Long-range order coexisting with long-range entanglement: Is Nd2Sn2O7 the first Coulombic antiferromagnet with a visible emergent gauge photon?

Exotic state of matter could coexist with conventional orders such that the gauge fields and fractionalized excitations could prevail in a seemingly ordered state. By digging out and examining old experiments, we identify the pyrochlore Nd2Sn2O7 as a Coulombic antiferromagnet. This novel and exotic state carries both antiferromagnetic order and emergent quantum electrodynamics with gapless gauge photon and fractionalized quasiparticles. The antiferromagnetic order and the gauge photon naturally explain the neutron diffraction result and the anomalously large T 3 specific heat, respectively. We propose the representative dynamical properties for the Coulombic antiferromagnet that can be examined by the inelastic neutron scattering measurements. If the experiment in Nd2Sn2O7 can be repeated, Nd2Sn2O7 might become the first Coulombic antiferromagnet with a visible emergent gauge photon and thus an unique system with both the long-range order and the long-range entanglement. We also expect this work to inspire interests in the search of exotic physics in ordered magnets.