New and Artificially Structured Electronic and Magnetic Materials

FRANCIS J.DI SALVO

New materials research is the primary route to the discovery of new physical or chemical phenomena and to the understanding of how the atomic, electronic, and bulk structures of materials lead to their observed properties. This research leads not only to fundamental discoveries, but also, not infrequently, to technological applications. Such research is interdisciplinary; many of the successful programs involve scientists and ideas from the fields of chemistry, electrical engineering, materials science, and physics. Indeed, the Materials Research Laboratories (MRL) program was instituted not only to give researchers from these disciplines shared access to expensive equipment but also to help overcome the traditional segregation of scientists in university departments, thus facilitating more and broader collaboration.

This chapter focuses on new materials, including superconductors, metals, semiconductors, and ionic conductors that exhibit novel electrical or magnetic properties. It assesses the status of research on such new materials in the United States and, at least in part, the MRL program, and makes recommendations for meeting the challenges of tomorrow. The topics chosen and ideas expressed here are those of a small group of chemists, physicists, materials scientists, and electrical engineers from university, government, and industrial laboratories.1 Although each of us has sought the opinions of our colleagues at work and at scientific meetings, we do not claim to represent the entire scientific community in new materials research. Our general consensus, similar to that reflected in previous reports,27 is that the health of new materials research in the United States, specifically in the areas of conducting and magnetic materials, is only fair. Some areas are doing rather poorly, and others relatively well.

This conclusion is based on an examination of major discoveries of new



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Advancing Materials Research New and Artificially Structured Electronic and Magnetic Materials FRANCIS J.DI SALVO New materials research is the primary route to the discovery of new physical or chemical phenomena and to the understanding of how the atomic, electronic, and bulk structures of materials lead to their observed properties. This research leads not only to fundamental discoveries, but also, not infrequently, to technological applications. Such research is interdisciplinary; many of the successful programs involve scientists and ideas from the fields of chemistry, electrical engineering, materials science, and physics. Indeed, the Materials Research Laboratories (MRL) program was instituted not only to give researchers from these disciplines shared access to expensive equipment but also to help overcome the traditional segregation of scientists in university departments, thus facilitating more and broader collaboration. This chapter focuses on new materials, including superconductors, metals, semiconductors, and ionic conductors that exhibit novel electrical or magnetic properties. It assesses the status of research on such new materials in the United States and, at least in part, the MRL program, and makes recommendations for meeting the challenges of tomorrow. The topics chosen and ideas expressed here are those of a small group of chemists, physicists, materials scientists, and electrical engineers from university, government, and industrial laboratories.1 Although each of us has sought the opinions of our colleagues at work and at scientific meetings, we do not claim to represent the entire scientific community in new materials research. Our general consensus, similar to that reflected in previous reports,2–7 is that the health of new materials research in the United States, specifically in the areas of conducting and magnetic materials, is only fair. Some areas are doing rather poorly, and others relatively well. This conclusion is based on an examination of major discoveries of new

