One-dimensionalization by Geometrical Frustration in the Anisotropic Triangular Lattice of the 5d Quantum Antiferromagnet Ca₃ReO₅Cl₂

We report on the emergence of antiferromagnetic spin chains from two-dimensionally aligned spins on the anisotropic triangular lattice (ATL) in the insulating calcium rhenium oxychloride Ca₃ReO₅Cl₂. The compound contains Re⁶⁺ ions, each with one unpaired electron in the d_xy orbital, which are arranged to form a spin-1=2 ATL with J A=J ∼ 0.32 and J = 41 K, where J and J A are magnetic interactions in and between the chains, respectively. In spite of the apparent two-dimensionality, we observe clear evidence of a gapless spin liquid that is characteristic of the spin-1=2 Heisenberg chain. This one-dimensionalization must be caused by geometrical frustration: growing antiferromagnetic correlations in every chain effectively cancel out interchain zigzag couplings at low temperatures.