Ring Exchange and Correlated Fermions

TL;DR

This paper reviews the role of multi-particle exchange interactions in strongly correlated fermion systems, highlighting experimental evidence of four-spin exchanges in high-temperature superconductors and discussing their effects on magnetic frustration.

Contribution

It generalizes the concept of pair exchange to higher-order permutations and connects theoretical models with recent experimental findings in cuprates.

Findings

Evidence of four-spin exchange interactions in high Tc cuprates.

Illustration of magnetic frustration due to competing exchange interactions.

Emphasis on the importance of multi-particle exchanges in strongly correlated systems.

Abstract

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 four-spin exchange interactions in the CuO2 plaquettes of high Tc cuprates are reported.