00:00:00 The work you will be presenting tonight

00:00:02 has been put together by the ITC

00:00:04 System Production Group

00:00:06 in partnership with the University of

00:00:08 Territory Research Center.

00:00:10 This project will be a faithful example of ITC's

00:00:12 commitment to technology.

00:00:14 Tom, you've got the bell.

00:00:18 You can play this in the hour, I can understand.

00:00:20 There's more than enough transparency here.

00:00:26 I think I do have some things to contribute

00:00:28 that you may be interested in.

00:00:30 This is the title.

00:00:32 As you've seen,

00:00:34 I want to make it very clear

00:00:36 that I'm filling in for

00:00:38 Ken Poppelmeyer, who is expecting

00:00:40 a child.

00:00:42 His wife is expecting a child momentarily.

00:00:44 And so,

00:00:46 you'll have to put up with me for tonight.

00:00:48 These are the

00:00:50 faculty

00:00:52 and research associates

00:00:54 at Northwestern.

00:00:56 Not only are these industrial labs

00:00:58 involved, but the universities, as you well know,

00:01:00 are getting into the game.

00:01:02 We also would be

00:01:04 remiss if we did not

00:01:06 plug the real workers,

00:01:08 the postdocs and graduate

00:01:10 assistants who are

00:01:12 going without sleep and sometimes without

00:01:14 food to get these

00:01:16 measurements made.

00:01:18 I would like to say

00:01:20 that a thorough study,

00:01:22 not just of Michelle

00:01:24 and Revol in 1984,

00:01:26 but of a whole

00:01:28 series of papers by

00:01:30 Revol and coworkers in the late 70s

00:01:32 and early 80s,

00:01:34 would probably have saved us all

00:01:36 a great deal of time.

00:01:38 And what Revol

00:01:40 does here, and I'm plotting this

00:01:42 instead of LA-203,

00:01:44 which would be the ceramic way

00:01:46 to approach the system,

00:01:48 LA-01.5 so that the

00:01:50 lanthanum barium copper and

00:01:52 lanthanum barium

00:01:54 oxide all jive

00:01:56 and match with the LA,

00:01:58 BACU and the compounds involved.

00:02:00 Also these compounds will then be

00:02:02 horizontal lines representing fixed

00:02:04 copper content. But

00:02:06 Revol and coworkers were able to find

00:02:08 actually four different phases

00:02:10 in these diagrams.

00:02:12 I won't say anything more about

00:02:14 this one because it's not a

00:02:16 perovskite derivative. It may actually be closer

00:02:18 to this ubiquitous green phase

00:02:20 that we'll be talking about.

00:02:22 But there are three perovskite derivative

00:02:24 or layer perovskite type structures.

00:02:26 The K2NiF4,

00:02:28 which is where the action's at

00:02:30 in the lanthanum barium and lanthanum

00:02:32 strontium copper oxides.

00:02:34 The

00:02:36 strontium typenate

00:02:38 related series,

00:02:40 which is listed here,

00:02:42 which only forms in the strontium case.

00:02:44 And then the

00:02:46 LA-3, BA-3,

00:02:48 1, 2, 6, 0, 14,

00:02:50 which is on the line,

00:02:52 as we'll see later,

00:02:54 where the yttrium

00:02:56 barium 2 Cu3 compound,

00:02:58 what we call the 1, 2, 3,

00:03:00 occurs.

00:03:02 Before I go on to the

00:03:04 yttrium system, I'd just like to make mention

00:03:06 of the fact that it's this

00:03:08 line where

00:03:10 Bednarz and Muller

00:03:12 found their superconductivity.

00:03:14 You can see that the substitution

00:03:16 of lanthanum with strontium

00:03:18 goes much farther than

00:03:20 the barium case, but both in this

00:03:22 range are superconducting.

00:03:24 Although we were

00:03:26 not active experimentally at the

00:03:28 time,

00:03:30 you've all seen these structures enough,

00:03:32 but Art Freeman, who does

00:03:36 electronic structure calculations,

00:03:38 local density theory calculations,

00:03:40 very quickly generated

00:03:42 with some coworkers

00:03:44 the structure for the K2NiF4

00:03:46 systems, and what you've

00:03:48 got here is charge density,

00:03:50 and you can see the

00:03:52 gross two-dimensionality. You see

00:03:54 the metallic

00:03:56 copper-oxygen layer, which is obviously

00:03:58 where the superconductivity is taking place,

00:04:00 and almost no charge

00:04:02 density here at all.

00:04:04 The second thing that

00:04:06 they were able to show, which I think is a real

00:04:08 tribute to where

00:04:10 band calculations are

00:04:12 now, and that is that they

00:04:14 were able to show a singularity

00:04:16 in the density of states just at

00:04:18 about the composition

00:04:20 in either the strontium or the

00:04:22 barium lanthanum systems

00:04:24 where the maximum Tc

00:04:26 is observed, and so that's

00:04:28 at this point here.

00:04:30 I'll show you some additional

00:04:32 calculations later on the yttrium system.

00:04:34 Now you'll see a lot of

00:04:36 triangles because I'm going to talk about

00:04:38 phase diagrams. This is the

00:04:40 again

00:04:42 taking the Revo series

00:04:44 and just putting these as lines across the

00:04:46 diagram. Where would we expect compounds

00:04:48 to occur? Fixed copper content.

00:04:50 You see the K2NiF4.

00:04:52 You see the strontium

00:04:54 titanates. You see the

00:04:56 La3Ba3Cu6O14.

00:04:58 These are all

00:05:00 Revo and co-worker

00:05:02 compounds from the La,

00:05:04 Ba, or Sr

00:05:06 CuO systems. We've also

00:05:08 had Ken Poppelmeier thought perhaps

00:05:10 this Rellis and Popper phase,

00:05:12 the 4,3,10,

00:05:14 might be a possibility.

00:05:16 And so

00:05:18 what we did,

00:05:20 where was Chu

00:05:22 working? Everybody has a

00:05:24 theory as to what Chu was really after.

00:05:26 I think

00:05:28 they were after K2NiF4

00:05:30 and this is

00:05:32 where Chu and co-workers

00:05:34 operated. As soon as

00:05:36 we got the

00:05:38 composition over the phone,

00:05:40 we went to work and I won't

00:05:42 bore you with any more of these but you can see

00:05:44 that we found several different

00:05:46 compositions very near the

00:05:48 Chu composition where you see the

00:05:50 resistance plummeting and

00:05:52 your 90 degree Kelvin superconductivity.

00:05:54 What

00:05:56 indeed is going on? Now my

00:05:58 colleague just preceding me showed you a phase

00:06:00 diagram nearly identical to

00:06:02 ours.