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Advancing Materials Research phenomena in materials in the last 20 years. Although we may have missed some important discoveries, the trends discussed here are accurate. Having examined the key elements in a discovery and where and how it took place, we looked for recent changes in these patterns that may portend further change. The following summary is in two parts, the first of which deals with bulk materials, and the second with artificially structured materials. BULK MATERIALS The table on page 163 lists some materials in which new phenomena have been discovered or in which a particular property has been considerably enhanced in the last decade or two. Also included are some materials in thin-film form. These can also be prepared in bulk form, but their usefulness arises from the ability to make films or wires (e.g., NbN and Nb3Sn). Since the compounds are stable in bulk form, they are included here. It is immediately evident from this table that the new compounds—that is, their composition and crystal structure—were often discovered by one group, usually not in the United States, and that the new phenomena were discovered by another group, most often in the United States. In a few cases the compounds were discovered as a result of seeking the particular property (e.g., Fe14Nd2B or RbAg4I5). In some cases the structure type has been known for a long time, but the particular compound had never been examined in a way that revealed something new. In those cases the table shows the structure type rather than a reference to its discovery. The discovery of metal insulator transitions in 1946 did not trigger immediate worldwide interest; rather, that field blossomed in the mid-1950s and again in the late 1960s, and important advances are continuing today. The organization of the table is somewhat misleading in the following ways. First, the understanding and the synthesis method to discover particular materials have most often been developed in programs that produced materials with rather ordinary properties. Second, by focusing on properties, we ignore the most common motivations for the original synthesis, which are usually to develop a new chemistry, or to make materials with new or unusual structures, or to elucidate reaction mechanisms particularly when new preparative conditions or techniques are discovered. The purpose for such studies is usually to understand the relation between chemical bonding and crystal structure. Indeed, many of the compounds listed in the table were discovered by “accident” rather than by design, while researchers were examining the phases that result from new combinations of elements. These searches were often empirical, motivated by the expectation that new structures would result, the exact nature of which was not certain. Since there is as much art as science in the consistent discovery of new materials, the scientists who most frequently find new

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Advancing Materials Research Materials in Which New Phenomena Have Been Discovered in the Past Two Decades Phenomenon Prototype Compound Structure Type or Where-When Compound Was Reported Where-When Phenomenon Was Reported Charge density waves 2H-TaSe2 Holland-19648 U.S.-19749 England-197410 Sliding charge density waves NbSe3 France-197511 U.S.-197612 Polymeric conductors (CH)x·AsF5 Japan-197413 U.S.-197714 Organic charge transfer conductors TTF-TCNQ TCNQ-U.S.-196315 TTF-U.S.-197016 U.S.-197317 High-field superconductors PbMo6S8 France-197118 U.S.-197219 Magnetic superconductors SmRh4B4 USSR-197220 U.S.-197721 Mixed-valence compounds SmB6 Germany-193222 France-193223 USSR-196525 SmS Italy-196124 U.S.-197026 Heavy fermions CeCu2Si2 UBe13 ThCr2Si2 U.S.-194927 Germany-197928 Switzerland/U.S.-198329 Semimagnetic semiconductors Cd1–xMnxTe EuO ZnS U.S.-196131,32 Poland-197830 – Large coercive force magnets Fe14Nd2B U.S.-198433 Japan-198434 – – Spin glasses Au1–xMnx Au U.S.-197235 Metal insulator transitions V2O2 Si:P Al2O3 Si France-194636 U.S.-195537 Intercalation compounds LixTiS2 CnK Germany-196538 Germany-192639 U.S.-197640 U.S.-196641 Superionic conductors RbAg4I5 U.S.-196742 England-196743 – – Hydrogen storage interstitials FeTi-H LaNi5-H CsCl CaCu5 U.S.-196744 Holland-197345 Technologically developed superconductors (films) Nb3Sn NbN ß-W NaCl U.S.-198146,47 Japan-198548 Germany- 198349 High-dielectric-constant microwave resonators Ba2Ti9O20 Holland-195850 U.S.-197451

