Spin liquids with fractional spin excitations occur in frustrated magnetic crystals, like ZnCu 3 (OH) 6 Cl 2 (herbertsmithite), and in α-RuCl 3. Fractional spin-1/2 excitations have also been observed in spin-1 quantum spin chains. Spin ice in Dy 2 Ti 2 O 7 and Ho 2 Ti 2 O 7 has fractionalized spin freedom, leading to deconfined magnetic monopoles. They should be contrasted with quasiparticles such as magnons and Cooper pairs, which have quantum numbers that are combinations of the constituents. The most celebrated may be quantum Hall systems, occurring at high magnetic fields in 2D electron gas materials such as GaAs heterostructures. Electrons combined with magnetic flux vortices carry current. Graphene exhibits charge fractionalization.