Oral History Transcript — Dr. Robert Maurer
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Robert Maurer; December 22, 1981
ABSTRACT: Opinions on the physics of color centers and point defects. Comments on the contributions of Yakov Frenkel, Robert Pohl, Nevill Mott, and others; developments in real crystals, rectification, and color centers.
Szymborski:would be very grateful if I could register what you just said
I would trace something like -- maybe there were more -- three lines: One began with an attempt to understand the perfect crystal. The basic properties, like cohesive energy, thermal coefficient of expansion, in other words what we might call, thermodynamic properties, and to develop a quantum theory of the electron in solids, and through them, through their wave functions and interaction understand things like cohesive energy. That, I think, really began with people like Wigner, Seitz and Slater, but later great many people -- Peierls [???] contributed to it. Amongst the people who were involved in this was Mott, who really moved out -- that’s what I would think is the second line of theory -- and became extremely active in applying all of this to a tremendously wide range of properties in both metals and insulators and then semiconductors, attempting to understand electrical conduction, optical properties etc., with tremendous achievements.
But there was still another line, which had great limitations, which in the early stage rally was the most appropriate. It was concerned primarily with defects in crystals, and it begins with people like Frenkel and Schottky, and then Wagner, and - perhaps most active exponent -- Pohl. Within this central—European activity there were two lines of motion. People like Schottky and Wigner were interested in applying what they could of thermodynamics and statistical— mechanics to really very wide variety of problems [???]
Szymborski:So, this was more phenomenological approach?
Maurer:I would think of it as a very classical approach.
Szymborski:Pohl was not very much interested in quantum physics...
Maurer:You know, to a very large extend, neither was Wagner. If you look at Wagner’s work you find that it’s really all centered in work you find that it’s really all centered in more or less classical thermodynamics and statistical mechanics. Pohl brought to it almost the same point of view. With actually less emphasis on.... Pohl almost treated most problems as single particle problems. I don’t think he had the depth of people like Wagner but he had this intuitive sense of what was likely to be important. He was really an experimentalist who scorned theory. The idea that somebody would do an experiment and rely on somebody else to interpret it in term of theory was anathema to Pohl. (In his opinion) you were not a real physicist unless you both conceive the idea, did the experiment and you interpret it.
Szymborski:But, finally, as far as I know, he was happy that Mott became interested in interpreting his experiments.
Maurer:Well, things got out of hand for people like Pohl. They really could not do the interpretation. They had cut themselves off from the modern and really sophisticated theory, and theorists. But, they were awfully good within their 1imitation and they were awfully good at taking information and boiling it down into a very, very simple, remarkably good approximation. They had this nose for smelling what the next experiment ought to be. We have a lot of theorists around who solve a problem, but as far as they are concerned it’s an end of it. They don’t really ask themselves “where do we go from here”? -- except perhaps in a sense of how can we refine a theory. But Pohl always was asking “now, when we learned this much, what need we do, next? And what does this last experiment and its interpretation tells us about what to do tomorrow?” He was really a hunter like a weasel or a ferret. Pick more than Hilsch had this attitude. One of the people who came from this group, is Pick’s group in Stuttgart, is Fritz Luty. If you look at interpretations that Luty has done of things like alkalia halides crystals containing lithium, or structure of the impurity center it smacks [???], you see the Pohl tradition right away.
On the other hand Luty is not a man to ignore the sophisticated, new theory. That is something that changes with the time. You must remember Pohl was really a physicist of the 30s.
Szymborski:He stated even much earlier with his work on photoelectric effect with Pringsheim...
Maurer:Pringsheim was here during the latter part of the war. He started that group at Argonne. He was an old man, and I remember, when I came here I ‘49 it was just about that time, or little earlier, that Pringsheim started a group to work on Alkali halides at Argonne. He recruited a couple of young physical chemists -- Delbecq and Yuster. I remember we were very skeptical about that. At the first place, I thought Pringsheim was really over the hill, much too old to do anything. I did not know Delbecq and Yuster, but I was pretty sure they had literally no background, and they were plunging into what was a terribly complicated area of optical and luminescent problems. It took only a couple of years before I realized that I made a mistake. They were really good (laughs). I don’t know how much Pringsheim had to do with it except to get the group started.
So, these were the three lines. In the first line, I must say Slater was just as important as Wigner and Seitz. This should be Wigner—Seitz—Slater. Then there is a whole group around Mott -Gurney, Jones … They started from the same basis as Wigner did. And there is this German group which [???]. Remakable type equation here.
Szymborski:In a sense, everything goes back to this German group.
