  Inelastic neutron scattering measurement is performed on a breathing
pyrochlore antiferromagnet Ba3Yb2Zn5O11. The observed dispersionless
excitations are explained by a crystalline electric field (CEF) Hamiltonian of
Kramers ion Yb3+ of which the local symmetry exhibits C3v point group symmetry.
The magnetic susceptibility previously reported is consistently reproduced by
the energy scheme of the CEF excitations. The obtained wave functions of the
ground state Kramers doublet exhibit the planer-type anisotropy. The result
demonstrates that Ba3Yb2Zn5O11 is an experimental realization of breathing
pyrochlore antiferromagnet with a pseudospin S = 1/2 having easy-plane
anisotropy.
