00:00:00 How's that for magic, huh?

00:00:10 Hal Wang received his Ph.D. degree from the University of Minnesota in 1981 under Professor

00:00:16 Lu Pinolet in the field of inorganic and organometallic chemistry.

00:00:21 After that, he worked with Professor Ken Suslick at the University of Illinois-Urbana in the

00:00:26 area of sodochemistry.

00:00:28 He has joined Jack Williams' group at the Argonne National Laboratory in 1983.

00:00:33 His current research interests include synthetic metals and solid-state chemistry.

00:00:38 Tonight, Hal will speak to us on the synthesis and characterization of high-Tc metal oxide

00:00:45 superconductors.

00:00:46 Please, Hal.

00:01:12 First, I apologize that I don't have slides ready.

00:01:18 I had to use an overhead projector.

00:01:23 I hope the latter is big enough for you to see.

00:01:27 Basically, I will be focused on two systems.

00:01:31 That is the Lansman-Strongstein copper oxide and the Yitran-Berry copper oxide.

00:01:38 I will go into very detail about how to make those materials.

00:01:43 I will be discussing how to use a solution technique to make those materials.

00:01:48 Although most of the lab have been using powder metallurgy, however, I would like to quote

00:01:55 one word from a material scientist at Argonne.

00:01:59 Basically, what he said is, if those materials are going to be made in industry, it will

00:02:07 be made by solution technique.

00:02:09 That's one of the excuses we use the solution technique.

00:02:14 Basically, chemists like to work with wet chemistry.

00:02:18 I will go through several solution techniques with you.

00:02:23 Then, later on, I will be talking about crystal growth, how to grow single crystal of those

00:02:29 metal oxides.

00:02:31 Later on, I will be talking about Yitran-Berry copper oxide and show you some structure of

00:02:36 those materials.

00:02:42 This is just basically a quick review.

00:02:48 The first inorganic superconductor was found around 1911.

00:02:52 It's mercury.

00:02:54 It moved all the way up to 1973.

00:02:58 The highest Tc is around 23 degrees Kelvin.

00:03:03 Since 1973, there are several different kinds of superconductors that have been studied.

00:03:10 Most of those materials have been studied in quite detail by physicists.

00:03:14 There are some superconducting polymers, like SNX.

00:03:19 There is metal oxide.

00:03:22 There are several other exotic metallic superconductors.

00:03:27 Our group has been involved with organic superconductors.

00:03:32 All those systems suffer one disadvantage.

00:03:35 Their Tc, in general, is much below 23 degrees Kelvin.

00:03:42 I just give you one example since we are working on the organic superconductor as well.

00:03:47 The organic superconductor, up to date, the highest Tc is around 8 degrees Kelvin under pressure.

00:03:55 Please skip the next one.

00:03:57 Just skip this one.

00:04:00 Next.

00:04:06 What I will be talking about today is basically focused on two compounds.

00:04:13 The first one is perovskite.

00:04:18 It has a general formula, ABO3.

00:04:21 A typical example would be KNiO3 or lanthanum copper oxide.

00:04:28 If you dope the lanthanum with barium, we call this a one-to-one contrast.

00:04:35 Basically, if you sum up the lanthanum and barium, it would be one.

00:04:39 That is the ratio of lanthanum and barium to copper.

00:04:42 We call that a one-to-one.

00:04:44 Another system is the double or layer perovskite.

00:04:47 That has a general formula as A2BO4.

00:04:53 A typical example would be K2NiO4 and La2CuO4.

00:04:59 If you dope those systems, you make a two-to-one contrast.

00:05:07 I am just going to go through this quickly.

00:05:09 The red line here indicates a unit cell.

00:05:13 This is KNiO3.

00:05:16 It is a perovskite structure.

00:05:18 Basically, it is cubic.

00:05:20 Next.

00:05:23 This is another schematic drawing.

00:05:25 It shows you all those perovskites.

00:05:27 Each square here stands for a copper with six oxygens.

00:05:35 This is the metal in the center.

00:05:37 Next.

00:05:43 This one is the K2NiO4 structure.

00:05:47 Basically, it has a very similar feature,

00:05:50 although those two layers, this is one layer

00:05:54 and this is another layer, those two layers

00:05:56 are separated by nano-cadmium.

00:05:59 Next, please.

00:06:05 This is a different view.

00:06:07 Basically, from this view, you can see the layer structure.

00:06:12 Those are the copper oxide layer.

00:06:18 Again, they separate by those metal cadmium.

00:06:22 Next.

00:06:26 The whole issue was really stirred up

00:06:28 by Reynolds and Mueller.

00:06:31 They published in one paper.

00:06:34 Basically, they give a formula,

00:06:36 bearing X less than 5 minus X, copper 5.

00:06:40 If you divide everyone by 5,

00:06:42 that's a one-to-one compound.

00:06:45 Later on, this compound was identified.

00:06:48 Actually, it's not a superconducting phase.

00:06:51 However, they do have the superconducting phase.

00:06:54 That's nicely shown in this graph.

00:06:58 Basically, the resistivity goes down

00:07:01 and you see a broad hump.

00:07:03 That's a fairly general feature

00:07:05 if your sample is impure,

00:07:07 mixed with those one-to-one compounds.

00:07:11 It goes through a nice transition

00:07:13 and based on the graph,

00:07:16 it did go to a zero conductivity.

00:07:18 Next.

00:07:23 This is the powder pattern quoted

00:07:25 from Bednor-Mueller's paper.

00:07:28 Basically, the powder pattern

00:07:31 shows there are two components.

00:07:34 The one labeled with red dots,

00:07:36 those are the one-to-one compounds

00:07:38 and was identified as impurity.

00:07:40 The other one, the two-to-one,

00:07:42 was identified as the superconducting phase

00:07:45 first by a Japanese group, Uchida et al,

00:07:49 and then later also by the same group,

00:07:51 Bednor-Mueller.

00:07:56 Now I'd like to talk about

00:07:58 the solution technique

00:07:59 how to make those materials.

00:08:02 The oxalate route

00:08:06 was the first procedure

00:08:08 that appeared in the literature.

00:08:10 The idea is very straightforward.

00:08:13 Basically, you take metal nitrate

00:08:17 in the right ratio

00:08:18 mixed with potassium oxalate

00:08:22 and in principle,

00:08:23 you get a nice precipitation of metal oxalate.

00:08:26 However, there's a problem.

00:08:29 Once you isolate the metal oxalate,

00:08:32 we call those blue powder,

00:08:34 and then you can go through

00:08:35 the firing condition

00:08:37 and eventually turn those materials

00:08:39 into a dark powder or powder form.

00:08:42 One problem is basically

00:08:45 if you look at the barrier analysis,

00:08:47 the barrier is very low,

00:08:49 and if you look at strontium analysis,

00:08:52 strontium is very low.

00:08:54 I would like to mention

00:08:56 that this is not just our own problem

00:08:59 because we jumped into the field

00:09:01 about four months ago.

00:09:03 I want to mention that

00:09:05 even Bednor and Mueller,

00:09:07 they do see the same kind of situation.

