What is the current highest temperature super conductor?

[superpaul3000]

I've read that as of now the highest Tc is 254 K ((Tl4Ba)Ba2Ca2Cu7O13+). Is that correct? My source is superconductors.org, can that site be trusted? It seems like the record has been rapidly increasing in the past decade. If there are any experts out there, do you think we will reach a Tc of 294 K anytime soon?

 

[0xDEADBEEF]

No , the substance you are reporting has not been verified independently. Many claims have been made for many temperatures, unless someone verifies it, I'd be very skeptical. The record for normal pressure is around 138K maybe a few degrees more under very high pressure. There has been no progress in raising the critical temperature for the last 10 years. We will probably need a new material class, and those are normally found by accident. Because theory is not very good in predicting these things. But the main thing to keep in mind is, that to transport appreciable current you must cool a superconductor far below the critical temperature. Room temperature is not enough 330K or so would be what one would aim for so it can be used without cooling.

 

[Mu naught]

I wouldn't say room-temperature is the only superconductor that is viable. I think a material which can be mass-produced and has good workability that can operate under liquid nitrogen would have many many applications.

 

[superpaul3000]

But the main thing to keep in mind is, that to transport appreciable current you must cool a superconductor far below the critical temperature. Room temperature is not enough 330K or so would be what one would aim for so it can be used without cooling.

I thought that was only for certain types of superconductors like the cuprate ones. I read that some of the lower Tc superconductors suddenly transition to zero resistance at Tc. Is that correct?

 

[ZapperZ]

I thought that was only for certain types of superconductors like the cuprate ones. I read that some of the lower Tc superconductors suddenly transition to zero resistance at Tc. Is that correct?

Er... ALL superconductors make an abrupt drop in resistivity at Tc, not just the cuprates. That is a significant property of a superconductor - a "phase transition".

The point that is being made here is that, for most practical application it isn't just enough that Tc is high. That isn't the issue anymore because we have already many compounds that becomes superconducting above LN2 temperature. The issue here is that (i) the current density for many of these material is rather small to carry any appreciable current for long-distance transport (ii) the flux migration can cause significant hindrance to transport (iii) quenching on certain parts of the material due to Hc1 being rather low, etc...

To overcome these effects, one still has to go to a considerably lower temperatures than just getting to below Tc.

Zz.

 

[superpaul3000]

Thanks ZapperZ, that's some useful info. So what are the BEST known superconductors based on these additional factors? (by best I mean most useful to practical applications)

 

[Minich]

MgB2 and graphite

I wouldn't say room-temperature is the only superconductor that is viable. I think a material which can be mass-produced and has good workability that can operate under liquid nitrogen would have many many applications.

May be the most mass produced superconductor in future: graphite?

Let us see:
MgB₂ and graphite are very similar in electronic structure
see the book: Electronic structure: basic theory and practical methods / Richard M. Martin. 2004

The simple hexagonal form of graphite
consists of these planes stacked with hexagons over one another in the three-dimensional
simple hexagonal structure, Fig. 4.2. This is also the structure of MgB 2 which is illustrated
in 4.6. The boron atoms form graphene-like planes in the simple hexagonal structure and
the Mg atoms occupy sites in the centers of the hexagons between the layers. Since each
Mg atom provides two valence electrons, the total electron valence count per cell is the
same for graphite and MgB2. Thus we can expect the band structures to be closely related
and the bands near the Fermi level to be similar.

4 relevant pages from this book are here (only for teaching use):
http://love.minich.ru/s/p068.djvu
http://love.minich.ru/s/p069.djvu
http://love.minich.ru/s/p070.djvu
http://love.minich.ru/s/p071.djvu
There is a pic from page 48 (p069.djvu)
[URL]http://love.minich.ru/s/mgb2-graphite-electron-band.jpg[/URL]
MgB₂ to the left, graphite to the right (in red by Minich hand)

We know experimental Chapnic-Kikoin rule, that materials with negative low temperature Hall coefficient as a rule are not superconductors:
http://iopscience.iop.org/0305-4608/13/5/011

[URL]http://www-physics.ucsd.edu/~jorge/tcvshall.gif[/URL]
Negative Hall coeff elements to the left.

As we can see MgB₂ and graphite also obey Chapnik-Kikoin rule. But!

It may be very usefull to experiment with doped graphite so as to get Fermi level cross (a little bit) sigma graphite band, to get sigma band in graphite partially unoccupied and get hole conductivity (positive Hall coefficient). Graphite can absorb many gases and liquids.
Sigma band is stronger in graphite, than in MgB₂ so we can hope to get Tc more than 39K.

It would be very interesting to test the Chapnik-Kikoin rule for "doped" graphite. May be we have a chance get from unsuperconductive graphite superconductive one.

 

[Minich]

Graphite superconductor

Excuse, i missed an appropriate reference.
Graphite-like superconductors are already obtained:
T.E.Weller et al., Nature Phys. 2005, 1, 39
C6M (M = Yb or Ca).

and
http://www.nature.com/nphys/journal/v1/n1/full/nphys135.html
Tc is about >10K
Who knows where can i get band picture for invented "doped" graphite? Hall coefficient?

http://www.nature.com/nphys/journal/v1/n1/fig_tab/nphys119_F1.html

See Г A region!