00:06:04 What we have done here

00:06:06 this is the superconductor

00:06:08 which we will call 1,

00:06:10 2, 3, yttrium, barium,

00:06:12 copper.

00:06:14 This is the ubiquitous green

00:06:16 phase and I'll mention

00:06:18 why it's ubiquitous in just a minute.

00:06:20 What I would like to suggest

00:06:22 is that

00:06:24 as opposed to

00:06:26 resistivity, the

00:06:28 susceptibility, and here were some

00:06:30 susceptibility plots that go along

00:06:32 with those resistivity plots

00:06:34 that I just mentioned previously,

00:06:36 the diamagnetic signal

00:06:38 is a much better indicator

00:06:40 of superconductivity and it is

00:06:42 for the following reason.

00:06:44 If we go back

00:06:46 to the

00:06:48 phase diagram just with the dotted lines

00:06:52 and we superimpose this on top

00:06:54 of the other diagram, I'm not sure if I'm going to be able to

00:06:56 do that very well at all.

00:06:58 If you think about the fact that

00:07:00 this is the superconductor and we're operating

00:07:02 quite a ways away,

00:07:04 you have to be very near

00:07:06 this joint

00:07:08 to have any substantial fraction of

00:07:10 superconductor. Therefore, if you're

00:07:12 looking for a resistivity effect,

00:07:14 the only place it will be seen on this entire line

00:07:16 is just very, very

00:07:18 close to the x.

00:07:20 So, in other words, of all the

00:07:22 other compositions, this was probably the only one

00:07:24 that was superconducting.

00:07:26 In contrast to that,

00:07:28 if one is looking at diamagnetic

00:07:30 signal intensity as opposed

00:07:32 to the resistivity,

00:07:34 one could expect

00:07:36 in a very, very large

00:07:38 fraction

00:07:40 of the diagram, and I'm kind of jumping

00:07:42 ahead, but let me grab it.

00:07:44 If I just superimpose this,

00:07:46 one could expect

00:07:48 diamagnetic signal anywhere

00:07:50 in the red-hatched region.

00:07:52 And the reason being,

00:07:54 and I might only have 5% there,

00:07:56 but I'm still going to see a diamagnetic signal,

00:07:58 barring, of course,

00:08:00 screening effects and things like that.

00:08:02 Let me just give you a demonstration

00:08:04 of phase diagram work by

00:08:06 diamagnetic susceptibility.

00:08:08 The dark dots,

00:08:10 including the 1, 2, 3 compound,

00:08:12 represent some

00:08:14 diamagnetic signal intensity.

00:08:16 The open circles indicate

00:08:18 where you do not have any.

00:08:20 Here, for instance,

00:08:22 is the diamagnetic,

00:08:24 or this is the diamagnetic signal intensity

00:08:26 as we cross

00:08:28 the

00:08:30 copper oxide

00:08:32 Y2BACO5 join,

00:08:34 not counting

00:08:36 this one far point,

00:08:38 open point, but you see two open

00:08:40 circles and then five closed circles.

00:08:42 And here's precisely

00:08:44 what the normalized,

00:08:46 I don't think I have that

00:08:48 right, do I?

00:08:50 I have the normalized

00:08:52 diamagnetic signal intensity.

00:08:54 Here is the join.

00:08:56 You see very little signal intensity,

00:08:58 if at all, and all of a sudden you have

00:09:00 diamagnetic signal intensity. In addition,

00:09:02 now you can see the difference between

00:09:04 the susceptibility

00:09:06 and the resistivity.

00:09:08 These circle points indicate

00:09:10 where, in addition, we've got superconductivity.

00:09:12 So this just tells you

00:09:14 that Chu was very lucky

00:09:16 to even hit it going across that line.

00:09:18 But the diamagnetic

00:09:20 signal intensity is quite a good indicator

00:09:22 of what's going on.

00:09:24 Now, of course, since that time

00:09:26 we've really

00:09:28 and I understand now

00:09:30 about six other groups across the country

00:09:32 have really gone to work

00:09:34 and here is very close

00:09:36 to the completed

00:09:38 950 degrees C

00:09:40 isothermal subsolidus phase diagram.

00:09:44 Those are surrounding terms.

00:09:46 This is the

00:09:48 1-2-3 superconductor.

00:09:50 There is another

00:09:52 perovskite-like compound.

00:09:54 Several people reported it is

00:09:56 the 1-3-2.

00:09:58 We had this controversy before.

00:10:00 Was it 1-2-3 or 2-1-3?

00:10:02 Well, here's an additional confusing factor.

00:10:04 The superconductor is

00:10:06 1-heatrium-2-barium-3-copper.

00:10:10 Interestingly enough,

00:10:12 and I've lost the transparency now,

00:10:14 but don't worry about it,

00:10:16 is the K2NiF4 line.

00:10:18 On that line is an additional phase.

00:10:20 It's not superconducting.

00:10:22 It is opaque. It is black.

00:10:24 We have hopes of doing some things to it

00:10:26 to see if we can get superconductivity.

00:10:28 So if you'd like to look at another compound,

00:10:30 there is another ternary compound.

00:10:32 There are three compounds in the system.

00:10:34 I'd just like to make a point

00:10:36 to plug optical microscopy

00:10:38 because

00:10:40 we run to the X-ray diffractometer,

00:10:42 we run to the electron microscope,

00:10:44 and one of the simplest tools

00:10:46 is to just crush these up in a mortar and pestle

00:10:48 and sprinkle them in a little oil

00:10:50 between a cover slip and a glass slide

00:10:52 and look at them under cross polarizer.

00:10:54 Just look at them

00:10:56 under transmitted light.

00:10:58 And the reason I say that

00:11:00 is that there are two colored phases in the system.

00:11:02 These down here

00:11:04 are white,

00:11:06 but this is a

00:11:08 beautiful blue-green

00:11:10 and this is a very sharp green.

00:11:12 I should have brought vials with me.

00:11:14 What this means is that

00:11:16 even more so

00:11:18 than the X-ray diffraction patterns

00:11:20 which can be very complicated and difficult

00:11:22 to sort out,

00:11:24 if I simply put an overlay here,

00:11:26 you'll see what I mean by the ubiquitous green.

00:11:28 Anywhere within the green

00:11:30 patched area in the diagram,

00:11:32 we can detect

00:11:34 the ubiquitous green phase.

00:11:36 And maybe they are in a very small fraction.

00:11:38 But this indeed indicates

00:11:40 that all roads lead there.

00:11:42 In other words, that all those joins

00:11:44 radiate back to the Y2VAC05.

00:11:46 I've just learned

00:11:48 by phone from Dave Clark

00:11:50 that there is a surprise

00:11:52 binary compound, as of yet

00:11:54 not reported,

00:11:56 at this end of the system

00:11:58 which explains why we were having difficulties.