00:09:09 This is a quote from their publication

00:09:12 that when they tried to make

00:09:15 a one-to-one compound

00:09:16 using methanin 0.94 and barium 0.06,

00:09:21 instead of getting barium as 0.06,

00:09:24 their analysis indicated

00:09:26 barium is way low.

00:09:27 It's 0.01.

00:09:29 So we looked at a problem.

00:09:32 In their paper, basically,

00:09:34 they noticed a problem,

00:09:36 and then they mentioned

00:09:37 that their solution technique

00:09:39 needs to be perfect,

00:09:42 needs to be improved.

00:09:43 So I'd like to point out

00:09:45 several possible causes.

00:09:46 One is if you look at

00:09:48 the solubility of those

00:09:50 lanthanum-barium oxalate

00:09:52 versus the lanthanum-barium carbonate,

00:09:55 the oxalate is at least

00:09:57 four times more soluble

00:09:59 than the carbonate.

00:10:00 And then the situation gets even worse.

00:10:03 In the case of lanthanum,

00:10:05 first, lanthanum oxalate precipitates,

00:10:08 and then in the presence

00:10:10 of excess oxalate,

00:10:12 that's what you have to use,

00:10:14 potassium oxalate,

00:10:15 in order to get those materials

00:10:16 precipitated out.

00:10:18 The lanthanum oxalate

00:10:20 forms double sort

00:10:21 and precipitates out

00:10:23 with acrylide oxalate and hydrate.

00:10:28 Those double sort formations

00:10:31 basically compete with

00:10:33 barium and strontium oxalate

00:10:35 precipitation.

00:10:36 Next.

00:10:41 So because there are so many problems

00:10:43 with oxalate,

00:10:44 we decided we better forget

00:10:46 about the oxalate.

00:10:47 We start working on the carbonate.

00:10:49 And at first trial, again,

00:10:51 we used metal nitrate

00:10:52 mixed with potassium carbonate.

00:10:55 Again, this is the analysis

00:10:57 for strontium.

00:10:59 They've done by ICP analysis

00:11:01 that's inductively coupled plasma

00:11:04 or atomic emission spectrum.

00:11:07 This is what we try to put in.

00:11:10 For instance, the strontium

00:11:11 should be 1.09%.

00:11:13 We only get 50% incorporation.

00:11:18 So we start thinking about

00:11:20 turn out the answer

00:11:21 is very straightforward.

00:11:24 Those metal nitrate,

00:11:26 that's the strontium

00:11:27 copper nitrate,

00:11:29 those aqueous solution

00:11:30 are strongly acidic.

00:11:32 So there's a very basic problem

00:11:34 that simply the carbonate

00:11:36 has been protonated

00:11:37 instead of chelated

00:11:39 to those individual metal cations.

00:11:42 So the way to get around it

00:11:44 is to use neutralized solution.

00:11:47 We use potassium hydroxide.

00:11:50 And you don't want to use

00:11:52 ammonium water

00:11:53 because those ammonium

00:11:54 forms complex with copper.

00:11:57 There's a simple way to tell

00:11:59 that if your precipitation

00:12:01 is near completion,

00:12:03 basically just look at the filtrate.

00:12:06 If the filtrate still has

00:12:07 a light blue color,

00:12:09 that means you are losing,

00:12:11 most likely you are losing copper.

00:12:13 So if you're doing the right thing,

00:12:16 the filtrate should be

00:12:17 totally colorless.

00:12:20 Once you neutralize the solution

00:12:22 to pH around seven

00:12:24 and then you're tossing

00:12:25 potassium carbonate,

00:12:27 those compounds precipitate out

00:12:29 nicely as metal hydroxyl carbonate.

00:12:32 And then you have to

00:12:35 wash those down

00:12:36 and doing repeat centrifuge

00:12:38 and eventually filtration.

00:12:40 That process take quite some time

00:12:43 and that's probably

00:12:44 one of the disadvantage.

00:12:46 Just basically the washing

00:12:47 takes quite a while.

00:12:50 Those are the heat treatment

00:12:52 we've been using.

00:12:54 Basically follow another procedure

00:12:56 developed at Argonne

00:12:58 Material Science Commission.

00:13:00 It has been calcined

00:13:01 at 820 degree for two hour

00:13:03 and then sintering

00:13:05 and then finally oxygen annealing.

00:13:07 And if you're not,

00:13:08 if you look at analysis

00:13:10 of those final product

00:13:13 after you go through the hydrolysis,

00:13:15 after you go through

00:13:16 all those pH balance technique,

00:13:19 the analysis indicate

00:13:21 it's fairly close

00:13:22 to what you want to put in.

00:13:24 For instance,

00:13:27 when X equals to 0.2,

00:13:29 we get about 98% incorporation.

00:13:33 That's as good as you can get.

00:13:35 That's within our instrument resolution.

00:13:38 And some of the number

00:13:40 is slightly low.

00:13:41 For instance,

00:13:42 the 0.15 is 2.99.

00:13:45 It should be 3.3.

00:13:47 Those are,

00:13:49 one possible reason is

00:13:51 while you're washing

00:13:52 those metal hydroxy carbonate,

00:13:54 you don't want to wash it too much

00:13:57 because basically once you wash too long,

00:14:00 all those carbonate

00:14:01 has a finite solubility

00:14:03 so they have to go to solution.

00:14:06 Next.

00:14:11 Okay.

00:14:12 So those are about

00:14:13 the co-precipitation technique.

00:14:15 There's another solution technique

00:14:17 that has been developed

00:14:19 at Argonne

00:14:20 by two ceramicists,

00:14:22 Brian Freidenmayer

00:14:24 and Roger Popol.

00:14:32 Basically,

00:14:33 most of you might find

00:14:35 this geofoundation technique

00:14:37 is kind of interesting.

00:14:39 What you want to do

00:14:40 is again,

00:14:41 you can mix

00:14:42 three different nitrates.

00:14:44 You can either mix it with nitrate

00:14:46 or you can put in

00:14:47 individual metal

00:14:48 and simply add nitric acid to it.

00:14:51 And then you mix it

00:14:52 with ethylene glycol

00:14:53 and citric acid.

00:14:57 And those are the percentage

00:14:59 you can add it.

00:15:01 And then you have to

00:15:02 go through those steps.

00:15:05 The first step is to heat up

00:15:07 to 90 degrees

00:15:09 for several hours.

00:15:11 And during the heating process,

00:15:14 nitrous oxide gas,

00:15:17 it's a brown smoke

00:15:18 coming out.

00:15:20 And at the same time,

00:15:21 the ethylene glycol

00:15:23 starts to polymerize.

00:15:26 And then by the end

00:15:28 of the two hours,

00:15:30 you let the solution

00:15:31 slowly cool to room temperature.

00:15:34 And during the cooling process,

00:15:37 the whole mixture

00:15:38 sets into a gel.

00:15:42 And those metal ion

00:15:44 is very finely suspended

00:15:47 in those gel.

00:15:49 And then the next step

00:15:50 is the most interesting step.

00:15:52 You basically take those gel

00:15:54 and put it into a Pyrex beaker

00:15:56 you can heat up on a hot plate.

