Unravelling competing microscopic interactions at a phase boundary: a single crystal study of the metastable antiferromagnetic pyrochlore Yb$_{2}$Ge$_{2}$O$_{7}$

Abstract: We report inelastic neutron scattering measurements from our newly synthesized single crystals of the structurally metastable antiferromagnetic pyrochlore Yb${2}$Ge${2}$O${7}$. We determine the four symmetry-allowed nearest-neighbor anisotropic exchange parameters via fits to linear spin wave theory supplemented by fits of the high-temperature specific heat. The exchange parameters so-determined are strongly correlated to the values determined for the $g$-tensor components, as previously observed for the related Yb pyrochlore Yb${2}$Ti${2}$O${7}$. To address this issue, we directly determined the $g$-tensor from electron paramagnetic resonance of 1% Yb-doped Lu${2}$Ge${2}$O${7}$, thus enabling an unambiguous determination of the exchange parameters. Our results show that Yb${2}$Ge${2}$O${7}$ resides extremely close to the classical phase boundary between an antiferromagnetic $\Gamma_5$ phase and a splayed ferromagnet phase. By juxtaposing our results with recent ones on Yb${2}$Ti${2}$O$_{7}$, our work illustrates that the Yb pyrochlore oxides represent ideal systems for studying quantum magnets in close proximity to classical phase boundaries.