00:12:00 And so we'll probably have this diagram

00:12:02 finished up also in a couple of days.

00:12:06 I want to say just a few more things

00:12:08 now about

00:12:10 the structure and about the calculations

00:12:12 that Art is doing.

00:12:14 I think you've seen the Hazen or Hazen

00:12:16 finger structure.

00:12:18 And also

00:12:20 we have a copy

00:12:22 of the argon

00:12:24 structure.

00:12:26 Very, very similar.

00:12:28 The square planar

00:12:30 copper with the

00:12:32 oxygen. But what

00:12:34 Art Freeman and students and

00:12:36 postdocs and colleagues have been

00:12:38 doing is, again,

00:12:42 the electronic structure calculations.

00:12:44 And I don't know if I can do justice to this.

00:12:46 The difference

00:12:48 between, this is the Hazen

00:12:50 structure, but it assumes an oxygen

00:12:52 structure which we don't agree with.

00:12:54 An oxygen 6.

00:12:56 If that were true, the charge density

00:12:58 would be very reminiscent of

00:13:00 what is in the K2NiF4

00:13:02 where very strong

00:13:04 scattering occurs.

00:13:06 We don't think that that, our

00:13:08 thermograms agree with a

00:13:10 higher oxygen content, closer to 7.

00:13:14 And here's where

00:13:16 you have a material scientist,

00:13:18 a ceramist, telling a bunch of chemists

00:13:20 what a physicist has calculated.

00:13:24 So,

00:13:26 this is

00:13:28 now for the

00:13:30 argon structure.

00:13:32 What you're looking at is, this is the C-axis.

00:13:34 This is in one direction

00:13:36 of the unit cell. I'm going to put these side-by-side

00:13:38 to make them really confusing.

00:13:40 And you've got to

00:13:42 remember that this is 90 degrees.

00:13:44 In other words, if I could bend it, like a book.

00:13:46 You're looking at it this way,

00:13:48 and you're looking at it this way.

00:13:50 I just did that with the transparencies.

00:13:52 But the point is, is that you do

00:13:54 see still layering, which is interesting.

00:13:56 But you see the

00:13:58 one-dimensionality

00:14:00 along the copper-oxygen-1.

00:14:02 And if you remember

00:14:04 some of the diagrams, you see the

00:14:06 one-dimensionality in the

00:14:08 very base of the unit cell.

00:14:10 And so, what do these mean?

00:14:12 Well, definitely two-dimensionality.

00:14:14 And we'll see that in some of the

00:14:16 micrographs that I'm going to show.

00:14:18 As to reasons yet

00:14:20 for the superconductivity,

00:14:22 it's a little too early to tell.

00:14:24 But we are now beginning to calculate

00:14:26 these structures, the electronic

00:14:28 band and bond picture,

00:14:30 and

00:14:32 are making progress.

00:14:34 Now, I would like to

00:14:36 throw one thing

00:14:38 from the phase diagram work

00:14:40 that may be of some interest

00:14:42 to you, and that is

00:14:44 this whole issue that was

00:14:46 raised way back at the beginning

00:14:48 on the basis of the

00:14:50 Chu et al. work.

00:14:52 And that was the issue of, gee, is it

00:14:54 interfacial, is it bulk, etc.?

00:14:56 I don't agree that it's bulk.

00:14:58 But let me just show you a very interesting

00:15:00 effect. I showed you

00:15:02 the diamagnetic signal

00:15:04 across this series.

00:15:06 Now I'm going to show you

00:15:08 the diamagnetic signal across this series.

00:15:10 And by

00:15:12 lever rule, one would argue

00:15:14 that the maximum

00:15:16 content of the 1-2-3 superconductor

00:15:18 would occur right at the joint.

00:15:20 And I'm going to go down on either side.

00:15:22 And lo and behold,

00:15:24 that

00:15:26 happens to the right.

00:15:28 That is where I'm equilibrating

00:15:30 the 1-2-3 compound

00:15:32 with the BaCuO2 phase

00:15:34 in this ubiquitous green.

00:15:36 Green's everywhere.

00:15:38 But right at the phase boundary,

00:15:40 it tells us the phase boundary's there.

00:15:42 So again, diamagnetic signals give us the phase boundary

00:15:44 in the system. But there's a precipitous

00:15:46 drop.

00:15:48 What does it mean?

00:15:50 Well, you could have interfacial effects.

00:15:52 You could have

00:15:54 a screening effect.

00:15:56 If you have a large fraction

00:15:58 of superconductor, weak links

00:16:00 can give you an outer layer

00:16:02 of superconductor, which can exclude lines

00:16:04 of flux from the interior, which would explain

00:16:06 why these are hot.

00:16:08 But yet these fall very nicely

00:16:10 with a lever rule

00:16:12 argument as to how much

00:16:14 superconductor I would have.

00:16:16 What I am tempted to suspect

00:16:18 and will be my transition

00:16:20 to the last

00:16:22 well, actually transition to

00:16:24 the last transparency that I have.

00:16:26 What I am tempted to suspect

00:16:28 is that

00:16:30 one has

00:16:32 a slightly different composition of 1-2-3.

00:16:34 We've said this is a point compound.

00:16:36 But is it?

00:16:38 Is it possible that

00:16:40 the oxygen content, the copper-2-copper-3

00:16:42 ratio

00:16:44 of the 1-2-3 equilibrium

00:16:46 in this phase triangle

00:16:48 is different than in this phase triangle?

00:16:50 That indicates to me that some point defect

00:16:52 studies need to be done.

00:16:54 Now, this is

00:16:56 a messy slide, and we love

00:16:58 to do this type of thing at the universities.

00:17:00 And that is

00:17:02 just to show you how

00:17:04 we're set up and how we're operating.

00:17:06 This is our superconductor group.

00:17:08 I've shown you

00:17:10 Art Freeman's theory calculations.

00:17:12 This is guiding us in terms

00:17:14 of selecting new systems

00:17:16 and substitutions. It's also

00:17:18 beginning to explain, hopefully,

00:17:20 the types of physics that are going on.

00:17:22 Much of what I've talked about is

00:17:24 phase work. Synthesis, structure

00:17:26 determination, others have spoken about

00:17:28 phase relationships. I've intimated

00:17:30 that defect studies need to be done.

00:17:32 We're all participating

00:17:34 down here in the characterization

00:17:36 of all sorts of techniques.

00:17:38 This is just what we've been doing,

00:17:40 but you could probably make a list

00:17:42 as long as my arm of either current

00:17:44 or potential things that people are doing.

