00:01:00 Let me introduce tonight's program by reading you, it's very unusual for me to read a book

00:01:29 and it's very unusual that you get an opportunity to open an American Chemical Society meeting

00:01:34 and particularly a technical meeting by using information from the Wall Street Journal.

00:01:40 I've been wanting to do this for a very long time.

00:01:43 So let me introduce tonight's program to reading to you some of the excerpts from the editorial page

00:01:50 of the Wall Street Journal of last Monday.

00:01:54 And I quote,

00:01:55 Bored by our anamok, had enough of insider trading investigations, take heart.

00:02:03 While politicians and prosecutors dance atop the heads of legal pins,

00:02:08 science has just presented two of the most dazzling discoveries of our time,

00:02:13 superconductivity and a grand and puzzling supernova.

00:02:18 Researchers seeking superconductivity have been scientists, men of La Mancha,

00:02:24 dreaming an impossible dream.

00:02:26 The dream was the possibility of creating materials that would transmit electricity at room temperature

00:02:32 without losing power into the transmitting material.

00:02:36 Superconducted electricity, for instance, could be transmitted from coast to coast without any loss of energy.

00:02:43 In the past year, scientists believe that they have learned how to achieve superconductivity

00:02:48 at higher temperatures using new ceramic-like materials.

00:02:53 The practical applications could be immense.

00:02:56 At a scientific colloquium in New York on March 18th,

00:03:00 Bell Labs physicist Bertram Batlogg stood before a hall crammed with excited colleagues

00:03:05 and spoke six words that may become history's signature for the superconductivity revolution.

00:03:11 I think our life has changed.

00:03:14 Electric motors may become much smaller, perhaps a tenth of their current size.

00:03:19 Electric cars become a reasonable option.

00:03:22 The Japanese have been working on a new railway system or railway design

00:03:28 that uses electromagnets to levitate the train slightly above the tracks and propel it at super speeds.

00:03:35 Superconductivity could make such trains an affordable reality.

00:03:39 A breakthrough in this area would also impact some planned major facilities.

00:03:44 For example, if superconducting magnets could be run at liquid nitrogen temperatures rather than liquid helium,

00:03:49 the cost of the supercollider could possibly be reduced by 50%.

00:03:54 However, much hard work in these areas lies ahead, of course.

00:03:58 It is rare, though, for knowledge to hurdle progress forward on such a vast scale.

00:04:03 The progress began last April with a successful superconduction experiment

00:04:07 by physicist K. Alex Muller at an IBM laboratory in Zurich.

00:04:12 Researchers at the University of Alabama at Huntsville and the University of Houston

00:04:16 then offered the composition of the material slightly

00:04:19 and achieved even higher and better superconductivity results.

00:04:23 Physicists all over the world dropped their research to take up the challenge

00:04:27 of achieving superconduction at or near room temperature.

00:04:32 By now, a rapid headway has been made in laboratories in the United States, Europe, Japan, and China.

00:04:39 This kind of thing, an MIT scientist remarks, you see once in your lifetime.

00:04:44 It is so unusual to have such a fast-breaking science area

00:04:48 in which chemistry will ultimately play such a pivotal role.

00:04:52 The latter part of these comments are my own, not the Wall Street Journal's.

00:04:56 It is immensely exciting to host this tutorial.

00:05:03 We are very fortunate to have some of the principal players in this new area to address us tonight.

00:05:09 They have responded on very short notice, and we are grateful.

00:05:13 At this point, I'd like to introduce Dr. Valerie Cook from AMTT Laboratories.

00:05:18 They're laboratories who will introduce the speakers.

00:05:22 Valerie deserves most of the credit for arranging this tutorial,

00:05:26 and I would like to thank her publicly for doing the legwork that made it possible.

00:05:30 Val, the program is all yours.

00:05:42 I'll tell you, I thought getting speakers to come on short notice was tough.

00:05:46 I thought this was a little tougher.

00:05:49 All of us as scientists have noted specific reactions or observed phenomena

00:05:54 occurring at exponential rates in our laboratories,

00:05:57 but rarely have advances occurred in the field of science at this rapid rate.

00:06:01 But this is what is happening in the area of high-temperature superconductivity.

00:06:06 Until 1986, the highest temperature observed for superconducting behavior

00:06:11 was 23 degrees Kelvin, and research, as Mary said,

00:06:15 was exclusively within the realm of physicists.

00:06:18 In the fall of 1986, results from IBM Zurich were published

00:06:23 that reported superconductivity at 30 degrees Kelvin.

00:06:27 Subsequent work carried out in various laboratories throughout the world

00:06:31 verified these findings, and then the race was on.

00:06:35 In the last three months, superconductivity has been reported

00:06:40 first at 39 degrees K, then 50, 59, 90, and as of two weeks ago,

00:06:47 according to the New York Times, at 220 degrees K.

00:06:52 The key to this rapid growth in high-temperature superconductivity

00:06:56 has been the identification of a new class of superconducting materials,

00:07:01 one which I believe chemists will find very exciting.

00:07:06 At present, the physical basis on how these new superconductors work

00:07:11 is not well understood, and the structural determinants are not known.

00:07:16 Solving these problems will require both a chemical and physical understanding

00:07:20 that can be only achieved, I believe, when chemists, physicists,

00:07:25 and material scientists work together.

00:07:28 The technological impact that high-temperature superconductors

00:07:32 will have on all of our lives is so profound that the popular press,

00:07:36 such as the New York Times, Time Magazine, and I understand

00:07:39 an excellent article in Businessweek, to name a few,

00:07:42 have printed extensive articles on this topic.

00:07:46 Condescending of the potential impact of these materials

00:07:50 and the importance of understanding their properties,

00:07:52 coupled with the insight chemists could bring to bear on this problem,

00:07:57 this evening's program has been organized.

00:08:00 At the risk of boring those of you who are familiar with the field,

00:08:04 but try to keep all of you on board,

00:08:07 I have asked that our speakers speak on a general plane.

