Spin Dynamics and Unconventional Coulomb Phase in Nd2Zr2O7

Coulomb phases form a novel exotic state of matter which, because of frustration, lacks long range order, yet is described by a local organizing principle. Spin ice is the emblematic example of this physics: in a lattice where magnetic ions occupy the vertices of corner sharing tetrahedra (pyrochlore lattice), the combination of strong Ising anisotropy along local <111> axes with ferromagnetic interactions leads to the so called ice rule (2 spins point in and 2 spins point out in each tetrahedron). In the presence of quantum fluctuations, which can be introduced through transverse couplings (as opposed to Ising coupling terms at play in classical Coulomb phases), quantum spin ice and more generally U(1) quantum spin liquids can be stabilized. However, if too large, the transverse terms are expected to stabilize ordered phases. In this context, the question whether classical ordered phases may be stabilized out of Coulomb phases via a Higgs mechanism, has become an important issue. The Nd2Zr2O7 pyrochlore magnet is an excellent candidate to explore this physics: its ground state is known to be antiferromagnetically ordered in the so-called all-in all-out (AIAO) state, but its paramagnetic phase above the ordering temperature could be a novel example of Coulomb phase. Recently, it was proposed that the transition from this possible Coulomb phase towards the AIAO phase could be driven by a Higgs mechanism. Through a careful neutron scattering study of the dynamics and correlations below and above the transition temperature TN, we confirm the coulombic nature of the phase above TN, yet its organizing principle is different from canonical spin ice. In addition, in this Coulomb phase, the spin dynamics contains features typical of the low temperature AIAO phase. Our observations suggest that the transition arises in the thermal regime of the Coulomb phase and is likely not associated to a Higgs mechanism.