Quantum entanglement of XY-type spin dimers in 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 intra-dimer exchange interaction shows strong XY-type anisotropy and the ground state of BaCe${}_2$ZnS${}_5$ is in an entangled state $(\mathrm{\mid }\uparrow \uparrow >-\mathrm{\mid }\downarrow \downarrow >)\mathrm{/}\sqrt{2}$. 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.