Speaker: Nick Butch, NIST
One of the most interesting differences between spin triplet superconductors and the conventional spin variety is the two-component triplet order parameter that allows spin up and down electrons to couple with different strength. Such nonunitary superconductors, in which spin up and down components have different gaps and an intrinsic spin polarization, are ideal platforms for studying topological phenomena. So far, the only established examples of nonunitary pairing include the superfluid 3He in high magnetic fields, known as the A1 phase, as well as ferromagnetic superconductors. It is an intriguing question whether nonunitary pairing can happen in the absence of a magnetic field - external or internal - thus spontaneously breaking time reversal symmetry. Here we report the discovery of novel nonunitary spin-triplet superconductivity in UTe2, which closely resembles the ferromagnetic superconductors with dramatically enhanced transition temperature and upper critical field, and a paramagnetic normal state. UTe2 exhibits the crucial ingredients of a nonunitary triplet superconducting state, namely: an extremely large, anisotropic upper critical field Hc2, temperature independent NMR Knight shift in the superconducting state that can only be due to triplet pairing, and a large residual normal electronic density of states indicating that half of the electrons remain ungapped. In other words, a spin up superfluid coexists with a spin down Fermi liquid. This discovery yields a new platform for encoding information using topological excitations and for manipulation of spin-polarized currents.