  Solid-solutions of the "soft" quantum spin ice pyrochlore magnets Tb2B2O7
with B=Ti and Sn display a novel magnetic ground state in the presence of
strong B-site disorder, characterized by a low susceptibility and strong spin
fluctuations to temperatures below 0.1 K. These materials have been studied
using ac-susceptibility and muSR techniques to very low temperatures, and
time-of-flight inelastic neutron scattering techniques to 1.5 K. Remarkably,
neutron spectroscopy of the Tb3+ crystal field levels appropriate to at high
B-site mixing (0.5 < x < 1.5 in Tb2Sn2-xTixO7) reveal that the doublet ground
and first excited states present as continua in energy, while transitions to
singlet excited states at higher energies simply interpolate between those of
the end members of the solid solution. The resulting ground state suggests an
extreme version of a random-anisotropy magnet, with many local moments and
anisotropies, depending on the precise local configuration of the six B sites
neighboring each magnetic Tb3+ ion.