00:16:01 Those gel would decompose

00:16:03 into brown powder.

00:16:04 If the temperature gets too hot,

00:16:07 the gel might burn.

00:16:09 And we have seen one way

00:16:12 those ceramics,

00:16:17 ceramic canisters

00:16:18 handle those materials.

00:16:19 You can even light the gel

00:16:21 with a match.

00:16:23 And then the flame

00:16:24 just burns through those gel.

00:16:27 And after that,

00:16:30 you get a nice brown powder

00:16:32 and then you can do the board meal

00:16:34 and then go through

00:16:35 the same heat treatment.

00:16:39 Okay, those are the ICP analysis

00:16:42 based on the ICP analysis

00:16:45 for compound made

00:16:46 by geoformation technique.

00:16:50 Basically, if you look at

00:16:52 the overdose number,

00:16:53 for instance,

00:16:54 just focus on the strontium level,

00:16:57 2.19 and you get 2.36,

00:17:02 3.3, 3.0, 4.4 and 4.6.

00:17:06 They are fairly close.

00:17:08 There are two good references

00:17:10 from Journal of American Ceramic Society

00:17:15 and another one is Polymetallurgy.

00:17:17 They describe this kind of procedure

00:17:19 in very detail.

00:17:21 So those are two good references.

00:17:23 Next, please.

00:17:25 And this is a conductivity curve,

00:17:28 probably one of our best run.

00:17:31 And those are sample made

00:17:33 by geoformation technique.

00:17:35 You get a nice linear decrease

00:17:37 in the resistivity

00:17:39 and eventually it goes

00:17:40 through a very sharp transition.

00:17:42 The material do go to a zero resistivity.

00:17:46 Next.

00:17:49 I just want to point out,

00:17:51 once you get your stoichiometry right,

00:17:55 another thing you have to be careful

00:17:57 is when you do the fire heat treatment

00:18:01 or the oxygen annealing.

00:18:04 There's a study done by a Japanese group,

00:18:06 Uchida et al,

00:18:08 and basically they indicate

00:18:10 using different oxygen pressure,

00:18:13 they see quite different conductivity behavior.

00:18:18 They use 0.1 torr oxygen pressure

00:18:24 and they get a nice conductivity curve.

00:18:26 Otherwise, at lower oxygen pressure,

00:18:29 they get those brown hum

00:18:31 that indicate another impurity phase

00:18:33 that's growing.

00:18:35 Next, please.

00:18:38 Okay, so much about the solution technique.

00:18:41 I want to mention a few words

00:18:42 about the crystal growth.

00:18:45 By the time we start working on this field,

00:18:48 we don't really know if we can

00:18:50 actually get single crystal

00:18:52 of those materials.

00:18:53 It's actually quite challenging

00:18:55 to find some way to get single crystal.

00:18:59 So you cannot use typical melted

00:19:02 and then let it be crystallized.

00:19:04 You cannot use those techniques.

00:19:06 You have to go through other

00:19:07 high-temperature techniques.

00:19:09 We try to use Froude's technique.

00:19:11 And Froude's growth is quote from a book.

00:19:15 Froude's growth is the growth of crystal

00:19:18 from molten salt at high temperature.

00:19:22 Basically, the idea is very similar

00:19:24 to recrystallization,

00:19:27 but instead of using aqueous

00:19:29 or organic solvent,

00:19:31 you take a Froude's material

00:19:33 and you heat up to 900 or 1,000 degrees.

00:19:40 Okay, those are the conditions

00:19:42 we've been trying.

00:19:44 What we have done is we make

00:19:46 those brown powder

00:19:48 as lanthanum 1.85, strontium 0.15.

00:19:51 So we go through all the solution technique

00:19:54 and eventually get a brown powder.

00:19:58 We tried several.

00:19:59 The first one we tried is lead fluoride

00:20:01 at 1,000 degrees.

00:20:03 And it turned out the material

00:20:04 is totally decomposed.

00:20:06 The product isolate was identified

00:20:08 as lanthanum fluoride by powder X-ray.

00:20:12 And then later on, we tried a mixture

00:20:13 of lead fluoride and lead oxide,

00:20:16 and it turned into lanthanum oxide fluoride.

00:20:21 So we decided we're going to get rid

00:20:23 of all the fluoride

00:20:25 and try just simply just the lead oxide.

00:20:29 So we tried lead oxide

00:20:30 mixed in with a very small amount

00:20:32 of lead dioxide.

00:20:35 Turned out the technique did work,

00:20:37 and we get many small X-ray-sized crystals.

00:20:42 So far, the crystal size has been limited

00:20:46 to about this is the typical dimension.

00:20:49 So it's suitable for X-ray single-crystal study.

00:20:53 However, we're still trying

00:20:54 to improve the technique

00:20:57 as far as getting larger crystal.

00:21:01 Okay, again, this is a single-crystal structure

00:21:03 of this compound.

00:21:05 All I want to point out here is

00:21:07 if you focus on the copper,

00:21:09 or if you look at here,

00:21:13 each copper cooperate unit,

00:21:17 it's a 4 plus 2 bipyramidal structure.

00:21:20 Basically, it has four short copper-oxygen bonds

00:21:25 and then two long copper-oxygen bonds.

00:21:30 Next.

00:21:33 One problem we have for these crystal blobs

00:21:37 is you can use one technique,

00:21:40 and you can use a C over A ratio.

00:21:43 C is the long axis in the previous slide,

00:21:46 and A is the short axis.

00:21:49 You can use a C over A ratio

00:21:52 to identify what's the doping level in your sample.

00:21:57 So if those dots were obtained

00:22:01 based on X-ray powder technique,

00:22:04 you can index the powder and assign C,

00:22:09 and you can capture the C over A ratio.

00:22:12 Basically, it shows a relationship

00:22:14 between...

00:22:16 It has a curve

00:22:18 between the C over A ratio

00:22:20 versus your doping level.

00:22:23 What we have found is our single-crystal

00:22:26 has a lower doping level.

00:22:28 It actually is somewhere around 0.08.

00:22:30 It's somewhere around this part.

00:22:33 And we measured the TC of those single-crystal,

00:22:37 and those single-crystal do superconduct.

00:22:41 The properties were measured by RF penetration.

00:22:43 That's one of the techniques

00:22:45 you can determine superconductivity.

00:22:47 And the TC was around 8.5.

00:22:52 So that's another thing we're trying to improve.

00:22:54 We're trying to incorporate

00:22:56 the right amount of strontium level.

00:22:59 We start from strontium 0.15.

00:23:01 However, we end up getting strontium 0.08.

00:23:05 So under the condition we are using,

00:23:07 there's a thermodynamic equivalent

00:23:09 to determine how much strontium you can do,

00:23:12 even if you start from a different strontium level.

00:23:16 This is another single-crystal X-ray study

00:23:20 done by Eric Geiser and Mark Dino.

00:23:24 Basically, if you focus on one of the bright refractions

00:23:28 at a lower temperature,

00:23:30 you see the bright peak start to split

00:23:32 into several components.

00:23:34 And this is just another bright refraction,

00:23:37 and it splits into several peaks.

