A Novel Strongly Spin-Orbit Coupled Quantum Dimer Magnet: Yb₂Si₂O₇

The quantum dimer magnet (QDM) is the canonical example of 'quantum magnetism'. This state consists of entangled nearest-neighbor spin dimers and often exhibits a field-induced 'triplon' Bose-Einstein condensate (BEC) phase. I will discuss a new QDM in the strongly spin-orbit coupled, distorted honeycomb-lattice material Yb₂Si₂O₇. Single crystal neutron scattering, specific heat, and ultrasound velocity measurements reveal a gapped singlet zero field ground state with sharp, dispersive excitations. We find a field-induced magnetically ordered phase reminiscent of a BEC phase, with exceptionally low critical fields of H_c1 ~0.4 T and H_c2 ~1.4 T. Using inelastic neutron scattering we observe a Goldstone mode that persists throughout the entire field-induced magnetically ordered phase, suggestive of the spontaneous breaking of U(1) symmetry expected for a triplon BEC. However, in contrast to other well- known cases of this phase, the high-field (H > 1.2T) part of the phase diagram in Yb₂Si₂O₇ is interrupted by an unusual regime signaled by a change inthe field dependence of the ultrasound velocity and net magnetization, as well as the disappearance of a sharp anomaly in the specific heat. These measurements raise the question of how anisotropy in strongly spin-orbit coupled materials modifies the field induced phases of QDMs.