  We report neutron scattering and muon spin relaxation measurements (muSR) on
the pyrochlore antiferromagnet Yb2Ge2O7. Inelastic neutron scattering was used
to probe the transitions between crystal electric field levels, allowing us to
determine the eigenvalues and eigenvectors appropriate to the J=7/2 Yb3+ ion in
this environment. The crystal electric field ground state doublet in Yb2Ge2O7
corresponds primarily to m_J = +/- 1/2 with local XY anisotropy, consistent
with an S_eff = 1/2 description for the Yb moments. muSR measurements reveal
the presence of an ordering transition at T_N = 0.57 K with persistent weak
dynamics in the ordered state. Finally, we present neutron diffraction
measurements that reveal a clear phase transition to the k = (000) Gamma_5
ground state with an ordered magnetic moment of 0.3(1) mu_B per Yb ion. We
compare and contrast this phenomenology with the low temperature behavior of
Yb2Ti2O7 and Er2Ti2O7, the prototypical S_eff = 1/2 XY pyrochlore magnets.

Strongly Correlated Electrons