00:23:39 And you can plot it out three-dimensional.

00:23:41 And this is a single peak

00:23:43 and a low-temperature start for the cell.

00:23:47 The 295K run was actually done

00:23:51 after the 101K,

00:23:53 so the process is totally reversible.

00:23:56 However, by our...

00:23:58 Remember, this is a very small single-crystal,

00:24:01 and by our X-ray technique,

00:24:03 we would not be able to solve the X-ray structure.

00:24:10 The work is actually done by Kava et al.

00:24:14 at Bayer Lab.

00:24:15 It's very nice work.

00:24:16 And they use a neutron powder.

00:24:19 And basically, what they identify is,

00:24:22 at low temperature,

00:24:23 those materials go through a phase transition.

00:24:26 At higher temperature, it's tetragonal.

00:24:28 This is tetragonal.

00:24:30 This is a very schematic

00:24:32 and somewhat oversimplified drawing

00:24:35 just to indicate what's happened

00:24:37 at a low temperature is

00:24:39 it changed to an osirombic cell,

00:24:41 and the unicellular area is almost twice

00:24:43 compared to the tetragonal phase.

00:24:46 Next.

00:24:50 Okay, now I'm going to talk about

00:24:52 the yttrium-barium copper oxide system.

00:24:56 The system was first studied by Chu and Wu et al.,

00:25:00 and they gave a formula

00:25:02 as yttrium 0.6, barium 0.4,

00:25:07 copper oxide.

00:25:09 And this is a two-to-one they're trying to make.

00:25:12 This is another one-to-one compound

00:25:14 they're trying to make.

00:25:17 The first thing we did is we tried solution technique,

00:25:20 trying to put in those kind of ratio,

00:25:23 and it turned out that didn't work

00:25:25 because the stoichiometry is off.

00:25:30 Again, it's by Bellet et al.

00:25:33 They indicate the phase is really a one-to-three,

00:25:37 or if you look at it, it's a three-to-three material.

00:25:42 And so we tried powder metallurgy technique,

00:25:47 and basically those were,

00:25:50 those are the heat treatment condition

00:25:53 which follow Bellet's procedure,

00:25:55 and it works very nicely.

00:25:57 This is a typical powder pattern you can get

00:26:01 after it goes through those heat treatment.

00:26:04 Basically, we have seen two phase.

00:26:07 One is the major superconducting phase,

00:26:11 or also rhombic space group.

00:26:14 And the other one is so-called green phase

00:26:17 because in the initial Chu's publication,

00:26:20 they mentioned the compound is green.

00:26:23 Turn out green compound is one of the impurities.

00:26:26 There could be more than one green compound in the system.

00:26:31 The green compound has a formula Y2BACUO5.

00:26:36 It's reminiscent of Y2CU2O5,

00:26:40 and those materials are magnetic,

00:26:43 so you can use ESR to identify the impurity.

00:26:47 Basically, if you do a quantitative,

00:26:51 putting same amount of material,

00:26:53 you can monitor the noise level,

00:26:55 and you can sort off.

00:26:57 That's another way you can identify the impurity.

00:27:02 This is a single crystal structure

00:27:04 done by Hazen and Larry Finder.

00:27:06 It's published in the last ACA meeting.

00:27:11 Basically, what they see is it's a tetragonal.

00:27:17 Unistel, and there are two barium, one yttrium,

00:27:21 and there are some oxygen.

00:27:23 There are some vacant oxygen they cannot locate.

00:27:26 All the copper oxygen layers are puckled,

00:27:30 and there's a one-fret layer.

00:27:32 However, this material is tetragonal.

00:27:34 It's not also rhombic.

00:27:36 Next.

00:27:38 This is a neutron powder diffraction

00:27:45 done by Mark Vino, Jim Jorgensen, Dave Hintz,

00:27:49 at Argonne, at the IPMS facility.

00:27:54 Basically, one interesting feature

00:27:58 is the oxygen at the 1-6 and the 5-6 position

00:28:06 along the z-axis has been totally refined,

00:28:10 and has been refined close to one occupancy.

00:28:16 There are oxygen missing on the basal

00:28:20 and the top layer.

00:28:24 Next.

00:28:28 This is a different view.

00:28:30 We put three unistel together.

00:28:35 Those are barium, and yttrium will be underneath here,

00:28:39 and there will be another yttrium on top.

00:28:43 Basically, this indicates there's a layer

00:28:45 of those copper oxide.

00:28:47 You can look at each copper.

00:28:50 This is copper.

00:28:51 Each copper is surrounded by five oxygen.

00:28:56 That's one, two, three.

00:28:58 Another one did not show in the fifth.

00:29:01 It's a 4 plus 1 square pyramidal structure.

00:29:05 There's one layer, and that's another layer.

00:29:08 Those two layers were interconnected

00:29:10 by a COO2 layer.

00:29:15 Next.

00:29:18 This is, again, a different schematic drawing.

00:29:22 You can view the layer better in this drawing.

00:29:27 You can basically see the layer around here,

00:29:29 another layer around here,

00:29:31 and the yttrium sits in the center.

00:29:36 It might raise a question that if you replace the yttrium

00:29:39 with some other metal,

00:29:42 what effect you're going to have.

00:29:48 Okay, just skip this one.

00:29:55 This is work done by Dick Hinks et al,

00:29:57 and I believe Dr. Imler will be talking a lot more

00:30:01 about the substitution.

00:30:03 I just want to mention a few words.

00:30:05 When you're putting in different lanthanum elements

00:30:08 to replace the barium,

00:30:10 there's a whole bunch of material.

00:30:11 They all superconduct around 93 to 95 degree Kelvin.

00:30:16 That also fits with the total structure.

00:30:19 Both structures indicate lanthanum-strong yttrium

00:30:23 sits in the middle, and those are nice.

00:30:26 As far as the metal ratio,

00:30:28 those are nice 1, 2, 3 stoichiometry material.

00:30:31 Compared to the lanthanum-strong copper oxide,

00:30:34 those are solid solution,

00:30:36 and those are, if you look at the metal ratio,

00:30:40 those are nice stoichiometric material.

00:30:43 So you see a lot of TC variation,

00:30:46 but you do not see the TC variation

00:30:52 in this type of system.

00:30:56 Okay, as a summary, my conclusion is

00:30:59 we indicate this type of structure,

00:31:02 that's a yttrium-barium copper oxide,

00:31:05 and then the other type,

00:31:06 that's the lanthanum-strong zinc copper oxide.

00:31:09 They both indicate superconductivity.

00:31:12 So as a chemist, one must expose some question to ourselves

00:31:16 that we have to go into literature

00:31:18 and find out some similar structure.

00:31:22 There's a lot of possible candidates,

00:31:25 so it's possible by this time next year

00:31:29 that probably a lot more new superconductors will be made.

00:31:33 So finally, I would like to thank the chairman

00:31:40 for the invitation,

00:31:41 and I would like to thank all the colleagues

00:31:46 at Argonne National Lab.

00:31:48 I would like to thank our funding agents

00:31:50 at the Department of Energy.

00:31:52 Thank you for your attention.

