A Brief Introduction to Quasiparticles in Frustrated Magnets

Conventional magnets typically undergo a phase transition as temperature crosses the characteristic interaction energy scale, separating a disordered paramagnetic phase from an ordered one. The resulting behaviour has been the focus of attention and the setting of countless important discoveries over the past century in physics. Over the last few decades, an even richer plethora of phenomena has been uncovered in presence of frustration, when the competition between energy terms suppresses the transition temperature and opens the door to a new regime: A `spin liquid’ where the magnetic degrees of freedom remain disordered and yet `strongly correlated’, when the temperature becomes substantially smaller than the characteristic interactions. These new phases of matter can give rise to unusual and interesting phenomena, encompassing emergent symmetries and topological order, and excitations that can fractionalise the properties of the very constituent elements that make up the system. In this talk we will review some of the basic concepts relating to frustrated magnets and spin liquids. We will focus in particular on how one can model and understand their elementary excitations, both at the classical and quantum mechanical level.