Cool heads and open minds: dealing with the potentially vanilla S-wave superconductor Sr2RuO4

by Remko Fermin, third year PhD candidate at Leiden University working on correlated electron matter with a focus on unconventional superconductivity

Last year, Andrew Mackenzie, section director at the Max Planck Institute for Chemical Physics of Solids in Dresden, published ‘A personal perspective on the Unconventional Superconductivity in Sr2RuO4[1], which for me contains two important virtues of the natural sciences.

In order to explain these properly, first a small introduction on superconductivity is needed. When a material becomes superconducting it loses all of its electrical resistance and starts to expel magnetic fields. This was first discovered roughly a century ago (in Leiden). Fifty years later, most superconductors were relatively well understood and since the 1980s a new class of superconductors, labelled unconventional, was discovered. The latter are special since they have a pairing symmetry which is non S-wave. If we would compare physics to food it would correspond to an ice cream flavor that is not vanilla, but something less generic. Normally, if a new unconventional superconductor is discovered, its ‘flavor’ will be established in a few years. There is one exception: Sr2RuO4.

In 1996, two years after establishing superconductivity in this material, its flavor was proposed to be chocolate (actually equal-spin chiral P-wave) which was exciting since chocolate was not found in any other material so far and provided exotic features. Many tried to prove this flavor both theoretically and experimentally. One big leap forward came with an experiment finding non-reduced spin susceptibility below the critical temperature, or to stay in food terms, the presence of cocoa powder. Over the years, some found more traces of cocoa. However, others found traces of fruit, so nobody agreed on the exact flavor. In 2019 everything changed, when during a follow-up study it was found that the outcome of the cocoa powder experiment was caused by an artifact. When the author of the original work, Kenji Ishida, was confronted by these new results, he returned to the lab himself, redid his original experiment and confirmed the artifact.

This is the first of the virtues: admitting you are wrong and showing in detail why this is the case. After starting their major, all physics students are familiar with responses like: “You study physics? That must mean that you are really smart!” Many people think that physicists are a bunch of geniuses in their labs with their difficult formulas, and I fear that some physicists actually start to believe in that picture themselves. Besides, people with a career in academia naturally are the ones publishing and therefore those who receive recognition. Hearing you are right all the time makes it difficult to admit that sometimes you are not. Ishida did not commit any fraud, he was just wrong and it is admirable that he admits this.

The second virtue is keeping “cool heads and open minds”, to use Mackenzie’s own words. Since the idea of an equal-spin chiral (chocolate flavored) superconductor was so exciting to many physicists, the absence of cocoa powder has come as a disappointment to some. For many years people have been thinking about experiments to confirm that Sr2RuO4 is a chiral P-wave superconductor and many theoreticians have worked on Sr2RuO4 in the chiral P-wave framework. It is a virtue to keep the vanilla flavor options open as well, even though more uncommon ones might speak to the imagination and are potentially more impactful.

So, how does that leave us with Sr2RuO4? No cocoa powder means no chocolate flavor? Everyone who has ever eaten a bar of cheap chocolate knows this is not true. The proposed flavors now range from chiral D-wave (strawberry-mint) to degenerate D- plus G-wave (orange-cardamom), or simply to vanilla S-wave. Generic or not, having a taste of Sr2RuO4 has become all the more interesting for all involved.

[1] Mackenzie, A.P. A Personal Perspective on the Unconventional Superconductivity of Sr2RuO4. J Supercond Nov Magn 33, 177–182 (2020).