Re: Little's high temp supercon - numbers

At the risk of bringing down the wrath and ire of Uncle Al,
he wrote:

Commercial superconductors quench by 25 K and run at 4 K. MgB2 is
highest temp BCS supercon, Tc = 39 K. High temp ceramic supercons
have Tc between 92 K (YBCO) and about 140 K
([Hg_0.8Tl_0.2Ba]_2Ca_2Cu_3O_8.33). There is no theory describing
them. They cannot be usefully fabricated.

First, one should be careful about saying that MgB2 is the
highest BCS superconductor. The high Tc materials do obey aspects
of BCS theory, in the sense of having cooper pairs, a well-defined
macroscopic phase, the "correct" BCS relationship between Tc
and the measured superconducting gap, etc. The pairing mechanism
isn't phonon-mediated, and this is almost certainly what Uncle Al
means.

Saying that there is no theory describing high Tc compounds is
pejorative, though accurate in the sense that there is still, after
20 years, no good agreement about the underlying mechanism.
The March issue of Nature Physics had a great set of articles
about this, and I blogged about it a bit here: nanoscale.blogspot.com.
However, the problem is really that there are *many* theoretical
ideas about those compounds, and it's very hard to determine
which are correct.

As for useful fabrication, these folks would take issue with
the idea that they can't be usefully fabricated:
http://www.amsuper.com/products/htsWire/faq.cfm
http://www.accel.de/pages/high_tc_current_leads.htm
http://www.trithor.de/
http://www.prysmian.com/en_42/cables_systems/energy/innovation/hts_power_system.jhtml
etc.

Stanford Department of Physics' William A. Little proposed excitonic
high temperature superconductors: decorated polyacetylenes,
[-C(Ar)=(Ar)C-]n,

Phys. Rev. A 134 1416 (1964)

This reference is wrong. It's Phys. Rev. 134, A1416-A1424 (1964).

Replace BCS large mass phonons (Debye temperature) with small mass
excitons possessing characteristic energies around 2 eV or 23,000 K.
Exciton-mediated electron pairing suggests supercon Tc substantially
exceeding 300 K.

In 1964 these polymers could not even be modeled. In 2006 they can be
trivially run in a desktop PC and trivially synthesized.

Many many people have thought about this. Uncle Al basically
implies with his writing that no one has paid attention to Little's
work or tried to synthesize similar compounds. On the contrary -
there has been an enormous amount of work on this. This paper has
been cited 843 times, according to the science citation index.
This was the main motivation behind the huge volume of organic
superconductivity work in the 1970s, as well as the development
of polyacetylene by people like Heeger, MacDiarmid, and Shirakawa
(chemistry Nobel, 2000).

I can't name names of the top of my head, but people have definitely
tried, in the intervening 42 years, to synthesize these compounds
and examine them for superconductivity. As far as I know, no one
has found a single example of any compound that conclusively
exhibits excitonic superconductivity.

So, Al, if you want to go ahead and make these compounds yourself,
go for it, but it's not unexplored territory.

.