Aharon Kapitulnik
Stanford University
A Chiral superconductor is an unconventional superconducting material with distinctive topological properties, in which time-reversal symmetry is broken. A natural place to search for chiral superconductors is among superconductors in which the pair wavefunction possesses internal degrees of freedom, often reflecting a complex order parameter that breaks time reversal symmetry (TRSB). While TRSB must conform to the point group symmetry and the representations of each material system, its discrete nature originating from a multi-component order parameter may also allow for vestigial phases valid in the vicinity of the superconducting transition.
In this talk we will discuss recent data on several unconventional, topological superconductors where the unique crystal structure, intrinsic magnetism, and/or topological effects result in a unique superconducting state that exhibits TRSB. The main experimental tool is the Zero-area Sagnac interferometer that we developed in our laboratory for the past fifteen years and is able to measure polar Kerr effect (PKE) with nanoradians resolution. Besides its exquisite sensitivity, an important feature of this apparatus is the ability to reject (by symmetry) all reciprocal effects. Starting with UPt3, we show that the onset of PKE below a temperature T_{Kerr} that coincides with the low temperature B-phase superconducting transition temperature T_{c-}~480mK. In contrast, no change in Kerr effect is observed through either the high temperature A-phase superconducting transition at T_{c+}∼550mK. These results indicate that TRS is broken only in the B-phase. We continue our discussion with results on the nearly ferromagnetic compound UTe2, where the strong paramagnetic susceptibility results in a strong normal state effect in the presence of magnetic field, and play and important role in nucleating ferromagnetism in the vortex state. The fact that UTe2 is orthorhombic restrict the order parameter to only one-dimensional representation, which to explain the time-reversal symmetry breaking require a two, nearly degenerate, order parameter state. Indeed, specific heat measurements confirm such two transitions.