Spin dynamics and unconventional Coulomb phase in Nd$_2$Zr$_2$O$_7$

TL;DR

This study explores the spin dynamics in Nd$_2$Zr$_2$O$_7$, revealing a persistent spin ice mode above the magnetic transition and identifying a novel Coulomb phase with unconventional magnetic correlations.

Contribution

It provides the first neutron scattering evidence of a Coulomb phase coexisting with spin ice dynamics in Nd$_2$Zr$_2$O$_7$, highlighting unconventional spin excitations and correlations.

Findings

Persistence of spin ice mode above $T_N$

Development of spin ice correlations in the paramagnetic phase

Observation of a Coulomb phase with unconventional features

Abstract

We investigate the temperature dependence of the spin dynamics in the pyrochlore magnet Nd$_2$Zr$_2$O$_7$ by neutron scattering experiments. At low temperature, this material undergoes a transition towards an "all in - all out" antiferromagnetic phase and the spin dynamics encompass a dispersion-less mode, characterized by a dynamical spin ice structure factor. Unexpectedly, this mode is found to survive above $T_{\rm N} \approx 300$ mK. Concomitantly, elastic correlations of the spin ice type develop. These are the signatures of a peculiar correlated paramagnetic phase which can be considered as a new example of Coulomb phase. Our observations near $T_{\rm N}$ do not reproduce the signatures expected for a Higgs transition, but show reminiscent features of the "all in - all out" order superimposed on a Coulomb phase.