title: Higgs transition from a magnetic Coulomb liquid to a ferromagnet in Yb₂Ti₂O₇.
abstract: 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 fractionalized 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 Yb(2)Ti(2)O(7). Polarized 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 depolarization 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, whereas the ground state shows a nearly collinear ferromagnetism with gapped spin excitations.
authors.affiliation: Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan. ljchang@mail.ncku.edu.tw
authors.forename: Lieh-Jeng
authors.initials: LJ
authors.lastname: Chang
authors.forename: Shigeki
authors.initials: S
authors.lastname: Onoda
authors.forename: Yixi
authors.initials: Y
authors.lastname: Su
authors.forename: Ying-Jer
authors.initials: YJ
authors.lastname: Kao
authors.forename: Ku-Ding
authors.initials: KD
authors.lastname: Tsuei
authors.forename: Yukio
authors.initials: Y
authors.lastname: Yasui
authors.forename: Kazuhisa
authors.initials: K
authors.lastname: Kakurai
authors.forename: Martin Richard
authors.initials: MR
authors.lastname: Lees