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. We report on a new QDM in the strongly spin-orbit coupled, distorted honeycomb-lattice material Yb2Si2O7. Our 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 Hc1 ~0.4 T and Hc2 ~1.4 T. Using inelastic neutron scattering we observe a Goldstone mode (gapless to within /delta/E = 0.037 meV) 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 in the field dependence of the ultrasound velocity and 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.