A New Triangular Lattice Quantum Antiferromagnet in the Strong Spin-Orbit Coupling Regime

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 ongoing hunt for exotic quantum materials. In this talk, we present a new candidate material to realize such physics [1]. We report a combination of thermodynamic, magneto-elastic and neutron scattering experiments on single crystals to determine the phase diagram in axial magnetic fields and propose a minimal model Hamiltonian. The material 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 = 0.22 K and a highly complex H − T phase diagram, including three different incommensurate states. Spin-waves in the high-field polarized regime are well described by a Heisenberg-like triangular lattice Hamiltonian with a potential sub-leading bond-dependent term. We discuss the candidate magnetic structures in all phases and propose two mechanisms that could explain the field-dependent incommensurability, requiring either a small ferromagnetic Kitaev term or a tiny magneto-elastic J − J′ isosceles distortion driven by pseudospin-lattice coupling. This compound is the first in an extended family of quantum triangular lattice magnets, providing a new playground to study the interplay of geometric frustration and spin-orbit effects.