00:08:12 To maintain continuity to tonight's proceedings

00:08:15 and to conclude at a reasonable hour, remember 1 o'clock is our deadline,

00:08:19 I ask that you hold your questions for the panel discussion,

00:08:22 which will be held at the end after all the speakers have presented their work.

00:08:27 Before we start, I too would like to thank some people.

00:08:30 I would like to express my appreciation to the ECS president, Mary Good,

00:08:34 and to ECS staff personnel.

00:08:37 In particular, I would like to identify Chris Pruitt and Barbara Hodgson,

00:08:40 who took care of all the facilities and everything in this wild dash.

00:08:44 They were tremendous.

00:08:46 Above all, I would like to thank our speakers,

00:08:49 who on very short notice, as Mary has told you, agreed to come

00:08:53 and leave their laboratories, coming away from the cutting edge of science,

00:08:57 and come and share with us the excitement of high-temperature superconductivity.

00:09:01 And now, with no further ado, we will start with our first speaker.

00:09:06 For your information, the list of speakers, as you saw in the notice, is not the list.

00:09:12 The order in which they're going to occur tonight,

00:09:14 someone came up to me and thought it was neat that they were all alphabetical,

00:09:17 and that's the way they're going to go.

00:09:19 That's not the case.

00:09:21 For your information, Robert Dynes will speak first,

00:09:25 then Art Slate, if you wish to mark on yours,

00:09:29 then Don Murphy,

00:09:33 Hal Wang,

00:09:36 Ed Engler,

00:09:39 Thomas Mason,

00:09:41 and we have an addition.

00:09:43 This was put together very fast,

00:09:45 and I had trouble getting a hold of one of the people I wanted to be here tonight,

00:09:49 but it is connected.

00:09:50 And if I got a hold of her, Angela Stacey will speak at the end.

00:09:56 Bob Dynes is a director of the Chemical Physics Research Laboratory

00:10:00 at AT&T Bell Laboratories.

00:10:02 He is a fellow of the American Physical Society

00:10:05 and sits on review committees at Los Alamos National Laboratory,

00:10:09 Argonne National Laboratory,

00:10:11 the Institute for Theoretical Physics,

00:10:13 and several universities.

00:10:16 Bob is a research physicist whose interests are in the area of properties of materials

00:10:21 and low temperatures.

00:10:22 He has worked in the area of superconductivity for 20 years.

00:10:27 Tonight, Bob will speak to us on superconductivity.

00:10:31 What's the fuss all about?

00:10:33 Robert Dynes.

00:10:35 What I hope to do for you this evening is to convey to you

00:10:50 the level of excitement that exists in the physics community

00:10:54 on these high-temperature superconductors.

00:10:58 Why physicists are appearing in centerfolds in Business Week, for example.

00:11:04 Well, it's the best we can do. What the hell?

00:11:09 Why we're not sleeping these nights,

00:11:12 and why people are calling the New York Times

00:11:18 rather than sending their articles to physical review letters.

00:11:24 Actually, what I really hope to do is first get your attention

00:11:28 and then give you a little bit of a tutorial

00:11:31 on what we do know about superconductivity,

00:11:34 as physicists point to superconductivity,

00:11:37 and why these materials are probably different

00:11:41 from what we generally understand with superconductors.

00:11:47 First, I'll try to get your attention by showing you a series of slides.

00:11:53 Is that focused?

00:11:55 Okay. All right.

00:11:59 Until recently, that is, until the last less than a year,

00:12:05 it was generally believed, or it was generally understood,

00:12:09 this empirical relationship that I'm showing you here.

00:12:12 This is a plot of the maximum critical temperature

00:12:16 that had been achieved with superconductors as a function of time,

00:12:20 starting with the year 1911,

00:12:22 when Cameron Leonis first discovered the phenomenon of superconductivity,

00:12:26 up to and including 1973,

00:12:30 the highest TC known to man and woman,

00:12:34 that is, niobium-3-germanium,

00:12:37 which was first discovered at Westinghouse

00:12:40 and then, let me be parochial,

00:12:42 then shortly thereafter at Bell Laboratories.

00:12:49 It's only an empirical relationship,

00:12:51 but every one of us in the business sort of knew this curve

00:12:55 and we were grieving by the observation

00:12:59 that nothing had been discovered since 1973.

00:13:02 And it sort of bothered us,

00:13:05 and some of us actually spent some time

00:13:07 going through theoretical calculations

00:13:09 worrying about maximum TCs,

00:13:11 and I'll get back to that in a few minutes, myself included.

00:13:14 Then Ben Norston Mueller, IBM Zurich,

00:13:20 announced in a European journal

00:13:26 a possible superconductivity close to 30 degrees.

00:13:32 That's shown as this, I think, red or orange dot.

00:13:38 And the first I heard about it,

00:13:40 and the first most of the people in the U.S. heard about this,

00:13:43 was at the MRS meeting in Boston

00:13:47 the first week of December, 1986.

00:13:50 And amazingly enough, it was announced to us

00:13:52 by Kitazawa from the University of Tokyo,

00:13:56 who had earlier seen the work from IBM Zurich.

00:14:01 It's a comment on our times, I'm afraid.

00:14:04 And they had gone back and isolated the particular phase

00:14:08 that Mueller and Ben Norston had first observed

00:14:16 And at that point, all hell broke loose in the U.S.

00:14:20 And within a few weeks, we at AT&T, Bell Laboratories,

00:14:25 and in several other places, I'm glad to understand,

00:14:32 I was supposed to slip on it,

00:14:37 with some reasonably small substitution,

00:14:43 as is shown in the next point up there,

00:14:47 up to close to 40 degrees.

00:14:49 At that point, I was delirious,

00:14:51 because I immediately went and put this on this curve.

00:14:54 We actually used to call this a Matias plot.

