One of the most intriguing phenomena in strongly correlated systems is the fractionalization of quantum numbers — familiar examples include the spin-charge separation in one-dimensional metallic systems, the fractionalization of the electron in fractional quantum Hall states or the emergence of monopoles in spin ice.
In this talk, I will discuss the fractionalization of magnetic moments in a certain class of Mott insulators, in which the emergent degrees of freedom are Majorana fermions that form an (almost) conventional metal. The origin of such a dichotomous state is elucidated by a family of exactly solvable models of frustrated quantum magnets in three dimensions, which might be realized in a class of recently synthesized Iridate compounds. These models thereby provide the first analytical tractable examples of long sought-after quantum spin liquids with a spinon Fermi surface and even an entire new class of quantum spin liquids — a so-called Weyl spin liquid, in which the fractionalized degrees of freedom form a topological semi-metal.