1,289 research outputs found
Photoemission, inverse photoemission and superconducting correlations in Hubbard and t--J ladders: role of the anisotropy between legs and rungs
Several experiments in the context of ladder materials have recently shown
that the study of simple models of anisotropic ladders (i.e. with different
couplings along legs and rungs) is important for the understanding of these
compounds. In this paper Exact Diagonalization studies of the one-band Hubbard
and t-J models are reported for a variety of densities, couplings, and
anisotropy ratios. The emphasis is given to the one-particle spectral function
A(q,\omega) which presents a flat quasiparticle dispersion at the chemical
potential in some region of parameter space. This is correlated with the
existence of strong pairing fluctuations, which themselves are correlated with
an enhancement of the bulk-extrapolated value for the two-hole binding energy
as well as with the strength of the spin-gap in the hole-doped system. Part of
the results for the spectral function are explained using a simple analytical
picture valid when the hopping along the legs is small. In particular, this
picture predicts an insulating state at quarter filling in agreement with the
metal-insulator transition observed at this special filling for increasing rung
couplings. The results are compared against previous literature, and in
addition pair-pair correlations using extended operators are also here
reported.Comment: 13 pages, 9 figs, LateX, submitted to The European Physical Journal
Relevance of Cooperative Lattice Effects and Correlated Disorder in Phase-Separation Theories for CMR Manganites
Previous theoretical investigations of colossal magnetoresistance (CMR)
materials explain this effect using a ``clustered'' state with preformed
ferromagnetic islands that rapidly align their moments with increasing external
magnetic fields. While qualitatively successful, explicit calculations indicate
drastically different typical resistivity values in two- and three-dimensional
lattices, contrary to experimental observations. This conceptual bottleneck in
the phase-separated CMR scenario is resolved here considering the cooperative
nature of the Mn-oxide lattice distortions. This induces power-law correlations
in the quenched random fields used in toy models with phase competition. When
these effects are incorporated, resistor-network calculations reveal very
similar results in two and three dimensions, solving the puzzle.Comment: RevTeX 4, 4 figure
Bound states of holes in an antiferromagnet
The formation of bound states of holes in an antiferromagnetic spin-1/2
background is studied using numerical techniques applied to the
Hamiltonian on clusters with up to 26 sites. An analysis of the binding energy
as a function of cluster size suggests that a two hole bound state is formed
for couplings larger than a ``critical'' value . The symmetry of
the bound state is \dx2y2. We also observed that its ``quasiparticle'' weight
(defined in the text), is finite for all values of the coupling
. Thus, in the region the bound state of two
holes behaves like a quasiparticle with charge , spin , and \dx2y2
internal symmetry. The relation with recent ideas that have suggested the
possibility of d-wave pairing in the high temperature cuprate superconductors
is briefly discussed.Comment: 12 pages and 3 figures (3 postscript files included), Report
LPQTH-93/0
Magnonic Crystal Theory of the Spin-Wave Frequency Gap in Low-Doped Manganites
A theory of three-dimensional (3D) hypothetical magnonic crystal (conceived
as the magnetic counterpart of the well-known photonic crystal) is developed
and applied to explain the existence of a spin-wave frequency gap recently
revealed in low-doped manganites by neutron scattering.
A successful confrontation with the experimental results allows us to formulate
a working hypothesis that certain manganites could be regarded as 3D magnonic
crystals existing in nature.Comment: 5 pages, 3 figures, submitted to PR
Charge density correlations in t-J ladders investigated by the CORE method
Using 4-site plaquette or rung basis decomposition, the CORE method is
applied to 2-leg and 4-leg t-J ladders and cylinders. Resulting range-2
effective hamiltonians are studied numerically on periodic rings taking full
advantage of the translation symmetry as well as the drastic reduction of the
Hilbert space. We investigate the role of magnetic and fermionic degrees of
freedom to obtain the most reliable representation of the underlying model.
Spin gaps, pair binding energies and charge correlations are computed and
compared to available ED and DMRG data for the full Hamiltonian. Strong
evidences for short-range diagonal stripe correlations are found in periodic
4-leg t-J ladders.Comment: Computation of Luttinger liquid parameters (charge velocity and
charge correlation exponent) adde
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
Bogoliubov Quasiparticle Excitations in the Two-Dimensional t-J Model
Using a proposed numerical technique for calculating anomalous Green's
functions characteristic of superconductivity, we show that the low-lying
excitations in a wide parameter and doping region of the two-dimensional
model are well described by the picture of dressed Bogoliubov
quasiparticles in the BCS pairing theory. The pairing occurs predominantly in
-wave channel and the energy gap has a size
between quarter and half fillings. Opening of
the superconducting gap in the photoemission and inverse-photoemission spectrum
is demonstrated.Comment: 6 pages, RevTe
Photoemission Spectra in t-J Ladders with Two Legs
Photoemission spectra for the isotropic two-leg t-J ladder are calculated at
various hole-doping levels using exact diagonalization techniques. Low-energy
sharp features caused by short-range antiferromagnetic correlations are
observed at finite doping levels close to half-filling, above the naive Fermi
momentum. These features should be observable in angle-resolved photoemission
experiments. In addition, the formation of a d-wave pairing condensate as the
ratio J/t is increased leads to dynamically generated spectral weight for
momenta close to where the -order parameter is large.Comment: 9 pages, RevTex, to be published in Phys. Rev. B (RC
Rapid Suppression of the Spin Gap in Zn-doped CuGeO_3 and SrCu_2O_3
The influence of non-magnetic impurities on the spectrum and dynamical spin
structure factor of a model for CuGeO is studied. A simple extension to
Zn-doped is also discussed. Using Exact Diagonalization
techniques and intuitive arguments we show that Zn-doping introduces states in
the Spin-Peierls gap of CuGeO. This effect can beunderstood easily in the
large dimerization limit where doping by Zn creates ``loose'' S=1/2 spins,
which interact with each other through very weak effective antiferromagnetic
couplings. When the dimerization is small, a similar effect is observed but now
with the free S=1/2 spins being the resulting S=1/2 ground state of severed
chains with an odd number of sites. Experimental consequences of these results
are discussed. It is interesting to observe that the spin correlations along
the chains are enhanced by Zn-doping according to the numerical data presented
here. As recent numerical calculations have shown, similar arguments apply to
ladders with non-magnetic impurities simply replacing the tendency to
dimerization in CuGeO by the tendency to form spin-singlets along the rungs
in SrCuO.Comment: 7 pages, 8 postscript figures, revtex, addition of figure 8 and a
section with experimental predictions, submmited to Phys. Rev. B in May 199
Optical Conductivity of the Two-Dimensional Hubbard Model
Charge dynamics of the two-dimensional Hubbard model is investigated.
Lanczs-diagonalization results for the optical conductivity and
the Drude weight of this model are presented. Near the Mott transition, large
incoherence below the upper-Hubbard band is obtained together with a remarkably
suppressed Drude weight in two dimensions while the clearly coherent character
is shown in one dimension. The two-dimensional results are consistent with
previous results from quantum Monte Carlo calculations indicating that the Mott
transition in this two-dimensional model belongs to the universality class
characterized by the dynamical exponent of .Comment: 4 pages LaTeX including 2 PS figures, to appear in J. Phys. Soc. Jp
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