00:17:46 But before I talk

00:17:48 about this last step,

00:17:50 what I'd like to do is turn off the

00:17:52 transparency and turn on the slides.

00:17:56 I just have two slides.

00:17:58 We may need to dim the lights.

00:18:00 If you can advance it,

00:18:02 because I don't have the...

00:18:04 Again, we've already seen

00:18:06 one

00:18:08 high-resolution

00:18:10 electron micrograph. This is from

00:18:12 Mars.

00:18:14 Again, you can see

00:18:16 the barium and yttrium

00:18:18 spacing and that

00:18:20 triple-layer structure.

00:18:22 I think what's interesting is the next

00:18:24 slide. What we did is we

00:18:26 melted

00:18:28 the 1, 2, 3 superconductor.

00:18:30 Once you

00:18:32 scan through the next two or three,

00:18:34 you get everything in there.

00:18:36 This, I believe, is the green

00:18:38 phase.

00:18:40 This is the, I believe,

00:18:42 C-low. If you'll now go back

00:18:44 two slides,

00:18:46 you see there we go. That's a superconductor.

00:18:48 I noticed

00:18:50 the argon work.

00:18:52 You saw something that was, what,

00:18:54 0.14 millimeters by 0.14 millimeters

00:18:56 by 0.009 millimeters.

00:18:58 To me, this is confirming

00:19:00 the two-dimensionality.

00:19:02 The index says this is 1 to 2 to 3

00:19:04 yttrium to barium to copper.

00:19:06 If I can have the slides off,

00:19:08 I'll just make my conclusions

00:19:10 on the basis of this

00:19:12 slide right here.

00:19:14 That is that I think that

00:19:16 in spite of some of the signal victories

00:19:18 in film growth, etc.,

00:19:20 that there's a great deal that needs to be done

00:19:22 in processing. Certainly, in order

00:19:24 to facilitate some of the

00:19:26 very fancy physics experiments

00:19:28 that need to be done, we need to grow

00:19:30 large single crystals.

00:19:32 As has been shown already

00:19:34 tonight, that's not straightforward.

00:19:36 Secondly, sintering.

00:19:38 Sintering in such a textured

00:19:40 structure that we can pass

00:19:42 these enormous amps per square

00:19:44 centimeter is not

00:19:46 a completely straightforward thing.

00:19:48 Then finally, film growth.

00:19:50 I think you can see that what we're talking about

00:19:52 theory, just in terms

00:19:54 of phase work.

00:19:56 We looked at one system tonight, yttrium to barium

00:19:58 to copper oxygen.

00:20:00 What lies hidden in all of the phase

00:20:02 diagrams for all of those

00:20:04 rare-earth

00:20:06 substitutions that we've seen.

00:20:08 Then finally, processing.

00:20:10 As I said on Channel 7 in Chicago,

00:20:12 these are ceramic

00:20:14 materials. They are brittle.

00:20:16 You don't just do with them

00:20:18 as you typically do with metals.

00:20:20 There are some very interesting ceramic

00:20:22 processing techniques

00:20:24 that need to be brought to bear in the problem.

00:20:30 Thank you very much. I'm very happy

00:20:32 to be here. Amazed that there are still

00:20:34 people left in the audience. It's not

00:20:36 a problem for me. I'm from the West Coast.

00:20:38 It's only quarter after 11.

00:20:40 In fact, I haven't changed my watch yet, so it's still

00:20:42 only quarter after 10.

00:20:44 Anyway, I hope to tell you

00:20:46 some different things. We learned a lot

00:20:48 about the crystal structure and the crystal chemistry.

00:20:50 I would like to tell you what our group

00:20:52 has been doing at Berkeley

00:20:54 in collaboration with many other groups on campus

00:20:56 in measuring some of the

00:20:58 properties of these materials

00:21:00 and what that's leading us to

00:21:02 in terms of what the mechanism

00:21:04 of the superconductivity is.

00:21:06 The title is Determination of the

00:21:08 Mechanism of High-Temperature Superconductivity.

00:21:10 I don't mean to imply that we know it.

00:21:12 I just want to tell you where we are.

00:21:14 In order to do that,

00:21:16 I want to address two questions.

00:21:18 One is, does the entire sample

00:21:20 superconduct? We've heard a lot

00:21:22 tonight that would indicate

00:21:24 that this is bulk superconductivity.

00:21:26 I would like to show you what

00:21:28 measurements you do to prove that.

00:21:30 The couple that I need to go through

00:21:32 are to establish that the resistivity

00:21:34 really is zero, not

00:21:36 at the detection limit of

00:21:38 our typical 4-probe

00:21:40 conductivity measurement, which is

00:21:42 roughly 10 to the minus 8 ohms

00:21:44 centimeters. 10 to the minus 8 is not

00:21:46 zero. I want to talk a little

00:21:48 bit about magnetic measurements.

00:21:50 Superconductors are perfect diamagnets

00:21:52 and they ought to show a volume

00:21:54 susceptibility of minus 1 over 4

00:21:56 pi emus per cc.

00:21:58 Then I would like to show you

00:22:00 how we show that these materials have

00:22:02 persistent currents and what we know about

00:22:04 the persistent currents. Once I've

00:22:06 gone through that and convinced you

00:22:08 that we really do know that these are bulk

00:22:10 superconductors, I want to ask

00:22:12 a question that was brought up

00:22:14 in the first talk tonight.

00:22:16 Are these BCS-type

00:22:18 superconductors? BCS theory

00:22:20 really does describe the conventional

00:22:22 superconductors very well.

00:22:24 What does it predict

00:22:26 for these materials and

00:22:28 how do some of the measurements that we've made

00:22:30 show whether the BCS

00:22:32 theory applies or not?

00:22:34 Again, I have that 30 Kelvin maximum

00:22:36 which was really believed for a number

00:22:38 of many years now that we

00:22:40 could never go above 30 Kelvin.

00:22:42 Let me begin

00:22:44 by just pointing out a few

00:22:46 things about the resistivity

00:22:48 of these samples.

00:22:50 You've all seen these graphs before.

00:22:52 This is the lanthanum

00:22:54 strontium copper oxide. I'm going to focus

00:22:56 on that because we haven't had time to complete

00:22:58 all the measurements that we've

00:23:00 done on this material and the yttrium

00:23:02 barium compound. What I want

00:23:04 to point out here is that for this particular

00:23:06 sample there is a very sharp

00:23:08 drop in the resistivity

00:23:10 at 40 Kelvin.

00:23:12 I want to show you something interesting about the scale.

00:23:14 If you haven't noticed by now,

00:23:16 everyone is plotting

00:23:18 a linear resistance scale.

