Re: Time until room-temp. superconductivity

Hello Shaun!

When high temperature superconductivity was found in the mid- to late-
1980's, it's fair to say that it knocked the theory of superconductivity 
into the proverbial cocked hat. That is, the BCS theory predicted that the
maximum temperature for superconductivity should not be that high.  So 
obviously the theory needed extension or revision, and that effort is still 
going on now. There are several contenders for an expanded theory (BCS is
still OK for the lower-T effects), but it is probably safe to say that none
of them is a clear "winner" today, so the theoretical foundations are weak.
With respect to your question, it makes it kind of difficult to know where
we're going, because we really don't know where we are.

And, you should be aware that predictiona of future technological trends, by 
Mad Scientists or anyone else, tend to be abysmally inaccurate. Whatever I 
say here must be considered with a *highly* skeptical attitude.

The web has a lot of information of use. Try http://www.google.com with the
keywords:  thallium, calcium, copper, oxide, superconductivity.  This gets a
manageable number of hits (a few hundred) on one of the highest temperature
superconductors (125K at 1 atm, up to ca. 160K at high pressures). Unfortu-
nately, I found a couple of pseudoscientific kook sites on the list as well,
so have your "BS Meter" running as you browse the selections.  An intro-
ductory (HA! All of them are as dense as neutronium!) text on superconduc-
tivity could be useful reading as well, if difficult.

Forward! You submitted the question/rope, so it's time for this Mad
Scientist to answer/go hang himself.

Your target temperature of 223K (-50C) is "only" 63K more than the present 
value of 160K at high pressure.  It may be that there is still a copper-
oxide perovskite-type compound "out there" that will go higher, perhaps at 
extreme pressures.  If so, I'd expect that the amount of activity aimed at 
these materials will probably discover it within 20 years. I don't expect
such a material to be very useful however, and it may only achieve this
kind of result inside a diamond anvil press. 

Pretty much everything else I will say falls to the realm of speculation. 

   A.  The dimensionality of the superconductivity may have something to do 
with all this. That is, BCS theory works in 3-D lattices. The high-Tc 
superconductivity is essentially in Cu-O planes, and thus closer to 2-D. 
This Mad Scientist notes four things: 

		1. higher pressures tend to raise the critical temperature. This
              may mean that flattening the slighly puckered Cu-O planes 
              (which occurs at higher pressures) may lead to a "better" two-
              dimensional electron gas in those planes and thus, MAYBE, a 
              better (e.g. higher temperature) 2-D superconductivity.
		2. The limb I'm out on is getting REALLY thin!
		3. If you want some *totally* unsupported speculation, it may be
              that 2D superconductivity is "maxed" but 1-D superconductivity 
              (nanotubes???) could be possible at higher temperatures.
		4. The limb just broke!

1-4 is based on the notion that the dimensionality constrains the electrons, 
and thus in some sense prevents or removes some of the randomness due to 
thermal noise.  

   B. The theorists will finally figure out what's going on. Assuming they
do, then there is a chance (note BCS didn't help much finding the copper-
oxide materials, so it's not a given we'll get it right on round 2, either)
that the new theory will show a way to try to extend the temperature range. 

   C.  Thermal noise will forbid much higher temperatures. One must assume
there is some energy gap between the superconducting and non-superconducting
states. This gap must not be so small as to allow thermal noise to break up
the superconducting state of the electrons. But since thermal noise gets
worse with higher temperatures, we have to continually find higher gap 
materials (note I am not referring to the valence-conduction bandgaps of 
insulators and semiconductors, but something akin to the BCS energy gap for 
the Cooper electron pairs vs. unpaired electrons). At some point, thermal 
noise will put an end to things, but exactly what that temperature is, 
nobody knows (and with my luck, tomorrow some astrophysicist is going to 
discover super-conductivity in the hyper-hot cores of neutron stars with 
ultra-intense magnetic fields...).

Coming back down to earth again, it may be fun to speculate but the reality
of the matter is that we're in the dark on this one. The start of the chain 
of discovery leading to the current "champion" superconductors was, more or 
less, something of a lucky break.  Theory is behind experiment in this area, 
and offers no guide for us. So we'll stumble around, and maybe something new 
and dramatic will again show up, but I'd bet against it. Aside from the 
gradual development of copper-oxide superconductors up to 223K, I'd say 
there's very little chance to make it with any alternative material.