00:32:06 Our next speaker will be Ed Egbert,

00:32:09 who is a manager of the Materials Science Department

00:32:12 at IBM's Olmhudin Research Center.

00:32:15 Egbert has a Ph.D. in Chemistry at Princeton University

00:32:19 and then joined IBM in 1973

00:32:22 and spent a good part of his research career

00:32:25 working on organic metals and superconductors until recently.

00:32:30 Now he's fully converted

00:32:32 and is now working totally on inorganic superconductors.

00:32:35 Ed.

00:32:40 First, before I start,

00:32:42 I'd like to also thank Valerie Cook

00:32:45 for putting this program together.

00:32:47 I think it's important that we get this field

00:32:50 in front of the chemical community

00:32:53 so that you can react to it

00:32:55 and perhaps share in some of the excitement we have here,

00:32:57 and I hope we can leave you with just a little bit of that.

00:33:00 Also, I'd like to thank Mary Good

00:33:02 and the ACS leadership and its organization

00:33:06 for responding so quickly

00:33:07 and being flexible enough to allow this to happen.

00:33:10 Thank you.

00:33:18 Let me try to not cover the same ground

00:33:23 that the other speakers have,

00:33:25 but maybe emphasizing different points.

00:33:31 This field, as you know,

00:33:33 is a rather new one

00:33:36 from the perspective of improving TC.

00:33:39 There's always been a lot of work going on

00:33:41 in superconductivity,

00:33:43 and this paper, as you all know now,

00:33:46 by George Bednorz and Alex Mueller

00:33:49 at the IBM Zurich Research Laboratory,

00:33:52 really changed many of our lives

00:33:55 in a way that we could have not expected at the time.

00:33:58 I certainly didn't when this paper appeared,

00:34:01 and it was only with the amazing improvements

00:34:04 upon this material

00:34:05 that we began to take really serious note of it.

00:34:08 At that time, they had raised TC

00:34:11 from a value which had stayed pretty stagnant

00:34:14 for about 10 years

00:34:17 for about 23 degrees

00:34:19 up to a value in the mid-30s.

00:34:22 What I'd like to try to do

00:34:24 is to give you a little bit of their perspective

00:34:27 and how they arrived at it.

00:34:29 You know, how did they stumble upon this?

00:34:31 It certainly was somewhat of an empirical

00:34:35 go-out-and-test material,

00:34:37 but they were guided by the fact

00:34:39 that they were guided by some very nice chemistry

00:34:42 that had gone on prior to that.

00:34:45 These are things that are cited in this paper.

00:34:47 It's a very nice paper to read.

00:34:49 It may not be totally correct in hindsight,

00:34:52 but this is the considerations they used.

00:34:54 One is the work that we heard from Sleight

00:34:57 and his co-workers at DuPont,

00:34:59 was that the barium lead-bismuth oxide,

00:35:03 while today we might not think that's a high TC,

00:35:07 it was a surprising material.

00:35:10 That is because, as we heard from the first talk,

00:35:13 that in BCS theory,

00:35:15 we expect that TC should scale

00:35:18 with the density of states at the Fermi surface

00:35:21 and the value of the electron-phonon coupling.

00:35:24 That was considered to be small

00:35:26 from various measurements in this material,

00:35:28 that is, the density of states.

00:35:30 Therefore, it seemed that it must have

00:35:32 a strong electron-phonon interaction,

00:35:35 that is, the electrons that are going to give us

00:35:37 the Cooper pairs for superconductivity

00:35:40 are going to interact with the lattice in a strong way.

00:35:47 Now, one of the things that Alex Mueller was interested in,

00:35:52 and this was touched on in some of the earlier talks,

00:35:55 is that we'd like to be near an unstable situation

00:35:58 where there's a lattice distortion.

00:36:00 Now, there are a variety of ways you can conceive

00:36:02 of doing that chemically,

00:36:04 and one of the things that he wanted to explore

00:36:06 was the Jahn-Teller effect.

00:36:08 He wanted to get materials that are on the verge of distorting.

00:36:12 This would be a means of trying to enhance

00:36:15 the coupling of the electrons to this lattice distortion

00:36:18 and hopefully raise TC.

00:36:20 So he certainly was trying to raise TC in this work.

00:36:23 He started off with ion compounds,

00:36:25 that is, ion 4 and nickel 3.

00:36:28 And about the time that he was starting to explore

00:36:31 the copper 2 ions,

00:36:33 they became aware of some very nice work

00:36:35 that's been going on in France by Michel and Ravel,

00:36:38 which had explored a whole range

00:36:41 of the lanthanum copper oxides,

00:36:43 in which they were able to take these materials

00:36:46 systematically by doping with the alkaline earths

00:36:51 from semiconductors to metallic materials.

00:36:53 And certainly you'd like to start off

00:36:55 with a metallic material

00:36:57 when you're looking for superconductivity.

00:36:59 And so in the spring of 86,

00:37:02 they found a composition

00:37:04 of lanthanum barium copper oxide in the mid-30s

00:37:07 that was superconducting.

00:37:09 And after that, everything broke out.

00:37:11 Many groups jumped in.

00:37:13 There was immediate progress

00:37:15 in December-January time frame.

00:37:17 A number of papers appeared.

00:37:19 And the structure was determined,

00:37:21 as you've seen already, to be this

00:37:23 layered perovskite shown here,

00:37:26 in which doping with M

00:37:29 being either barium, strontium, calcium,

00:37:32 would give you superconducting materials.

00:37:34 It was the strontium in a range

00:37:36 of between 0.1 and 0.2

00:37:38 that gave the best values.

00:37:45 I won't go into this in much detail.

00:37:47 You've seen that a lot.

00:37:49 I just wanted to illustrate

00:37:51 the two types of structures,

00:37:53 because I'm going to relate it back

00:37:55 to the yttrium compounds,

00:37:57 which caught most of our attention.

00:37:59 The basic perovskite structure here,

00:38:01 ABX3,

00:38:03 is the idealized cubic structure,

00:38:05 and we have a modification of that.

00:38:07 We still have the octahedrally

00:38:09 coordinated coppers,

00:38:11 but it's a lower-dimensionality structure.

00:38:13 What led him to yttrium?

00:38:15 One of the things that he specializes in

00:38:17 is applying pressure to superconductors.

00:38:19 When you apply pressure,

00:38:21 most superconductors transition to decrease.

00:38:23 In these materials, however,

00:38:25 he was able to raise it above 50 degrees

00:38:27 by applying pressure.

00:38:29 So there was the thought,

00:38:31 could he use smaller atoms here,

00:38:33 replacing the large lanthanum,

00:38:35 perhaps, with yttrium,

00:38:37 and mimic, perhaps,

00:38:39 an internal pressure?

00:38:43 One of the first things he tried

00:38:45 was to look at various combinations,

00:38:47 and the true composition here

00:38:49 was effectively the same

00:38:51 composition that Bedrock's and Mueller did,

00:38:53 with different dopants.

00:39:03 Here, he had a much higher amount

00:39:05 of barium in the material,

00:39:07 but as he fully realized,

00:39:09 and all of us who tried to repeat this work,

00:39:11 is he had a multiple-phase material.

