2,510 research outputs found
Spectral Density of the Two-Impurity Anderson Model
We investigate static and dynamical ground-state properties of the
two-impurity Anderson model at half filling in the limit of vanishing impurity
separation using the dynamical density-matrix renormalization group method. In
the weak-coupling regime, we find a quantum phase transition as function of
inter-impurity hopping driven by the charge degrees of freedom. For large
values of the local Coulomb repulsion, the transition is driven instead by a
competition between local and non-local magnetic correlations. We find evidence
that, in contrast to the usual phenomenological picture, it seems to be the
bare effective exchange interactions which trigger the observed transition.Comment: 18 pages, 6 figures, submitted to J. Phys.:Condens. Matte
Fourth-Order Perturbation Theory for the Half-Filled Hubbard Model in Infinite Dimensions
We calculate the zero-temperature self-energy to fourth-order perturbation
theory in the Hubbard interaction for the half-filled Hubbard model in
infinite dimensions. For the Bethe lattice with bare bandwidth , we compare
our perturbative results for the self-energy, the single-particle density of
states, and the momentum distribution to those from approximate analytical and
numerical studies of the model. Results for the density of states from
perturbation theory at agree very well with those from the Dynamical
Mean-Field Theory treated with the Fixed-Energy Exact Diagonalization and with
the Dynamical Density-Matrix Renormalization Group. In contrast, our results
reveal the limited resolution of the Numerical Renormalization Group approach
in treating the Hubbard bands. The momentum distributions from all approximate
studies of the model are very similar in the regime where perturbation theory
is applicable, . Iterated Perturbation Theory overestimates the
quasiparticle weight above such moderate interaction strengths.Comment: 19 pages, 17 figures, submitted to EPJ
Do oil and gas platforms off California reduce recruitment of bocaccio (Sebastes paucispinis) to natural habitat? An analysis based on trajectories derived from high-frequency radar
To investigate the possibility that oil and gas platforms may reduce recruitment of rockfishes (Sebastes spp.) to natural habitat, we simulated drift pathways termed “trajectories” in our model) from an existing oil platform to nearshore habitat using current measurements from high-frequency (HF) radars. The trajectories originated at Platform Irene, located west of Point Conception, California, during two recruiting seasons for bocaccio (Sebastes paucispinis): May through August, 1999 and 2002. Given that pelagic juvenile bocaccio dwell near the surface, the trajectories estimate transport to habitat. We assumed that appropriate shallow water juvenile habitat exists inshore of the 50-m isobath. Results from 1999 indicated that 10% of the trajectories represent transport to habitat, whereas 76% represent transport across the offshore boundary. For 2002, 24% represent transport to habitat, and 69% represent transport across the offshore boundary. Remaining trajectories (14% and 7% for 1999 and 2002, respectively) exited the coverage area either northward or southward along isobaths. Deployments of actual drifters (with 1-m drogues) from a previous multiyear study provided measurements originating near Platform Irene from May through August. All but a few of the drifters moved offshore, as was also shown with the HF radar-derived trajectories. These results indicate that most juvenile bocaccio settling on the platform would otherwise have been transported offshore and perished in the absence of a platform. However, these results do not account for the swimming behavior of juvenile bocaccio, about which little is known
Low-lying excitations of the three-leg spin tube using the density-matrix renormalization group method
Using the (dynamical) density-matrix renormalization group method, we study
the low-energy physics of three-leg antiferromagnetic Heisenberg model where
the periodic boundary conditions are applied in the rung direction. We confirm
that the spin excitations are always gapped as long as the intra-ring couplings
form a regular triangle. From precise finite-size-scaling analyses of the spin
gap and dimerization order parameter, we also find that the spin gap is
collapsed by very small asymmetric modulation of the intra-ring couplings.
Moreover, the dynamical spin structure factors on the intra- and inter-leg
correlations are calculated. It is demonstrated that the low-lying structure of
the inter-leg spectra is particularly affected by the asymmetric modulation.Comment: 10 pages, 8 figure
Analytical and Numerical Treatment of the Mott--Hubbard Insulator in Infinite Dimensions
We calculate the density of states in the half-filled Hubbard model on a
Bethe lattice with infinite connectivity. Based on our analytical results to
second order in , we propose a new `Fixed-Energy Exact Diagonalization'
scheme for the numerical study of the Dynamical Mean-Field Theory. Corroborated
by results from the Random Dispersion Approximation, we find that the gap opens
at . Moreover, the density of states near the gap
increases algebraically as a function of frequency with an exponent
in the insulating phase. We critically examine other analytical
and numerical approaches and specify their merits and limitations when applied
to the Mott--Hubbard insulator.Comment: 22 pages, 16 figures; minor changes (one reference added, included
comparison with Falicov-Kimball model
Dynamics in two-leg spin ladder with a four-spin cyclic interaction
We study two-leg Heisenberg ladder with four-spin cyclic interaction using
the (dynamical) density-matrix renormalization group method. We demonstrate the
dependence of the low-lying excitations in the spin wave, staggered dimer
order, and scalar-chirality order structure factors on the four-spin cyclic
interaction. We find that the cyclic interaction enhances spin-spin
correlations with wave vector around momentum .
Also, the presence of long-range order in the staggered dimer and
scalar-chirality phases is confirmed by a -function peak contribution
of the structure factors at energy .Comment: 4 pages, 4 figure
Phase diagram of the one-dimensional half-filled extended Hubbard model
We study the ground state of the one-dimensional half-filled Hubbard model
with on-site (nearest-neighbor) repulsive interaction () and
nearest-neighbor hopping . In order to obtain an accurate phase diagram, we
consider various physical quantities such as the charge gap, spin gap,
Luttinger-liquid exponents, and bond-order-wave (BOW) order parameter using the
density-matrix renormalization group technique. We confirm that the BOW phase
appears in a substantial region between the charge-density-wave (CDW) and
spin-density-wave phases. Each phase boundary is determined by multiple means
and it allows us to do a cross-check to demonstrate the validity of our
estimations. Thus, our results agree quantitatively with the renormalization
group results in the weak-coupling regime (), with the
perturbation results in the strong-coupling regime (), and with
the quantum Monte Carlo results in the intermediate-coupling regime. We also
find that the BOW-CDW transition changes from continuous to first order at the
tricritical point and the BOW
phase vanishes at the critical end point .Comment: 4 pages, 5 figure
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