Figure 1: Crystal structure of κ-(BEDT-TTF)₂Cu₂(CN)₃.

(a) Molecular arrangement in a two-dimensional magnetic layer. The dotted ellipsoid and arrow stand for a molecular dimer, which constitutes a triangular lattice, and an S=1/2 spin on the dimer, respectively. (b) Two-dimensional layered structure viewed in the crystallographic a*−c plane. The symbols θ and θ₀ define the field angle in the torque measurements, and θ′ and α define the field angle in the calculation of the magnetic susceptibility (see the ‘Methods’ section). The yellow, grey, blue and red ellipsoids represent the sulfur, carbon, nitrogen and copper atoms, respectively. The hydrogen atoms are omitted for clarity.