I did a literature review on that superconductivity for my capstone course. Superconductivity celebrated its 100th anniversary last year. The first person to discover superconductivity was Onnes in 1911 and it was observed in mercury when cooled below its critical temperature of 4K using liquid helium. Later other superconducting materials were found but it took more than half a century after its first discovery before a successful theory was developed. The BCS theory (named after the physicists who discovered it- Bardeen, Cooper and Shrieffer) . The theory predicts that below the metals critical temperature, electrons (fermions) with opposite spins form Cooper pairs(bosons) which flow without any resistance-Cooper pairs form a superfluid/BEC. I not sure how in detail how this works but we will learn about it later in the semester.
But the fact is that everyone thought they understood superconductivity until high temperature superconductors (higher than the theoretical limit of BCS theory~30K) were discovered by Bednorz and Muller in 1986. Bednorz and Muller found a copper oxide material superconducting at 35K. Just a few months later, another group found material in the same class to superconduct at 93K. Interestingly copper oxides are insulators at room temperature. As you would expect, the physics community (and probably the public as well) would go crazy at that time. It was an exciting time to do condensed matter physics. BCS theory fails to explain high temperature superconductivity.
Even now, there no theory explains high temperature superconductivity. I say this cautiously because there are theories out there but not one that everyone can agree on to fully explain this phenomenon as good as BCS theory explains low temperature superconductivity. If it piques your interest, the two most prominent ones are the resonant valence bond (RVB) theory and spin fluctuation theory. Currently the highest critical temperature stands at 135 K, recorded in 1993. Apart from high temperature and BCS superconductors, there are many other exotic superconductors like organic superconductors researched at UQ. In 2008, a new family of high temperature superconductors were discovered - iron pnictides.
It is interesting to mention that all the people I mentioned so far have a Nobel prize, and as every condensed matter physicists out there knows there is certainly one waiting for the person to explain high temperature superconductivity.
That's quite a post!
ReplyDeleteThe current highest critcal temperature of 135 K was from 1993, as you posted. I assume it's harder to find higher critical temperatures, but it's been almost 10 years...
Dry ice is at 195 K, so it's not too far off...
It's interesting that you cautiously point out that there are no theories to explain this.
I would wonder if it has already been solved, or at least started quite well, but just in a different context...or language.
A lot of our studies have analogous problems, e.g. Penrole with his tiling from maths games to the local 5-fold symmetry of quasicrystals.
Even if there was already a model out there somewhere, there would still need to be an interpretation and (experimentally) testing it!
1993 was actually just under 20 years ago ;) Which just makes it even more surprising that very little progress has been made since then!
ReplyDeleteI remember a past lecturer of mine saying how he was actually at the conference where they released the findings for the superconductor working at 93K. Apparently they had buckets full of liquid nitrogen on all of the table there, and allowed everywhere there to test it out themselves.
I seem to recall him saying that everyone was just pouring the liquid nitrogen all over the tables and trying it on them, but he might have been exaggerating for effect there! In any case, it was a very exciting time for physics.
I think we should make a distinction between the regular superconductors we see at nitrogen temperature (I'm sure we've seen this in lecture or a talk or something?) and the organic ones which are at 35K at the moment.
DeleteGood thing I have matlab or mathematica if I ever want to cacluate anything...