  Ba3Yb2Zn5O11 is unique among breathing pyrochlore compounds for being in the
nearly decoupled limit where inter-tetrahedron interactions are weak, hosting
isolated clusters or "molecular magnet" like tetrahedra of magnetic ytterbium
(Yb3+) ions. In this work, we present the first study carried out on
single-crystal samples of the breathing pyrochlore Ba3Yb2Zn5O11, using a
variety of magnetometry and neutron scattering techniques along with
theoretical modeling. We employ inelastic neutron scattering to investigate the
magnetic dynamics as a function of applied field (with respect to both
magnitude and direction) down to a temperature of 70 mK, where inelastic
scattering reveals dispersionless bands of excitations as found in earlier
powder sample studies, in good agreement with a single-tetrahedron model.
However, diffuse neutron scattering at zero field and dc-susceptibility at
finite field exhibit features suggesting the presence of excitations at
low-energy that are not captured by the single tetrahedron model. Analysis of
the local structure down to 2 K via pair distribution function analysis finds
no evidence of structural disorder. We conclude that effects beyond the single
tetrahedron model are important in describing the low-energy, low temperature
physics of Ba3Yb2Zn5O11, but their nature remains undetermined.
