Kerr rotation in the unconventional superconductor Sr2RuO4

The interpretation of Kerr rotation measurements in the superconducting phase of Sr2RuO4 is a controversial topic. Both intrinsic and extrinsic mechanisms have been proposed, and it has been argued that the intrinsic response vanishes by symmetry. We focus on the intrinsic contribution and clarify several conflicting results in the literature. On the basis of symmetry considerations and detailed calculations, we show that the intrinsic Kerr signal is not forbidden in a general multiband system but has a rich structure in the near-infrared regime. We distinguish different optical transitions determined by the superconducting gap (far infrared) and the interorbital coupling of the normal state (near infrared). We argue that the low-frequency transitions do not contribute to the Hall conductivity, while only the interorbital transitions in the near-infrared regime contribute. Finally, we discuss the difficulties to connect the calculations for the optical Hall conductivity to the experimental measurement of the Kerr angle. We will compare different approximations which might lead to conflicting results.