Higgs transition from a magnetic Coulomb liquid to a ferromagnet in   Yb_2Ti_2O_7
  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. 
Lieh-Jeng Chang, Shigeki Onoda, Yixi Su, Ying-Jer Kao, Ku-Ding Tsuei,   Yukio Yasui, Kazuhisa Kakurai, Martin Richard Lees
cond-mat.str-el cond-mat.stat-mech