Spin gap antiferromagnets: Materials and phenomena

Abstract

There are several interacting spin systems which have a gap in their spin excitation spectra. The gap does not occur due to anisotropies present in the system but is quantum mechanical in origin. We give a brief overview on different types of spin gap (SG) antiferromagnets, the models proposed to describe their physical properties and experimental realizations of such systems. Our special focus is on exactly-solvable models and rigorous theories which provide the correct physical picture for the novel phenomena exhibited by SG systems.