Ring exchange and correlated fermions

The concept of exchange in strongly-correlated fermions is reviewed with emphasis on the generalization of the Heisenberg pair exchange to higher order n-particle permutations. The 'frustration' resulting from competing ferromagnetic three-spin exchange and antiferromagnetic two- and four-spin exchanges is illustrated on a two-dimensional model system: solid 3He films. Recent experimental results proving the presence of significant four-spin exchange interactions in the CuO2 plaquettes of high Tc cuprates are reported.