00:14:57 And I put this on this Matias plot

00:15:00 and projected that we had reached the year 2025,

00:15:05 as is shown by the arrow down at the bottom.

00:15:10 Well, you know, you can only bask in glory for so long.

00:15:14 And this is the same curve now,

00:15:18 with the announcement from Paul Chu and his collaborators

00:15:22 at the University of Houston and Huntsville.

00:15:29 Only a few weeks later, they announced a superconductor

00:15:33 with a critical temperature above 90 degrees.

00:15:37 And then, in a matter of hours,

00:15:40 after the particular set of ingredients,

00:15:45 not the concentrations, but the set of ingredients were known,

00:15:49 there were several laboratories,

00:15:51 and I couldn't possibly count them,

00:15:53 but there were several laboratories

00:15:55 that had reproduced the results from the University of Houston.

00:15:59 And within days, the particular crystal structure

00:16:05 and the right concentrations in the phase had been isolated.

00:16:09 And it was clearly demonstrated that bulk superconductivity,

00:16:14 that is, the entire phase was going superconducting,

00:16:17 at above 90 degrees, about 93, 94 degrees Kelvin.

00:16:22 We now had a superconductor above liquid nitrogen temperature.

00:16:27 And that, it turns out, is very important for practical reasons.

00:16:33 And that's, I think, probably most simply demonstrated in this next slide,

00:16:39 where I have listed several cryogens, several cryogenic fluids,

00:16:45 and simply the latent heat of those cryogenic fluids.

00:16:49 I don't have dollars associated with that,

00:16:51 but I'll translate that for you if you like.

00:16:54 Until these materials, what we did was cool materials

00:17:00 that we knew were going to be superconductors,

00:17:03 or we wanted to be superconductors.

00:17:05 We cooled them with liquid helium.

00:17:07 Even niobium and 3-germanium,

00:17:09 we weren't crazy enough to cool them with liquid hydrogen.

00:17:12 If you look at that, if you look at those numbers,

00:17:16 you realize that the latent heat of liquid nitrogen

00:17:21 is substantially higher, of order 50 times higher

00:17:29 than that of liquid helium.

00:17:32 Translated in real terms, it means that if you take a 100-watt light bulb

00:17:36 and you put it in liquid nitrogen,

00:17:38 it takes a couple seconds to boil off a cubic centimeter.

00:17:42 It's only something like 20 milliseconds to boil off

00:17:45 a cubic centimeter of liquid helium.

00:17:49 Put another way, in terms of cost,

00:17:51 the cooling power per dollar of liquid nitrogen

00:17:57 compared to liquid helium is about a factor of 1,000.

00:18:01 And so it starts to become very important.

00:18:05 And if you start looking at other cryogens,

00:18:08 I've even been so folly as to use water down at the bottom

00:18:13 as a potential cryogen.

00:18:20 I'm a low-temperature physicist.

00:18:22 I had a dilution refrigerator in my laboratory,

00:18:26 and my technician looked at me about a month ago and said,

00:18:29 what the hell are we going to do with this?

00:18:32 As we start cooling things down to liquid nitrogen,

00:18:34 and start measuring them rather than 5 milliKelvin.

00:18:40 Anyway, I hope what that does is illustrate for you

00:18:43 the very practical reason why physicists

00:18:48 and electrical engineers are getting very excited

00:18:52 about superconductors.

00:18:55 Now hopefully I've gotten your attention.

00:18:58 I'm going to go back and describe for you a little bit

00:19:01 about what superconductivity is

00:19:05 and what we know about traditional superconductors.

00:19:09 For those of you who understand all this, I apologize.

00:19:12 This is going to be pretty dull for the next few minutes.

00:19:14 But let me start at the beginning

00:19:18 and tell you what a lot of you know.

00:19:21 And that is that if you start cooling down a metal

00:19:24 and you look at the resistance of a metal,

00:19:27 depending upon whether you've chosen an alloy to look at

00:19:30 or whether you've chosen an elemental material

00:19:34 like niobium or lead or tin or something,

00:19:38 the temperature dependence of the resistance

00:19:40 will look rather different.

00:19:42 If you choose a very clean, pure material,

00:19:45 you'll get a curve like that bottom curve

00:19:48 where the resistance as you cool down decreases,

00:19:51 goes down rather rapidly,

00:19:53 and then it levels off at the bottom.

00:19:55 If you look very carefully,

00:19:57 you'll see that I drew the asthmatode

00:19:59 going into a finite value at low temperatures.

00:20:02 On the other hand, an alloy,

00:20:04 something that has some disorder and mixtures

00:20:07 and dislocations and defects, etc., etc.,

00:20:09 an alloy will show very little temperature dependence

00:20:13 to the resistance.

00:20:15 And that comes about because the electrons

00:20:18 are scattering off defects and impurities

00:20:21 and interstitials, dislocations, etc.,

00:20:24 in the alloy.

00:20:27 If you look a little more carefully

00:20:30 at what causes electron scattering in a metal,

00:20:34 there are a myriad of scattering mechanisms,

00:20:38 and I've shown you some pretty pictures

00:20:41 of only three of them,

00:20:43 the three dominant scattering mechanisms in metals.

00:20:46 The top one is scattering off the lattice vibrations.

00:20:50 At finite temperatures, of course,

00:20:52 the lattice is vibrating,

00:20:54 the phonons are propagating through the material,

00:20:56 and the electrons in their block states

00:20:58 will scatter off the vibrations

00:21:00 and cause a change in energy and momentum

00:21:03 of the electron,

00:21:05 resulting in losses, in dissipation,

00:21:07 in a change of the energy and momentum of the electron.

00:21:10 That shows up as resistance.

00:21:12 At lower temperatures,

00:21:14 but still a temperature-dependent phenomenon,

00:21:16 electrons can scatter off other electrons.

00:21:19 And as shown in the middle part of this slide,

00:21:24 this is also temperature-dependent

00:21:26 and goes to zero at t equals zero

00:21:29 because of the Pauli exclusion principle.

