Magneto-optical Faraday effect in spin-liquid candidates

Abstract

We propose an experiment to use the magneto-optical Faraday effect to probe the dynamic Hall conductivity of spin-liquid candidates. Theory predicts that an external magnetic field will generate an internal gauge field. If the source of conductivity is in spinons with a Fermi surface, a finite Faraday rotation angle is expected. We predict the angle to scale as the square of the frequency rather than display the standard cyclotron resonance pattern. Furthermore, the Faraday effect should be able to distinguish the ground state of the spin liquid, as we predict no rotation for massless Dirac spinons. We give a semiquantitative estimate for the magnitude of the effect and find that it should be experimentally feasible to detect in both κ-(ET) 2 Cu 2 (CN) 3 and, if the spinons form a Fermi surface, herbertsmithite. We also comment on the magneto-optical Kerr effect and show that the imaginary part of the Kerr angle (circular dichroism) may be measurable.

Low-Energy Excitations in Quantum Spin Liquids Identified by Optical Spectroscopy

“…A possible scenario featuring gapless spin excitations has been proposed by Lee and collaborators [38], based on a U(1) gauge theory of the Hubbard model, suggesting a spinon Fermi surface, which should produce metallic-like low-temperature behavior of both the specific heat and the thermal conductivity. In addition, the coupling of such spinons with an internal gauge field has been predicted to contribute to the optical conductivity [12,39,40] and even produce the magneto-optical Faraday effect [41]. This intriguing possibility motivated our present investigations on the electrodynamic behavior of quantum spin liquids.…”

“…Considering that a two-magnon process is equivalent to four spin-1 2 excitations, one might speculate whether spin-charge coupling directly maps spinons via the charge response. Spectroscopic studies under magnetic field may elucidate the nature of these low-energy excitations [41].…”

Floquet engineering correlated materials with unpolarized light

“…Electromagnetic gauge fields do interact with the neutral spinons, albeit often with significantly lower amplitudes than electrons. Gapless spin liquids are predicted to have power-law behavior of the optical conductivity [70], with some evidence in herbertsmithite and others [71,72], and liquids may have signatures in the magneto-optical Faraday or Kerr effects [73].…”

Polarization as a tuning parameter for Floquet engineering: magnetism in the honeycomb, square, and triangular Mott insulators

“…Discontinuities associated with phase transitions should be observable in optical measurements, and magnetic excitations can be followed 73 . Spin liquids have neutral low energy spinon excitations that couple only weakly to the external gauge field, but gapless spin liquids have been predicted to have power law behaviors in optical conductivity [74][75][76] , and spin liquids, in general, may have signatures in the magneto-optical Faraday or Kerr effects 77 .…”