Geometrically frustrated antiferromagnets

Some insulating solids contain magnetic moments (spins) on a regular lattice of sites. These are formed when electrons become localized due to electron-electron interaction effects. These spins predominantly interact with each other via antiferromagnetic `exchange' interactions between neighbours, giving rise in many cases to long range antiferromagnetic order below transition temperatures of order the exchange energy scale. However, if the geometry of the lattice forces various exchange interactions to compete with each other, the spins can evade this ordering tendency down to temperatures much lower than the exchange scale, forming a `spin liquid'. I will describe this physics and talk about our work on impurity effects in such spin liquids.