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.Quantum dimer magnets (QDMs) represent the simplest cases of quantum magnetism, where entanglement is a required ingredient for even a qualitative understanding of the phase. In a QDM, entangled pairs of spins form S tot = 0 dimers and result in a non-magnetic ground state. The excited states of these entangled spins can be treated as bosons, called triplons, which can undergo Bose-Einstein condensation (BEC) as their density is tuned by an applied magnetic field. This BEC state is a magnetic field-induced long range ordered phase, which occupies a symmetric "dome" in the field vs. temperature phase diagram with two temperature-dependent critical fields, H c1 (T ) and H c2 (T ). The vast majority of the previously studied QDMs are based on 3d transition metal ions with "bare" (spin-only) S = 1/2 or S = 1 angular momentum, resulting in simple Heisenberg or XXZ spin interaction Hamiltonians, and high critical fields set by the relatively high energy scale of exchange interactions [1-6].Lanthanide-based magnetic materials with spin-orbit coupled pseudo-spin 1/2 (S eff = 1/2) angular momenta can also exhibit quantum phases, and these are often directly analogous to their traditional 3d transition metal ion counterparts. However, entirely new phases are possible due to the anisotropic exchange in these materials [7][8][9][10][11][12]. In the lanthanide series, Yb 3+ has been of particular interest as it can generically host interactions leading to quantum fluctuations irrespective of the Crystal Electric Field (CEF) ground state doublet composition [13]. Indeed, various quantum phases have been discovered in Yb-based systems [14][15][...
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