  The ytterbium pyrochlore magnets, Yb2B2O7 (B = Sn, Ti, Ge) are well described
by S_eff = 1/2 quantum spins decorating a network of corner-sharing tetrahedra
and interacting via anisotropic exchange. Structurally, only the non-magnetic
B-site cation, and hence, primarily the lattice parameter, is changing across
the series. Nonetheless, a range of magnetic behaviors are observed: the low
temperature magnetism in Yb2Ti2O7 and Yb2Sn2O7 has ferromagnetic character,
while Yb2Ge2O7 displays an antiferromagnetically ordered Neel state at low
temperatures. While the static properties of the ytterbium pyrochlores are
distinct, inelastic neutron scattering measurements reveal a common character
to their exotic spin dynamics. All three ytterbium pyrochlores show a gapless
continuum of spin excitations, resembling over-damped ferromagnetic spin waves
at low Q. Furthermore, the specific heat of the series also follows a common
form, with a broad, high-temperature anomaly followed by a sharp
low-temperature anomaly at T_C or T_N. The novel spin dynamics we report
correlate strongly with the broad specific heat anomaly only, remaining
unchanged across the sharp anomaly. This result suggests that the primary order
parameter in the ytterbium pyrochlores associated with the sharp anomaly is
"hidden" and not simple magnetic dipole order.
