54 research outputs found
Enhanced Bound State Formation in Two Dimensions via Stripe-Like Hopping Anisotropies
We have investigated two-electron bound state formation in a square
two-dimensional t-J-U model with hopping anisotropies for zero electron
density; these anisotropies are introduced to mimic the hopping energies
similar to those expected in stripe-like arrangements of holes and spins found
in various transition metal oxides. In this report we provide analytical
solutions to this problem, and thus demonstrate that bound-state formation
occurs at a critical exchange coupling, J_c, that decreases to zero in the
limit of extreme hopping anisotropy t_y/t_x -> 0. This result should be
contrasted with J_c/t = 2 for either a one-dimensional chain, or a
two-dimensional plane with isotropic hopping. Most importantly, this behaviour
is found to be qualitatively similar to that of two electrons on the two-leg
ladder problem in the limit of t_interchain/t_intrachain -> 0. Using the latter
result as guidance, we have evaluated the pair correlation function, thus
determining that the bound state corresponds to one electron moving along one
chain, with the second electron moving along the opposite chain, similar to two
electrons confined to move along parallel, neighbouring, metallic stripes. We
emphasize that the above results are not restricted to the zero density limit -
we have completed an exact diagonalization study of two holes in a 12 X 2
two-leg ladder described by the t-J model and have found that the
above-mentioned lowering of the binding energy with hopping anisotropy persists
near half filling.Comment: 6 pages, 3 eps figure
Dynamical Properties of Two Coupled Hubbard Chains at Half-filling
Using grand canonical Quantum Monte Carlo (QMC) simulations combined with
Maximum Entropy analytic continuation, as well as analytical methods, we
examine the one- and two-particle dynamical properties of the Hubbard model on
two coupled chains at half-filling. The one-particle spectral weight function,
, undergoes a qualitative change with interchain hopping
associated with a transition from a four-band insulator to a two-band
insulator. A simple analytical model based on the propagation of exact rung
singlet states gives a good description of the features at large . For
smaller , is similar to that of the
one-dimensional model, with a coherent band of width the effective
antiferromagnetic exchange reasonably well-described by renormalized
spin-wave theory. The coherent band rides on a broad background of width
several times the parallel hopping integral , an incoherent structure
similar to that found in calculations on both the one- and two-dimensional
models. We also present QMC results for the two-particle spin and charge
excitation spectra, and relate their behavior to the rung singlet picture for
large and to the results of spin-wave theory for small .Comment: 9 pages + 10 postscript figures, submitted to Phys.Rev.B, revised
version with isotropic t_perp=t data include
The ground state of the two-leg Hubbard ladder: a density--matrix renormalization group study
We present density-matrix renormalization group results for the ground state
properties of two-leg Hubbard ladders. The half-filled Hubbard ladder is an
insulating spin-gapped system, exhibiting a crossover from a spin-liquid to a
band-insulator as a function of the interchain hopping matrix element. When the
system is doped, there is a parameter range in which the spin gap remains. In
this phase, the doped holes form singlet pairs and the pair-field and the "" density correlations associated with pair density fluctuations decay as
power laws, while the "" charge density wave correlations decay
exponentially. We discuss the behavior of the exponents of the pairing and
density correlations within this spin gapped phase. Additional one-band
Luttinger liquid phases which occur in the large interband hopping regime are
also discussed.Comment: 14 pages, 18 figures, uses Revtex with epsfig to include the figure
Phase Diagram of the 1D Anderson Lattice
We map out the phase diagram of the one--dimensional Anderson lattice by
studying the ground state magnetization as a function of band--filling using
the density matrix renormalization group technique. For strong coupling, we
find that the quarter--filled system has an S=0 ground state with strong
antiferromagnetic correlations. As additional electrons are put in, we find
first a ferromagnetic phase, as reported by M\"{o}ller and W\"{o}lfle, and then
a phase in which the ground state has total spin . Within this
phase, we find RKKY oscillations in the spin--spin correlation functions.Comment: REVTEX manuscript with 5 Postcript figures included in uu file.
Submitted to Phys. Rev.
Electronic Correlations Near a Peierls-CDW Transition
Results of a phenomenological Monte carlo calculation for a 2D
electron-phonon Holstein model near a Peierls-CDW transition are presented.
