344 research outputs found
Generic mixed columnar-plaquette phases in Rokhsar-Kivelson models
We revisit the phase diagram of Rokhsar-Kivelson models, which are used in
fields such as superconductivity, frustrated magnetism, cold bosons, and the
physics of Josephson junction arrays. From an extended height effective theory,
we show that one of two simple generic phase diagrams contains a mixed phase
that interpolates continuously between columnar and plaquette states. For the
square lattice quantum dimer model we present evidence from exact
diagonalization and Green's function Monte Carlo techniques that this scenario
is realised, by combining an analysis of the excitation gaps of different
symmetry sectors with information on plaquette structure factors. This presents
a natural framework for resolving the disagreement between previous studies.Comment: 4 pages, 5 figure
Phase diagram of the two-dimensional t--J model at low doping
The phase diagram of the planar t--J model at small hole doping is
investigated by finite size scaling of exact diagonalisation data of NXN
clusters (up to 26). Hole-droplet binding energies, compressibility and static
spin and charge correlations are calculated. Short range antiferromagnetic
correlations can produce attractive forces between holes leading to a very rich
phase diagram including a liquid of d-wave hole pairs (for ), a
liquid of hole droplets (quartets) for larger J/t ratios ()
and, at even larger coupling J/t, an instability towards phase separation.Comment: 3 pages, latex, 5 postscript figures, uuencode
Breathers and Raman scattering in a two-leg ladder with staggered Dzialoshinskii-Moriya interaction
Recent experiments have revealed the role of staggered Dzialoshinskii-Moriya
interaction in the magnetized phase of an antiferromagnetic spin 1/2 two-leg
ladder compound under a uniform magnetic field. We derive a low energy
effective field theory describing a magnetized two-leg ladder with a weak
staggered Dzialoshinskii-Moriya interaction. This theory predicts the
persistence of the spin gap in the magnetized phase, in contrast to standard
two-leg ladders, and the presence of bound states in the excitation spectrum.
Such bound states are observable in Raman scattering measurements. These
results are then extended to intermediate Dzialoshinskii-Moriya interaction
using Exact Diagonalizations.Comment: RevTeX 4, 14 pages, 11 EPS figure
Spin Gaps in Coupled t-J Ladders
Spin gaps in coupled - ladders are investigated by exact
diagonalization of small clusters up to 48 sites. At half-filling, the
numerical results for the triplet excitation spectrum are in very good
agreement with a second order perturbation expansion in term of small
inter-ladder and intra-ladder exchange couplings between rungs
(). The band of local triplet excitations moving
coherently along the ladder (with momenta close to ) is split by the
inter-ladder coupling. For intermediate couplings finite size scaling is used
to estimate the spin gap. In the isotropic infinite 4-chain system (two coupled
ladders) we find a spin gap of , roughly half of the single ladder
spin gap. When the system is hole doped, bonding and anti-bonding bound pairs
of holes can propagate coherently along the chains and the spin gap remains
finite.Comment: 11 pages, 5 figures, uuencoded form of postscript files of figures
and text, LPQTH-94/
Recent progress in the truncated Lanczos method : application to hole-doped spin ladders
The truncated Lanczos method using a variational scheme based on Hilbert
space reduction as well as a local basis change is re-examined. The energy is
extrapolated as a power law function of the Hamiltonian variance. This
systematic extrapolation procedure is tested quantitatively on the two-leg t-J
ladder with two holes. For this purpose, we have carried out calculations of
the spin gap and of the pair dispersion up to size 2x15.Comment: 5 pages, 4 included eps figures, submitted to Phys. Rev. B; revised
versio
Doped two-leg ladder with ring exchange
The effect of a ring exchange on doped two-leg ladders is investigated
combining exact diagonalization (ED) and density matrix renormalization group
(DMRG) computations. We focus on the nature and weights of the low energy
magnetic excitations and on superconducting pairing. The stability with respect
