Detecting Symmetry Fractionalization by Magnetic Impurities*

In contrast to various symmetry-breaking orders in Landau's paradigm, quantum spin liquids are long-range entangled many-spin ground states that cannot be characterized by local order parameters. A longstanding question is how to unambiguously detect the fractionalized excitations in quantum spin liquids, such as spinons that carry half spin each. Here we show that Kondo impurities can be used to sharply distinguish different symmetry fractionalization of spinons in a gapped quantum spin liquid. We also show the sharp experimental signatures of these spin liquids in the presence of dilute magnetic impurities.