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Advancing Materials Research materials with novel structures characteristically have a broad background in synthesis. Third, the table does not show the methodology of the “synthesis loop” that is often established after a new property is discovered or upon organizing a research program aimed at enhancing a particular property. In such a loop there is a close coupling between the synthesizers and the characterizers of materials. One measurement leads to ideas about new compounds of a similar structure, which leads to more and broader measurements, which leads to more synthesis, and so on. Although the synthesis practiced in this loop is often “derivative chemistry” (that is, not a search for completely new materials), it usually leads to a material in which the novel property is optimized for detailed research studies (for example, uncomplicated by other phenomena) or for some technological application. It is particularly in this synthesis loop that the interdisciplinary nature of materials research is apparent and necessary. The establishment and maintenance of such a loop are enhanced when the collaborators are at the same institution or in the same building and optimized when they are in the same group. The trend in novel solid-state synthesis is toward ternary and quaternary compounds with complex crystal structures. Methods of synthesis other than brute force (for example, high temperatures) are needed and are beginning to be developed. Low-temperature methods should allow the preparation of many new compounds and structure types that are unstable at high temperatures. Indeed, materials prepared at or near room temperature may be metastable. Such materials are likely to assume greater technological importance. For example, materials with high superconducting transition temperatures, Tc, are difficult to prepare because their structures are often unstable at high temperatures. Low-temperature methods may be the most successful route to the preparation of superconductors with even higher Tc’s. In part because of this emerging trend, a closer contact between the methods of solid-state synthesis and those of inorganic and organic chemistry will lead to many new discoveries. In some areas there is hope of developing a “rational synthesis” similar to the approach of organic chemistry. Such rational approaches are seen in the Zintl principle52 or in cluster condensation ideas.53 However, a rational synthesis for any solid-state phase is a challenge of considerable magnitude, since the bonding in solid-state compounds covers the spectrum of types—metallic, covalent, and ionic. New phenomena will be discovered both in these new complex phases and in known phases as they are examined under new or extreme conditions such as ultralow temperatures and high pressures. That the new compounds listed in the table were with few exceptions synthesized originally in Europe is no surprise. Inorganic chemistry, especially solid-state chemistry, is strong in France and Germany. In the United States, synthesis of solid-state compounds has been considered out of date

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Advancing Materials Research and a little dull, and few academic departments have even one professor involved in synthesis of new solid-state compounds. The MRL program addresses only part of the materials synthesis needs at universities. By and large, the synthesis done at MRLs is an important part of the synthesis loop, doing “derivative chemistry” on materials that are already of scientific or technological interest. This is an important job, and it should be continued. However, from the point of view of discovering new compounds with new physical or chemical properties, the MRL program is not enough. The MRLs could play a leading role in establishing interdisciplinary groups—for example, solid-state science rather than solid-state physics or solid-state chemistry—concerned with the preparation of novel phases and the examination of their properties. Industrial materials research in solid-state compounds also suffers from insufficient attention to the synthesis of novel materials. In the past this mode of operation—European scientists discover the compound, U.S. scientists discover the phenomena—may have been considered enough justification for ignoring the search for totally new compounds. However, after years of watching scientists in the United States take advantage of discoveries made elsewhere, the European materials science community is reorganizing its mode of research and broadening its interests. The change is most advanced in France, where many of the synthesis groups have been expanded to include physical measurement. Those groups discover not only the material but the phenomena as well. Their first publication will no longer report just the synthesis and structure of the new material, but will report electrical, magnetic, thermal, and other measurements. If such a group cannot make a particular measurement, it will find a scientist in France to collaborate with. This change is now orchestrated by Centre Nationale de Recherche Scientifique (CNRS). In France it is not uncommon to find university groups headed by one professor and including both solid-state chemists and solid-state physicists, some of whom are permanent staff members paid through CNRS. Such mixed groups are a way to optimize the search-and-discovery process in new-materials research. The traditional structure of university departments in the United States is a liability in trying to produce such groups, but some way to establish, encourage, support, and expand such efforts is clearly needed. If this is not done, the discovery of new phenomena in new compounds by scientists in the United States will occur less and less frequently. In Japan the situation is also beginning to change. Little novel synthesis was done in Japan in the past. Researchers often adopted topics already popular in the United States. Indeed, they often studied materials synthesized by collaborators in the United States. Now, however, they realize that they need an active new-materials synthesis program. For example, the Institute of Solid State Physics in Tokyo has as a top priority the establishment of a first-rate solid-state synthesis program. When such materials programs are