Here the zero Matsubara frequency part of the phonon action is dominant and we
approximated it by a phenomenological form that as an Ising-like Peierls-CDW
transition. The resulting model is studied on a 32 by 32 lattice. The single
particle spectral weight A(k,\omega), the density of states N(\omega), and the
real part of the conductivity \sigma_1(\omega) all show evidence of a pseudogap
which develops in the low-energy electronic degrees of freedom as the
Peierls-CDW transition is approachedComment: 14 pages, 7 figure
Microscopic description of d-wave superconductivity by Van Hove nesting in the Hubbard model
We devise a computational approach to the Hubbard model that captures the
strong coupling dynamics arising when the Fermi level is at a Van Hove
singularity in the density of states. We rely on an approximate degeneracy
among the many-body states accounting for the main instabilities of the system
(antiferromagnetism, d-wave superconductivity). The Fermi line turns out to be
deformed in a manner consistent with the pinning of the Fermi level to the Van
Hove singularity. For a doping rate , the ground state is
characterized by d-wave symmetry, quasiparticles gapped only at the
saddle-points of the band, and a large peak at zero momentum in the d-wave
pairing correlations.Comment: 4 pages, 2 Postscript figure
The spin and charge gaps of the half-filled N-leg Kondo ladders
In this work, we study N-leg Kondo ladders at half-filling through the
density matrix renormalization group. We found non-zero spin and charge gaps
for any finite number of legs and Kondo coupling . We also show evidence
of the existence of a quantum critical point in the two dimensional Kondo
lattice model, in agreement with previous works. Based on the binding energy of
two holes, we did not find evidence of superconductivity in the 2D Kondo
lattice model close to half-filling.Comment: 4 pages, 1 table, 3 fig
Ground State Properties of the Doped 3-Leg t-J Ladder
Results for a doped 3-leg t-J ladder obtained using the density matrix
renormalization group are reported. At low hole doping, the holes form a dilute
gas with a uniform density. The momentum occupation of the odd band shows a
sharp decrease at a large value of k_F similar to the behavior of a lightly
doped t-J chain, while the even modes appear gapped. The spin-spin correlations
decay as a power law consistent with the absence of a spin gap, but the pair
field correlations are negligible. At larger doping we find evidence for a spin
gap and as x increases further we find 3-hole diagonal domain walls. In this
regime there are pair field correlations and the internal pair orbital has
d_x^2-y^2 - like symmetry. However, the pair field correlations appear to fall
exponentially at large distances.Comment: 14 pages, 11 postscript figure
Haldane-Gapped Spin Chains as Luttinger Liquids: Correlation Functions at Finite Field
We study the behavior of Heisenberg, antiferromagnetic, integer-spin chains
in the presence of a magnetic field exceeding the attendant spin gap. For
temperatures much smaller than the gap, the spin chains exhibit Luttinger
liquid behavior. We compute exactly both the corresponding Luttinger parameter
and the Fermi velocity as a function of magnetic field. This enables the
computation of a number of correlators from which we derive the spin
conductance, the expected form of the dynamic structure factor relevant to
inelastic neutron scattering experiments, and NMR relaxation rates. We also
comment upon the robustness of the magnetically induced gapless phase both to
finite temperature and finite couplings between neighbouring chains.Comment: 32 pages, 8 figures; published version includes additions discussing
the robustness of the magnetically induced gapless phase to ordering between
chains as well as the relationship between the spin-1 chains and spin-1/2
ladders in the presence of a magnetic fiel
The transition between hole-pairs and four-hole clusters in four-leg tJ ladders
Holes weakly doped into a four-leg \tj ladder bind in pairs. At dopings
exceeding a critical doping of four hole clusters are
observed to form in DMRG calculations. The symmetry of the ground state
wavefunction does not change and we are able to reproduce this behavior
qualitatively with an effective bosonic model in which the four-leg ladder is
represented as two coupled two-leg ladders and hole-pairs are mapped on hard
core bosons moving along and between these ladders. At lower dopings,
, a one dimensional bosonic representation for hole-pairs
works and allows us to calculate accurately the Luttinger liquid parameter
\krho, which takes the universal value \krho=1 as half-filling is
approached
- …