Superconductivity Press Release

Friday's links post at Uncertain Principles included a pair of press releases, one from Princeton and the other from the NSF, about an article published Friday in the journal Science. I should make it clear that I have not read that article nor am I an expert on superconductivity other than what most nuclear theorists know about BCS and Bogolyubov theory from the nuclear structure perspective. I'm just putting on my critical reading hat here.

My comments here are in response to a question in the comments to "explain what the heck that article about superconductivity is about". My reply is here because it requires a picture:

Click on the image for the larger version; comments below the fold.

Short answer: This is an experimental paper whose main point is that they now have clear experimental results that say the mechanism for high-Tc superconductivity is significantly different from that of the BCS theory that applies to low-Tc superconducting materials. The reason you don't see much of a discussion or explanation of the mechanism is that smart experimentalists know better than to dabble in theory! Just the facts, ma'am.

Longer answer: The essence of their result is that there is an anti-correlation between electron behavior in this material at temperatures well above and well below the transition temperature. What is red becomes blue and what is blue becomes red. My middle picture shows those regions as purple, and helps clarify that they really do appear at exactly the same place. It is not perfect (a perfect anti-correlation would have the same color everywhere), but it is quite striking.

Their point is the simple one that this behavior is quite different from what is seen in BCS materials, providing a strong argument for a different model and a specific result that such a model would have to explain. Along the way, they provide additional evidence that electron pairing definitely plays a role in high-Tc materials just as it does in low-Tc materials. (This is not news.)

I'll add that this strikes me as a positive step. The problem has always been that BCS theory predicts an upper limit on the transition temperature between the normal and superconducting state. A crude way to put it is that the interaction between the "Cooper pairs" of electrons and the lattice is through phonons (a quantum particle of sound), and this is messed up by the random vibrations of the lattice at temperatures above something like 20 to 30K. (Compare that to the 90K transition temp for the YBCO material found by Wu and Chu in 1987 and you see the problem.) Showing that it is not like BCS, and thus does not require phonons, might send theory off in a more productive direction. (If you don't know what I mean by Cooper pairs, take a look at the Wikipedia article on superconductivity. It does a decent job.)

About making the picture:
I took the jpeg photos in one of the press releases and simply combined them by overlaying the cold one (below Tc) over the warm one (above Tc) with an opacity of 50%. Their color coding must have been normalized in each one, because 50% seemed visually optimal.

Materials comment: They used a compound of Sr, Bi, Ca, Cu, and O. The original YBCO material is Y, Ba, C, and O. If you look at a periodic table, you will see that variations in the recipe (there are many others) take place around certain columns in the table.

Side remark: I still remember the excitement in February 1987 when the Chu and Wu paper leaked out of PRL to the NYTimes. Entire experimental research programs changed direction within weeks, and everyone with a theoretical hammer started pounding away whether they were used to working on a nail or a screw. The huge response compared to the invisible paper by Bednorz and Muller a year earlier must have been partly because Wu and Chu pushed the transition temperature well above 77K (Liguid Nitrogen temperature) where superconductivity becomes more practical, and partly because the german group did not put out a big press release.