2026: Half a Superconductor

In an ordinary superconductor, electrons pair up with opposite spins. The pair carries no spin, and a magnetic field breaks it apart. In a spin-triplet superconductor the pairs carry spin~1 --- and in the rare nonunitary case they carry a net spin polarization, so the two spin species get different energy gaps. In the extreme version, one spin species is fully gapped while the other is not gapped at all: half the electrons are superconducting and half are still a normal metal, in the same material at the same time. 

The candidate materials are UTe2, UPt3 and 4Hb-TaS2. The same states are predicted to host three-dimensional Majorana fermions in the bulk. Nobody has yet demonstrated nonunitary pairing convincingly, because it is not obvious what experiment would show it. That is the open problem.

This project attacks it with about the simplest experiment there is: push current from a normal metal into the superconductor through a point contact and measure the tunneling conductance G=dI/dV. An electron entering the superconductor can come back out as a hole --- Andreev reflection --- and how often that happens depends directly on the gap structure. Our job will be to work out what the resulting curves look like.