  Existence of spinons is the defining property of quantum spin liquids. These
exotic excitations have (fractionalized) spin quantum number and no electric
charge, and have been proposed to form Fermi surfaces in the recently
discovered organic spin liquid candidates. However direct probes for them are
still lacking. In this paper we propose to experimentally identify the spinons
by measuring the spin current flowing through the spin liquid candidate
materials, which would be a direct test for the existence of spin-carrying
mobile excitations. By the nonequilibrium Green function technique we evaluate
the spin current through the interface between a Mott insulator and a metal
under a spin bias, and find that different kinds of Mott insulators, including
quantum spin liquids, can be distinguished by different relations between the
spin bias and spin current, In the end we will also discuss relations to
experiments and estimate experimentally relevant parameters.
