Crystals and Ultrasound Old-Fashioned Materials Science
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Crystals and Ultrasound old-fashioned materials science Albert Migliori and Zachary Fisk tone Age, Bronze Age, Iron Age, for bronze were so remarkable that make the study of new materials excit- the age of exploration and the many, many others spent lifetimes ad- ing, risky, difficult, and what seems Ssearch for gold, the industrial rev- vancing available technology to im- best described as fun. olution and the steel that made it possi- prove that material to the point where Essential to the richness of the field ble, and, of course, the present age of the fully developed and vastly im- is an understanding of the laws of electronics—totally dependent on tiny proved copper alloys were to the first physics and chemistry that enable de- chunks of ultrapure, single-crystal sili- bronze nugget as that nugget was to the tection and realization of new proper- con—it seems that entire chapters in stone it replaced. ties of matter. But despite having its human history are strongly connected Just as important as the production roots in basic science, condensed-mat- to certain special materials that enabled of high-performance bronzes were the ter physics is viewed as a kind of ap- massive bursts of what has come to be parallel developments necessary to un- plied science: After all, we know all of called progress. derstand completely the properties of the fundamental physical laws that de- But the pursuit of human progress is such alloys. And so goes the story for scribe the individual atoms in solids. not the driving force behind physicists many other materials. The entire busi- The interesting fact, however, is that studying new materials today. We have ness of materials science, from the first the collective properties of many atoms much simpler motivations, more akin to discovery of a new substance to the full assembled together are too complex to the curiosity of the scientist who four development of its properties, is a be predicted from first principles. For thousand years ago first noticed the tremendously attractive intellectual example, the sharp transition from non- brilliant luster revealed by scratching puzzle of sufficient complexity to inter- magnetic to magnetic behavior in iron the surface of the glob of bronze pulled est almost anyone. Moreover, the in- at 1043 kelvins cannot be predicted from a state-of-the-art, charcoal-driven, credible importance of new materials, from a calculation of the properties of high-temperature, controlled-atmos- the qualitative changes in human life an assemblage of a few or even a few phere furnace. That scientist had, by that they produce, their intrinsic physi- hundred iron atoms. Such calculations serendipity, produced some sort of very cal attractiveness, their appeal to the in- indicate no abrupt transition to ferro- imperfect alloy. The appearance and tellect, and our total inability to foresee magnetism but only a gradual change the properties of even that poor excuse how important their impact will be from weakly magnetic to more strongly 182 Los Alamos Science Number 21 1993 Crystals and Ultrasound magnetic. Nobel laureate P. W. Ander- empirically and produce structures that ative idea led to their discovery. son has most clearly laid out this idea in are largely only what nature allows; The unpredictable effects found his article “More is Different”: He that is, the materials are microscopical- when large numbers of various atoms makes the point that phenomena arising ly homogeneous and theoretically in- are assembled thus provide both moti- as more and more particles interact are tractable. The payoff from this old- vation and justification for the empiri- not simple extrapolations of the behav- fashioned work is the discovery of cally based search for new physics and ior of a few particles. Further, we can- completely new phenomena, which chemistry through the study of new ma- not, nor do we want to, solve the most often arise when the number of atoms terials. Fortunately, the production of basic equations for the many-atom sys- is large (on the order of 1018). any single new material is often accom- tem because those equations are too A particular innovation that arose plished by a very few scientists work- complex and provide too much informa- from the “large-number effect” coupled ing with a small budget and a limited tion. No one cares where a particular with “old-fashioned” intuition was the collection of inexpensive equipment. atom is at a particular time in the mag- inadvertent discovery by K. Mueller in The demonstration of superconductivi- netic powder of a cassette tape. What is 1987 that certain cuprates (namely, ty in heavy-fermion compounds by F. important is the overall magnetization those copper-oxide compounds that are Steglich at Universität zu Köln, and the of millions of atoms. Thus most of the doped with transition-metal and other discovery of the high-temperature su- useful theoretical descriptions of solids