Neutron Scattering Study in Breathing Pyrochlore Antiferromagnet Ba₃Yb₂Zn₅O₁₁

Inelastic neutron scattering (INS) study on breathing pyrochlore antiferromagnet Ba₃Yb₂Zn₅O₁₁ is presented. Observed crystalline electric field (CEF) excitations are explained by a Hamiltonian of Kramers ion Yb³⁺ of which the local symmetry exhibits C_3v point group symmetry. The magnetic susceptibility is consistently reproduced by the energy scheme of the CEF excitations. The INS spectra in the low-energy range are quantitatively explained by spin-1/2 single-tetrahedron model having XXZ anisotropy and Dzyaloshinskii-Moriya interaction. This model has a two-fold degeneracy of the lowest-energy state per tetrahedron and well reproduces the bulk properties at T ≥ 0.5K. At lower temperatures, however, we observe a broad maximum in the heat capacity around 63 mK, demonstrating that a unique quantum ground state is selected due to extra perturbations.