00:39:13 This is the x-ray part of the fraction pattern.

00:39:15 At least three phases

00:39:17 appeared to be present,

00:39:19 and only a small fraction of the material

00:39:21 actually went superconducting.

00:39:23 And having a material that was so categorically different,

00:39:25 up to 90 degrees,

00:39:27 it was all kinds of speculations.

00:39:29 And the theorists were having a field day,

00:39:31 interfacial effects, metastable phases,

00:39:33 and whatnot.

00:39:45 Well, as soon as we had seen

00:39:47 Chew's preprint in March,

00:39:49 we, of course, like many other groups,

00:39:51 went off to make it.

00:39:53 And we noticed something that he'd noticed.

00:39:55 Material's not homogeneous.

00:39:57 It has black and green material in it,

00:39:59 and we applied

00:40:01 microscopic

00:40:03 analytical techniques,

00:40:05 like electron microprobe

00:40:07 and transmission electron microscopy,

00:40:09 to try to get at the approximate atomic ratios.

00:40:11 And also applying

00:40:13 x-ray diffraction techniques

00:40:15 and varying the preparation techniques,

00:40:17 we were able to see some enhancement of lines

00:40:19 which we could index

00:40:21 to perovskite-like structures.

00:40:23 And so, considering this,

00:40:25 we decided to look at

00:40:27 our best guesses at the ratios.

00:40:29 It was a 2 to 1 to 1,

00:40:31 or a 1 to 2 to 3.

00:40:33 You see, we differ from the AT&T group.

00:40:35 They go 2 to 1 to 3.

00:40:37 We're here 1 to 2 to 3.

00:40:39 So don't get confused.

00:40:41 Anyway,

00:40:43 lo and behold,

00:40:45 when we cooked these up,

00:40:47 the green material,

00:40:49 as expected,

00:40:51 was the 2 to 1 to 1.

00:40:53 It was insulating,

00:40:55 and the black material now

00:40:57 that came out

00:40:59 was a metallic compound.

00:41:05 You'll hear more of this

00:41:07 in the talk later on,

00:41:09 but I'll just show this

00:41:11 to illustrate

00:41:13 the phase diagram

00:41:15 approximately for this superconducting material.

00:41:17 Chu started here in blue.

00:41:21 And so, when he formed this material,

00:41:23 he did get superconducting material,

00:41:25 but it wasn't the predominant material.

00:41:27 It was this green phase,

00:41:29 and also you'd expect some copper oxide to form.

00:41:31 However,

00:41:33 if you're in that

00:41:35 triangular region,

00:41:37 almost anything we have made

00:41:39 will have some superconductivity.

00:41:41 If you're outside that region,

00:41:43 you'll only get non-superconducting materials.

00:41:51 Now, when you make

00:41:53 the yttrium compound

00:41:55 with this 1 to 2 to 3 stoichiometry,

00:41:57 you get a much simpler x-ray diffraction pattern.

00:42:01 This is an orthorhombic pattern.

00:42:05 We can index this pattern

00:42:07 to a 3-fold elongation

00:42:09 of the idealized cubic perovskite.

00:42:13 Now, let me show this.

00:42:15 I won't go again repeating

00:42:17 what you've seen already,

00:42:19 but let me give you some different features

00:42:21 and show you some different aspects about it.

00:42:23 This is the same type of unit cell

00:42:25 you've seen from the AT&T presentation

00:42:27 that Don Murphy gave.

00:42:31 We have an ordering

00:42:33 of the barium-yttrium-barium triplets

00:42:35 in the unit cell.

00:42:37 I'm going to show you here.

00:42:39 I hope it can come out.

00:42:43 This is a high-resolution

00:42:45 transmission electron

00:42:47 micrograph

00:42:49 showing, in fact, these triplet pairs.

00:42:51 Let's see where we can focus.

00:42:53 You can see here,

00:42:55 barium-yttrium-barium.

00:42:57 Notice that

00:42:59 these triplets are

00:43:01 interspersed with lighter regions.

00:43:03 Those are regions of low

00:43:05 charge density.

00:43:07 In fact, it's quite consistent

00:43:09 with the picture

00:43:11 that we heard that is emerging

00:43:13 on the structure of these,

00:43:15 is that there is a collapse.

00:43:17 In fact, these bariums move inward,

00:43:19 collapse inward around the yttrium structure.

00:43:21 In fact, they move about three-tenths of an angstrom,

00:43:23 we would estimate from our measurements,

00:43:25 in towards the yttrium

00:43:27 and leaving

00:43:29 an expansion of the barium

00:43:31 at a distance here.

00:43:33 Also, this region here, as we heard earlier,

00:43:35 has a partial oxygen

00:43:37 occupancy.

00:43:41 I'm going to turn to some of the

00:43:43 preparative procedures, but this question

00:43:45 of where the oxygen vacancies are

00:43:47 and how much oxygen in it

00:43:49 is a very important factor in these materials,

00:43:51 one that I think we still have a lot to learn.

00:44:01 Now, the preparation

00:44:03 of these materials is really quite easy.

00:44:05 I think

00:44:07 once you know the tricks,

00:44:09 I think almost anyone can make it.

00:44:11 You start

00:44:13 with powders

00:44:15 of the pure

00:44:17 constituents, yttrium oxide,

00:44:19 barium carbonate, copper oxide.

00:44:21 We start with barium carbonate because

00:44:23 the barium oxide is fairly hydroscopic

00:44:25 and barium carbonate is a finer

00:44:27 powder. You can ball mill

00:44:29 or use a mortar and pestle

00:44:31 and mix them thoroughly

00:44:33 and then go, as you saw earlier,

00:44:35 through a heating process.

00:44:37 We've heated 900 degrees

00:44:39 in oxygen using aluminum

00:44:41 bolts. In silica

00:44:43 bolts, we find an extensive reaction.

00:44:45 We typically go through

00:44:47 a regrinding just to make sure that we're going to get

00:44:49 single-phase material

00:44:51 and then press them into

00:44:53 pellets and sit through them for a few hours

00:44:55 to get something durable that we can do

00:44:57 measurements on. Now one thing

00:44:59 that I think hasn't been really well

00:45:01 appreciated is that it's this last

00:45:03 step that's very critical.

00:45:05 If you don't do that right,

00:45:07 you're not going to get the material that you'd

00:45:09 like to get. It's a very important

00:45:11 step and it relates back to these oxygen

00:45:13 deficiencies.

00:45:15 Let me show you this graphically.

00:45:17 If you're patient

00:45:19 and

00:45:21 you allow your oven to cool down

00:45:23 slowly from your

00:45:25 tempering temperature, you get

00:45:27 a plot

00:45:29 such as this.

00:45:31 You see the decrease in resistivity

00:45:33 and then right around

00:45:35 the point for the transition,

00:45:37 in this case, starting around 95 degrees,

00:45:39 you see the rapid drop

00:45:41 and then

00:45:43 zero resistance here around 92 degrees

00:45:45 Kelvin.

00:45:47 In our early days

00:45:49 of making these materials, we weren't

00:45:51 always so patient.