Maurer:Everything really goes back to people like Frenkel and Schottky. Frenkel had very, very early understanding of what there are in the crystals, the basis for the cohesion of the atoms, for the structure and the defects.
Szymborski:You did not know him?
Maurer:I never met him. Schottky I knew. We had a meeting at Gottingen, ‘49 I guess, and Schottky was there, and there were a couple of very bright guys from Berlin -- Stasiw and Teltow [???], who did a lot of very good work. ‘I remember, my German was little better than it is now, but when Stasiw talked, and it was one of the first talks, I could not understand him. And I thought “Oh, God, my German....” And I mentioned this to Becker, and Becker roared and said “we can’t understand him -- he is a Ukrainian [???]” Schottky was a very meticulous man. He had a pattern, and he had to follow it. He gave one talk on defects. It was...quale [???] because he had a pile of notes like that and he was going clearly to go just all through. It was going to take a long, long time. And he had beautifully drawn figures which he carefully reproduced on the blackboard. They were colored figures, and he got completely stopped at one point. He was drawing a figure on the board and he looked around, and he stopped, and he could not go on. He turned, and looked at the audience, and said: “Wo ist die schone grune kreide?” (Laughs) -- Without green chalk he was lost. And somebody had to leave the room, we waited ten minutes, and came back with a piece of green chalk, and Schottky could continue.
Szymborski:One of my problems will be to acknowledge Russian contribution to the field. It’s not very easy, because they are not very helpful.
Maurer:I think the real problem is that we in the United States paid very little attention to the Russians. They published very little, a few scraps which we could not really make much of it. The people who probably were more in contact were the Europeans -- the French and the Germans.
I remember, that was some time in the sixties, I was in India and at the University of Delhi (I was an advisor to a summer school there) and a graduate student came to me who was doing a thesis on alkali halides (one of the people who was here, Jaina took the idea back to India). He was doing some work which was really based on a Russian paper. And that was one of the first Russian papers that I really looked at in depth. I looked at it and I had a funny feeling that it was wrong. It was dealing
with color centers, an attempt to explain the lack of coloration by cadmium vapors.
(Tells about the Russian paper)
Besides Frenkel, one of these significant Russians in these early days was Davydov. There was not enough work in the 50s, published in an understandable form, to take it very seriously. An effect would be announced, but there would not be any background, that you could judge the experimental technique, anything like that.
When did you realize that defects play such an important role in determining real crystals properties?
Maurer:In 1940 I started working with von Hippel and he got me working on dielectric breakdown of glasses and conduction mechanisms in glasses and dielectric breakdown of crystals, primarily quartz. And the first thing that happened was that, looking at conduction in glasses I found out from previous work that it was due to interstitial material primarily, in typical glasses, sodium. So that one was looking at the migration of a particular ion, through the glass and one could begin to think of it in terms of the typical ionic conduction in liquids, with the exception that in liquids it was more like a gaseous problem of interparticle collisions whereas in a glass you undoubtedly had to think of it in terms of a hopping motion from one trapping site to another, of the sodium. But that actually did not play a great role. It directed my attention toward diffusion processes, but did not really make me terribly alert to the role of defects, because a glass is an amorphous material and it is hard to tell what a defect is and what isn’t. What happened was that I was looking around for other problems to investigate and I was particularly interested in electronic phenomena in crystals. One of-the things that knew about was the importance of the cuprous oxide rectifier as a device. I started to look through the literature and I found that a group of Germans, most of them under the influence of Carl Wagner, had studied the electronic conduction in a series of solids, things like lead oxide, tin oxide, and what they had primarily done was to examine the fact of equilibrium pressure of oxygen on the conduction process. The conductive of these materials, they are typically prepared in form of thin films, changed very much with the ambient pressure. So people studied it. And they related that fact to the concepts of Frenkel and Schottky defects. What you were doing was changing the stoichiometry of the crystals [???]From purely thermo Dynic considerations Wagner had derived equations for how the conductivity should depend upon the pressure of oxygen. It worked pretty well.
In the course of looking around for some interesting problem to work on I thought that these oxides problems were rather complicated, and furthermore, in many cases took relatively high temperatures. My resources were very limited at that point because I was supposed to be working on dielectric breakdown and I had to do anything else [???] of my time schedule. But I read into a very early work of Baedeker on cuprous Iodide. That was a material with which you worked in a very low temperature. [???] (Covered by Lillian Hoddeson’s interview) That really got me interested in the question of defects.
Now, on your question on color centers. I don’t really think I was much aware of color centers and the work of Pohl’s group until about 1940. I was then working in von Hippels laboratory. [???] Von Hippel came from Germany and was very aware of work of Pohl’s group and furthermore, about that time, I got hold of Mott-Gurney book. And also there was this Seitz’s book which I remember has a little bit of it.