00:21:32 And it's these top two scattering mechanisms,

00:21:36 which are the dominant scattering mechanisms

00:21:38 responsible for the temperature dependence

00:21:40 that I showed you in the previous slide.

00:21:43 Finally, the world is real,

00:21:45 and we don't make perfect crystals,

00:21:48 and there are interstitials, impurities,

00:21:52 dislocations, etc.,

00:21:54 and electrons scatter off those,

00:21:56 block states scatter off those impurities and dislocations,

00:22:00 and that's a temperature-independent scattering process

00:22:04 and results in a temperature-independent resistance

00:22:08 as the alloy that I showed you in the previous view graph.

00:22:13 The net result of all this dirt,

00:22:15 of all this scattering, is resistance.

00:22:18 That is, if you put a current through a material,

00:22:21 it will measure a voltage.

00:22:24 And that implies then that there's dissipation,

00:22:27 there's power being dissipated in that material,

00:22:29 just as there's power being dissipated

00:22:31 in this obnoxious light that's up here

00:22:35 that's making me about 10 degrees hotter than the rest of you.

00:22:40 Heating and losses, all right?

00:22:44 On the other hand,

00:22:48 Kamerlingh Onnes, in the year 1911,

00:22:52 was looking at these processes,

00:22:54 he was looking at, trying to understand

00:22:57 what was the source of resistance in metals,

00:23:02 and he was looking at single-crystal mercury,

00:23:06 largely because it's easy to grow single-crystals of mercury,

00:23:10 and as he lowered the temperature,

00:23:14 of course, the light allows for the first to liquefy helium,

00:23:18 so he had a head start on the rest of the world in this regard.

00:23:21 As he lowered the temperature,

00:23:23 he discovered to his horror at the time

00:23:26 that the resistance went down many orders of magnitude,

00:23:30 somewhere around 4.2 degrees.

00:23:32 He worried about that,

00:23:33 because that's a liquefaction point of liquid helium.

00:23:36 It was a coincidence at the time,

00:23:37 and like a responsible scientist,

00:23:40 he wrote down there,

00:23:42 this is actually lifted from his paper,

00:23:44 he wrote down there,

00:23:45 10 to the minus 5 ohms, rather than 0 ohms.

00:23:49 He put up with a lot of flack at the time,

00:23:53 because, of course,

00:23:54 everyone knew that electrons were scattering in these materials,

00:23:58 and there was no such thing as a perfect conductor.

00:24:01 Well, it turns out that there is such a thing as a perfect conductor,

00:24:04 it's a superconductor,

00:24:05 and Carolingius was indeed the first person

00:24:08 to demonstrate superconductivity in a material.

00:24:14 Since that time, since the first discovery,

00:24:16 until the microscopic description,

00:24:19 which I'll get to in a minute,

00:24:21 which was in 1956,

00:24:23 there was a lot of experimental work

00:24:27 and a lot of phenomenology associated with superconductivity.

00:24:31 But suffice it to say that what the essence of it is,

00:24:36 is that there is simply no resistance.

00:24:38 The resistance absolutely goes to zero.

00:24:41 There's no dissipation,

00:24:43 and there are no losses.

00:24:45 And a favorite example of physicists,

00:24:48 and in fact one of the ways it was first demonstrated

00:24:51 that there were no losses,

00:24:52 was to take a loop, a wire,

00:24:57 induce a current in that wire

00:25:00 while it was in liquid helium,

00:25:01 while it was superconducting,

00:25:03 and walk away from it,

00:25:04 and come back later and ask

00:25:06 whether that current was still going around in that wire.

00:25:08 And the answer was yes,

00:25:11 as long as you did the experiment properly.

00:25:14 The current was still going around in the wire,

00:25:16 and the current was still the same current

00:25:18 as the one that you induced.

00:25:20 Be it an hour, a day, a week, a month, a year later,

00:25:24 that current was still going around in that loop.

00:25:28 Now that's something that has intrigued physicists

00:25:33 since the discovery of superconductivity,

00:25:37 and is a little counterintuitive,

00:25:41 to say the least.

00:25:42 It also implies one of the first applications

00:25:44 of superconductivity.

00:25:46 If you have that loop,

00:25:47 and you have a circulating current,

00:25:51 then there's a magnetic field

00:25:53 coming out of the board.

00:25:55 It's a right-hand rule.

00:25:56 You have a magnetic field coming out of the screen.

00:25:59 And so you can well imagine

00:26:01 winding a solenoid with a lot of loops,

00:26:05 bringing the wire back on itself,

00:26:08 inducing a current in that solenoid,

00:26:10 and you have a very, very stable

00:26:13 and long-lasting magnetic field.

00:26:15 And that, of course,

00:26:16 is an application of superconductivity,

00:26:19 which has long been used in laboratories,

00:26:24 and in fact, it's finally reaching the,

00:26:28 well, I'm not sure you'd call

00:26:31 anemometromography consumer market,

00:26:33 but nonetheless, it's reaching the outside world,

00:26:37 superconducting magnets.

00:26:40 Persistent currents are things

00:26:42 that have fascinated physicists

00:26:44 for a very long time.

00:26:47 Now, it wasn't until the year 1956

00:26:50 that superconductivity

00:26:52 was really fully described,

00:26:54 and it was a major triumph

00:26:56 of many-body theory,

00:26:58 many-body solid-state physics,

00:27:00 to describe how you can get

00:27:03 this macroscopic phenomenon

00:27:05 from microscopic interactions.

00:27:07 And I'll try to walk this through with you

00:27:10 to try to give you a feeling

00:27:12 for how you can get

00:27:14 microscopic interactions

00:27:16 and translate that

00:27:17 to a macroscopic phenomenon,

00:27:19 that is, a persistent current in a wire

00:27:21 that can be extremely, extremely large.