00:45:53 We grabbed the materials and pulled them out of the ovens

00:45:55 rapidly and this is what you

00:45:57 get.

00:45:59 This is a sister pellet. Two pellets

00:46:01 in the same oven made the same way.

00:46:03 Both single phase materials.

00:46:05 Clearly, if you want a superconductor

00:46:07 bubble liquid nitrogen, you better pay attention

00:46:09 to this property.

00:46:11 Also, we've talked about oxygen being

00:46:13 important.

00:46:15 If you

00:46:17 anneal pellets

00:46:19 under the same conditions as this

00:46:21 one here,

00:46:23 you can see that in oxygen

00:46:25 we have a lower partial pressure of oxygen.

00:46:27 In air, we have a lower partial

00:46:29 pressure of oxygen.

00:46:31 You also get a depression of the transition

00:46:33 and it's boiling.

00:46:35 I think these results are

00:46:37 adding weight to the fact that

00:46:39 a lot of us suspect that

00:46:41 not only the amount of oxygen in that structure

00:46:43 is important, but also

00:46:45 where those vacancies go.

00:46:47 I don't think at this stage we've sorted that out.

00:46:49 It is a handle we have on these materials

00:46:51 and one that we need to explore.

00:46:59 Knowing the

00:47:01 structure of the

00:47:03 high temperature superconductor,

00:47:05 you can then go about trying to make

00:47:07 new materials, hopefully ones that

00:47:09 represent systematic

00:47:11 variations on that

00:47:13 structure. You only have

00:47:15 to go to the periodic table and

00:47:17 find what you can order quickly

00:47:19 and try it.

00:47:21 I'm going to tell you

00:47:23 some of the different types of changes

00:47:25 to give you an example.

00:47:27 I think all of the groups involved

00:47:29 are pretty much mixing anything they can get

00:47:31 their hands on. No one has preconceived notions

00:47:33 of what will work and what won't work.

00:47:37 The yttrium,

00:47:39 one can

00:47:41 replace it with a variety of trivalent

00:47:43 ions. You saw some of the substitutions.

00:47:45 I'll talk about some of them

00:47:47 myself. And then one can make

00:47:49 replacements in the battery

00:47:51 and also in the copper.

00:47:55 Before I do that,

00:47:57 let me

00:47:59 try to tell you how

00:48:01 we're going to give you values of

00:48:03 transition temperatures.

00:48:05 I think that's important because

00:48:07 of all the different reports that are out there

00:48:09 about the onsets of superconductivity

00:48:11 and high temperature

00:48:13 conditions.

00:48:15 This is a

00:48:17 material in which

00:48:19 we could draw a linear line.

00:48:21 I don't know if I'm getting it right.

00:48:25 I'm going to put that down a little bit.

00:48:27 Okay. Almost.

00:48:31 I don't want to prejudice too much.

00:48:33 But the point is, if you extrapolate

00:48:35 from higher temperature here,

00:48:37 I might argue that

00:48:39 something's happening around 120 degrees.

00:48:41 It's dropping.

00:48:43 And I think

00:48:45 there have been some reports that

00:48:47 in fact are of that type.

00:48:49 The data I'm going to report to you

00:48:51 is done this way.

00:48:53 We'll

00:48:55 look a little closer to the transition

00:48:57 and pick a midpoint.

00:49:03 Now for these granular materials,

00:49:05 maybe Tc is higher if you get

00:49:07 the single crystal. But for right now,

00:49:09 we're picking the point where we see this maximum

00:49:11 drop in the resistivity.

00:49:13 And we'll pick a transition

00:49:15 width to give you some idea about the quality

00:49:17 of that material

00:49:19 using the value that represents sort of the

00:49:21 10 to 90 percent

00:49:23 width value.

00:49:31 And this summarizes

00:49:33 some of the data.

00:49:35 Now in the interim

00:49:37 substitutions,

00:49:39 we've had no luck with scandium.

00:49:41 Lanthanum also

00:49:43 has been a dud, although I was

00:49:45 interested to see that.

00:49:47 Don Murphy had reported, I think,

00:49:49 he had found it going.

00:49:51 We've never got it to be a clean single-phase

00:49:53 material, and perhaps

00:49:55 we just have to work on the chemistry.

00:49:57 Cerium is not surprising.

00:49:59 It tends to form plus-4 oxidation states.

00:50:01 Praseodymium

00:50:03 is a very interesting case, and I'm going to come back to that.

00:50:05 That's an insulator, not even a metal.

00:50:07 And then we start in the

00:50:09 rare-earth family,

00:50:11 and neodymium

00:50:13 is sort of marginal, right?

00:50:15 It has a Tc of the order of 45 degrees

00:50:17 Kelvin

00:50:19 and a very broad transition.

00:50:21 The

00:50:23 subsequent

00:50:25 materials going on from samarium

00:50:27 all the way down to ytterbium all have

00:50:29 fairly high and reasonably sharp transitions.

00:50:31 There's a few

00:50:33 broader transitions here,

00:50:35 and that may well be just in the processing of the material.

00:50:37 But it does seem that you can replace

00:50:39 the

00:50:41 yttrium with ions

00:50:43 that actually span a reasonable

00:50:45 size difference, and also,

00:50:47 very interesting, some of them

00:50:49 also have very high

00:50:51 magnetic moments.

00:50:53 So I think right off the bat it suggests that the

00:50:55 rare-earths are not playing a vital

00:50:57 role in the superconductivity.

00:50:59 Lutetium

00:51:01 is a puzzle.

00:51:03 Perhaps it's just a matter of the

00:51:05 processing conditions,

00:51:07 but the thing that puzzles us is that

00:51:09 lutetium is single-phase as far as the X-ray is concerned,

00:51:11 and yet it has a very depressed

00:51:13 transition.

00:51:15 Now, in the barium substitutions,

00:51:17 as far as I'm aware, certainly in our hands,

00:51:19 we haven't had any

00:51:21 complete substitution of the barium

00:51:23 work. We have tried and tried and tried.

00:51:25 So have many other groups.

00:51:27 Strontium, for example,

00:51:29 has not gone superconducting

00:51:31 in our hands, neither has calcium.

00:51:33 The interesting thing,

00:51:35 however, was that the combination of strontium

00:51:37 and calcium does go. Nice, sharp

00:51:39 transition around 85 degrees.

00:51:41 So I think we're beginning to get

00:51:43 some puzzles here that,

00:51:45 from a chemist's point of view, may enable us

00:51:47 to sort out some of the

00:51:49 structure-property relationships in these

00:51:51 materials.

00:51:53 In copper substitution,

00:51:55 it's still the only one that works. I think we've all

00:51:57 tried most of the transition metals,

00:51:59 but in many of the other transition metals,

00:52:01 you have to worry about magnetism,

00:52:03 and you also have to worry about higher oxidation states,

00:52:05 and that means you have to run these reactions

00:52:07 under more control

00:52:09 and a wider range of conditions.

00:52:15 Let me

00:52:17 show you,

00:52:19 go back to this praseodymium compound.

