150 research outputs found
Efficiency of organic scintillators for fast neutrons
Organic scintillator detection efficiencies for fast neutron
Predictions for Impurity-Induced Tc Suppression in the High-Temperature Superconductors
We address the question of whether anisotropic superconductivity is
compatible with the evidently weak sensitivity of the critical temperature Tc
to sample quality in the high-Tc copper oxides. We examine this issue
quantitatively by solving the strong-coupling Eliashberg equations numerically
as well as analytically for s-wave impurity scattering within the second Born
approximation. For pairing interactions with a characteristically low energy
scale, we find an approximately universal dependence of the d-wave
superconducting transition temperature on the planar residual resistivity which
is independent of the details of the microscopic pairing. These results, in
conjunction with future systematic experiments, should help elucidate the
symmetry of the order parameter in the cuprates.Comment: 13 pages, 4 figures upon request, revtex version
Mammal communities are larger and more diverse in moderately developed areas
Developed areas are thought to have low species diversity, low animal abundance, few native predators, and thus low resilience and ecological function. Working with citizen scientist volunteers to survey mammals at 1427 sites across two development gradients (wild-rural-exurban- suburban-urban) and four plot types (large forests, small forest fragments, open areas and residential yards) in the eastern US, we show that developed areas actually had significantly higher or statistically similar mammalian occupancy, relative abundance, richness and diversity compared to wild areas. However, although some animals can thrive in suburbia, conservation of wild areas and preservation of green space within cities are needed to protect sensitive species and to give all species the chance to adapt and persist in the Anthropocene.
DOI: https://doi.org/10.7554/eLife.38012.00
The mechanism of hole carrier generation and the nature of pseudogap- and 60K-phases in YBCO
In the framework of the model assuming the formation of NUC on the pairs of
Cu ions in CuO plane the mechanism of hole carrier generation is
considered and the interpretation of pseudogap and 60 K-phases in
. is offered. The calculated dependences of hole
concentration in on doping and temperature
are found to be in a perfect quantitative agreement with experimental data. As
follows from the model the pseudogap has superconducting nature and arises at
temperature in small clusters uniting a number of
NUC's due to large fluctuations of NUC occupation. Here and
are the superconducting transition temperatures of infinite and finite
clusters of NUC's, correspondingly. The calculated and
dependences are in accordance with experiment. The area between
and corresponds to the area of fluctuations
where small clusters fluctuate between superconducting and normal states owing
to fluctuations of NUC occupation. The results may serve as important arguments
in favor of the proposed model of HTSC.Comment: 12 pages, 7 figure
Superconductivity in the Cuo Hubbard Model with Long-Range Coulomb Repulsion
A multiband CuO Hubbard model is studied which incorporates long-range (LR)
repulsive Coulomb interactions. In the atomic limit, it is shown that a
charge-transfer from copper to oxygen ions occurs as the strength of the LR
interaction is increased. The regime of phase separation becomes unstable, and
is replaced by a uniform state with doubly occupied oxygens. As the holes
become mobile a superfluid condensate is formed, as suggested by a numerical
analysis of pairing correlation functions and flux quantization. Although most
of the calculations are carried out on one dimensional chains, it isComment: LATEX, 14 pages, 4 figures available as postcript files or hard copy,
preprint ORNL-CCIP/93/1
Polarons as Nucleation Droplets in Non-Degenerate Polymers
We present a study of the nucleation mechanism that allows the decay of the
metastable phase (trans-cisoid) to the stable phase
(cis-transoid) in quasi one-dimensional non-degenerate polymers within the
continuum electron-phonon model. The electron-phonon configurations that lead
to the decay, i.e. the critical droplets (or transition state), are identified
as polarons of the metastable phase. We obtain an estimate for the decay rate
via thermal activation within a range of parameters consistent with
experimental values for the gap of the cis-configuration. It is pointed out
that, upon doping, the activation barriers of the excited states are quite
smaller and the decay rate is greatly enhanced. Typical activation energies for
electron or hole polarons are eV and the typical size for a
critical droplet (polaron) is about . Decay via quantum nucleation is
also studied and it is found that the crossover temperature between quantum
nucleation and thermal activation is of order . Metastable
configurations of non-degenerate polymers may provide examples for mesoscopic
quantum tunneling.Comment: REVTEX 3.0, 28 PAGES, 3 FIGURES AVAILABLE UPON REQUEST, PITT 94-0
Effective three-band model for double perovskites
We start from a six-band model describing the transition-metal t2g orbitals
of half-metallic double perovskite systems, such as Sr2FeMoO6, in which only
one of the transition metal ions (Fe) contains important intratomic repulsion
Ufe. By eliminating the Mo orbitals using a low-energy reduction similar to
that used in the cuprates, we construct a Hamiltonian which contains only
effective t2g Fe orbitals. This allows to treat exactly Ufe, and most of the
Fe-Mo hopping. As an application, we treat the effective Hamiltonian in the
slave-boson mean-field approximation and calculate the position of the
metal-insulator transition and other quantities as a function of pressure or
on-site energy difference.Comment: 8 pages, 3 figure
Quantum Monte Carlo Evidence for d-wave Pairing in the 2D Hubbard Model at a van Hove Singularity
We implement a Quantum Monte Carlo calculation for a repulsive Hubbard model
with nearest and next-nearest neighbor hopping interactions on clusters up to
12x12. A parameter region where the Fermi level lies close to the van Hove
singularity at the Saddle Points in the bulk band structure is investigated. A
pairing tendency in the symmetry channel, but no other channel,
is found. Estimates of the effective pairing interaction show that it is close
to the value required for a 40 K superconductor. Finite-size scaling compares
with the attractive Hubbard model.Comment: 11 pages, REVTex, 4 figures, postscrip
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