00:27:24 This was explained to us

00:27:27 by Barty, Cooper, and Schrieffer

00:27:29 in the year 1956,

00:27:31 and it goes something like this.

00:27:33 If you take a lattice,

00:27:35 which I've sort of drawn up there

00:27:38 as point-positive charges

00:27:40 with its background C

00:27:42 of negative electron charge,

00:27:44 and an electron is charging

00:27:46 through that lattice,

00:27:48 as is illustrated by whatever color

00:27:50 that line is going through there.

00:27:52 I can't see up here.

00:27:54 That's why I'm sort of staggering.

00:27:56 This light is completely blinding me.

00:27:59 Why not?

00:28:02 Anyway, as the electron

00:28:04 is propagating through,

00:28:06 what I've tried to depict there

00:28:08 is that the lattice actually

00:28:10 responds a little bit,

00:28:12 and you end up with a strain field

00:28:14 that follows the electron.

00:28:16 So the ions in the lattice

00:28:18 actually relax a little bit

00:28:20 as the electron goes through the solid.

00:28:22 Now, the timescale for that relaxation

00:28:24 is simply 1 over the phonon frequency.

00:28:27 Phonon frequencies are typically

00:28:29 10 to the 12th hertz or so.

00:28:31 So the timescale for this relaxation process

00:28:34 is about 10 to the minus 12 seconds.

00:28:37 Now, a second electron will come along.

00:28:39 I hope this is an overlay here.

00:28:41 Yeah, a second electron will come along,

00:28:43 and the second electron is going

00:28:45 sort of going the other direction.

00:28:47 And it can come by at a time

00:28:49 which is later than that first electron,

00:28:51 as long as it's within

00:28:53 the 10 to the minus 12 seconds.

00:28:55 And it will see the lattice polarization

00:28:58 but it really won't see the first electron

00:29:01 because the first electron

00:29:03 will be long gone.

00:29:05 And I'll get to some numbers

00:29:07 at the bottom of the slide in a minute.

00:29:10 And so the net result

00:29:12 is that that second electron

00:29:14 will see an attractive interaction

00:29:16 to the first electron

00:29:18 via the lattice relaxation

00:29:20 associated with the polarization cloud

00:29:22 that the first electron

00:29:24 generates.

00:29:26 Now, when does that work,

00:29:28 what timescale does that work,

00:29:30 and what length scale does that work at?

00:29:33 Now, if I can see that,

00:29:35 the timescale for this lattice

00:29:37 to relax and then to follow

00:29:39 and then relax back again

00:29:41 is something like about

00:29:43 10 to the minus 12 seconds,

00:29:45 as I suggested earlier.

00:29:47 The range of interaction,

00:29:49 that is, how far these two electrons

00:29:51 can stay in the lattice

00:29:53 and how far these two electrons

00:29:55 can still see each other

00:29:57 via this lattice relaxation

00:29:59 is a long distance.

00:30:01 And that's where we go

00:30:03 from the microscopic

00:30:05 to the macroscopic.

00:30:07 The distance can be as far

00:30:09 as a micron.

00:30:11 And the way that happens

00:30:13 is the first electron

00:30:15 goes zooming through

00:30:17 at its Fermi velocity,

00:30:19 which is about

00:30:21 a micron.

00:30:23 Later, it sees this lattice cloud

00:30:25 and there's a net attraction.

00:30:27 The length scale,

00:30:29 the first electron has gone

00:30:31 of order a micron

00:30:33 by the time the second electron

00:30:35 sees that.

00:30:37 And so it doesn't see

00:30:39 the Coulomb repulsion anymore

00:30:41 because it's well outside

00:30:43 any screening lengths.

00:30:45 So there's a very weak

00:30:47 Coulomb repulsion.

00:30:49 So what happens is

00:30:51 that the electrons on this length scale

00:30:53 are bound together.

00:30:55 And the binding energy

00:30:57 is given by the balance

00:30:59 between this electron

00:31:01 full-on attraction

00:31:03 and the Coulomb repulsion.

00:31:05 The Coulomb repulsion becomes very weak

00:31:07 because of this very long distance

00:31:09 over which they look at each other.

00:31:13 And there's a second issue

00:31:15 which is very important

00:31:17 when we compare these microscopics

00:31:19 to the macroscopics,

00:31:21 and that is that over this length scale

00:31:23 there are something like a million

00:31:25 to ten million other electrons

00:31:27 that are also engaged

00:31:29 in a similar dance.

00:31:31 So over this distance

00:31:33 of typically a micron

00:31:35 or between a thousand electrons

00:31:37 and a micron,

00:31:39 there are something like

00:31:41 a million electrons

00:31:43 that are dancing coherently

00:31:45 almost from a microscopic

00:31:47 individual particle interaction

00:31:49 to a macroscopic interaction

00:31:51 where all ten to the sixth of them

00:31:53 have to know what each other is doing

00:31:55 at some level.

00:31:57 That is, you end up with a coherence length

00:32:01 where the electrons

00:32:03 know what each other is doing,

00:32:05 which is of the order

00:32:07 of a micron or so.

00:32:09 Now, you can think of this thing at the bottom

00:32:11 that I drew, this blue box at the bottom

00:32:13 as a wire,

00:32:15 and this yellow ball

00:32:17 is a coherence length.

00:32:19 And there are about a million electrons

00:32:21 inside there that know what each other is doing,

00:32:23 and if I violate those electrons

00:32:25 by taking an unpaired electron

00:32:27 and putting it in there,

00:32:29 all the rest of them have to respond

00:32:31 to that one electron that I put in.

00:32:33 So you can think of this

00:32:35 as a coherence state

00:32:37 which has to respond

00:32:39 in some collective way

00:32:41 to a perturbation

00:32:43 that I put into the system.

