paper

Order by disorder and gauge-like degeneracy in quantum pyrochlore antiferromagnet

arXiv:cond-mat/0509005 · doi:10.1103/PhysRevLett.96.047201

Abstract

The (three-dimensional) pyrochlore lattice antiferromagnet with Heisenberg spins of large spin length is a highly frustrated model with an macroscopic degeneracy of classical ground states. The zero-point energy of (harmonic order) spin wave fluctuations distinguishes a subset of these states. I derive an approximate but illuminating {\it effective Hamiltonian}, acting within the subspace of Ising spin configurations representing the {\it collinear} ground states. It consists of products of Ising spins around loops, i.e has the form of a lattice gauge theory. The remaining ground state entropy is still infinite but not extensive, being for system size . All these ground states have unit cells bigger than those considered previously.

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