00:23:20 We know that resistivity

00:23:22 changes orders of magnitude

00:23:24 and we really do need to show

00:23:26 that that zero is zero

00:23:28 and not some small finite value

00:23:30 if we want to determine

00:23:32 that the whole sample is superconducting.

00:23:34 I'll have a little bit more to say

00:23:36 about whether that value really is zero

00:23:38 but the one other thing

00:23:40 that I want to mention is that

00:23:42 this transition looks very sharp

00:23:44 and the resistivity transitions always

00:23:46 look sharp because they pick out

00:23:48 one link in the material which

00:23:50 superconducts. So as a chemist

00:23:52 if I want to characterize this whole material

00:23:54 resistivity falls short

00:23:56 in determining whether the entire

00:23:58 sample is superconducting.

00:24:00 All I know from this measurement is that

00:24:02 there is one link between

00:24:04 the leads in my four point probe

00:24:06 analysis. Now in contrast

00:24:08 this is the susceptibility for

00:24:10 the same sample that I just showed you

00:24:12 the resistivity on. Now

00:24:14 superconductors above

00:24:16 the transition temperature

00:24:18 in a very small field, in this case

00:24:20 12 gauss, show nearly

00:24:22 zero susceptibility. It's just

00:24:24 too small a field to detect anything.

00:24:26 And as you decrease

00:24:28 below the transition temperature

00:24:30 you see a very diamagnetic signal.

00:24:32 Now the one thing I want

00:24:34 to point out about this particular sample

00:24:36 and I think this is just coincidental

00:24:38 our other samples haven't been so good

00:24:40 the value that we measure

00:24:42 is that of minus one over

00:24:44 four pi. The volume susceptibility

00:24:46 converts to that of a perfect

00:24:48 diamagnet and I have

00:24:50 a hundred percent Meissner because this is

00:24:52 known as the Meissner effect.

00:24:54 But what you notice right off the bat

00:24:56 with this sample is that the transition

00:24:58 is not very sharp

00:25:00 and it's also at a lower temperature

00:25:02 than the resistivity.

00:25:04 So what you need to realize is that

00:25:06 when you look at the bulk of the sample

00:25:08 it may not be as good as you expected.

00:25:10 Now we made this particular

00:25:12 sample by mixing

00:25:14 the oxides, grinding

00:25:16 and we call the shake and bake

00:25:18 methods also heat and beat

00:25:20 if you want. We obviously didn't

00:25:22 heat and beat enough

00:25:24 and we're not very homogeneous.

00:25:26 And so I'd like to

00:25:28 just show you that that is the case

00:25:30 that we can make the magnetic transition

00:25:32 which is looking at the bulk of the

00:25:34 material sharper by going to

00:25:36 other synthesis methods

00:25:38 or by grinding longer.

00:25:40 And I think you'll realize

00:25:42 that on this graph the transition is

00:25:44 sharper and I won't go into the details

00:25:46 of the method that we used but we used

00:25:48 the solution precipitation method

00:25:50 which got the materials mixed

00:25:52 on an atomic scale before we

00:25:54 heated them. So rather than

00:25:56 having particles of oxide where these

00:25:58 three cations have to diffuse

00:26:00 over long distances, in this

00:26:02 case we've got them homogeneous

00:26:04 before we begin.

00:26:06 But I don't want to dwell too much on synthesis

00:26:08 because I'd like to tell you a little bit more about

00:26:10 the measurements. At this point

00:26:12 from the magnetism there's

00:26:14 no question that we have a bulk

00:26:16 superconductor but we'd like to

00:26:18 really show that the resistance

00:26:20 of the sample is

00:26:22 zero. And we've done the following

00:26:24 measurement with John Clark

00:26:26 in the physics department

00:26:28 some of you may know John Clark

00:26:30 for the development of the squids

00:26:32 the superconducting quantum interference

00:26:34 devices. And this experiment

00:26:36 utilizes a squid

00:26:38 to show what the upper limit

00:26:40 of the resistivity of one of

00:26:42 these superconducting samples is.

00:26:44 Now the way we do the experiment is

00:26:46 as follows. We take a hollow

00:26:48 cylinder of the lanthanum strontium

00:26:50 copper oxide. It's got a

00:26:52 hole in the middle and we put

00:26:54 this cylinder in

00:26:56 a coil of niobium

00:26:58 and now we

00:27:00 use a squid detector

00:27:02 that's at the bottom

00:27:04 of the silicon wafer here

00:27:06 which is just there as a detector

00:27:08 don't worry about how it works

00:27:10 it's there to measure

00:27:12 the mutual inductance

00:27:14 between the niobium coil

00:27:16 and the superconducting

00:27:18 cylinder. Now if you

00:27:20 put a current through the

00:27:22 coil, through the niobium coil

00:27:24 because you create a magnetic

00:27:26 field you induce a current

00:27:28 in the superconductor

00:27:30 this is known as a shielding

00:27:32 current. And

00:27:34 with the squid we can detect that

00:27:36 that shielding current has been established

00:27:38 and we can detect how long

00:27:40 it persists. And the measurement

00:27:42 was made over only a few

00:27:44 minutes although I imagine people could

00:27:46 spend years seeing whether these

00:27:48 things persisted. But what

00:27:50 we find is that the

00:27:52 current does not decay

00:27:54 and we can put an upper limit by

00:27:56 the lifetime of this current

00:27:58 that we've induced in the cylinder

00:28:00 we put an upper limit on the resistivity

00:28:02 of 10 to the minus 17

00:28:04 well now we're getting lower

00:28:06 in resistivity. So

00:28:08 we are getting closer to zero. We do

00:28:10 believe that these samples do have zero

00:28:12 resistance. Now again

00:28:14 the resistance has a problem

00:28:16 and that is that it only depends

00:28:18 on one closed path

00:28:20 we only need one loop around

00:28:22 the cylinder roughly

00:28:24 9 millimeters in length to give

00:28:26 us this effect. And it doesn't tell

00:28:28 us that the entire sample

00:28:30 is linked up. So although we

00:28:32 know this is a bulk superconductor

00:28:34 I just want to point out

00:28:36 with one further experiment

00:28:38 that it is difficult to get

00:28:40 these superconducting rings

00:28:42 to link up in a continuous path.

00:28:44 And the experiment

00:28:46 that we've done is along the same

00:28:48 lines but now we've just used

00:28:50 a conventional magnetometer.

00:28:52 We're taking again

00:28:54 a superconducting ring of this sample

00:28:56 and to measure to see

00:28:58 if we get currents around the ring

00:29:00 we place the ring

00:29:02 in a 50 gauss field

00:29:04 so that the magnetic field lines

00:29:06 are threading the loop

00:29:08 and then we take this ring in the 50

00:29:10 gauss field, we cool it

00:29:12 and then we turn off the field.

