Chapter 2 Quantum Phase Transitions

Quantum phase transition (QPT) occurs at zero temperature where thermal fluctuations are absent and instead the transition is driven by quantum fluctuations which are tuned by variations in some nonthermal parameters, such as pressure, magnetic field, or chemical composition, as demanded by Heisenberg’s uncertainty principle. Although QPT occurs at zero temperature, its influence expands to a broad nonzero temperature regime of quantum criticality, enabling us to study these phenomena through variety of experimental techniques. This chapter considers QPTs in quantum magnets, presenting a brief introduction to several well-studied systems and reviewing experimental examples for spin dimers, geometrically frustrated magnets, heavy fermions, itinerant magnets, and transverse field Ising systems, with focus on neutron scattering techniques utilized to deliver a more comprehensive picture of the fundamental physics behind the QPT in these systems.