Szymborski:So, you don’t remember that this was such an important problem in solid state physics at this time?
Maurer:Well, I think it was, but I was very young and much focused on another area. You must remember that during the thirties nobody really talked about the solid state physics. There were just people doing many different things. One of the largest issues concerned photoelectric and thermionic properties. It was reaching the end of period in the end of thirties, but people like Lee BuBridge, had made his reputation on studying photoelectric and thermionic properties. Nottingham at MIT was one of the prime people in the field. The earlier work prior to DuBridge’s was little bit messy. People like DuBridge introduced really good vacuum techniques for that period, appreciated the necessity of the clean surface...
Szymborski:I think Robert Millikan’s work in this field was also quite important.
Maurer:Millikan’s works was very early, but remember that Millikan was not really interested in properties of solids. He was interested in sodium photoelectric effect from the point of view of checking Einstein’s ideas. [???]I don’t think the vacuum was better that 10-6... He never measured work function of sodium or any other material.
Dushman and Richardson had put together, independently, thermionic equation. There was a great deal of activity in studying the properties of polycrystalline surfaces. In photoelectric effect very important was introduction of Drude and Sommerfeld theory of metals. Particularly the idea of the Fermi distribution, and a Fermi level. Great deal of DuBridge’s work was devoted to showing that temperature effect on the photoelectric effect was properly described by the Fermi function. It’s sort of forgotten now, but at this time it was probably the very best, almost the only experimental reason for believing in the Fermi function. People studied the energy distraction of photoelectrons emitted as the function of the temperature of the surface.
Szymborski:Historically photoelectric effect was somehow connected with defects, for that’s what Pohl had been doing before he turned to internal photo effect and ionic crystals.
Maurer:You are talking about two very different photoelectric phenomena. When you talk about internal photoelectric effect it’s what we call photoconductivity in insulators or semiconductors.
K. S.: Was DuBridge also interested in photoconductivity?
Maurer:Not as far as I know. Only in the surface phenomena. In 1934 he was getting out of the business. About everything that could be done had been done. It was still not possible to prepare a really clean surface and know it’d clean. [???] Nuclear physics was becoming very exciting and just at the time I began to work in 34—35 with BuBridge he was building a small cyclotron and there was just a remnant -- we were about four people: Marvin Man [???] (I took over his apparatus), Al Hale, who became eventually the provost of MIT, was working there with him, Leroy Apker [???] There was one aspect what you might call Solid state physics but it was only one. There were a large number of other activities going on; one of these was for example all this business of dielectric breakdown which had important technological applications. People measured things like dielectric breakdown getting very scattered results, almost impossible to interpret them. There was a work in Germany that I mentioned on the effect of gases like oxygen on the conductivity of crystals, and there was -- and this was like DuBridge’s work on photoelectric effect -- Pohl’s work was one of the consistent well developed programs for studying crystals. That was one of its great strengths. The other great strength was that Pohl had the sense to work with rather simple materials and to study those problems which he thought -- and he turned to be correct -- were susceptible to a relatively simple interpretation.
They studied, for example, the production of defects in alkali halides by soaking them in alkali vapor. And they studied the changes in optical properties, the changes in electrical properties. They did this in a consistent fashion. Actually Pohl started with photoconductivity in diamond, but he gave that up rather quickly. (...) As soon as he started the work with alkali halides he found he had a more useful material to deal with. Despite the fact that it was not an element but a compound. He found that there was a wealth of properties that could be studied, and that could be studied systematically. So, this was an important group. Other people around who were doing things -- one of the old, old, really old problems was diffusion in solids, particularly metals. And if you go back to the turn of the century you will find there were people who were studying the diffusion of one metal in another. They were using chemical techniques and in general, by hindsight, not really being able to get to
do very well because the techniques were too poor. What you really wanted to study from the fundamental point of view was self-diffusion. And the only thing they could do there -- some interesting things were done -- was the discovery of radioactive materials and particularly radioactive lead. A few people in Germany studied self- diffusion in materials like lead and were able to get some preliminary, but interesting results. That sort of things went on slowly, because, for example, of its importance of a technical sort in Haber process for fixing nitrogen [???] This process was developed in Germany, I guess during World War One, and plants blew up because hydrogen under pressure diffused into steel and enbrittled it and the embrittlement of steel by hydrogen was an interesting problem that various people worked on. There was a man at the University of Pennsylvania who did a long series of experiments on diffusion of hydrogen in iron and in nickel. Very complicated problem... They are
still working on it (laughs).
