This intrigued me in the last (or second last?) lecture. The question is; why do Copper, Silver, and Gold, three very good conductors not superconduct?
Well, it seems that it is due to them not being able to form Cooper pairs, required by BCS theory. These elements all have a free conduction electron which they can easily use to conduct well. However, they all have a FCC lattice structure which prevents eletron-phonon coupling required to form a cooper pair.
For a basic overview, I thought this site was pretty cool: http://www.superconductors.org/
It's aimed at beginners, but is surprisingly concise, and has some nice, comprehensive tables of superconducting materials.
On another note, A&M says in one of the footnotes that amorphous Bismuth superconducts at higher temperatures than crystalline Bismuth. This is all kinds of crazy! I decided to have a quick look around, and I found a couple of papers:
http://prl.aps.org/pdf/PRL/v22/i11/p526_1
These guys talk about the phonon spectrum of Bismuth and Gallium. To be perfectly honest, I don't really understand what the phonon spectrum is (perhaps some series of resonances within the crystal?). If anyone can shed some light on this is would be great. Speaking of shedding light, this paper:
http://iopscience.iop.org/0305-4608/5/11/034/pdf/0305-4608_5_11_034.pdf
looks at the optical properties of both amorphous and crystalline Bismuth films. I haven't had time to properly read this one yet, but it seems pretty interesting. Also, I'm sure it'll have a reference to a paper talking about electronic properties of the two types as well. In saying that, this paper is not particularly thoroughly referenced, which is slightly irritating. These guys;
http://iopscience.iop.org/0305-4608/11/3/013/pdf/0305-4608_11_3_013.pdf
talk about Bismuth films, both amorphous and crystalline, which are pretty interesting. Also, this paper is from 1980, as opposed to 1968 and 1975 for the last two, so maybe they have a better idea about the theory. However, the first ceramic superconductors weren't around until 1986, and the first with Tc>77K were discovered a year later. I guess what I'm trying to say is that these guys were in for a surprise...
Subscribe to:
Post Comments (Atom)
I like your first link on superconductivity.
ReplyDeleteIt makes Chapter 34 A&M easier to understand...although I think this chapter has been the clearest in the whole book.
The bismuth thing took me a bit to figure out, but yeah, that it a bit strange. I would expect that electrons can move around more freely in the amorphous form. Hence you would want a lower temperature so there is less thermal motion/contributing to the loss...
To enlighten you, by looking at the axes, I would suspect a phonon spectrum is like a light/photon spectrum - have the intensity distribution over all wavelengths (or energies, eavenumbers)?
Just a little surprise...
Yeah, I'm beginning to understand why those three metals have their own distinct group as the noble metals now, there seems to be quite a few characteristics that really separate them from the other plebeian elements. Of course, when gold for example is included in an alloy these unique traits disappear and superconducting can still occur (http://authors.library.caltech.edu/7063/1/JOHprb75.pdf)
ReplyDeleteAs for the bismuth issue, whilst your suggestion Ann does make sense, why is bismuth the exception to the rule then? Quite strange. From doing a little bit of reading, bismuth is one of the few metals with a lower density in liquid form than in metal. As the amorphous form can be somewhat based on the liquid form (from memory anyway), perhaps this might be part of an explanation? Not sure.
Also, whilst it would have been a surprise, they probably would have been more surprised afterwards to learn that we haven't really improved on that result in the last 40 years or so!