Hierarchy of energy scales and field-tunable order by disorder in dipolar-octupolar pyrochlores

TL;DR

This paper investigates the complex hierarchy of energy scales and quantum effects in dipolar-octupolar pyrochlore magnets under a strong magnetic field, revealing distinct chain phases and their implications for experimental observations.

Contribution

It introduces a detailed analysis of quantum fluctuations in chain phases of dipolar-octupolar pyrochlores and derives an effective 2D Ising model explaining their behavior.

Findings

Identification of two types of chain phases with classical degeneracy.

Mapping of the ground-state phase diagram using spin wave and perturbation theory.

Connection of theoretical results with experimental observations in Nd2Zr2O7 and Ce2Sn2O7.

Abstract

Dipolar-octupolar pyrochlore magnets in a strong external magnet field applied in the [110] direction are known to form a `chain' state, with subextensive degeneracy. Magnetic moments are correlated along one-dimensional chains carrying effective Ising degrees of freedom which are noninteracting on the mean-field level. Here, we investigate this phenomenon in detail, including the effects of quantum fluctuations. We identify two distinct types of chain phases, both featuring distinct subextensive, classical ground state degeneracy. Focussing on one of the two kinds, we discuss lifting of the classical degeneracy by quantum fluctuations. We map out the ground-state phase diagram as a function of the exchange couplings, using linear spin wave theory and real-space perturbation theory. We find a hierarchy of energy scales in the ground state selection, with the effective dimensionality of the system varying in an intricate way as the hierarchy is descended. We derive an effective two-dimensional anisotropic triangular lattice Ising model with only three free parameters which accounts for the observed behavior. Connecting our results to experiment, they are consistent with the observation of a disordered chain state in Nd$_2$Zr$_2$O$_7$. We also show that the presence of two distinct types of chain phases has consequences for the field-induced breakdown of the apparent $U(1)$ octupolar quantum liquid phase recently observed in Ce$_2$Sn$_2$O$_7$.