Then there were people who worked on luminescence. Turns out, that almost everything luminesces (laughs). And there was absolutely no real theory up till the thirties. So, people took data, they did not appreciate the role of impurities in Producing luminescence...
Szymborski:Who was pioneer in this field in the States?
Maurer:I would have to go back and get one of my older books. There is a book by Pringsheim on luminescence which gives references.
There is a long history there. Just as for instance, there is a long history of the subject of rectification. And that was another topic, which was really the beginning of the semiconductor physics. The fact that, for instance, that nickel particles were used as detectors in some of the early radios, they called them coherers, and it was discovered that galena, zinc sulfide, was a rectifier.
The two important industrial rectifiers were selenium and cuprous oxide. Westinghouse had a large program of studying cuprous oxide. In latter part of the thirties a theory of rectification emerged as a result of applying quantum mechanics -- work of Wilson, and Mott, and people in Germany (I have really forgotten the names). But all of this, each area was done in isolation of the others and there was not very much of a connection between them. The connections just began to be made during the middle thirties as a result of application of quantum mechanics to crystals, the idea of the Bloch wave function, the work of people like Seitz, Wigner and Slater, and very much the work of Mott and his coworkers in applying these ideas to a wide variety of problems ranging from rectification and semiconducting to dielectric breakdown. And it was really this application of basic theory which led people to realize that there was a common basis for the optical properties, for the electronic conduction properties, rectification properties, even to a certain extent mechanical properties, at least more thermodynamic properties like specific heats and thermal coefficient. This is what provided unification of the field and led people to begin to talk about solid state physics.
Szymborski:To which extent the impurities and defects played the same unifying role? People probably started to realize that in all these fields’ real properties of crystals were determined by small amounts of these “irregularities”.
Maurer:That came rather late. Let me give an example. I was at the University of Pennsylvania in 1942 with Seitz. And at that time Seitz gave a course to the metallurgists on dislocations. This was, you know, a completely new stuff to them. And meant was that the idea that the mechanical properties were determined by a defect which was a primitive defect, a really basic defect, not accidental cracks or micro fissures, but something that was intrinsic part of a crystal -- that was still to a lot of people new, as late as 1942. Now, the information had been growing during the late thirties and I really do not know what the date of Cottrell’s book is, for example, but that came later I think after the war.
Let me give another example. After the war I went back to Carnegie in 1946 and started work on self-diffusion in alkali halide crystals. We were interested in defects. It was known that the electrical conductivity of sodium chloride crystals, for example, is due to the sodium ions. That was all measured in Germany by people like Wagner. The work of Pohl had really established the fact that there were chlorine inn vacancies, in other words that you had Schottky defects in a crystal. Soon after I started this Wagner came to the United States -- he was brought by the US Army -- and I talked with him in Pittsburg and told him about what we were doing. He immediately perked up [???] because he around 1942 in Germany had started to do the same sort of thing with a man named Hantelman, and that work had been interrupted by the war. As soon as he heard what we were doing he said “Oh, I have to get this stuff published”. He did. The point is that as early as 1942 Wagner had been aware of the fact that you can learn things about defects by studying ionic conductivity. He did not have technique for self-diffusion. We were measuring both ionic conductivity and self-diffusion.
The primitive state of ideas at that time is illustrated by this: we found, for example, that at relatively high temperatures, over 5500 the ionic conductivity and the self-diffusion coefficient was a very consistent function of temperature. You could measure at one crystal and another crystal and you got the same result. But when you went below about 500 you found that the ionic conductivity at given temperature like 3000 may vary by a factor of 2 or 4 from crystal to crystal. The same thing was true of self-diffusion. That was not entirely new. People had observed that. But it was still debated at that time whether it was due to a fundamental type of defect or rather that was due to micro fissures, micro cracks. That was not really understood. I think the work that was done by Dillon Mapother on the sodium chloride was the first real data that demonstrated that erratic results were due to accidental variations in the quantity of impurities in the crystal. That was when the things really started becoming very sensible.
At the same time I had a student, for example, Howard Etzel, and he studied conductivity of sodium chloride as a function of temperature and deliberate addition of impurities. We decided that if impurities were the trouble-maker the thing to do was to put some deliberate impurities and control the amount of them, and measure the amount of impurities. It was not a really new idea. We got it all from Wagner. Wagner had talked about these things and he even done a little bit. Wagner never did a great deal that was consistent and programmed. He did quick and dirty experiment and then would do something else. But he had all these ideas which were just based upon his understanding of Schottky and Frenkel’s work. But Etzel measured the ionic conductivity of sodium chloride as a function of temperature and concentration of cadmium, the divalent cation. We applied the theory that people like Wagner had developed and we were able to show not only that one was varying the concentration of positive ion vacancies by putting cadmium in but that sodium ion vacancies associated forming a kind of molecule to a certain extent with the cadmium ion. You had equilibrium between associated ion vacancies and free ion vacancies. From the data one could determine the binding energy of these associations, of these complexes as we called them.
