93 research outputs found
Planar pyrochlore: a strong-coupling analysis
Recent investigations of the two-dimensional spin-1/2 checkerboard lattice
favor a valence bond crystal with long range quadrumer order [J.-B. Fouet et
al., preprint cond-mat/0108070]. Starting from the limit of isolated
quadrumers, we perform a complementary analysis of the evolution of the
spectrum as a function of the inter quadrumer coupling j using both, exact
diagonalization (ED) and series expansion (SE) by continuous unitary
transformation. We compute (i) the ground state energy, (ii) the elementary
triplet excitations, (iii) singlet excitations on finite systems and find very
good agreement between SE and ED. In the thermodynamic limit we find a ground
state energy substantially lower than documented in the literature. The
elementary triplet excitation is shown to be gapped and almost dispersionless,
whereas the singlet sector contains strongly dispersive modes. Evidence is
presented for the low energy singlet excitations in the spin gap in the
vicinity of j=1 to result from a large downward renormalization of local
high-energy states.Comment: 4 pages REVTeX4 including 3 figures: published version (minor
changes
Finite temperature properties of the two-dimensional SU(2) Kondo-necklace
We analyse several thermodynamic properties of the two-dimensional Kondo
necklace using finite-temperature stochastic series expansion. In agreement
with previous zero-temperature findings the model is shown to exhibit a quantum
critical point (QCP), separating an antiferromagnetic from a paramagnetic
dimerized state at a critical Kondo exchange-coupling strength . We evaluate the temperature dependent uniform and staggered structure
factors as well as the uniform and staggered susceptibilities and the local
'impurity' susceptibility close to the QCP as well as in the ordered and
quantum disordered phase. The crossover between the classical, renormalized
classical, and quantum critical regime is analyzed as a function of temperature
and Kondo coupling.Comment: 4.2 pages, 6 figure
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