Optical evidence for time reversal symmetry breaking of the eutectic Sr₂RuO₄-Ru (3-Kelvin) phase in Sr₂RuO₄ crystals

Sr₂RuO₄ (SRO) is a strongly correlated metal with possible p-wave superconductivity below 1.5 K. Besides its intriguing bulk superconductivity, a filamentary superconducting phase nucleating at Tc≈3K in eutectic SRO-Ru at SRO-Ru interface, known as the 3K phase of SRO, has also attracted attention due to its interesting features. Tunneling spectroscopy studies have observed sharp zero bias conductance peak (ZBCP) in SRO-Ru below 3K, indicating possible Andreev surface bound states and unconventional pairing in the 3K phase. Recent theoretical studies suggested the onset of ZBCP is related to a second superconducting phase transition into a time-reversal symmetry breaking phase, with additional component introduced into the order parameter at a temperature below Tc. With a low temperature Sagnac scanning microscope based on a zero-loop fiber-optics Sagnac interferometer, we have performed high resolution magneto-optic Kerr effect (MOKE) measurements on SRO-Ru interface in the eutectic SRO-Ru system, at various locations away from the Ru inclusions. Evidence of time-reversal symmetry breaking below the Tc of the 3K phase and its location dependence will be presented.