Z 2 vortex crystal candidate in the triangular S = 1/2 quantum antiferromagnet

The prospect of merging the paradigms of geometric frustration on a triangular lattice and bond anisotropies in the strong spin-orbit coupling limit holds tremendous promise in the search for exotic quantum materials. Here we identify a new candidate system to realize such physics, the organic quantum antiferromagnet (CD₃ND₃)₂NaRuCl₆. We report a combination of thermodynamic, magneto-elastic and neutron scattering experiments on single-crystals to determine the phase diagram in axial magnetic fields H∥c and propose a minimal model Hamiltonian. (CD₃ND₃)₂NaRuCl₆ displays an ideal triangular arrangement of Ru³⁺ ions adopting the spin-orbital entangled j _eff = 1/2 state. It hosts residual magnetic order below T _N = 0.23 K and a highly unusual H - T phase diagram including three different incommensurate states. Spin-waves in the high-field polarized regime are described by a Heisenberg triangular lattice Hamiltonian with a potential sub-leading bond dependent anisotropy term J _±±. We argue that the multi-q ground state in zero magnetic field is a prime candidate for hosting the $Z_2$ vortex crystal proposed on the triangular Heisenberg-Kitaev model. (CD₃ND₃)₂NaRuCl₆ is the first member in an extended family of quantum triangular lattice magnets, providing a new playground to study the interplay of geometric frustration and spin-orbit effects.