title: Order by quantum disorder in Er2Ti2O7.
abstract: Here we establish the systematic existence of a U(1) degeneracy of all symmetry-allowed Hamiltonians quadratic in the spins on the pyrochlore lattice, at the mean-field level. By extracting the Hamiltonian of Er(2)Ti(2)O(7) from inelastic neutron scattering measurements, we then show that the U(1)-degenerate states of Er(2)Ti(2)O(7) are its classical ground states, and unambiguously show that quantum fluctuations break the degeneracy in a way which is confirmed by experiment. The degree of symmetry protection of the classical U(1) degeneracy in Er(2)Ti(2)O(7) is unprecedented in other materials. As a consequence, our observation of order by disorder is unusually definitive. We provide further verifiable consequences of this phenomenon, and several additional comparisons between theory and experiment.
authors.affiliation: Department of Physics, University of California, Santa Barbara, California 93106-9530, USA.
authors.forename: Lucile
authors.initials: L
authors.lastname: Savary
authors.forename: Kate A
authors.initials: KA
authors.lastname: Ross
authors.forename: Bruce D
authors.initials: BD
authors.lastname: Gaulin
authors.forename: Jacob P C
authors.initials: JP
authors.lastname: Ruff
authors.forename: Leon
authors.initials: L
authors.lastname: Balents