  The synthesis and characterization of a new Yb-based material Ba3Yb2Zn5O11
are reported. This material is identified as the first model system of a
pseudospin-1/2 quantum antiferromagnet on "breathing" pyrochlore lattice
characterized by an alternating array of small and large Yb tetrahedra. Despite
dominant antiferromagnetic interactions J ~ 7 K, a large amount of magnetic
entropy (25%) remains at 0.38 K, indicating that each small Yb tetrahedron
forms a unique doubly degenerate singlet state. These results are well
described by the Heisenberg pseudospin-1/2 single tetrahedron model.
