  In a class of frustrated magnets known as spin ice, magnetic monopoles emerge
as classical defects and interact via the magnetic Coulomb law. With
quantum-mechanical interactions, these magnetic charges are carried by
fractionalised bosonic quasi-particles, spinons, which can undergo
Bose-Einstein condensation through a first-order transition via the Higgs
mechanism. Here, we report evidence of a Higgs transition from a magnetic
Coulomb liquid to a ferromagnet in single-crystal Yb2Ti2O7. Polarised
neutron-scattering experiments show that the diffuse [111]-rod scattering and
pinch-point features which develop on cooling are suddenly suppressed below T_C
~ 0.21 K, where magnetic Bragg peaks and a full depolarisation of the neutron
spins are observed with thermal hysteresis, indicating a first-order
ferromagnetic transition. Our results are explained on the basis of a quantum
spin-ice model, whose high-temperature phase is effectively described as a
magnetic Coulomb liquid, while the ground state shows a nearly collinear
ferromagnetism with gapped spin excitations.
