Detection of spinons via spin transport

The existence of deconfined spinons is the defining property of quantum spin liquids. These exotic excitations have a (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. Here 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's 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. We also discuss relations to experiments and estimate experimentally relevant parameters.