impurities) are high-temperature super- perconductors by K. Müeller at IBM rely on statistical averages to predict conductors, that is, they become super- Zurich are typical examples of small, measurable quantities. There is, then, a conducting at temperatures well above successful efforts. However, underly- theoretical gap between our knowledge absolute zero (above 30 kelvins). ing all of the apparently small efforts is of the basic laws for each atom and our Mueller knew that those cuprates are a powerful information and technology ability to predict what large numbers of quite unusual solids. He also knew that base easily accessible to those working interacting atoms do. in many materials, if a smaller atom is within the umbrella of a large research In the last few years some attempts deliberately substituted for a larger laboratory or university. That access is have been made to control macroscopic atom at the center of each unit cell in crucial; without it small groups of sci- material properties by constructing arti- the crystal lattice, then when the solid entists, driven by whatever odd notions ficial materials through the deposition cools the smaller atom and its cloud of motivate them, could not succeed. of sequential layers of carefully con- electrons would not have a stable rest- trolled composition and thickness. At- ing spot in the center of its symmetric tempts are being made also to produce cage. As a result it must move to some The Remarkable Effects nanostructures, materials assembled other position in the unit cell and spon- of Impurities atom by atom through such processes taneously break the crystal symmetry. as molecular-beam epitaxy and chemi- The effect (called a Jahn-Teller insta- Also important to success is a clear cal or vapor deposition. Both ap- bility) is driven by large symmetric ar- focus on some particular set of material proaches produce very small, but not rays of atoms and is only poorly under- properties. Many groups today are fo- quite microscopic, building blocks, or stood in detail (“large numbers” + “in- cused on modifying the electronic and repeated units. Such designed materi- tuition” = “mysterious phenomena”). magnetic properties of solids by doping als are applicable to the production of Mueller’s brilliant conjecture was that, them with impurities and studying the ef- practical devices as well as the study of in electrical conductors, the distortion fects of those impurities as the solids are quantum physics, and they have proper- of the crystal lattice resulting from the cooled to low temperatures. Often the ties that are roughly predictable from Jahn-Teller instability would produce presence of impurities causes the solids theory. Still, the theoretical tools need- new material properties provided the to undergo a drastic change in some ed to predict their material properties energy associated with the lattice dis- physical property as they cool, from con- necessarily include statistics and tortion is comparable to one of the en- ducting to superconducting, from non- macroscopic approximations. ergy scales of the conduction electrons. magnetic to magnetic, from paraelectric Recent work on “materials by de- As it turned out, the marvelous super- to ferroelectric, and so on. These drastic sign” contrasts strongly with the work conducting properties of the cuprates changes are called phase transitions. of what one could call “old-fashioned” were not attributable to Mueller’s con- It is a curious finding that some materials scientists, who work semi- jecture, but nevertheless his very cre- seemingly intrinsic properties of solids 1993 Number 21 Los Alamos Science 183 Crystals and Ultrasound owe their existence to duce single crystals small amounts of im- New Materials of germanium and purities. For example, silicon with the im- Systems with highly off-the-shelf tungsten correlated electrons such as purity content kept high metal is typically hard Tc superconductors to the unprecedented and brittle. When and heavy-fermion low level of 1 part superconductors pains are taken to re- per million—and move the various trace they used every impurities dissolved in piece of technology nominally pure tung- available to reach sten, it becomes quite their goal. soft. The change in Along the way, hardness is so pro- New when state-of-the-art nounced that the purity Physics was inadequate, of a tungsten sample their team developed Novel electronic, can be reliably esti- structural, and new methods of mated with an ordinary magnetic phenomena growing and charac- Dremel grinding tool. terizing crystals, Technology New Tools The very pure metal is Transfer such as zone-refine- softer than annealed New Resonant ment, a simple copper, and its x-ray ultrasound method for remov- superconductors spectroscopy pattern becomes New magnets ing impurities while blurred, no doubt be- New techniques for a crystal is growing. nondestructive cause of the large de- testing In the end they were formations that easily able to make the occur in its soft condi- first transistors.