Isotopic disorder and the quantum phase transition in an xy antiferromagnet

LiErF₄ is a model system for dipole-coupled XY antiferromagnetism. A magnetic field perpendicular to the xy plane induces a quantum phase transition to a disordered state. We characterize this transition with ac susceptibility and heat capacity measurements and find that the quantum phase transition presents two qualitatively different regimes. (i) In the high temperature regime (T > 150 mK), the transition sharpens as temperature decreases. (ii) In the low temperature regime (T < 150 mK), the transition broadens as temperature decreases. We demonstrate that this surprising behavior is explained by isotopic disorder. Erbium has a natural abundance of 23% Er-167, the only stable isotope with non-zero nuclear moment. In the low temperature regime, these nuclear spins polarize and produce random field disorder, leading to suppression of the critical divergence of the electronic spin susceptibility. LiErF₄ thereby permits the controlled study of the hypersensitivity of quantum phase transitions to disorder.