Title: A novel strongly spin-orbit coupled quantum dimer magnet: Yb2Si2O7
Abstract: 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 have found a new QDM in the strongly spin-orbit coupled, distorted honeycomb-lattice material Yb2Si2O7 . I will first explain the physical picture leading to BEC in QDMs, and then talk about how our single crystal neutron scattering, specific heat, and ultrasound velocity measurements have been used to reveal a very unusual instance of this. In Yb2Si2O7, we have found a gapped singlet zero field ground state with sharp, dispersive spin excitations, and a field-induced magnetically ordered phase reminiscent of a BEC phase. However, compared to other material realizations of this type of phase, the critical fields are exceptionally low (Hc1 ~0.4 T and Hc2 ~1.4 T). Using inelastic neutron scattering, we observed a Goldstone mode (gapless to within the energy resolution of the instrument, 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.