In 1994 superconductivity was discovered in strontium ruthenate (Sr2RuO4). This attracted considerable interest because it had a perovskite crystal structure, just like the cuprates. Furthermore, it was a stoichiometric compound and so not plagued by impurities like the cuprates.
In 1998, things got more interesting when NMR Knight shift measurements were interpreted as evidence for triplet superconductivity.
Analogues were made with triplet Cooper pairing in superfluid 3He mediated by ferromagnetic spin fluctuations.
Triplet pairing is associated with odd-parity (spatial) and time-reversal symmetry breaking. Evidence for the latter was claimed from muon spin relaxation (muSR) and the polar Kerr effect.
There are subtle questions about whether a bulk sample of a triplet superconductor exhibits spontaneous magnetisation. Leggett discussed this in an Appendix of his textbook. It turns out the magnetisation probably only exists on the edges.
Aside. The metallic phase is of interest because (unlike the cuprates) it is a Fermi liquid. More recently, it has been argued to be a Hund's metal.
Fueled by hype about topological quantum computing, the past two decades have seen even greater interest in the material due to proposals that it may be a topological superconductor. See for example, this paper.
Now we come to the disappointment. It turns out that the original Knight shift measurements were flawed, probably due to a problem with thermometry.
Recent, careful Knight shift measurements suggest spin-singlet pairing. They were described in a Physics Today article by Alex Lopatka in 2021, An unconventional superconductor isn’t so odd after all. The article describes all the intricacies and challenges of these measurements. Stuart Brown is to be commended for persisting with this problem.
What about the Kerr effect and muSR measurements suggesting time-reversal symmetry breaking?
The polar Kerr effect involves rotation of the plane of polarisation of the electromagnetic radiation by an angle of 65 nanoradians! There is only one group in the world (at Stanford) that can detect these ultra-minute rotations.
This post was stimulated by a helpful colloquium at UQ given recently by James Annett. He has worked on strontium ruthenate for many years and is a co-author of a relevant review article.
Update. 23 April. James Annett pointed out to me that the authors for the 1998 NMR published a paper in 2020 which acknowledges that their original paper was incorrect.
Interesting post. The 1998 paper should probably be retracted, indeed. That said, there is no issue with it simply being wrong. It appears to have been an honest and careful study, and the authors continued working on the material for years without identifying the heating issue (rather than a thermometry problem), which seems to have been both subtle and difficult to detect. This is an experimental error. I would not place this story in the category of “overselling results to get into Nature,” which you often criticize. Not every Nature paper is oversold, even if there are arguably too many that are.
ReplyDeleteThe more interesting sociological aspect may be that no one attempted to reproduce the experiment until Stuart Brown (whose initial goal, as I understand it, was not even to replicate those measurements). Experiments tend to be repeated when there are doubts, and here there were few: the authors were highly respected experimentalists, and the measurements themselves are technically demanding at low temperature.
The issue of time-reversal symmetry breaking is quite different. I have not followed that literature closely, but I have seen a few cases—for example μSR studies in kagome metals and pnictides—where I found the evidence rather unconvincing. It is somewhat concerning to see how readily such claims can be taken up and propagated by the community (even though some may ultimately be correct). In this case, the sociological dimension may lie more in a collective dynamic, where critical scrutiny is relaxed because these narratives help support appealing interpretations of one’s own results.
Thanks for your helpful and detailed comments. Your points are well taken.
ReplyDeleteI think a problem that this highlights is the all-or-nothing nature of retraction. There are strong (perverse) career incentives not to retract a high profile paper that presumably is highly cited. If an honest mistake is discovered decades after a paper is published, there should be some way to add a notice to the paper online pointing out the error without any of those career costs. Neither retraction nor an erratum seem quite fit for this purpose, imho.
ReplyDeleteThis is not just to be nice to the authors - but because we want to encourage people to point out such mistakes prominently. In this case it's high enough profile that I doubt many people in the field don't know about it. But in smaller cases it's easy to miss such developments.
https://undark.org/2026/04/09/opinion-journal-fact-checkers/
ReplyDeleteScientific Journals Need Dedicated Fact-Checkers
"When a doctoral student at the Australian National University was preparing a review on systems for harvesting water from the air and purifying water through desalination, she ran into something strange. The student, who chemist Brett Pollard was co-supervising, had been looking into a bunch of academic papers that cited concentrations of lithium, potassium, sodium, calcium, and magnesium that indicated the purity and safety of drinking water. The World Health Organization and U.S. Environmental Protection Agency were cited as the sources for those values.
But there was one problem: The WHO and EPA had never produced any standards for these metals, according to Pollard and colleagues. So how had such guidelines and standards been circulating in literature for several years, unchecked and being cited again and again? “In a lot of instances, they’ve just made up values as far as I can tell,” Pollard told me. (Neither the EPA nor the WHO responded to specific questions about where such numbers may be coming from by press time.) “In a lot of instances, they’ve just made up values as far as I can tell,”
"In a lot of instances , they have just made up values as far I can tell". This is the issue which Elizabeth Bik science integrity specialist has been saying for a long time.
Thanks for this update. I had not followed the story of Sr2RuO4 for a number of years, and this was very informative. On the reproducibility aspects of the story, you may be interested in our new policy paper: https://doi.org/10.1103/27h6-yghn. It's interesting to me to note that the more recent work correcting the error in NMR Knight shift experiment is not published in Nature as the original.
ReplyDelete