The most famous example is that of three quarks confined by the strong nuclear force to form protons or neutrons. As this force becomes stronger with increasing separation, free quarks do not exist and can only be observed indirectly. An analogous confinement process is predicted in condensed matter systems known as spin ladders. Ladders consist of two parallel chains of spin-1/2 ions magnetically bonded together. A remarkable feature of a single chain is that the individual electrons separate into independent spin and charge parts. The spin parts, or spinons, have spin 1/2. By coupling two chains together to form a ladder, spinon pairs form composite spin 0 or 1 particles in a way analogous to how protons and neutrons emerge from quarks. This phenomenon was investigated on the spin-ladder compound CaCu2O3 where the interchain coupling is weak enough to allow the observation of confinement effects. Using Maps, the excitation spectrum was measured and found to agree well with the theoretical model, confirming the confinement picture.
B Lake (Helmholtz Zentrum Berlin, Technical University Berlin), AM Tsvelik (Brookhaven National Laboratory, USA), S Notbohm (Helmholtz Zentrum Berlin, St Andrew's University), DA Tennant (Helmholtz Zentrum Berlin, Technical University Berlin), TG Perring (ISIS), M Reehuis (Helmholtz Zentrum Berlin), C Sekar (Periyar University), G Krabbes, B Büchner (IFW Dresden)
Research date: January 2010
Contact: Prof B Lake, Bella.firstname.lastname@example.org
Further reading: B Lake et al., Nat Phys 6 (2010) 50-55