6 research outputs found
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
Enhancement of Antiferromagnetic Correlations Induced by Nonmagnetic Impurities: Origin and Predictions for NMR Experiments
Spin models that have been proposed to describe dimerized chains, ladders,
two dimensional antiferromagnets, and other compounds are here studied when
some spins are replaced by spinless vacancies, such as it occurs by
doping. A small percentage of vacancies rapidly destroys the spin gap, and
their presence induces enhanced antiferromagnetic correlations near those
vacancies. The study is performed with computational techniques which includes
Lanczos, world-line Monte Carlo, and the Density Matrix Renormalization Group
methods. Since the phenomenon of enhanced antiferromagnetism is found to occur
in several models and cluster geometries, a common simple explanation for its
presence may exist. It is argued that the resonating-valence-bond character of
the spin correlations at short distances of a large variety of models is
responsible for the presence of robust staggered spin correlations near
vacancies and lattice edges. The phenomenon takes place regardless of the long
distance properties of the ground state, and it is caused by a ``pruning'' of
the available spin singlets in the vicinity of the vacancies. The effect
produces a broadening of the low temperature NMR signal for the compounds
analyzed here. This broadening should be experimentally observable in the
structurally dimerized chain systems
, , , and
, in ladder materials such as , in the
spin-Peierls systems and , and in several others since it
is a universal effect common to a wide variety of models and compounds.Comment: 18 pages revtex with 26 figures include
Teoria de ESR de momentos magnéticos diluídos e localizados em sólidos
Orientador: Gaston Eduardo BarberisTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Não informadoAbstract: Not informed.DoutoradoFísicaDoutor em Ciência