Surface plasmons and reflectance of U(1) spin liquid states with large spinon Fermi surfaces

In this paper we study the optical properties of U(1) spin liquids with large spinon Fermi surfaces based on an expression for the bulk optical conductivity obtained through the Ioffe-Larkin composition rule. We show that the optical conductivity of U(1) spin liquids at energies above the charge gap has a unique feature that distinguishes them from ordinary insulators. In particular we propose the existence of a long-life surface plasmon mode propagating along the interface between a linear medium and the spin liquid at frequencies above the charge gap, which can be detected by the widely used Kretschmann-Raether three-layer configuration. The experiment provides a test that distinguishes between ordinary insulators and spin liquid states with large spinon Fermi surfaces.