00:29:14 Now when we turn off the field

00:29:16 the flux lines which are threading

00:29:18 the loop can't escape because

00:29:20 of the superconducting currents which

00:29:22 are going around the ring. And so

00:29:24 those, because there are flux

00:29:26 lines trapped in this ring

00:29:28 we measure a net

00:29:30 magnetic moment in this case.

00:29:32 So we are quite happy, we take this ring

00:29:34 we do this experiment

00:29:36 we measure a large magnetic moment

00:29:38 and we thought this meant that we had

00:29:40 a nice current going around

00:29:42 the ring. Then we did one

00:29:44 experiment too many, this is always the problem.

00:29:46 We took this ring

00:29:48 and we put it in the other direction

00:29:50 so that there was no field

00:29:52 going through the loop

00:29:54 and we did the same experiment

00:29:56 and we measured the same magnetic moment.

00:29:58 The only

00:30:00 conclusion you can make with that

00:30:02 is that this sample contains

00:30:04 many tiny loops

00:30:06 and there aren't very many paths

00:30:08 going over long distances.

00:30:10 So one of the real materials

00:30:12 issues which is going to come up

00:30:14 and I'm sure it can be overcome

00:30:16 but it's going to be an issue

00:30:18 is that we've got to worry about

00:30:20 how we connect these superconducting

00:30:22 grains up so that we get

00:30:24 continuous loops.

00:30:26 And just to show you that

00:30:28 these grains that we've centered together

00:30:30 and by the way this was a very

00:30:32 dense sample which we looked at

00:30:34 it was roughly 90% dense

00:30:36 still has a lot of junk

00:30:38 or crud at the boundaries

00:30:40 and just to show you that that really is

00:30:42 the case, that these materials

00:30:44 do have a lot of impurities in them

00:30:46 I've taken this sample

00:30:48 of lanthanum strontium copper oxide

00:30:50 and done a temperature program

00:30:52 desorption, very common measurement

00:30:54 to do in the field

00:30:56 of catalysis.

00:30:58 If I heat this sample at 1 degree

00:31:00 per second in a helium flow

00:31:02 and monitor what comes off the sample

00:31:04 with a mass spectrometer

00:31:06 I observe a lot of water

00:31:08 which desorbs as well as

00:31:10 a lot of CO2.

00:31:12 The water comes off at a

00:31:14 temperature well above

00:31:16 the boiling point

00:31:18 of water which would indicate that the

00:31:20 water is not physisorbed water

00:31:22 but present as hydroxyl groups.

00:31:24 The CO2 probably

00:31:26 indicates that there are carbonates

00:31:28 present in the material.

00:31:30 Now this is not a one shot experiment

00:31:32 we can do this experiment and get rid

00:31:34 of all the water, all the CO2

00:31:36 let this sample sit in air

00:31:38 for a day and repeat

00:31:40 the experiment and we'll find that it

00:31:42 picks up more water and CO2.

00:31:44 So we do need to worry about how we

00:31:46 seal these samples in terms of

00:31:48 these kinds of contaminants which are

00:31:50 obviously creating some resistive

00:31:52 boundaries between the

00:31:54 grains.

00:31:56 We've done

00:31:58 the same experiment on the

00:32:00 yttrium barium compound although I'm

00:32:02 not going to show a lot of data on the yttrium

00:32:04 barium. This one is quite interesting

00:32:06 because if you look at the mass

00:32:08 spec output not only do you

00:32:10 see water coming off but at a relatively

00:32:12 low temperature oxygen

00:32:14 diffuses out of this material

00:32:16 and it's already been pointed out

00:32:18 during this talk that we ought to worry

00:32:20 about what temperature and oxygen

00:32:22 pressure we anneal this material

00:32:24 well this would indicate that

00:32:26 already at 800 Kelvin

00:32:28 there's a lot of O2 coming

00:32:30 out and we might think about

00:32:32 annealing these samples at a temperature

00:32:34 lower than that or near there

00:32:36 so that we get, we control

00:32:38 the amount of oxygen in the samples.