This Etzel paper was the first I think really convincing, definitive paper on this subject. One result was that it got quoted in the literature for over ten years. Everybody who worked somewhere in that field put [???] himself going back and referring to Etzel. I had students who made more difficult theses but I do not think I ever got one whose work was quoted over as long a period. Eventually people made better measurements (Laughs). But, you see, in 1946 and between 1946 and 1949 we were really just giving an experimental basis for these ideas of Frenkel and Schottky and Wagner and establishing the role of the defects. So it came very late. I would say, by l94O the ideas were all around.
Szymborski:Would you say that about this time the main center of research in this field moved from Germany to the States. Many people of your generation. -- I mean also Fred Seitz, for instance, -- got their inspiration from German work. It changed later people started coming front Germany to this country to learn what is really going on in physics.
Maurer:As a result of the popularization of the idea of dislocation, for example, by people like Seitz, and its application by people like Zener, (he wrote a little book on inelasticity -- it must have been in early fifties) the whole subject of interpreting the properties, the mechanical properties of metals in terms of dislocations got a big push in this country. I think the Germans picked up - people like Seeger -- very, very fast. But, remember, they had come out of Germany that was destroyed, and furthermore Germany that had not, even before the war, been particularly strong in theory because of the influence of Nazi attitudes and the disappearance of German theorists. You must also remember something else that is that in pre-war Germany there was this magnificent group of theorists but they were all interested in very basic phenomena. We would call it atomic physics, quantum mechanics, and they, on a whole, looked upon work done on solids by people like Pohl, for example, as too messy. You could not get your teeth at what you think fundamental. I think, Pauli, for example, was one of the people who applied quantum mechanics to specific heat (I think it was Pauli) and. paramagnetic but this was sort of sport, thing that you did. You could not really take it too seriously -- solids were too complicated. It was all right, there were X-ray people who could study the structure of crystals, but what you got beyond that kind of things -- mechanical properties were messy, optical properties were messy. —The only thing that you could measure, that was sort of consistent, were things like electronic conductivity of metals, and temperature coefficient. There was not any hope of understanding it (...) by theory... So the Germans were very much lagging [???] in theory after the war, but they picked up the ideas that were developed by in theory in this country.
Szymborski:How the field of F-centers research grew in this country? When you started working on it who was other people you could talk to?
Maurer:There was a man named Jordan [???] Markham, very important, who was originally at the applied physics laboratory at Johns Hopkins U. He got interested in alkali halide crystals, and defects, and color centers. There were people like David Dexter, who was here and who got interested in that in Rochester when he left. He died just a few years ago. He got interested, like people like Seitz, in the interpretation of the F-center optical absorption, and the optical properties of the crystals. It came a little bit later -- it was in the fifties -- but people like Charlie Slichter, and people in Berkeley realized that you could use spin resonance very effectively to study the paramagnetic trapped electron in the F-center. Then there was this marvelous work that was done -- what is the name associated with it? Kip? Kittel? People worked out very thoroughly the symmetry of the wave function of the trapped electron in an F-center. They showed, for example, that the ground state was not a simple s-state but had an admixture of g-state which reflected the cubic symmetry of the lattice; On the ionic conductivity, and defect work there was not very much that you could work with -- there was a man at Yale, Nowick, who started to work on it, and there was Smoluchowski at Carnegie. He tried very hard to understand the growth of centers under X-nirradiation. Very, very tough problem... Sometimes you are smart -- and I decided to keep away from that problem. They learned a lot, but the whole process is incredibly complicated. But in the, early days one of the people who picked up the ideas was the man named Friauf at University of Kansas. He did some of the best work on ionic conductivity and self—diffusion. (Story about simultaneous publication)
Subject opened up very quickly. Among the people you could talk to, were a few people who worked on inelastic properties and defects in metals. There was David Lazarus here, and there were several other people around the country working on diffusion in metals.
