  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 Yb$_2$Si$_2$O$_7$. 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 $H_{c1} \sim 0.4$ T and $H_{c2} \sim 1.4$
T. Using inelastic neutron scattering in an applied magnetic field 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
($\mu$$_0$$H\geq1.2$T) part of the phase diagram in Yb$_2$Si$_2$O$_7$ 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.