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Advancing Materials Research in place, they will be formidable, since the Japanese already tend to work in collaborative interdisciplinary groups, especially in industry. To build a leadership position in novel solid-state synthesis requires a more concerted effort by funding agencies to emphasize such research. Increased funding is necessary to train more students and to provide the equipment and tools necessary to compete with European research groups. This need for increased funding is largely due to years of neglect when little equipment for such solid-state studies was purchased by even the few researchers in the field. Standard tools of the trade include apparatus for handling air-sensitive materials, high-temperature and high-vacuum ovens, and x-ray diffraction facilities and associated computers. Also to be included are the myriad apparatus for characterizing these compounds: electrical and magnetic measurements, solid-state nuclear magnetic resonance, thermal analysis from perhaps as low as 0.1 K to as high as 1500 K, dielectric and optical measurements, electron paramagnetic resonance, and so on. Easy access to such equipment is needed, perhaps not directly in a solid-state synthesis group, but in a departmental or MRL facility or through close collaboration within the university. At most universities such a collection of accessible apparatus does not exist. Although more such facilities are needed, cost will prohibit a large number. Many researchers will have to collaborate with scientists and institutions that have such equipment. Such a buildup is also impeded by the paucity of scientists in the United States who are familiar with solid-state synthesis techniques. There are, however, glimmers of hope. In recent American Chemical Society meetings the solid-state synthesis sessions have been attracting scientists from other branches of chemistry as well as some physicists.54 The preparation of compounds in film form, especially by “physical” methods such as sputtering or evaporation, to produce stable phases and (less frequently) metastable phases, is better established in university and industrial laboratories. Work on such techniques is getting a boost because of current technological needs and because of its connection with research in artificially structured materials. In summary, if we wish to maintain our leadership in the discovery of new phenomena in solid-state compounds, our approach must change, perhaps drastically. The health of novel-state synthesis, with the exception of organic synthesis, is poor. Too few university professors are in the field; the number of U.S. groups is smaller than the number in Europe. Finally, some mechanism must be found to establish and support many more truly interdisciplinary groups in the university setting; that is, we must continue further down the path that MRLs have already initiated. ARTIFICIALLY STRUCTURED MATERIALS Artificially structured materials are a class of new materials with intentionally produced spatial variations in composition. Many of these materials

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Advancing Materials Research are not in thermal equilibrium but are kinetically stable. For example, in thermal equilibrium the concentration of dopants in a semiconductor would be uniform, but at the temperature of preparation and especially at room temperature the bulk diffusion process is so slow that variations in concentration are essentially permanent. The methods used to produce such materials are dominated by this consideration, so that the growth of the material takes place well below any melting point of the material(s) in question. The common techniques include evaporation onto a substrate (e.g., deposition of polycrystalline metals onto any substrate), molecular beam epitaxy, chemical reaction of a feed gas to deposit a solid on a substrate (e.g., chemical vapor deposition or metal organic chemical vapor deposition), and growth of a material from a saturated solvent (e.g., liquid-phase epitaxy). These techniques are usually employed to produce films microns thick rather than bulk material. The overwhelming majority of research in this area concerns semiconductor materials, especially Group III–V materials such as gallium arsenide. The organization of and motivation for such research is considerably different than for bulk compounds. Whereas high-quality research in materials science and in semiconductor physics is an important outcome of the research, the research is primarily motivated by technological needs in high-speed devices and in lightwave communications (e.g., solid-state lasers, detectors, and integrated optical systems). These materials are new in a different sense than bulk materials are, in that their crystal structure and bulk electrical properties have long been well known. What is new is that their composition can be controlled and varied on length scales as short as 5–10 angstroms to produce materials with new properties. (In the remainder of this section the word “structure” denotes compositional changes and not the crystal structure.) The previously mentioned growth techniques, as well as related methods such as sputtering, can be used to deposit almost any kind of inorganic material on flat or variously shaped surfaces. Depending on the materials in question and on the deposition conditions, the films produced may be homogeneous or inhomogeneous (on length scales from tens of angstroms to microns). This presentation is limited to Group III–V semiconductors produced by molecular beam epitaxy (MBE), since the majority of research on films and epitaxial layers, as well as much of the scientific excitement, has been centered on these materials. Although this discussion is limited to the MBE technique, other techniques, such as chemical vapor deposition, or hybrid methods, such as gas-source MBE, may also produce materials that are very similar in structure and electronic properties. Compound semiconductor MBE began with research at industrial55 and government56 laboratories in the United States in the late 1960s and early 1970s. It especially caught on after the discovery that high-purity epitaxial materials with atomically smooth interfaces could be produced by this method.57

