Quantum Entanglement of XY-Type Spin Dimers on the Shastry-Sutherland Lattice.

We report a comprehensive study on the origin of the enigmatic disordered ground state within the Shastry-Sutherland lattice, BaCe_{2}ZnS_{5}, at low temperatures. The magnetization and heat capacity data show a lack of magnetic ordering down to 73 mK. We deploy a localized spin dimer model which can accurately reproduce the dynamic structure factor of the neutron data, magnetization, and heat capacity data. Remarkably, the intradimer exchange interaction shows strong XY-type anisotropy and the ground state of BaCe_{2}ZnS_{5} is in an entangled state (|↑↑⟩-|↓↓⟩)/sqrt[2]. This is in contrast to the singlet dimer state that is obtained for Heisenberg interactions. These results confirm that BaCe_{2}ZnS_{5} is in a quantum paramagnet state consisting of entangled spin dimer states.