$${{\mathbb{Z}}}_{2}$$ vortex crystal candidate in the triangular S = 1/2 quantum antiferromagnet

Abstract 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 (CD 3 ND 3 ) 2 NaRuCl 6 . 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. (CD 3 ND 3 ) 2 NaRuCl 6 displays an ideal triangular arrangement of Ru 3+ 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 $${{\mathbb{Z}}}_{2}$$ Z 2 vortex crystal proposed on the triangular Heisenberg-Kitaev model. (CD 3 ND 3 ) 2 NaRuCl 6 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.