2018/10/31 by Gavin Hester, H. S. Nair, T. Reeder +9 · 1 citation
Physics and Astronomy · #cond-mat.str-el
paper · pdf · doi:10.1103/physrevlett.123.027201
published as Phys. Rev. Lett. 123, 027201 (2019) · 6 pages, 4 figures, 12 pages of Supplemental Information, 15 Supplemental figures. Updated to published version
arxiv created 2019/07/09 · arxiv updated 2019/07/11
The quantum dimer magnet (QDM) is the canonical example of quantum magnetism. The QDM 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 ground state at zero field 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 in an applied magnetic field we observe a Goldstone mode (gapless to within δE = 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 (μ0H≥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.