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Advancing Materials Research At present, practically every research and development laboratory for high-technology industry in the United States and Japan, as well as some in Europe, is heavily involved in MBE (especially of Group III–V compounds) as well as a rapidly increasing number of government laboratories and of university groups in physics, electrical engineering, or materials science departments. The wide variety of structures that can be produced by MBE fall into two main classes—homoepitaxial and heteroepitaxial. Homoepitaxy is essentially the trivial case where a single-crystal film is grown on a single-crystal substrate of the same composition. This can be useful in producing defect-free layers or sharp doping interfaces in the same material (e.g., abrupt p-n junctions). Heteroepitaxy is the epitaxial growth of two or more different compositions in a single film. Heteroepitaxic growth is affected by the degree of lattice mismatch between the two materials that meet at an interface. The lattice mismatch is the percentage difference between atom spacings along a plane of the same crystallographic orientation as the interface plane in the respective bulk phases of the two materials. If the mismatch is small (less than a few tenths of a percent), an atomically coherent, defect-free interface can be prepared between two thick layers (typically microns) of different composition. If the mismatch is large, defects such as misfit dislocations are produced. Even in that case the crystal lattices of the two phases joined at the interface may have a definite relationship to each other, i.e., the interface can be coherent. Such defects usually degrade the electrical performance of such materials or of devices made in this way. A recent experimental advance, strained-layer epitaxy, first proposed theoretically many years ago,58 overcomes this lattice mismatch problem. When one or both of the material layers are made very thin (typically tens to hundreds of angstroms), the thin layer is strained (i.e., stretched or compressed) to the same interatomic spacing as the substrate and no defects are generated at the interface. This technique can be used to produce thick films if a superlattice of thin layers (i.e., multiple interleaved layers) is produced. Strained-layer superlattice epitaxy greatly expands the classes of semiconductors that can be considered for defect-free heteroepitaxial growth of materials with optimal electronic properties. Many semiconductor structures have been produced by MBE, and their complexity is increasing. The correct choice of materials permits the spatial variation of the band gap (perpendicular to the film surface) and the doping type (n or p) to be arbitrarily tailored.59 The band-edge discontinuities that occur between two different semiconductors can be modified, even removed, by properly grading the interface composition or by adding thin, heavily doped layers at the interface. By modifying the composition, the effective mass can also be independently controlled. The production of such artificially structured materials may be called “band-gap engineering.”60