We had contact with the British -- there was Mott’s group. In 1948 or 49 they had the first Bristol conference, and L went over there to Bristol. The British were pretty supercilious, they... but there was a very bright young chap there— Lidiard. He eventually came here as a post-doc, and he eventually stopped [???] with the problem. We also had a couple of Italians here -- Fumi, and Chiarotti. Lidiard and Fumi spent many years working out the energy associated with a defect in an alkali halide. In other words, how much energy do you have to provide to produce one Schottky defect in sodium chloride? The initial step had been taken by Mott and Littleton, and then their formulation of the problem always remained the basic procedure. But Lidiard and Fumi refined this and over period of 10-15 years (it turned out; if you want to do it right, to be a very, very difficult problem.) A Mott-Littleton procedure and (…) the initial results just ignored some of the critical effects. It is the usual
thing you have to add and sum up terms of different sign and this result is small compared with the size of those terms, very subtle effects ate terribly important. We were measuring things like the energy to form a Schottky defect to within -- I would say -- at least a tenth of eV. And the theory would be good (unclear) to better than half eV.
But those were people we could talk to, We had very good contact with the Gottingen group. Pick was here for a year, Pohl made a visit here; various times we went to Gottingen and talked to him. Hilsch visited us. I would guess that our primary contact on color centers was Pick [???] and Pohl group at that time. There was not a great deal else in this country. Delbecq and Yuster group up at Argonne, and man I had worked with at MIT -- Jim Shulman went to NRL. Then two of my students went there, they were both students from Carnegie, who got their degrees about ‘48 -- Klick and Howard Etzel.
Szymborski:Where are they now?
Maurer:Etzel ended up as deputy director of the Material Science division at the NSF. He spent last ten or fifteen years of his career there. He has retired from this position and is teaching at some small college in Virginia now. (Gives me his address). Because of the Navy interest in luminescence and phosphorescence they started to work on luminescence. They did some more basic work on just defects.
Eventually one of my other students, this was much later, Dale Compton went there and there was also a very good man named Rabin, who worked with Klick and Shulman. They did a lot of work on the optical properties of the alkali halide crystals, also on glasses. Because, for example, they developed radiation badge [???] which was glass, which when exposed to say gamma rays could then be put into a little holder and heated up and it would give luminescence, thermo luminescence, and it would give measure of how much radiation. It was a dosimeter.
(...) I would guess that during the fifties besides from Argonne and Oak Ridge Argonne and NRL, few isolated people like Markham -- who was a theorist at John Hopkins -- we were the largest single group. The Japanese got into the act. WE had a large number of them come here -- Kanzaki worked with me, Kobayashi came -- he actually worked with Fred Brown, and the result was that we established a lot of relationships with the Japanese. There was Mitsuno who worked with Fred Brown, and in late part of the fifties we had a vigorous exchange with Japanese people. I remember at one of my first trips to Japan I spent some time with Kawamura who at that time was one of the leading semi-conductor people in Japan, he just translated Shockley’s book “Electrons and holes in semi-conductors”. I think these people who were here formed a nucleus -- Kanzaki, Kobayashi, Suzuki, who worked with Koehler on metals -- they formed the gore of the Institute of Solid State Physics that was formed originally with a man named Muto, who was a molecular physicist at Tokyo.
Szymborski:What was the impact of low temperature, introduced among others by Lawson McKenzie of ONR, on the study of color centers?
Maurer:The technique had a great impact because, for instance, a lot of spin resonance work you had to do in a very low temperature. On the whole, I would say, in this area the low temperature techniques were not at that time intensely critical. It was primarily the spin resonance work that was affected. I better be a little more careful there -- it wouldn’t be impossible, for example, to study things like the H-center. The fact, that he had, for instance, the Collins machine here, liquid helium, that people like Kanzig could work on the very low temperature irradiation with X-rays on crystals and form the H-centers. The H-centers were really discovered because people went down to liquid helium temperatures, liquid hydrogen and liquid helium. If one hadn’t had liquid helium conveniently available that work could not have been done. The Germans, for example, Pohl’s group, had discovered that centers, that is how they named them, they were the ultraviolet centers, which turned out to be due to holes, trapped holes, but you see, they never worked below liquid nitrogen temperatures. (discussion about the overall number of color centers) F, F’, soon after M (mystery for a long time) -- all by Pohl’s group. Several others. R-centers, more and more aggregations. Then there are two hole centers, which turned out to be a very great surprise, because we had always anticipated that there ought to be an analog of the F-center (Draws a picture on a piece of paper) which would be that (draws), and when V-bands were first seen (by X-ray irradiation in low temperature) we thought that was it. That was why I got Kanzig started on doing the spin resonance due to this center, because if there was a hole there it ought to have a paramagnetic signal. And he found it. He got the signal, he found it, and the interpretation began to look a little difficult. It. didn’t look like a simple thing which you’d expected from a single hole trapped in an essentially a Coulomb center. And Kanzig and Castner analyzed these data and at the same time Delbecq and Yuster did. And it turned out not to be this at all. It’s really this (draws a picture) what you might call a chlorine molecular ion, chlorine molecule that has gained one electron. The hole neutralizes one of the electrons. It has this symmetry in the crystal, which is one of the reasons why you can spot where it is, because when you do the angle dependence of the spin resonance you can get the orientation of it. Now, Delbecq and Yuster always referred to this as the chlorine molecular ion, that was their name for it, and we always called it a VK-center (K for Kanzig -- laughs). Delbecq and Yuster would never call it a center because this would tend to throw too much credit toward Kanzig, and we never called it chlorine molecular ion, because that might throw too much credit toward Delbecq and Yuster. Their papers were essentially contemporaneous, they both had it, and regardless of what was the publication date. I think Kanzig’s data preceded Delbecq’s and Yuster’s but Kanzig went off to GE and took his data with him, and it took him some time to analyze them.
