by Dr.  Ling Wang

Department of Physics, Boston University

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Time: 15:30-16:30, Friday,  Feb. 22, 2008

Place: Physics Building 504

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Diluted Heisenberg antiferromagnets are of great interest to understand mechanism of high temperature superconductivity. We studied the low temperature dynamics of 2D Heisenberg antiferromagnets doped with non-mobile holes. At classical percolation point, diluted quantum
clusters exhibit classical percolation transition, but with a nontrivial dynamic exponent z=2D_f other than commonly expected value D_f from quantum rotor analogy, where D_f is the dimensionality of 2D percolating clusters. Quantum Monte Carlo simulation demonstrates that
the nature of this excitation is a noval localized excitation[1,2], which involves only a few sites. Immediately below percolation point, diluted single layer cluster restores its behavior of collective rotor excitation. If we completely suppress the sublattice imbalance by superposition two identical percolating clusters, diluted bilayer system at percolation point exhibits ordinary collective rotor excitations again with z=D_f. We side proof localized excitation nature through a study of classical dimer-monomer aggregation model and QMC simulation that demonstrate triplet excitation localization in Valence Bond basis.

[1] L.Wang, A.W.Sandvik, Phys.Rev.Lett. 97, 117204 (2006)
[2] L.Wang, A.W.Sandvik, unpublished

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