00:32:47 And so

00:32:49 I go then

00:32:51 from something that's microscopic,

00:32:53 from this basic electron lattice

00:32:55 interaction, to something that's

00:32:57 macroscopic, where I think of this

00:32:59 very large, long-range

00:33:01 wave function, which has

00:33:03 an amplitude,

00:33:05 which is the binding energy,

00:33:07 which is how much these energies are bound,

00:33:09 these electrons are bound together,

00:33:11 and a phase.

00:33:13 The phase of this wave function

00:33:15 is basically dictated

00:33:17 by,

00:33:19 if I push on one end

00:33:21 of this phase-coherent region,

00:33:23 the other end knows it.

00:33:25 So if I change

00:33:27 the phase of one end, a current

00:33:29 will flow to try to compensate

00:33:31 for that change of phase.

00:33:33 So I now have a macroscopic

00:33:35 wave function

00:33:37 which describes

00:33:39 the way this system would respond

00:33:41 to any perturbation.

00:33:45 Now,

00:33:49 what determines when this happens?

00:33:51 Again, it was

00:33:53 Barney and Cooper and Schrieffert

00:33:55 which told us when this happens,

00:33:57 and it's related

00:33:59 to the

00:34:01 lattice vibrational frequency,

00:34:03 which is given by this f,

00:34:05 and an exponential which has in it

00:34:07 the basic binding energy

00:34:09 associated with the electrons.

00:34:11 That is the

00:34:13 number of electrons, n of 0, which is the

00:34:15 electronic density of states, and v,

00:34:17 this coupling matrix element

00:34:19 associated with the electron-phonon interaction.

00:34:21 But the important issue

00:34:23 here, and I'll come back to that in a few minutes,

00:34:25 is that the lattice

00:34:27 vibration frequencies

00:34:29 in this mechanism

00:34:31 sets the energy scale

00:34:33 for the superconductivity.

00:34:35 And this is possibly where

00:34:37 things are different in these new oxides.

00:34:39 Some typical numbers,

00:34:41 if you put in typical numbers, the things that we

00:34:43 know and understand and have known and understood

00:34:45 for 20 years,

00:34:47 typical numbers are 10 to the 12 hertz for phonon

00:34:49 frequencies, which is an order

00:34:51 of 100 degrees Kelvin.

00:34:53 Actually, 10 to the 12 hertz is about 40 Kelvin.

00:34:55 The n of 0,

00:34:57 v, the basic binding energies

00:34:59 times the density of states

00:35:01 represents binding energy as a number, which is

00:35:03 between 0.2 and 0.4.

00:35:05 And that results in a critical

00:35:07 temperature somewhere between 1 and 10 Kelvin.

00:35:09 And almost every superconductor we know

00:35:11 and love is somewhere in that range,

00:35:13 with the exception of these extremes.

00:35:19 Now,

00:35:21 superconductivity can be destroyed several ways.

00:35:23 The most obvious way is to raise

00:35:25 the temperature, and

00:35:27 that's shown in the upper left

00:35:29 where I've plotted resistance as a function of temperature.

00:35:31 The resistance is 0, we reach a certain temperature,

00:35:33 the critical temperature, the resistance goes up.

00:35:35 It can also be destroyed

00:35:37 by the impression

00:35:39 of a very high magnetic field.

00:35:41 As we keep cranking up the magnetic field,

00:35:43 we reach a critical field

00:35:45 called HC2, for historical reasons.

00:35:49 And at that point, the resistance

00:35:51 pops normal again.

00:35:53 And finally, if we put too much current

00:35:55 through that superconductor,

00:35:57 we'll end up, at a critical current,

00:35:59 we'll end up with the resistance

00:36:01 going finite again

00:36:03 and dissipation.

00:36:05 Now, all three of those

00:36:07 simple curves

00:36:09 suggest applications,

00:36:11 which I'll get back to at the end of my talk.

00:36:13 The most obvious application

00:36:15 is, of course,

00:36:17 high magnetic fields.

00:36:19 Because

00:36:21 these critical currents,

00:36:23 these HC2s,

00:36:25 or these oxide superconductors

00:36:27 are now getting up towards

00:36:29 a magnet gauss, possibly.

00:36:31 Then, as

00:36:33 people started to mix metals together

00:36:35 forming alloys, they discovered a couple of things.

00:36:37 They discovered, one, that the

00:36:39 critical temperatures went higher

00:36:41 in alloys, and that the critical fields

00:36:43 went higher in alloys.

00:36:45 That's a subtlety associated

00:36:47 with the nature of

00:36:49 magnetic fluxes

00:36:51 in superconductors.

00:36:53 That's a subtlety which I don't want to go into

00:36:55 here, but suffice it

00:36:57 to say that at that point, people started thinking

00:36:59 about high field magnets.

00:37:03 As a function of time,

00:37:05 people then discovered

00:37:07 that intermetallic

00:37:09 compounds showed

00:37:11 even higher critical temperatures

00:37:13 and even higher critical fields,

00:37:15 and that brought in the

00:37:17 era of the A15s, vanadium-3-silicon,

00:37:19 niobium-3-tin,

00:37:21 lithium-3-germanium, etc.

00:37:23 That brought us up to 1973

00:37:25 and the present, the present being

00:37:27 early 1986.

00:37:29 And finally, these oxides,

00:37:31 the critical temperatures

00:37:33 are substantially higher,

00:37:35 and the

00:37:37 critical fields,

00:37:39 it turns out, are substantially

00:37:41 higher.

00:37:47 Of course, history doesn't

00:37:49 really evolve as simply as

00:37:51 people when they're sitting in a

00:37:53 chair after it's happened

00:37:55 and want to make

00:37:57 slides.

00:37:59 I'd like to write it down.

00:38:01 It turns out that

00:38:03 these oxides actually

00:38:05 have been around for

00:38:07 quite some time, and in fact

00:38:09 some of them we've known

00:38:11 have been superconductors for

00:38:13 some time.

