Er2Ti2O7: Evidence of Order by Disorder in a Frustrated Antiferromagnet

Er_2Ti_2O_7 has been suggested to be a realization of the frustrated < 111 > XY pyrochlore lattice antiferromagnet, for which theory predicts fluctuation-induced symmetry breaking in a highly degenerate ground state manifold. We present a theoretical analysis of the classical model compared to neutron scattering experiments on the real material, both below and above T_N = 1.173(2) K. The model correctly predicts the ordered magnetic structure, suggesting that the real system has order stabilised by zero-point quantum fluctuations that can be modelled by classical spin wave theory. However, the model fails to describe the excitations of the system, which show unusual features.