00:32:40 So these, not only

00:32:42 can there be

00:32:44 resistive material at the

00:32:46 interfaces but I think this last

00:32:48 experiment shows that we could

00:32:50 have non-stoichiometry in the oxygen

00:32:52 so we do need to worry about

00:32:54 these superconducting grains

00:32:56 how the oxygen content varies

00:32:58 through them. And one

00:33:00 last evidence that these samples

00:33:02 although they look homogeneous and they're both

00:33:04 superconductors may have problems

00:33:06 at the grains comes

00:33:08 another electron microscopy

00:33:10 picture, atomic resolution

00:33:12 this is for the lanthanum strontium

00:33:14 compound, you've already

00:33:16 seen the one for yttrium barium

00:33:18 this is an image now of

00:33:20 the material and you can

00:33:22 very clearly see the layers

00:33:24 due to the, of the lanthanum

00:33:26 of the potassium nickel

00:33:28 fluoride structure that you've seen the picture of

00:33:30 but what's very

00:33:32 typical of this sample, and this sample

00:33:34 is the one that gave us 100%

00:33:36 Meissner effect, what's very

00:33:38 typical throughout this material

00:33:40 is all of these dark regions

00:33:42 which would indicate intergross

00:33:44 there's a large intergrowth

00:33:46 in the center of the picture

00:33:48 it's just that all I'm trying to point out

00:33:50 is that there's a lot of room for

00:33:52 inhomogeneity and intergross in these

00:33:54 materials which need to be

00:33:56 worked out

00:33:58 especially if we want to get

00:34:00 currents over long distances

00:34:02 okay, so

00:34:04 those are the kinds of measurements you do

00:34:06 to show that this is a bulk superconductor

00:34:08 now I want to go back to BCS

00:34:10 theory, and unlike the first

00:34:12 talk where BCS theory was just

00:34:14 alluded to, I'm going to derive

00:34:16 it

00:34:18 the derivation is on this

00:34:20 slide

00:34:24 I think

00:34:26 this is a good cartoon

00:34:28 to just remind you what BCS

00:34:30 theory tells you

00:34:32 the first, the top slide shows

00:34:34 free electrons in a normal

00:34:36 metal, the free electrons

00:34:38 are free to move about the metal as

00:34:40 they wish, you notice

00:34:42 that they are not, they are independent

00:34:44 of each other, and they also

00:34:46 don't seem to care too much about where

00:34:48 the waves in the material are

00:34:50 and these waves are of course the lattice vibrations

00:34:52 every once in a while they

00:34:54 bump into, a big wave comes along

00:34:56 where they bump into each other and they're

00:34:58 scattered, and they fall off into the

00:35:00 ocean, and that of course is the

00:35:02 resistance, now in contrast

00:35:04 when you undergo superconducting

00:35:06 phase transition

00:35:08 everything calms down, everything

00:35:10 is in phase, the lattice

00:35:12 vibrations are coupled to the electrons

00:35:14 that's the main point that I want to get across

00:35:16 is that the electrons

00:35:18 are coupled to the lattice

00:35:20 vibrations, and they are coupled

00:35:22 to each other, not quite as

00:35:24 close as I show here, but

00:35:26 I just put them on the same surfboard

00:35:28 alright, so the BCS

00:35:30 theory, as we heard in the

00:35:32 first talk, then tells us

00:35:34 that superconductivity

00:35:36 has to do with an attractive

00:35:38 interaction between electrons

00:35:40 and is mediated by lattice vibrations

00:35:42 now that theory

00:35:44 tells us some parameters which

00:35:46 we can go and measure, and I'm just

00:35:48 going to show you three experiments

00:35:50 following these predictions, which tell

00:35:52 us how close we come to the predictions

00:35:54 of BCS theory

00:35:56 now in the first case, we've already

00:35:58 seen that the transition temperature

00:36:00 depends on the density of states

00:36:02 at the Fermi level, of course it's going to

00:36:04 depend on the number of electrons

00:36:06 which are in the highest occupied energy

00:36:08 levels, since these are the conduction

00:36:10 electrons, it's going to depend

00:36:12 on the Debye temperature, or the

00:36:14 stiffness of the lattice, and it's

00:36:16 going to depend on the electron-phonon

00:36:18 interaction, and to get

00:36:20 TC as high as possible, we want

00:36:22 to get those three parameters as large

00:36:24 as possible, and I'll show you

00:36:26 how you can use the heat capacity

00:36:28 to measure those parameters

00:36:30 and see whether they're in agreement with

00:36:32 these predictions

00:36:34 in the second case, BCS theory

00:36:36 predicts, although

00:36:38 it's not always observed, I'm not sure

00:36:40 whether it predicts this, it predicts something

00:36:42 about the energy gap, I guess is what I

00:36:44 should say, the

00:36:46 superconducting ground state

00:36:48 is separated from the normal

00:36:50 state of the

00:36:52 normal metallic state

00:36:54 by an energy gap, and that

00:36:56 energy gap, of course, is going to be

00:36:58 larger as the transition temperature

00:37:00 gets higher, so for these high

00:37:02 TC materials, we might expect

00:37:04 a larger energy gap

00:37:06 and the other thing is that

00:37:08 since this is a first-order phase transition

00:37:10 we expect to see an anomaly

00:37:12 in the specific heat, and

00:37:14 furthermore, BCS theory

00:37:16 predicts that the changes

00:37:18 in heat capacity divided by

00:37:20 the electronic specific heat

00:37:22 times TC should equal

00:37:24 1.43, and we've

00:37:26 measured that value and we can see how

00:37:28 close it comes to what's

00:37:30 predicted, so let me first

00:37:32 start with the heat capacity

00:37:34 and tell you what

00:37:36 we've observed

00:37:38 so this is the heat capacity

00:37:40 now of the lanthanum strontium

00:37:42 copper oxide, and

00:37:44 immediately you come up with a problem

00:37:46 what you measured, or

00:37:48 what you'd like to plot is C over T

00:37:50 versus temperature, notice

00:37:52 I only plot up to 5 Kelvin

00:37:54 and there's a reason for that, if you look

00:37:56 at C over T versus temperature

00:37:58 I've shown on the top there

00:38:00 that it depends on a term

00:38:02 that I really don't want to discuss

00:38:04 in these materials there's a nuclear hyperfine

00:38:06 term which comes in

00:38:08 but it really depends on gamma

00:38:10 which is the electronic specific heat

00:38:12 and it depends on the phonon

00:38:14 or the lattice contribution

00:38:16 to the specific heat

00:38:18 now there's an interesting problem here

00:38:20 most superconductors

00:38:22 go superconducting at low temperatures

00:38:24 where the lattice contribution

00:38:26 is small

00:38:28 but once you get above 20 Kelvin

00:38:30 the lattice contribution starts to just go

00:38:32 right through the roof, and now

00:38:34 you've got this heat capacity

00:38:36 which is rising extremely rapidly

00:38:38 with temperature and you're trying to look for

00:38:40 a little blip

00:38:42 now, we've made 30 gram samples

00:38:44 we've done 100% precision measurements

00:38:46 and we do not see

00:38:48 an anomaly in the specific heat

00:38:50 at the transition temperature

00:38:52 but all is not lost

00:38:54 because we can look at the heat

00:38:56 capacity as a function of magnetic field

00:38:58 and what I've plotted here

00:39:00 on the low temperature end

00:39:02 is to show you that if we make the measurement

00:39:04 in 0 kilogals

00:39:06 and then in 70 kilogals

00:39:08 that we clearly see an increase

00:39:10 in specific heat

00:39:12 very typical for superconductors

00:39:14 if you apply a magnetic field

00:39:16 you increase the

00:39:18 I don't know how to say this

00:39:20 the specific heat always increases

00:39:22 alright, now

00:39:24 what you'd really like to do

00:39:26 is get to completely the normal state

00:39:28 you'd like to compare 0 kilogals

00:39:30 to the normal state

