3,746 research outputs found
Non-perturbative many-body approach to the Hubbard model and single-particle pseudogap
A new approach to the single-band Hubbard model is described in the general
context of many-body theories. It is based on enforcing conservation laws, the
Pauli principle and a number of crucial sum-rules. More specifically, spin and
charge susceptibilities are expressed, in a conserving approximation, as a
function of two constant irreducible vertices whose values are found
self-consistently. The Mermin-Wagner theorem in two dimensions is automatically
satisfied. The effect of collective modes on single-particle properties is then
obtained by a paramagnon-like formula that is consistent with the two-particle
properties in the sense that the potential energy obtained from is
identical to that obtained using the fluctuation-dissipation theorem for
susceptibilities. The vertex corrections are included through constant
irreducible vertices. The theory is in quantitative agreement with Monte Carlo
simulations for both single-particle and two-particle properties. In the
two-dimensional renormalized classical regime, spin fluctuations lead to
precursors of antiferromagnetic bands (shadow bands) and to the destruction of
the Fermi-liquid quasiparticles in a wide temperature range above the
zero-temperature phase transition. The analogous phenomenon of pairing
pseudogap can occur in the attractive model in two dimensions when the pairing
fluctuations become critical. Other many-body approaches are critically
compared. It is argued that treating the spin fluctuations as if there was a
Migdal's theorem can lead to wrong predictions, in particular with regard to
the the single-particle pseudogap.Comment: Small changes to conform to published version. Main text 33 pages.
Appendices 16 pages. 11 PS figures epsf/Latex. Section on the single-particle
pseudogap can be read independentl
Spin susceptibility of interacting electrons in one dimension: Luttinger liquid and lattice effects
The temperature-dependent uniform magnetic susceptibility of interacting
electrons in one dimension is calculated using several methods. At low
temperature, the renormalization group reaveals that the Luttinger liquid spin
susceptibility approaches zero temperature with an infinite slope
in striking contrast with the Fermi liquid result and with the behavior of the
compressibility in the absence of umklapp scattering. This effect comes from
the leading marginally irrelevant operator, in analogy with the Heisenberg spin
1/2 antiferromagnetic chain. Comparisons with Monte Carlo simulations at higher
temperature reveal that non-logarithmic terms are important in that regime.
These contributions are evaluated from an effective interaction that includes
the same set of diagrams as those that give the leading logarithmic terms in
the renormalization group approach. Comments on the third law of thermodynamics
as well as reasons for the failure of approaches that work in higher dimensions
are given.Comment: 21 pages, latex including 5 eps figure
Fano Resonances in Mid-Infrared Spectra of Single-Walled Carbon Nanotubes
This work revisits the physics giving rise to the carbon nanotubes phonon
bands in the mid- infrared. Our measurements of doped and undoped samples of
single-walled carbon nanotubes in Fourier transform infrared spectroscopy show
that the phonon bands exhibit an asymmetric lineshape and that their effective
cross-section is enhanced upon doping. We relate these observations to
electron-phonon coupling or, more specifically, to a Fano resonance phenomenon.
We note that only the dopant-induced intraband continuum couples to the phonon
modes and that defects induced in the sidewall increase the resonance
probabilities.Comment: 5 pages, 4 figures and 1 Supplementary Information File (in pdf
Theory of single-particle properties of the Hubbard model
It is shown that it is possible to quantitatively explain quantum Monte Carlo
results for the Green's function of the two-dimensional Hubbard model in the
weak to intermediate coupling regime. The analytic approach includes vertex
corrections in a paramagnon-like self-energy. All parameters are determined
self-consistently. This approach clearly shows that in two dimensions
Fermi-liquid quasiparticles disappear in the paramagnetic state when the
antiferromagnetic correlation length becomes larger than the electronic thermal
de Broglie wavelength.Comment: 5 pages, latex, uuencoded figures, REVTEX Also available by direct
request to [email protected]
Neel order, ring exchange and charge fluctuations in the half-filled Hubbard model
We investigate the ground state properties of the two dimensional half-filled
one band Hubbard model in the strong (large-U) to intermediate coupling limit
({\it i.e.} away from the strict Heisenberg limit) using an effective spin-only
low-energy theory that includes nearest-neighbor exchange, ring exchange, and
all other spin interactions to order t(t/U)^3. We show that the operator for
the staggered magnetization, transformed for use in the effective theory,
differs from that for the order parameter of the spin model by a
renormalization factor accounting for the increased charge fluctuations as t/U
is increased from the t/U -> 0 Heisenberg limit. These charge fluctuations lead
to an increase of the quantum fluctuations over and above those for an S=1/2
antiferromagnet. The renormalization factor ensures that the zero temperature
staggered moment for the Hubbard model is a monotonously decreasing function of
t/U, despite the fact that the moment of the spin Hamiltonien, which depends on
transverse spin fluctuations only, in an increasing function of t/U. We also
comment on quantitative aspects of the t/U and 1/S expansions.Comment: 9 pages - 3 figures - References and details to help the reader adde
Variability of potato petiole nitrogen in response to nitrogen fertilizer, implications for variable management
Non-Peer ReviewedRecent increases in the cost of fertilizer nitrogen have prompted producers to assess the
potential to vary inputs in space and time to produce the highest marketable yield of
potatoes. A study was conducted from 2005 to 2007 near Brandon, Manitoba Canada, to
assess the spatial variability of potato yield in upper, middle and lower landforms on a
sandy loam soil in response to a range of nitrogen fertilizer rates and split application.
Petiole nitrogen, determined late in the growing season, was correlated with potato yield
and was used to assess nitrogen sufficiency through the growing season. Petiole nitrogen
varied with time during the growing season, from uniform levels in June across all
fertilizer treatments, to those which varied with fertilizer treatment in July and August.
Furthermore potato petiole nitrogen was higher in lower landforms during July and
August, where higher total and marketable yields were recorded. The potential for split
application of nitrogen in potatoes based on management zones or sensor readings will
have to be carefully assessed to account for temporal and spatial variability
Magnetic and pair correlations of the Hubbard model with next-nearest-neighbor hopping
A combination of analytical approaches and quantum Monte Carlo simulations is
used to study both magnetic and pairing correlations for a version of the
Hubbard model that includes second-neighbor hopping as a
model for high-temperature superconductors. Magnetic properties are analyzed
using the Two-Particle Self-Consistent approach. The maximum in magnetic
susceptibility as a function of doping appears both at finite
and at but for two totally different physical reasons. When
, it is induced by antiferromagnetic correlations while at
it is a band structure effect amplified by interactions.
Finally, pairing fluctuations are compared with -matrix results to
disentangle the effects of van Hove singularity and of nesting on
superconducting correlations. The addition of antiferromagnetic fluctuations
increases slightly the -wave superconducting correlations despite the
presence of a van Hove singularity which tends to decrease them in the
repulsive model. Some aspects of the phase diagram and some subtleties of
finite-size scaling in Monte Carlo simulations, such as inverted finite-size
dependence, are also discussed.Comment: Revtex, 8 pages + 15 uuencoded postcript figure
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