The Dipole-Octupole Quantum Spin Ice Candidate Ce2Zr2O7

Abstract

This thesis focuses on the cerium-based, insulating pyrochlore magnet Ce2Zr2O7. Of particular popularity in the condensed matter community are insulating rare-earth pyrochlores with chemical formula R2B2O7, with magnetism based on trivalent rare-earth ions, R3+, and where B4+ is a tetravalent transition metal ion. The magnetic R3+ ions occupy a sublattice of corner-sharing tetrahedra, which is one of the archetypes for geometric frustration and aids in the promotion of exotic magnetic phases at low temperature. In these rare-earth pyrochlores, the crystal electric field plays an important role in the description of the magnetic behavior. Our high-energy inelastic neutron scattering measurements of the crystal electric field excitations in Ce2Zr2O7 reveal a crystal electric field ground state with dipole-octupole symmetry. This dipole-octupole symmetry is particularly intriguing as it allows for novel types of quantum spin ice phases, including some which are based off of quantum-correlated magnetic octupoles rather than magnetic dipoles. Our low-energy neutron scattering measurements provide strong experimental evidence for a quantum spin ice phase at low temperature in Ce2Zr2O7. We fit the measured heat capacity, magnetic susceptibility, and neutron scattering signal from Ce2Zr2O7 to yield experimental estimates of the exchange parameters describing the magnetic interactions between Ce3+ ions in Ce2Zr2O7. The resulting parameter values, in accordance with the existing theory, provide evidence for a novel octupole-based quantum spin ice ground state in Ce2Zr2O7. Polarized neutron diffraction measurements were also performed on Ce2Zr2O7 and provide evidence for the significance of further-than-nearest neighbor interactions. This thesis concludes with an investigation of Ce2Zr2O7 in magnetic fields along the [1, -1, 0] and [0, 0, 1] crystallographic axes, using neutron scattering and heat capacity measurements, with results revealing a different polarized spin ice phase for each of these field directions.