Experimental Insights into Quantum Spin Ice Physics in Dipole–Octupole Pyrochlore Magnets

We review a key subset of the experimental studies that have recently focused on cubic pyrochlore magnets whose pseudospin-1 $/$ 2 degrees of freedom have mixed dipolar and octupolar character. We discuss how this comes about and how the character of the pseudospin-1 $/$ 2 can be experimentally determined. The minimal spin Hamiltonian for such magnetic insulators is known to give rise to a rich phase diagram with both disordered U(1) quantum spin ice (QSI) states and all-in–all-out (AIAO) noncollinear ordered states, each with dipolar and octupolar character. We focus primarily on experimental studies on two such single crystal systems, the $\TimesFont{J}$ = 5 $/$ 2 Ce2Zr2O7 and the $\TimesFont{J}$ = 9 $/$ 2 Nd2Zr2O7. We make the case that Ce2Zr2O7 is an excellent QSI ground-state candidate material, close to the border between QSIs with dipolar and octupolar symmetry. Nd2Zr2O7 exhibits an AIAO ordered phase, featuring an order parameter consisting of dipolar and octupolar magnetic moments. It is found to reside close to a QSI phase boundary and features dynamic fragmentation in its excitation spectrum.