1,473 research outputs found
Spin degrees of freedom and flattening of the spectra of single-particle excitations in strongly correlated Fermi systems
The impact of long-range spin-spin correlations on the structure of a flat
portion in single-particle spectra , which emerges beyond the point,
where the Landau state loses its stability, is studied. We supplement the
well-known Nozieres model of a Fermi system with limited scalar long-range
forces by a similar long-range spin-dependent term and calculate the spectra
versus its strength . It is found that Nozieres results hold as long as
. However, with changing its sign, the spontaneous magnetization is
shown to arise at any nonzero . The increase of the strength is
demonstrated to result in shrinkage of the domain in momentum space, occupied
by the flat portion of , and, eventually, in its vanishing.Comment: 7 pages, 15 figure
Spatial correlation properties of the anomalous density matrix in a slab of nuclear matter with realistic NN-forces
Spatial correlation characteristics of the anomalous density matrix
in a slab of nuclear matter with the Paris and Argonne v18 forces are
calculated. A detailed comparison with predictions of the effective Gogny force
is made. It is found that the two realistic forces lead to very close results
which are qualitatively similar to those for the Gogny force. At the same time,
the magnitude of for realistic forces is essentially smaller than the
one for the Gogny force. The correlation characteristics are practically
independent of the magnitude of and turn out to be quite close for the
three kinds of the force. In particular, all of them predict a small value of
the local correlation length at the surface of the slab and a big one, inside.
These results are in agreement with those obtained recently by Pillet at al.
for finite nuclei with the Gogny force.Comment: 19 pages, 13 figure
Damping effects and the metal-insulator transition in the two-dimensional electron gas
The damping of single-particle degrees of freedom in strongly correlated
two-dimensional Fermi systems is analyzed. Suppression of the scattering
amplitude due to the damping effects is shown to play a key role in preserving
the validity of the Landau-Migdal quasiparticle picture in a region of a phase
transition, associated with the divergence of the quasiparticle effective mass.
The results of the analysis are applied to elucidate the behavior of the
conductivity of the two-dimensional dilute electron gas in the
density region where it undergoes a metal-insulator transition.Comment: 7 pages, 6 figures. Improved and slightly extended version: new
paragraph about Hall effect + new Fig.
Mechanisms driving alteration of the Landau state in the vicinity of a second-order phase transition
The rearrangement of the Fermi surface of a homogeneous Fermi system upon
approach to a second-order phase transition is studied at zero temperature. The
analysis begins with an investigation of solutions of the equation
, a condition that ordinarily has the Fermi momentum as
a single root. The emergence of a bifurcation point in this equation is found
to trigger a qualitative alteration of the Landau state, well before the
collapse of the collective degree of freedom that is responsible for the
second-order transition. The competition between mechanisms that drive
rearrangement of the Landau quasiparticle distribution is explored, taking into
account the feedback of the rearrangement on the spectrum of critical
fluctuations. It is demonstrated that the transformation of the Landau state to
a new ground state may be viewed as a first-order phase transition.Comment: 16 pages, 10 figure
Latitudinal pattern in community-wide herbivory does not match the pattern in herbivory averaged across common plant species
The latitudinal herbivory hypothesis (LHH) predicts that plant losses to herbivores decrease from low to high latitudes. Although the LHH is a community-level hypothesis, it has been rarely tested with data on community-wide herbivory, i.e. the percentage of annual production of foliar biomass consumed by insects from all plant species at a given site. Therefore, we asked whether community-wide leaf herbivory follows the same latitudinal pattern as observed for an unweighted average of herbivory across common plant species. We selected 10 study sites in boreal forests from 60 to 69 degrees N along a 1,000-km long latitudinal gradient in NW Russia. We measured relative foliar losses to insect herbivores in seven woody plant species (jointly comprising over 95% of the community-wide above-ground biomass) and estimated their current-year foliar biomass. We averaged leaf herbivory for all seven species and calculated community-wide leaf herbivory by weighting the relative foliar losses of each plant species against the contribution of that species to the annual foliar biomass production. Leaf herbivory was five-fold higher in deciduous species than in conifers. Latitudinal patterns in herbivory varied from a significant poleward decrease in all deciduous species to a significant poleward increase in Norway spruce. Herbivory values, averaged across seven plant species, decreased with latitude and followed the pattern observed in deciduous plants due to their higher foliar losses compared with conifers. By contrast, community-wide herbivory did not change with latitude. This discrepancy emerged because the proportion of deciduous plant foliage in the community increased with increasing latitude, and this increase counterbalanced the simultaneous poleward decrease in losses of these species to insects. Synthesis. The herbivory measured by averaging relative losses of individual plant species and community-wide herbivory is likely to show different latitudinal patterns in various plant communities. The contributions of plant species to the total foliar biomass production should be taken into account in studies of spatial patterns of herbivory which test community-level hypotheses. This approach may provide new insight into macroecological research on biotic interactions and improve our understanding of the role of insect herbivores in ecosystem-level processes
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