00:52:21 I have here

00:52:23 the X-ray powder diffraction patterns

00:52:25 for the superconducting

00:52:27 yttrium barium copper oxide

00:52:29 and the

00:52:31 praseodymium material, which is insulated.

00:52:33 I think you can see that, in fact,

00:52:35 the patterns are almost identical

00:52:37 here.

00:52:39 What's different is that the orthorhombic

00:52:41 unit cell, which is

00:52:43 very much characterized by these,

00:52:45 I've picked out some of the splittings,

00:52:47 by splitting certain diffraction lines,

00:52:49 is nearly absent in the

00:52:51 praseodymium system.

00:52:53 In fact, you really can't index

00:52:55 this to a tetragonal unit cell.

00:52:57 It's still orthorhombic,

00:52:59 but the orthorhombic distortion is less.

00:53:01 Is that important

00:53:03 to superconductivity? Is that the reason

00:53:05 why this material is insulated?

00:53:07 We really don't know

00:53:09 at this point, but I think these are

00:53:11 some of the handles that chemists

00:53:13 can employ to try to

00:53:15 help physicists sort out

00:53:17 the important properties in this

00:53:19 class of material.

00:53:24 One of the

00:53:26 things that was a consequence

00:53:28 of finding out what is the structure

00:53:30 of the high-TC material

00:53:32 was that it immediately benefited

00:53:34 in a very important advance, which was

00:53:36 the preparation of thin films.

00:53:38 Now, I can see

00:53:40 two avenues in which this is going to go.

00:53:42 There's going to be the whole avenue of

00:53:44 ceramic processing. We saw that

00:53:46 in the AT&T talk, that there are

00:53:48 all kinds of things one can do to make

00:53:50 structures and shapes.

00:53:52 The other thing

00:53:54 is to make thin films and to use

00:53:56 the normal techniques of depositing thin films

00:53:58 that we've seen in

00:54:00 integration of

00:54:02 electronic circuits.

00:54:04 Knowing the right

00:54:06 stoichiometry, it was amazing how quickly

00:54:08 a group in Yorktown Heights

00:54:10 Research Laboratory there

00:54:12 could in fact make thin films

00:54:14 that also had almost identically good

00:54:16 superconducting properties.

00:54:18 That is, completely zero resistance

00:54:20 below 87 degrees Kelvin.

00:54:22 That, I think, is a

00:54:24 major step forward.

00:54:26 Prior to that, most of the data

00:54:28 that I have seen may have shown

00:54:30 a tendency to go down in resistivity,

00:54:32 but it extended almost down to zero degrees.

00:54:34 That is a very broad range.

00:54:36 This was done with

00:54:38 very little optimization.

00:54:40 I think it just illustrates

00:54:42 the durability of these materials.

00:54:44 If there's anything that I would like to

00:54:46 give you to take back

00:54:48 on these materials is the fact

00:54:50 that they seem to be ideally suited

00:54:52 for processing and

00:54:54 for manipulation

00:54:56 more so than we could expect.

00:54:58 This, for example, enables us now

00:55:00 to explore not only a variety

00:55:02 of interesting scientific questions,

00:55:04 but even to make devices.

00:55:08 Let me

00:55:10 summarize some of the points.

00:55:14 The structure of the superconductor is known.

00:55:16 There must have been a half a dozen

00:55:18 groups that simultaneously came up

00:55:20 with the structure.

00:55:22 I think all of us were working mad,

00:55:24 and all of us thought no one else in the world

00:55:26 could have solved it.

00:55:28 Then we found out that we were not alone.

00:55:30 I think we can make this material

00:55:32 in bulk quantities, very reliable.

00:55:34 You know the tricks now.

00:55:36 You have to worry about the quench rate.

00:55:38 You've got to have oxygen.

00:55:40 And you can make it

00:55:42 reproducibly.

00:55:44 I think the other groups

00:55:46 would confirm that also.

00:55:48 We have at least a dozen

00:55:50 distinctly different derivatives,

00:55:52 and then many other kinds of variations

00:55:54 on those derivatives.

00:55:56 I think that's going to give us

00:55:58 a lot of useful materials

00:56:00 to explore optimization.

00:56:02 We just saw one of that

00:56:04 in the Bell paper.

00:56:06 They raised the critical field.

00:56:08 That's the value

00:56:10 of having these derivatives.

00:56:12 It's not just getting high at TC,

00:56:14 but you have a lot of other properties

00:56:16 at critical currents and critical fields

00:56:18 that you have to optimize.

00:56:20 The thin films we think are very exciting,

00:56:22 and I think a lot of other groups

00:56:24 are beginning to make progress,

00:56:26 and certainly the search goes on.

00:56:28 Remember,

00:56:30 Chu's paper only appeared,

00:56:32 at least to the general public,

00:56:34 in the beginning of March,

00:56:36 so it's not a very old field.

00:56:38 Although a lot of us

00:56:40 feel older.

00:56:42 Let me acknowledge

00:56:44 my co-workers.

00:56:46 That list is growing,

00:56:48 but these are the people

00:56:50 who primarily contributed

00:56:52 to the work I discussed today.

00:56:54 Obviously, in this field,

00:56:56 you have to have chemists,

00:56:58 high-powered characterization,

00:57:00 certainly structure is an essential element,

00:57:02 and, of course, the physicists

00:57:04 to tell us what to do.

00:57:06 Without them, what would we do?

00:57:10 What we'd like to try to do

00:57:12 at the end of this talk,

00:57:14 you saw a very nice illustration

00:57:16 of levitation.

00:57:18 We have a variation on that

00:57:20 that we're going to set up

00:57:22 at the end of the talk,

00:57:24 and anyone who'd like to try

00:57:26 the hand at levitation

00:57:28 can come up and try it.

00:57:30 We simply have a magnet,

00:57:32 a string in which we attach

00:57:34 a conductor, and we're going to

00:57:36 cool it in liquid nitrogen,

00:57:38 and you'll see the levitation effect.

00:57:40 I think you can take a look at that.

00:57:42 Now, we were talking about

00:57:44 maybe these are things that you can do

00:57:46 as an undergraduate experiment.

00:57:48 Well, Paul Grant in our lab

00:57:50 is actually going to be this week

00:57:52 doing it in a high school lab

00:57:54 in California.

00:57:56 He estimated that for the cost of $1.60

00:57:58 you could run this experiment

00:58:00 in almost any typical American high school.

00:58:02 In a ceramics shop

00:58:04 in a high school.

00:58:06 You need a bar magnet.

00:58:08 You need a water and pestle.

00:58:10 You need to buy these chemicals

00:58:12 which are really not very exotic.

00:58:14 I think we can see a very elegant

00:58:16 demonstration of superconductivity

00:58:18 being carried out at a high school level.

00:58:20 I think that brings home to us

00:58:22 that these materials will be part of our society

00:58:24 and I think part of the way we live.

00:58:26 I think the important thing

00:58:28 is for us as scientists to think

00:58:30 about how to advance this field even faster

00:58:32 and how to get these things into

00:58:34 practical applications that will benefit all of us.

00:58:36 With that, I'd like to

00:58:38 thank all of you for staying here so late.

00:58:40 Thank you.

00:58:42 Applause