246 research outputs found
Spin-Spin Correlation Enhanced by Impurities in a Frustrated Two-leg Spin Ladder
We theoretically study a spin-spin correlation enhanced by non-magnetic
impurities in a frustrated two-leg spin ladder. The frustration is introduced
by the next-nearest-neighbor antiferromagnetic interaction in the leg direction
in the antiferromagnetic two-leg spin ladder. The spin-spin correlation
function around impurity site is calculated by the density-matrix
renormalization-group method. We find that the spin-spin correlation is
enhanced around impurity site with the wavenumber reflecting the frustration.
As increasing the frustration, the wavenumber is shifted from commensurate to
incommensurate. We discuss several experimental results on BiCu2PO6 in the
light of our theory.Comment: 5 pages, 4 figure
Quasi-Spin Correlations in a Frustrated Quantum Spin Ladder
The quasi-spin correlations in a frustrated quantum spin ladder with one-half
magnetization are theoretically studied by using the density-matrix
renormalization-group method and the quasi-spin transformation. In this model,
the frustration induces a gapless-to-gapful phase transition with a strong rung
coupling. The gapful state is observed as the one-half magnetization plateau in
the magnetization curve. In the magnetization-plateau state, we find that the
quasi-spin dimer has a large expectation value with long-ranged correlations.
This result does not only comes in useful to clarify the magnetization-plateau
state, but gives a crucial information to understand the magnetization curve of
the real compound BiCuPO, whose effective spin model corresponds to
ours.Comment: 7 pages, 2 figure
Origin of spatial variation of pairing gap in Bi-based high-Tc cuprates
Recently, scanning tunneling microscopy on Bi-2212 cuprate superconductor has
revealed a spatial variation of the energy gap that is directly correlated with
a modulation of the apical oxygen position. We identify two mechanisms by which
out-of-plane oxygens can modulate the pairing interaction within the CuO_2
layer: a covalency between the x^2-y^2 band and apical p-orbital, and a
screening of correlation U by apical oxygen polarization. Both effects strongly
depend on the apical oxygen position and their cooperative action explains the
experiment.Comment: 4 pages, 3 figures, revised version to appear in PR
Enhanced Pairing Correlations near Oxygen Dopants in Cuprate Superconductors
Recent experiments on Bi-based cuprate superconductors have revealed an
unexpected enhancement of the pairing correlations near the interstitial oxygen
dopant ions. Here we propose a possible mechanism -- based on local screening
effects -- by which the oxygen dopants do modify the electronic parameters
within the CuO_2 planes and strongly increase the superexchange coupling J.
This enhances the spin pairing effects locally and may explain the observed
spatial variations of the density of states and the pairing gap.Comment: 4 pages, 4 figures, minor revisions, to appear in PR
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