Dirac Strings and Magnetic Monopoles in the Spin Ice Dy ₂ Ti ₂ O ₇

TLDR

Spin ice on the pyrochlore lattice hosts networks of aligned dipoles forming Dirac‑string‑like solenoidal tubes whose terminations behave as magnetic monopoles, a scenario proposed as an emergent quasiparticle in condensed‑matter systems and reflected in the heat‑capacity behavior of a magnetic‑Coulomb gas. By applying a symmetry‑breaking magnetic field, the authors manipulate the density and orientation of these Dirac strings in Dy₂Ti₂O₇. Diffuse neutron scattering reveals the presence of these Dirac strings in Dy₂Ti₂O₇.

Abstract

While sources of magnetic fields - magnetic monopoles - have so far proven elusive as elementary particles, several scenarios have been proposed recently in condensed matter physics of emergent quasiparticles resembling monopoles. A particularly simple proposition pertains to spin ice on the highly frustrated pyrochlore lattice. The spin ice state is argued to be well-described by networks of aligned dipoles resembling solenoidal tubes - classical, and observable, versions of a Dirac string. Where these tubes end, the resulting defect looks like a magnetic monopole. We demonstrate, by diffuse neutron scattering, the presence of such strings in the spin-ice Dy2Ti2O7. This is achieved by applying a symmetry-breaking magnetic field with which we can manipulate density and orientation of the strings. In turn, heat capacity is described by a gas of magnetic monopoles interacting via a magnetic Coulomb interaction.