to this cyclic term of a remarkable resonant mode originating from a hole
pair-magnon bound state is examined. We also find that, near the zero-doping
critical point separating rung-singlet and dimerized phases, doping reopens a
spin gap.Comment: 5 pages, 7 figures, to appear in PR
Binding of holes and pair spectral function in the t-J model
Clusters of the two-dimensionnal t--J model with 2 holes and up to 26 sites
are diagonalized using a Lanczos algorithm. The behaviour of the binding energy
with system size suggests the existence of a finite critical value of J above
which binding occurs in the bulk. Only the d-wave pair field operator acting on
the Heisenberg GS has a finite overlap with the 2 hole ground state for all the
clusters considered. The related spectral function associated with the
propagation of a d-wave (spin singlet) pair of holes in the antiferromagnetic
background is calculated. The quasiparticle peak at the bottom of the spectrum
as well as some structure appearing above the peak survive with increasing
cluster size. Although no simple scaling law was found for the quasiparticle
weight the data strongly suggest that this weight is finite in the bulk limit
and is roughly proportional to the antiferromagnetic coupling J (for J<1).Comment: Report LPQTH-93/01, 18 pages (REVTEX), 8 postscript files include
Mixed Columnar-Plaquette Crystal of correlated fermions on the 2D pyrochlore lattice at fractional filling
We study a model of strongly correlated S=1/2 fermions on the planar
pyrochlore, or checkerboard, lattice, at fractional (1/8) filling. Starting
with the extended Hubbard model in the limit of strong Coulomb repulsion,
low-energy configurations can be mapped onto hard-core dimer configurations
whose dimers carry a spin degree of freedom. An effective Hamiltonian similar
to the kinetic term of a quantum dimer model on the square lattice which
rotates two parallel dimers (in a spin-singlet configuration) by 90 degrees
naturally emerges. We also introduce an additional term in the Hamiltonian, a
generalized dimer plaquette interaction, in order to realize a closer analogy
to the latter model. For a strong dimer plaquette attraction stabilizing a
columnar phase, a spontaneous dimerization takes place in the direction of the
columns of (spin-carrying) dimers. Using exact diagonalizations of
two-dimensional periodic clusters, the analysis of the low-energy spectrum and
of several types of correlation functions gives indeed evidence for a new type
of lattice symmetry breaking phase, the eight-fold degenerate Mixed
Columnar-Plaquette Crystal, and for a transition from this phase to a
Resonating-Singlet-Pair Crystal (found in previous studies) which restores the
rotational symmetry of the lattice. Similar conclusions and phase diagram are
also reached from a simple variational approach.Comment: 14 pages, 15 figure
Control of the finite size corrections in exact diagonalization studies
We study the possibility of controlling the finite size corrections in exact
diagonalization studies quantitatively. We consider the one- and two
dimensional Hubbard model. We show that the finite-size corrections can be be
reduced systematically by a grand-canonical integration over boundary
conditions. We find, in general, an improvement of one order of magnitude with
respect to studies with periodic boundary conditions only. We present results
for ground-state properties of the 2D Hubbard model and an evaluation of the
specific heat for the 1D and 2D Hubbard model.Comment: Phys. Rev. B (Brief Report), in pres
Li Induced Spin and Charge Excitations in a Spin Ladder
A lithium dopant in a cuprate spin ladder acts as a vacant (non-magnetic)
site accompanied by an extra hole bound by a Coulomb potential. We find that,
although the undoped ladder spin gap is not essentially altered by Li doping, a
dopant-magnon bound state appears within the gap. Contrary to the case of
Zn-doped ladders, we predict that, in the Li-doped ladder, the spin liquid
character is very robust against antiferromagnetism. We also predict the
spatial dependence of the density of states in the vicinity of the dopant which
could be measured by local spectroscopic probes.Comment: 4 pages, Submitted to Physical Review Letter
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