  Under certain experimental conditions, the deposition of C60 molecules onto
an atomically flat copper surface gives rise to the formation of corrugated
islands. This corrugation, which reflects a molecular displacement
perpendicular to the surface plane, presents an astonishing pattern: it is well
described by a frustrated Ising spin Hamiltonian whose thermodynamics is
compatible with a spin liquid about to transit towards an ordered zigzag state.
Here we study the statistical properties of such a molecular corrugation using
tools generally employed in frustrated magnetism. More specifically, the real
and reciprocal space analysis of pairwise molecule correlations allows us to
demonstrate that the C60/Cu system, in which magnetism is totally absent, has
all the characteristics of a triangular Ising antiferromagnet. Our results
indicate that the organization of two-dimensional matter, at the molecular
length scale, sometimes turns out to be particularly close to that encountered
in highly frustrated magnets.