00:38:15 The first one, to my knowledge,

00:38:17 was discovered in the

00:38:19 mid-60s, and that was strontium

00:38:21 titanate. It was

00:38:23 a semiconducting

00:38:25 superconductor.

00:38:27 It had a very low carrier density.

00:38:29 When you doped the strontium titanate with

00:38:31 niobium, tantalum, or, God forbid,

00:38:33 even hydrogen, what was discovered

00:38:35 was that it became a superconductor.

00:38:37 Now, its critical temperature didn't get much above

00:38:39 about 0.6 or 0.7

00:38:41 kelvin, so aside

00:38:43 from the intrinsic interest in why the

00:38:45 superconductor, there wasn't

00:38:47 really any practical interest

00:38:49 in it, although I remember the time

00:38:51 I was a student when

00:38:53 strontium titanate was

00:38:55 first discovered,

00:38:57 and people were chewing over why it was

00:38:59 a superconductor.

00:39:01 There really wasn't a satisfactory explanation

00:39:03 for why strontium titanate was a superconductor.

00:39:05 And then in the mid-70s,

00:39:07 and you may hear a little more of it from

00:39:09 parts later, in the mid-70s,

00:39:11 bismuth oxide was

00:39:13 discovered, and

00:39:15 I remember

00:39:17 at that time,

00:39:19 a lot of people, us included,

00:39:21 we were chewing on trying to understand why

00:39:23 that was a superconductor.

00:39:25 It had a critical temperature which was quite

00:39:27 respectable. It was about 11 degrees.

00:39:29 And again,

00:39:31 we really weren't satisfied as to why

00:39:33 as to whether

00:39:35 we understood whether it was a superconductor

00:39:37 or why it was a superconductor.

00:39:39 So there around, what we didn't recognize,

00:39:41 and I have to say,

00:39:43 was that there was a class

00:39:45 of materials there.

00:39:47 You know, you kid yourself, of course.

00:39:53 And so

00:39:55 in fact,

00:39:57 I expect that this is not

00:39:59 a complete list of

00:40:01 superconducting oxides. In fact, I know it's not

00:40:03 a complete list of superconducting oxides.

00:40:05 I'm afraid it's a parochial view.

00:40:07 And this starts

00:40:09 with what I suggested, which was the

00:40:11 early work on strontium titanate.

00:40:13 And then in the mid-70s,

00:40:15 barium lead bismuth oxide

00:40:17 really got people more interested

00:40:19 in these oxides.

00:40:23 Both because

00:40:25 they were higher critical temperatures.

00:40:27 Well, high, geez, it doesn't look high anymore.

00:40:29 13 Kelvin.

00:40:33 But also because they were very

00:40:35 interesting from a physicist's point of view.

00:40:37 The materials were either

00:40:39 superconductors or very close to metal insulator

00:40:41 transitions, and that was

00:40:43 interesting in its own right.

00:40:45 Then, of course, 1986, Ben Orson Mueller

00:40:47 with the lanthanum-barium

00:40:49 copper oxide, and then

00:40:51 I'm parochial on the next one,

00:40:53 the Bell Labs effort in lanthanum strontium

00:40:55 copper oxide, and then

00:40:59 the over-90

00:41:01 Kelvin work

00:41:03 of Chew and his collaborators, and then the rest

00:41:05 of the world following suit one weekend.

00:41:09 It was a hell of a weekend.

00:41:13 And then

00:41:15 I have down there etc.,

00:41:17 which are

00:41:19 all the reports

00:41:21 that have been,

00:41:23 all the press conferences that have

00:41:25 ensued from that time on.

00:41:27 What I have listed here are verified

00:41:29 superconductors.

00:41:31 It's not that I'm pessimistic,

00:41:33 I'm enormously optimistic

00:41:35 that we haven't seen the end of this,

00:41:37 and that's the reason for the etc.

00:41:41 What I have listed here

00:41:43 are superconductors that the community

00:41:45 basically agrees are now all

00:41:47 superconductors.

00:41:49 Most of the other

00:41:51 press conferences

00:41:53 and rumors, etc., that some of you have heard

00:41:55 may be right, they just haven't been

00:41:57 verified in other laboratories yet.

00:41:59 And so I haven't included them

00:42:01 in that view graph.

00:42:03 Now, let's talk about

00:42:05 limits. Where the hell is it going to end?

00:42:07 And I guess

00:42:09 I have to say I don't know

00:42:11 at this point. If I were giving

00:42:13 this talk a year ago, I'd say 30 degrees

00:42:15 is about as high as we're ever going to get.

00:42:17 Now that's honest. I really would have said that a year ago.

00:42:19 I'd say, I just don't see how we're going to get above 30 degrees.

00:42:23 But we're well above that,

00:42:25 and that has to

00:42:27 rattle you at the knees

00:42:29 a little bit and

00:42:31 make you start thinking that

00:42:33 perhaps there are other mechanisms

00:42:35 beside this electron-phonon interaction

00:42:37 that I described for you

00:42:39 several slides ago.

00:42:41 Remember what I told you,

00:42:43 that the critical temperature

00:42:45 scales with the

00:42:47 lattice vibration frequencies,

00:42:49 that's BCS theory, and then some

00:42:51 exponents, which are basically the coupling

00:42:53 constants.

00:42:55 Now, we and other people

00:42:57 went through detailed

00:42:59 numerical studies of the theory

00:43:01 ad nauseum in the 70s, and what

00:43:03 we concluded was that there was no limited

00:43:05 principle using the electron-phonon

00:43:07 interaction, but what happened was that nature couldn't

00:43:09 stand it. As you kept cranking up

00:43:11 the electron-phonon interaction,

00:43:13 finally nature would say, I can't take any more

00:43:15 and there'd be a structural phase transition.

00:43:21 So there really wasn't any

00:43:23 reason for optimism.

00:43:25 It really looked as if

00:43:27 we were almost at the limit.

00:43:29 And then

00:43:31 these oxides came along.

