Recent experiments on the organic compound $\kappa \mathrm{\text{−}}(\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}{)}_2{\mathrm{Cu}}_2(\mathrm{CN}{)}_3$ raise the possibility that the system may be described as a quantum spin liquid. Here we propose a pairing state caused by the “Amperean” attractive interaction between spinons on a Fermi surface mediated by the U(1) gauge field. We show that this state can explain many of the observed low temperature phenomena and discuss testable consequences.

• Received 12 July 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.067006