But that was a shocker. There apparently is not such a center as that (Shows a picture). And then when one went down to the liquid helium temperature one found a big band down there that one called an H-band.
Szymborski:Who discovered it?
Maurer:Who did? Kanzig, Markham, Delbecq and Yuster. I don’t know. It’s possible that somebody at Pohl’s group did. It’s very possible, because at Gottingen Hilsch had set up liquid helium plant, it was a primitive one, Joule-Thompson expansion one... The H-center is even more complicated....
It was long time afterwards and one of the people at NRL, Klick I think, established the fact -- I think it’s pretty well accepted today -- that when you X-ray you have chlorine ions like this and a chlorine ion catch a hole but it’s unstable and it gets injected and it becomes an interstitial chlorine. So that you have in a lattice, during your irradiation, you form interstitial chlorine. That was pretty shocking at first, the idea. Nobody really believed that there was enough room for an interstitial particle in alkali halides. But I think it is pretty well accepted today.
I think it was Klick—Schulman group and Rabin at NRL that introduced this idea and provided the evidence. But that came rather late. I would guess it was in the sixties.
Incidentally, in Germany there were other groups working on alkali halides in particular there was Smekal’s group -- but it was really in Vienna. You are unlikely to find this out if you read the Pohl literature, because the Germans would not refer to other people’s work in Germany. The most amusing thing, but can go through dozens and dozens of papers by Pohl and you almost (end of tape).
One of the interesting people in Pohl’s group was Smekula, Alexander Smekula. He had the width [???] to take some of the classical work on the dispersion and absorption in atomic system and apply it very intelligently to crystals. So, he derived the formulae which connected the concentration of F-centers with the optical absorption (not important)
Very good source of information are Seitz’s papers in Review of Modern Physic. He really refers to key papers of the Pohl group in particular. He was interested in the fifties and he pulled together ... What he did was, he was trying to apply what theory he could, and so he summarized all the experimental data to which he hoped he could apply theory. One result was these review articles. There were two of them.
Szymborski:How successful was he in theoretical explanation of these data?
Maurer:I would say, modest success. Two things happened: just as he was doing it the spin resonance came. The spin resonance work gave such detailed information on the structure of the F-centers, in particular, that, you know, (laughs), it was all there. The application of theory when you have that much information is very straight forward. He was not very successful when it came to trying to devise models. That was I think his biggest failure. But, you know the great importance of those papers and most review papers of utmost importance it was not really what he did, it was a fact that he pulled the subject together into coherent whole. The result of that was that a lot of people got interested and took it very seriously, started to do experiments did much the same thing with dislocations, with this little book, very elementary book and the lectures he gave on dis1ocations. The influence went far beyond what success he had. What he really did was, it was almost elementary, he brought to a surface all of the basic phenomena which were scattered throughout lots of papers, inadequately appreciated and so on. It probably would not have been as influential in this country if it had not been done just after the war when a lot of people were going back to their laboratories and interested in ‘now, what do we do?”
Well, I mentioned Smekal. Smekal’s work was nearly as large in total volume as Pohl’s, and on the whole Smekal’s work was not as incisive [???]. On the other hand Smekal brought ... he focused attention upon some things that Pohl people pretty much ignored. In particular the possible role of larger defects than these point defects. Smekal was one of those people who raised the issue that may be there are things like micro cracks. He kept that sort of thing alive. Which you would never know from Pohl’s papers.
If you look at Pohl’s papers they are very limited. They present the results, they give a minimum of information about the technique used (they always insisted; for example their crystals were the purest available) and they never told you what they did. The Pohl people kept their secrets. They were not at all like here in America where people exchange crystals, told one another, visited each other’s laboratories, looked at the apparatus. It was a bloody system in Germany. They did not tell anybody anything that they did not have to in order to get credit.