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Advancing Materials Research A particularly useful doping technique has also emerged from MBE— modulation doping.61 If a semiconductor is sandwiched between layers of another with lower electron affinity, the carriers produced by doping will fall into the “well” produced by the material with the greater electron affinity. If the well is thin (a few hundred angstroms or less), the carrier will occupy the lower quantum levels of the well. Furthermore, it will be able to move parallel to the well—i.e., in the plane of the film—without scattering from the impurities (dopants) used to produce them, because the impurities are outside the well. Modulation-doped semiconductors of this variety have the highest-mobility carriers ever produced in the given semiconductor (e.g., greater than 106 cm2/V-s at 1 K for electrons in GaAs).62,63 Besides the great array of structures and devices that have been and will be thought of, fundamentally new physical phenomena have been observed in some of the high-mobility materials. The quantized Hall effect, discovered first in silicon inversion layers64 (for which Klaus von Klitzing won the 1985 Nobel Prize in physics) and subsequently in GaAs heterostructures,65 is now fairly well understood.66 In the highest-mobility materials, the fractional quantized Hall effect (FQHE) has also been observed.67 It arises from interactions between carriers in two dimensions, which leads to a series of new ordered states,68 and is a subject of intense theoretical research. The solution of this problem is likely to bring a much deeper understanding of the nature of interacting electron systems, an understanding that will likely have importance in fields far beyond semiconductor physics. The future of semiconductor MBE research and development is bright. Band-gap engineering, superlattices and quantum wells with modulation doping, and strained-layer superlattices will produce rapid technological advances and possibly more fundamental discoveries like the FQHE. New materials advances likely in the future include in situ (ultrahigh-vacuum) processing of MBE wafers to produce devices without exposing the wafer to air. Whereas the dimensions of heterostructures can now be controlled only in the direction of growth, techniques such as in situ processing will eventually permit production of heterostructures whose dimensions are precisely controlled in the lateral direction as well. The kinds of structures and devices that would then be possible are perhaps limitless. Future possibilities for MBE preparation are not limited to Group III–V semiconductor systems but include novel metal superlattices and systems that contain mixtures of metals, semiconductors, and insulators. MBE or related techniques may also be used to produce metastable crystal structures in thin-film form. That is, these techniques may also be exploited to produce new solid-state compounds, such as high-temperature superconductors. It is at this juncture that the fields of solid-state synthesis and artificially structured materials obviously overlap. In fact, single-crystal superlattices of magnetic and nonmagnetic metals with structural perfection approaching that attainable

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Advancing Materials Research with semiconductor MBE have been reported.69 Also, single-crystal layers of magnetic metals have been grown on semiconducting substrates.70 It seems likely that new physical phenomena will emerge from such studies of novel materials prepared by MBE and related techniques. OBSTACLES AND ROUTES TO FURTHER DEVELOPMENT In view of the current possibilities and excitement in the field of artificially structured materials, can there also be problems? Although the field of MBE in the United States is stronger and attracts more research entrants than that of new bulk compounds, there are challenges. The first is being addressed by the electronics industry and is not directly influenced by the MRLs or even the broader engineering and scientific community, but it is important to the future of MBE and MBE research. The problem is that, although MBE and most device structures and concepts were developed in the United States,71 the Japanese have moved the technique into large-scale production of devices more rapidly than has the United States or Europe. The Japanese have demonstrated 1-K high-speed random-access memory chips using modulationdoped GaAs72 and are rapidly developing 4- and 16-K devices. Their heavy investment in MBE technology73 further suggests that they will soon threaten U.S. dominance in the invention of devices as well. We must meet this competition or the Japanese will dominate the market and sharply reduce the incentive for U.S. industry to fund further research. This situation could very well lead to an eventual decrease in funding from the National Science Foundation and other agencies for university research in MBE. The second problem is related to the first in an indirect way. MBE research is expensive. It requires ultrahigh-vacuum apparatus and techniques. A state-of-the-art machine, with the proper surface analytical tools, costs $500,000 to $1 million. The measurement of properties requires at least a well-equipped electronics laboratory and often access to ultralow-temperature and high-magnetic-field laboratories. Indeed, the high magnetic fields necessary for studying details of the FQHE are available only at the National Magnet Laboratory. Because of the expense of MBE research, not all university investigators who want to carry out MBE experiments or study MBE materials and devices can do so. There is a shortage of trained scientists to fill all the openings in MBE research and development. Despite the need for increased funding of research in artificially structured materials, the problem is larger than can be addressed by MRLs or the National Science Foundation alone. To meet the needs will require support at universities from all government agencies, from industrial research and development groups, and from universities themselves. Special efforts are needed to pull these resources together. The technologies that will emerge

