452 research outputs found
DMRG analysis of the SDW-CDW crossover region in the 1D half-filled Hubbard-Holstein model
In order to clarify the physics of the crossover from a spin-density-wave
(SDW) Mott insulator to a charge-density-wave (CDW) Peierls insulator in
one-dimensional (1D) systems, we investigate the Hubbard-Holstein Hamiltonian
at half filling within a density matrix renormalisation group (DMRG) approach.
Determining the spin and charge correlation exponents, the momentum
distribution function, and various excitation gaps, we confirm that an
intervening metallic phase expands the SDW-CDW transition in the weak-coupling
regime.Comment: revised versio
The impact of model building on the transmission dynamics under vaccination: Observable (symptom-based) versus unobservable (contagiousness-dependent) approaches
published_or_final_versio
Modeling the obesity epidemic: Social contagion and its implications for control
published_or_final_versio
Vaccination and clinical severity: Is the effectiveness of contact tracing and case isolation hampered by past vaccination?
published_or_final_versio
Phase Diagram of the ------ Model at Quarter Filling
We examine the ground-state properties of the one-dimensional Hubbard model
at quarter filling with Coulomb interactions between nearest-neighbors
and next-nearest neighbors . Using the density-matrix renormalization
group and exact diagonalization methods, we obtain an accurate ground-state
phase diagram in the - plane with three different phases: - and -charge-density-wave and a broad metallic phase
in-between. The metal is a Tomonaga-Luttinger-liquid whose critical exponent
is largest around , where and are frustrated,
and smallest, , at the boundaries between the metallic phase and
each of the two ordered phases.Comment: 4 pages, 5 figures, sumitted to PR
Peierls to superfluid crossover in the one-dimensional, quarter-filled Holstein model
We use continuous-time quantum Monte Carlo simulations to study retardation
effects in the metallic, quarter-filled Holstein model in one dimension. Based
on results which include the one- and two-particle spectral functions as well
as the optical conductivity, we conclude that with increasing phonon frequency
the ground state evolves from one with dominant diagonal order---2k_F charge
correlations---to one with dominant off-diagonal fluctuations, namely s-wave
pairing correlations. In the parameter range of this crossover, our numerical
results support the existence of a spin gap for all phonon frequencies. The
crossover can hence be interpreted in terms of preformed pairs corresponding to
bipolarons, which are essentially localised in the Peierls phase, and
"condense" with increasing phonon frequency to generate dominant pairing
correlations.Comment: 11 pages, 5 figure
Local density of states of the one-dimensional spinless fermion model
We investigate the local density of states of the one-dimensional half-filled
spinless fermion model with nearest-neighbor hopping t>0 and interaction V in
its Luttinger liquid phase -2t < V <= 2t. The bulk density of states and the
local density of states in open chains are calculated over the full band width
4t with an energy resolution <= 0.08t using the dynamical density-matrix
renormalization group (DDMRG) method. We also perform DDMRG simulations with a
resolution of 0.01t around the Fermi energy to reveal the power-law behaviour
predicted by the Luttinger liquid theory for bulk and boundary density of
states. The exponents are determined using a finite-size scaling analysis of
DDMRG data for lattices with up to 3200 sites. The results agree with the exact
exponents given by the Luttinger liquid theory combined with the Bethe Ansatz
solution. The crossover from boundary to bulk density of states is analyzed. We
have found that boundary effects can be seen in the local density of states at
all energies even far away from the chain edges
A Green's function decoupling scheme for the Edwards fermion-boson model
Holes in a Mott insulator are represented by spinless fermions in the
fermion-boson model introduced by Edwards. Although the physically interesting
regime is for low to moderate fermion density the model has interesting
properties over the whole density range. It has previously been studied at
half-filling in the one-dimensional (1D) case by numerical methods, in
particular exact diagonalization and density matrix renormalization group
(DMRG). In the present study the one-particle Green's function is calculated
analytically by means of a decoupling scheme for the equations of motion, valid
for arbitrary density in 1D, 2D and 3D with fairly large boson energy and zero
boson relaxation parameter. The Green's function is used to compute some ground
state properties, and the one-fermion spectral function, for fermion densities
n=0.1, 0.5 and 0.9 in the 1D case. The results are generally in good agreement
with numerical results obtained by DMRG and dynamical DMRG and new light is
shed on the nature of the ground state at different fillings. The Green's
function approximation is sufficiently successful in 1D to justify future
application to the 2D and 3D cases.Comment: 19 pages, 7 figures, final version with updated reference
Phase separation in the Edwards model
The nature of charge transport within a correlated background medium can be
described by spinless fermions coupled to bosons in the model introduced by
Edwards. Combining numerical density matrix renormalization group and
analytical projector-based renormalization methods we explore the ground-state
phase diagram of the Edwards model in one dimension. Below a critical boson
frequency any long-range order disappears and the system becomes metallic. If
the charge carriers are coupled to slow quantum bosons the Tomonaga-Luttinger
liquid is attractive and finally makes room for a phase separated state, just
as in the t-J model. The phase boundary separating repulsive from the
attractive Tomonaga-Luttinger liquid is determined from long-wavelength charge
correlations, whereas fermion segregation is indicated by a vanishing inverse
compressibility. On approaching phase separation the photoemission spectra
develop strong anomalies.Comment: 6 pages, 5 figures, final versio
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