00:39:32 of this material, but unfortunately

00:39:34 70 kilogals is the highest

00:39:36 field we have, and we can't apply

00:39:38 a field of 500 kilogals

00:39:40 to get to the normal state

00:39:42 so we're stuck with this data

00:39:44 but we can take this data

00:39:46 and analyze it

00:39:48 and if we take

00:39:50 the superconducting state

00:39:52 minus this mixed state

00:39:54 in other words I'm going to subtract the two graphs

00:39:56 that I just showed you

00:39:58 which were at 0 kilogals

00:40:00 and 7 kilogals

00:40:02 if you subtract the two of them

00:40:04 you get the graph that's shown there

00:40:06 notice the precision of these measurements

00:40:08 because these are done

00:40:10 on such large samples

00:40:12 and such a sensitive instrument

00:40:14 you do see the anomaly

00:40:16 in the specific heat

00:40:18 it's clearly there

00:40:20 we've seen it on a number of samples now

00:40:22 and this anomaly occurs

00:40:24 right at the point where

00:40:26 you see the transition

00:40:28 into the superconducting state

00:40:30 the heat capacity tells us

00:40:32 when we analyze this data

00:40:34 that 77% of the

00:40:36 material is superconducting

00:40:38 if you ever doubted this

00:40:40 the heat capacity doesn't lie

00:40:42 this is the best measure of

00:40:44 the amount of superconducting material

00:40:46 I compare that to the magnetism

00:40:48 for this particular material

00:40:50 which is only 35%

00:40:52 some of our samples are

00:40:54 exactly the same

00:40:56 this particular one the magnetism is low

00:40:58 but you should remember that magnetism

00:41:00 always gives you the lower limit

00:41:02 to the amount of material

00:41:04 superconducting

00:41:06 we can extract a gamma

00:41:08 which is the electronic specific heat

00:41:10 which has to do with density of states

00:41:12 remember that was one of the parameters

00:41:14 that we wanted to measure

00:41:16 we obtained a value of 1.9

00:41:18 now I remind you that we're trying

00:41:20 to compare this to BCS

00:41:22 it predicts 1.43

00:41:24 values of 2 are often observed

00:41:26 for other superconductors

00:41:28 so the parameters that we get

00:41:30 from the specific heat

00:41:32 are consistent at least

00:41:34 it doesn't say that this is the mechanism

00:41:36 but they're consistent with the predictions of BCS theory

00:41:38 now I just have

00:41:40 two more measurements

00:41:42 that I want to show you

00:41:44 one is that of the energy gap

00:41:46 this has been done

00:41:48 in collaboration with Paul Richards

00:41:50 and Alex Zettel

00:41:52 to measure the energy gap

00:41:54 we do a reflectivity experiment

00:41:56 but since the energy gap is so small

00:41:58 we don't use

00:42:00 UV vis like chemists like to do

00:42:02 but we use far infrared

00:42:04 radiation

00:42:06 and what you observe is that

00:42:08 if the sample is at 6 Kelvin

00:42:10 and the energy of the radiation

00:42:12 is less than the band gap energy

00:42:14 we see

00:42:16 perfect reflectance

00:42:18 so all the light is reflected

00:42:20 as long as the light has energy

00:42:22 less than the band gap

00:42:24 and as soon as the energy gets near the band gap energy

00:42:26 then we begin to absorb

00:42:28 and you see a decrease

00:42:30 in reflectance

00:42:32 now the other thing you notice is that

00:42:34 as you raise the temperature from 6 Kelvin

00:42:36 on up to 36 Kelvin

00:42:38 that the band gap

00:42:40 gets smaller and smaller

00:42:42 you expect this as you start to close the gap

00:42:44 and go to the normal state

00:42:46 and in the normal state there is no energy gap

00:42:48 well from this experiment

00:42:50 we've determined that this gap

00:42:52 is 60 wave numbers

00:42:54 the BCS value

00:42:56 is predicted to be 87 wave numbers

00:42:58 we're a little confused

00:43:00 as to why we're low

00:43:02 but I should mention

00:43:04 that other measurements

00:43:06 of the energy gap which were made by

00:43:08 tunneling experiments

00:43:10 are much closer to the BCS prediction

00:43:12 for now I'm not sure

00:43:14 why we're coming out with such a low

00:43:16 value for the gap compared to the

00:43:18 BCS theory

00:43:20 alright so far we're

00:43:22 we've got some numbers

00:43:24 to compare with and I think one very nice

00:43:26 measurement to end on

00:43:28 is elasticity measurements

00:43:30 not one that you might think of doing

00:43:32 right away, these are done

00:43:34 mainly by Lincoln Born a graduate

00:43:36 student and Alex Zettel in the physics department

00:43:38 and we know

00:43:40 that superconductors have

00:43:42 vibrations which

00:43:44 couple to the electrons

00:43:46 and what's typically observed in superconductors

00:43:48 is soft mode behavior

00:43:50 or a vibration which begins

00:43:52 to decrease in

00:43:54 energy so that it can couple to the electrons

00:43:56 now in this

00:43:58 measurement we've taken this lanthanum strontium

00:44:00 compound and measured the Young's

00:44:02 modulus or the stiffness of

00:44:04 this material and what

00:44:06 you observe is that as you decrease

00:44:08 the temperature at first the material

00:44:10 gets stiffer simply because it's

00:44:12 shrinking, there's a contraction as you

00:44:14 cool it down but then suddenly

00:44:16 as it's getting, it gets stiffer

00:44:18 for a while but then it starts to get

00:44:20 soft again and I

00:44:22 think this is evidence for a

00:44:24 structural instability, we don't

00:44:26 know how this relates to the superconductivity

00:44:28 but it's very typical

00:44:30 to observe such structural

00:44:32 instabilities

00:44:34 in materials which superconduct

00:44:36 the A15's for instance show these

00:44:38 structural instabilities

00:44:40 okay then in summary

00:44:42 I think

00:44:44 we've done a number of measurements

00:44:46 and I think you'll appreciate that

00:44:48 these are measurements that

00:44:50 chemists can do as well

00:44:52 we've established the

00:44:54 upper limit of the resistivity at

00:44:56 10 to the minus 17 ohm centimeters

00:44:58 the Meissner effect shows

00:45:00 that 20 to 100 percent

00:45:02 the range depending on how you

00:45:04 prepare the material

00:45:06 the sample is superconducting but I point

00:45:08 out that the Meissner effect is very

00:45:10 useful in determining whether

00:45:12 the samples are homogeneous

00:45:14 or how homogeneous they are

00:45:16 the heat capacity gives a value

00:45:18 for C over gamma Tc

00:45:20 of 1.9 which is consistent

00:45:22 at least with the BCS theory

00:45:24 and shows that the fraction superconducting

00:45:26 is nearly 75 percent

00:45:28 we've got an energy gap of 60

00:45:30 wave numbers and evidence for a structural

00:45:32 instability so now

00:45:34 you might say well these measurements at least

00:45:36 are consistent with the predictions of

00:45:38 BCS theory but what

00:45:40 what is so special

00:45:42 about this material and I think it's two

00:45:44 things one

00:45:46 possibility is that the

00:45:48 by temperature the density of states

00:45:50 and the electron photon coupling are just

00:45:52 right that's one

00:45:54 possibility but what's more important

00:45:56 I think to these oxides

00:45:58 just in speculating a little bit

00:46:00 is that the oxide matrix

00:46:02 with the barium and or

00:46:04 with the rare earth

00:46:06 and the alkaline earth provide

00:46:08 a matrix which don't

00:46:10 allow any distortions

00:46:12 to occur around the copper

00:46:14 one of the problems with high temperature

00:46:16 superconductivity in the past

00:46:18 has been that every time you thought you were going to get

00:46:20 one it distorted well for some

00:46:22 reason these oxides aren't distorting

00:46:24 and it will be very exciting to find

00:46:26 out how

00:46:28 what the real

00:46:30 cause of the high temperature

00:46:32 superconductivity is

00:46:34 thanks a lot

00:46:36 applause