Sagnac Interferometry and the Test of Broken Time Reversal Symmetry in Unconventional Superconductors
Symmetry is central to our understanding and description of natural phenomena. For example, conventional superconductors break only gauge symmetry, while a signature of an unconventional superconducting state is the breaking of additional symmetries. The breaking of time-reversal symmetry (TRS) is of particular interest since the condensate will have an overall magnetic moment due to either the spin or orbital (or both) parts of the pair wave function. However, this moment will be screened by the Meissner effect and is thus difficult to detect using conventional magnetic probes. To this end, we developed a new technique of detecting broken TRS using a Sagnac interferometer, in which left and right circularly polarized lights propagate in opposite directions in the Sagnac loop and interact with the sample. The two lights, being time-reversal mirror images of each other, will gain a difference in phase due to broken TRS in the sample. And this phase difference, usually referred to as Polar Kerr Effect (PKE) is measured with an unprecedented accuracy of 10 nano-radian at as low as mK temperatures using a Sagnac interferometer. In this talk, I shall review past results on chiral p-wave superconductor Sr2RuO4, d-wave superconductor UPt3, implying a broken time-reversal symmetry state in the superconducting state; as well as Measurements on high Tc superconductor YBCO crystal samples that showed broken TRS at temperatures tracking the so-called "Pseudo Gap" temperatures, marking what seems to be a true phase transition. And I will discuss a recent experiment on an artificially fabricated heterostructure of non-superconducting Bi and ferromagnetic Ni that is not only superconducting but also exhibit TRS signal below its Tc. A combination of experimental and theoretical work suggest that this heterostructure may host a new type of superconductivity that is mediated by magnetism and is d-wave in nature.