Szymborski:In your interview given to Lillian Hoddeson you mentioned about Dutton and Teegarden’s work on the thermo luminescence. Would you comment on it?
As usual idea was not new. I think it occurred to me because (if I recall) in dealing with uranium compounds (was it Spedding?) and other materials (someone at Wisconsin, I think) had observed that if you took a rock and heated it up you got the burst of luminescence. That was associated with the idea that under bombardment with radioactive materials incorporated in the rock you got electrons knocked out of their normal positions in the crystal and trapped. They were stable in low temperatures but if you heated the, crystal up the electrons would recombine and you got the energy released in form of luminescence. We were very familiar with the idea that irradiation stored energy in the crystals. During the war we worked with graphite from nuclear pile.
We were irradiating crystals in liquid nitrogen temperature we would form these V-bands. When we heated the crystals up V-bands almost entirely disappeared. (We were almost sure they were trapped holes). Some of the F-centers went down and we looked at it as the holes recombining with the electrons of F-centers. We said, well, now there is probably some luminescence, when these electrons and holes recombine. So, I put Dutton to work on this problem, and sure enough you could pick the thermo luminescence’ very easily.
There was a simple theory which related the temperature (draws a picture) and the energy U. We used that to determine the depth of the trap, the thermal depth of the trap. Our interpretation was incorrect, because we thought we were seeing the release of holes. What Delbecq and Yuster later showed what we had seen was the release of electrons and the electrons were combining with the holes? But these were the electrons which were being released not the holes. Holes were trapped much deeper. But Dutton started this and Teegarden worked on it also and eventually it became a quite general technique, which was particularly exploited by the Italians at Turin. In various ways. For example, they used this to study the polarization of material (abstract). The subject has become very important recently in connection with archeology and dating of clay.
Szymborski:Do you think the work on metals should also be included in the chapter on defects?
Maurer:Undeniably, the effort of people like Seitz and Lidiard, and one of Seitz’s students Hill Huntington to calculate the energy to form a vacancy in copper, relatively simple metal, was to me one of the major theoretical efforts which sort of brought to bear [???] and illuminated all of problems in making quantum mechanica1 calculations in metals. So, I would suggest you talk with them, and then you talk to Brown about the photographic process. Because that, the application to photography really came a little later. During the thirties, for instance, people in Kodak did nothing practically on silver halide crystals. Their whole effort was concentrated on colloidal systems because that to them was the critical thing. They were aware very early that sulphur was very important in getting the fast emulsion, but the role that the sulphur played was not understood. I do not think it was really until the post war. It started I think largely with the English.
One thing that because of my own background seems to me to be little bit missing is something that disappeared in the end of the thirties and was not revived until fifties and sixties. I am talking about photoelectric effect. That is really a surface problem. During the thirties the study of emission of electrons from the surfaces was a very great thing, partly because all radios used thermionic ally emitting cathodes and photoelectric cells were important sensors. The rise of interest in nuclear physics took a lot of physicists out of that area and then the war focused attention on silicon and germanium systems and it was not until well after the war that vacuum technique had vastly improved, and furthermore the theory of photo effect of emission processes, particularly photoemission have never been very satisfactory. That also improved. People like Spicer out at Stanford took it up and really turned it into a tool that gives you a great deal of information about the energy distribution of electrons (digression) (Photoelectric effect)
The point I am making is that reasonable fraction of American physicists that were working on solids were working on the surfaces of solids during the early thirties in particular, and that was at that point an important part of anything you might call solid state physics.
Szymborski:It is still not quite clear to me if we can really isolate the field of defects from that of dislocations.
Maurer:It cannot, but on the other hand the origin, of the dislocation idea is independent really on point defects. And the original applications to the strength of crystals and to growth and spiral dislocation all of that really goes quite, I think, independently of point defects. The most natural connection between the two is: point defects can coalesce to form dislocation and when dislocation moves they can shed the point defects.
The big difference between the point defects and dislocations is that a point defect is not really something that makes a crystal imperfect. It should be called an ideal crystal not a perfect crystal that has no defects. Point defects are there for thermodynamic reasons. They lower the free energy of the crystal and make it stable. It would not be stable without them. Whereas the dislocations -- there is very little entropy associated with then, and a crystal with dislocations is intrinsically thermodynamically unstable. If you could remove the dislocations you would actually lower the energy of the crystal. When you form a dislocation in crystal U -- TS goes up. So there is a very great energy between point defects and dislocation.