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 certain 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.