Confinement of fractional quantum number particles in a condensed-matter system

The concept of confinement states that in certain systems the constituent particles are bound together by an interaction for which the strength increases with increasing particle separation. One of the consequences of this is that these individual particles cannot be observed directly. The most famous example of confinement is found in particle physics where baryons and mesons are produced by the confinement of quarks. However, similar phenomena can occur in condensed-matter physics systems such as spin ladders that consist of two spin-1/2 antiferromagnetic chains coupled together by spin exchange interactions. Excitations of individual chains (spinons) carrying spin S= 1/2, are confined by even an infinitesimal interchain coupling. Most ladders studied so far cannot illustrate this process because the large strength of their interchain coupling suppresses the spinon excitations at all energy scales. Here we present neutron scattering experiments for a weakly coupled ladder material. At high energies the behaviour of this system approaches that of individual chains, but at low energies it is dominated by the integral spin excitations of strongly coupled chains.