat/9903227), see also cond-mat/000341
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