High Resolution Polar Kerr Effect Measurements of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$: 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 ${}^3\mathrm{He}\mathrm{\text{−}}A$.

Figure 1

Experimental setup and the polarization states at each locations: vertical, in-plane polarization; horizontal, out-of-plane polarization; solid line, beam 1; dashed line, beam 2.

Figure 2

Zero-field (earth field) measurement of Kerr effect (○) and $ab$-plane electrical resistance (dotted line). Dashed curve is a fit to a BCS gap temperature dependence.

Figure 3

Representative results of training the chirality with an applied field. (a) $+93\mathrm{Oe}$ field cool, then zero-field warm-up (○). The two solid squares represent the last two points just before the field was turned off. (b) $-47\mathrm{Oe}$ field cool, then zero-field warm-up (○). Dashed curves are fits to a BCS gap temperature dependence.