Spin Dynamics and Unconventional Coulomb Phase in Nd2Zr2O7

We investigate the temperature dependence of the spin dynamics in the pyrochlore magnet Nd2Zr2O7 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 TN ≈ 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 TN 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.