Discovery of a New Quantum Dimer Magnet in a Strongly Spin-Orbit Coupled Material*

Through the study of theoretical proposals for quantum spin liquids and high-temperature superconductivity several novel phases of matter have been discovered. One such state is the quantum dimer magnet, which exhibits a field-induced Bose-Einstein condensate (BEC) phase that occupies a symmetric dome in the field vs. temperature phase diagram. Many compounds based on 3d magnetic cations have been found that exhibit a quantum dimer state with a BEC phase. We have found a new realization in the distorted honeycomb-lattice material Yb₂Si₂O₇. Yb³⁺’s high spin-orbit coupling combined with crystal field effects results in pseudo-spin ½ angular momentum that leads to anisotropic exchange interactions between quantum spins. Our single crystal inelastic neutron scattering, specific heat, and ultrasound velocity measurements show the expected field-induced transition to a BEC-like phase, that occur at significantly lower critical fields than previously studied compounds due to the 4f magnetism. However, the high-field part of the BEC dome is interrupted by an unusual regime. Our results on Yb₂Si₂O₇ raise the question of how anisotropic exchange in strongly spin orbit coupled materials modifies the field induced phases of quantum dimer magnets.