00:43:33 And although they are close to structural

00:43:35 phase transitions, try as I

00:43:37 might, and putting numbers

00:43:39 as well as I understand

00:43:41 electron-phonon interaction,

00:43:43 I don't see how we can get a 100-degree superconductor

00:43:45 out of electron-phonon interaction.

00:43:47 It just doesn't seem reasonable.

00:43:49 So,

00:43:51 at last count, there were

00:43:53 eight mechanisms

00:43:55 that I know of from various theorists

00:43:57 for

00:43:59 these superconductors.

00:44:01 And they range from basically

00:44:03 every interaction that electrons have

00:44:05 in solids. Theorists have

00:44:07 now postulated that this is

00:44:09 responsible for superconductivity.

00:44:11 It's a hell of an exciting time for us

00:44:13 because there's

00:44:15 some really new physics going on here.

00:44:19 I think I'll skip that.

00:44:23 Now,

00:44:25 potential applications.

00:44:27 I think it's important to discuss this a little bit.

00:44:29 And I have two caveats there.

00:44:31 The first is, be careful of

00:44:33 overly-my English is really terrible-be careful

00:44:35 of overly enthusiastic physicists.

00:44:37 On the other hand, I'm an

00:44:39 overly enthusiastic physicist at the moment.

00:44:43 In the

00:44:45 days of the late 60s and early 70s

00:44:47 when people were worried about

00:44:49 applications of superconductivity,

00:44:51 and they went through all the

00:44:53 calculations of power transmission,

00:44:55 microcircuitry,

00:44:57 energy storage

00:44:59 with massive magnetic fields

00:45:01 and coils wrapped around mountains

00:45:03 and things like that where you would store

00:45:05 electric power, the equivalent of pumping

00:45:07 water up into a

00:45:09 lake and then dropping it

00:45:11 during the night time and then letting it run down

00:45:13 through turbines during the day, which is

00:45:15 massive superconducting costs.

00:45:17 People went through all those calculations. They've all been done.

00:45:19 And the

00:45:21 economics basically said, forget it.

00:45:23 Well,

00:45:25 the economics have changed.

00:45:27 And all of these

00:45:29 potential applications have to be looked at again.

00:45:31 High-field magnets are an

00:45:33 obvious application, and

00:45:35 I just don't see how that's not going to happen.

00:45:39 One that's not really talked about very much,

00:45:41 but one which

00:45:43 should be looked at, we are looking at,

00:45:45 are low-level sensors and detectors.

00:45:47 These things are marvelous infrared detectors.

00:45:49 Infrared

00:45:51 detectors working at liquid nitrogen temperature.

00:45:53 Now, if you're particularly perverse, you can think

00:45:55 of reasons why you would like infrared detectors

00:45:57 working at nitrogen temperatures.

00:46:01 High-speed electronics, one of the serious problems

00:46:03 associated with

00:46:05 taking silicon smaller and smaller and

00:46:07 smaller, is that ultimately

00:46:09 you're going to melt

00:46:11 your chips. And the

00:46:13 reason is that the power

00:46:15 dissipation becomes just too high.

00:46:17 You think about

00:46:19 how much current you're driving through the interconnects

00:46:21 between transistors,

00:46:23 and how much power you have

00:46:25 to dissipate every time you take

00:46:27 a MOSFET and you slam it from

00:46:29 high to low,

00:46:31 and you

00:46:33 start going sub-micron, and you realize

00:46:35 how much power is dissipated

00:46:37 in each switch.

00:46:39 You come to the realization that the power dissipation

00:46:41 becomes rather large,

00:46:43 and that's one of the very serious limitations

00:46:45 on

00:46:47 microcircuitry.

00:46:49 Well, people should rethink

00:46:51 that issue, because you can

00:46:53 imagine having interconnects where

00:46:55 power dissipation is the problem.

00:46:57 And finally, one of the most way-out

00:46:59 possibilities are transmission lines, where

00:47:01 you can imagine running one of these lines from

00:47:03 well, let's see, this isn't

00:47:05 the East Coast anymore, so you can imagine

00:47:07 one of these lines from hydroelectric

00:47:09 power plants and go back to Denver? No.

00:47:11 But to the Northeast,

00:47:13 for example.

00:47:15 And I think these things have to be looked at again.

00:47:17 These things are

00:47:19 potentially very serious and could

00:47:21 indeed change

00:47:23 how much we pay for oil.

00:47:27 Finally, where is it going? This is my last slide.

00:47:31 Obviously, there are going

00:47:33 to be some more new materials.

00:47:35 This is clearly not the end of it.

00:47:37 After all, we're just talking

00:47:39 about copper oxide, but I hope I've

00:47:41 convinced you already that

00:47:43 there were some early

00:47:45 titanates and some

00:47:47 bismuth oxides.

00:47:49 There

00:47:51 are undoubtedly

00:47:53 more.

00:47:55 And

00:47:57 I'm just hopeful

00:47:59 that we'll

00:48:01 be on top of them when they appear.

00:48:03 Higher critical temperatures, I think,

00:48:05 will happen. I don't believe

00:48:07 we've seen the end of that, either.

00:48:09 There's clearly some new physics. I hope

00:48:11 I've conveyed that to you, that the mechanisms

00:48:13 responsible for this phenomenon

00:48:15 are just not understood.

00:48:17 And applications, as we

00:48:19 learn how to make wires, tapes,

00:48:21 thin films, etc., etc., the applications

00:48:23 will follow from that.

00:48:25 These are very exciting times.

00:48:27 I believe we're seeing history

00:48:29 at this point.

00:48:31 How much it's going to change our lives, I don't think

00:48:33 any of us know yet.

00:48:35 It's one of those times

00:48:37 where if you're not in the business, you probably wish

00:48:39 you were, and if you are in the business,

00:48:41 you wish you'd get a good night's sleep.

00:48:43 Thank you very much.

00:48:45 applause

00:48:47 applause