High Resolution Polar Kerr Effect Measurements ofSr2RuO4: Evidence for Broken Time-Reversal Symmetry in the Superconducting State

The polar Kerr effect in the spin-triplet superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ was measured with high precision using a Sagnac interferometer with a zero-area Sagnac loop. We observed nonzero Kerr rotations as big as 65 nanorad appearing below $T_c$ in large domains. Our results imply a broken time-reversal symmetry state in the superconducting state of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, similar to ${}^3\mathrm{He}\mathrm{\text{−}}A$.