Experimental Insights into Ground State Selection of Quantum XY Pyrochlores

Extensive experimental investigations of the magnetic structures and excitations in the XY pyrochlores have been carried out over the last decade. Three families of XY pyrochlores have emerged: Yb$_2B_2$O$_7$, Er$_2B_2$O$_7$, and, most recently, $AA'$Co$_2$F$_7$. In each case, the magnetic cation (either Yb, Er, or Co) exhibits XY anisotropy within the local pyrochlore coordinates, a consequence of crystal field effects. Materials in these families display rich phase behavior and are candidates for exotic ground states, such as quantum spin ice, and exotic ground state selection via order-by-disorder mechanisms. In this review, we present an experimental summary of the ground state properties of the XY pyrochlores, including evidence that they are strongly influenced by phase competition. We empirically demonstrate the signatures for phase competition in a frustrated magnet: multiple heat capacity anomalies, suppressed $T_N$ or $T_C$, sample and pressure dependent ground states, and unconventional spin dynamics.