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Advancing Materials Research have a high probability of forming the basis for the electronics industry of the next century and are vital to our technological and military strength. At the university level, individual research groups are often too small to carry out MBE growth, structure and device design, and measurements of electronic properties. Intergroup collaboration, within and between institutions, will need to be supported by both government and industry. One approach could be to build specific ties between university groups and industrial or government laboratories. Such ties might include exchanges among institutions, mini-sabbaticals for faculty members and scientists, and arrangements whereby graduate students could perform their thesis research at industrial or government laboratories. This would allow university groups to be somewhat larger—thus producing more graduates with MBE backgrounds—and to have access to more and better equipment. It would also allow more industrial and government researchers to work in MBE (at low cost) and would help produce Ph.D. graduates who have the background needed to fill research and development positions. A more rapid diffusion of technical and scientific knowledge between these groups would also ensue, to the benefit of all. Another possible approach to these difficulties is to create several MBE centers at universities. This could allow the buildup of a core of experts who would collaborate with each other and with other scientists in the region to produce structures or devices on a proposal basis. Industry should also be a part of this effort, perhaps by partially funding the centers or on a fee-for-use basis. The size of the U.S. research effort in artificially structured materials is much larger than that in new solid-state synthesis. This difference in size is reasonable and necessary, since artificially structured materials are so closely related to electronics and telecommunications technologies. Indeed, research on artificially structured materials must continue to grow in the cooperative ways already outlined. In contrast, the discovery of new solid-state compounds in the United States has been almost nil. This field needs considerable attention by funding agencies if the United States is to continue to lead the world in the discovery of new phenomena in solids. At the proper equilibrium, the ratio of scientists pursuing novel solid-state synthesis to those pursuing artificially structured materials (or synthesis of materials already known to be technologically interesting) would be larger than it is today although still considerably less than unity. The nation needs to establish and support more solid-state synthesis groups. This might traditionally be thought of as solid-state chemistry, but such groups could also flourish in materials science, physics, or applied physics departments. In any case, some access to characterization of these new materials must be established. Indeed, the current success in artificially structured materials

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Advancing Materials Research is due to the excellent interdisciplinary and collaborative nature of much of the work, especially in industrial laboratories. In some cases it would be reasonable to have physical measurements available in a solid-state synthesis group. In others, collaborators in other groups or fields and at industrial and government laboratories could suffice. However, individual egos, university organizational structure, tenure procedures, and funding usually work against such collaborative arrangements. Other creative possibilities for encouraging the growth of solid-state synthesis have been proposed by the Solid State Sciences Committee of the National Research Council and more are needed. CONCLUSIONS The synthesis of new solid-state compounds in the United States has been a neglected field for decades. Rather, this country has focused on discovering new or enhanced properties in materials previously discovered—primarily in Europe. The high quality of such research in Europe, coupled with new interdisciplinary group organization and the emerging emphasis on new materials in Japan, makes it unlikely that the United States will continue to be the first to discover new properties or uses of materials. To maintain our position, it is imperative that the United States build a first-rate scientific presence in novel solid-state synthesis. The greatest scientific and technological impact of this effort will occur only with strong interaction between synthesizers and characterizes; that is, this should be an interdisciplinary research activity. The field of artificially structured materials, as typified by the MBE growth of semiconductors, is by comparison strong and robust. Challenges posed by the shortage of scientists trained in the field as well as a slow translation of research results into production can best be overcome by sharing and coordinating academic, industrial, and government resources in a national collaborative effort. Failure to do so could well result in Japanese companies dominating the technology that will most likely be the basis of advanced electronic devices in the twenty-first century, or probably even in the 1990s. NOTES 1.   A.Y.Cho and Arthur C.Gossard (molecular beam epitaxy), AT&T Bell Laboratories, Murray Hill, N.J.; John Corbett (solid-state chemistry), Department of Chemistry, Iowa State University of Science and Technology; Frank Y.Fradin (superconductivity, transport props), Materials Sciences and Technology Division, Argonne National Laboratory, Argonne, Ill.; M.Brian Maple (superconductivity, magnetism), Department of Physics, University of California, La Jolla; and Stephen von Molnar (physics of rare-earth compounds), Thomas J.Watson Research Center, IBM Corp., Yorktown Heights, N.Y. 2.   J.L.Warren and T.H.Geballe, “Research opportunities in new energy-related materials,”

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