This article will summarize the information gained from photoemission spectroscopy (PES) studies of the electronic structure of high temperature superconductor (HTSC) materials, with emphasis on distinguishing models for the normal state. Inverse photoemission spectroscopy (IPES) is a closely related measurement, and taken together PES and IPES yield the single-particle electronic structure, meaning the the spectrum to remove and add electrons, i.e., the electronic ionization and affinity spectrum. It is assumed that the sudden approximation holds, so that the photon event can be characterized by a photoemission cross section σ. The emission from a particular kind of electronic state i can often be identified because its cross section has a particular photon energy (hv) dependence σ (hv).

We will emphasize a few recent results especially pertinent to the conceptual issues posed by these materials, and we will divide the discussion between angle-integrated spectra and angle-resolved spectra. For angle-integrated spectra, electrons are collected from the sample surface in a large solid angle, while for angle-resolved spectra, electrons are collected in a well-defined direction. Angle resolution can be used to select emitted electrons of a particular value of crystal momentum k.

Magnetic recording can store digital information at an areal density well beyond the fundamental density-limit of optical systems. The central problems in achieving high trans-optical information density with magnetic recording reside primarily in the materials area. In addition to improved magnetic properties, such as head materials with high saturation magnetization and recording media with low noise and high coercivity, tribological considerations impose constraints on the ultimate density that will be attained. The exponential loss of signal due to spacing between the head and recording medium is inherent in all magnetic recording, necessitating the development of durable quasi-contact interfaces.

Magnetic recording is the universal technology for electronic information mass storage. Its presence is ubiquitous as audio tapes, VCRs, floppy disks, computer hard disks, credit cards, etc. Sales of magnetic recording products exceed $50 billion annually, with strong growth projected into the foreseeable future. A steady progression in storage density and corresponding reduction in cost has characterized all phases of magnetic recording; the literature is replete with historical and projective curves showing cost and density numbers. Computer disk file memories have doubled in areal density every 2.5 years for about the past 30 years. Although many competing technologies have been proposed through the years, none has been able to displace or even significantly impact magnetic recording. The first challenge was thermoplastic recording, followed by at least three resurrections of magneto-optic recording, bubbles, semiconductors, and various nonmagnetic optical storage devices.

The recent study by the National Research Council, entitled Materials Science and Engineering for the 1990s—Maintaining Competitiveness in the Age of Materials, has identified materials science and engineering as an intellectually exciting field crucial to the success of U.S. industries, economy, and defense. The study surveys the needs of eight industries (aerospace, automotive, biomaterials, chemical, electronics, energy, metals, and telecommunications), which employ more than seven million people and have sales in excess of $1.4 trillion. The survey shows the industries' critical requirements for new, improved, more economical materials and processes. Similar needs are documented in such public sector areas as defense, energy, transportation, space, and health. Despite the fact that their economic performance and technological leadership within the global market vary widely, these industries consistently identify materials science and engineering as vital to their ability to maintain or improve their international competitiveness. In every case, the survey indicates a clear need to produce and fabricate new and traditional materials with improved quality and economy. It is also evident that the United States currently leads in certain materials-related areas, but adequate resources, planning, and training are required to maintain this leadership role.

In terms of manpower and educational needs, the study emphasizes the need for more educated personnel to meet current and future opportunities, and recommends basing undergraduate courses and programs in materials science and engineering on the four basic elements of materials science and engineering—synthesis and processing, structure, properties, and performance — and on the relationships among them. The study further recommends that undergraduate materials education be centered in materials departments, and that these departments interact strongly with other science and engineering departments to develop interdisciplinary materials-related educational programs.

The Physical Chemistry Cluster (PCC), located in the Department of Physical Chemistry, University of Amsterdam, the Netherlands is a cooperative entity which directs a wide range of in-depth research. Extensive collaboration within the PCC offers a broad base of expertise to every participant. The PCC is situated in the inspiring environment of the Van't Hoff Laboratory, named for the Dutch chemist who received the first Nobel prize awarded for chemistry in 1901 for his research on valence-directions and stereochemistry of the carbon atom.

The PCC is composed of a number of different bodies, with the Department of Physical Chemistry serving as the pivot point. Figure 1 shows a diagram of the PCC organization which includes the University of Amsterdam (UvA); the foundation, Chemical Research Netherlands (SON); Institute of Mass Spectrometry (IMS); the Interaction of Matter with Photons group (IMF); Faculty of Physics (UvA); Laser Application and Information Center Amsterdam (LAICA); the LAICA Laser Technology Ventures, Ltd.; and the Laser Lease Center.

The Physical Chemistry Cluster finds its roots at the Laboratory of Physical Chemistry of the University of Amsterdam. The Laboratory for Physical Chemistry has an academie staff of about 18 members who cooperate with the faculty of physics of the University of Amsterdam. PCC research activities cover a broad field of physical chemistry. Here, researchers study the interaction of matter and photons to obtain information on spectroscopic properties and dynamics of excited states of isolated or clustered molecules in the gas or condensed phase.

The discovery by Bednörz and Müller of superconductivity in La20xBaxCuO4 initiated an intense and fruitful search for additional perovskite-based superconducting compounds. Following the discovery and characterization of a number of important new compounds, and the development of techniques to produce them in pure crystalline form, it becomes relevant to carefully measure the fundamental physical properties of well-characterized samples of these new materials. Ultimately the goal of this work is to illuminate the basic physics of these novel materials.

Traditionally many techniques, including critical field measurements, specific heat, penetration depth, magnetic resonance, and tunneling and infrared spectroscopies, have been useful for the study of superconducting elements and compounds. This article discusses the use of infrared measurements to study both cubic BaBiO3 based compounds and layered cuprates. We emphasize the work on the (ideally) stochiometric compound Y1Ba2Cu3O7, because its infrared properties have been obtained by a number of groups with good reproducibility. Infrared measurements are useful both in terms of their traditional role as a probe of the superconducting energy gap and for their ability to probe the dynamics of the normal state by measuring the conductivity vs. frequency in the crucial infrared range (~1 meV to 2 eV).

The presence and origin of elastic anomalies in superlattices, interfaces, composites, and nanocrystalline materials has been a subject of much interest and controversy in recent years. In particular, superlattices are being used as model systems to study the effect of interfaces on the mechanical properties of novel materials. Early claims of anomalously large enhancements of the biaxial and flexural moduli of Au/Ni and Cu/Ni superlattices (“supermodulus effect”) created considerable controversy and contradictory reports in the experimental and theoretical literature. To understand the mechanical properties of superlattices and their implication for other types of nanofabricated materials, it is important to look critically at the field.

Superlattices have been fabricated in different laboratories by a variety of preparation methods and have been characterized structurally and elastically to various degrees. Because of this, before addressing any sophisticated theoretical issues regarding elastic anomalies, it is important to understand in detail the experimental techniques and the possible pitfalls in the experimental determination of mechanical properties of thin films. Once the presence or absence of the effect is ascertained, the need arises to understand any possible experimental correlations with other structural and/or physical properties. These correlations can be used to address issues regarding the possible origins of the effect and their theoretical basis. It will be apparent from this article that slight changes in structural properties are correlated with relatively large elastic anomalies. Consequently, this points to the importance of precise, comprehensive, and detailed structural characterization. With our present understanding of the physics of superlattices, qualitative studies are no longer sufficient!

The importance of interfaces in determining the physical properties of technologically important materials — addressed in the Guest Editors' introduction to the articles in this issue — has stimulated theoretical efforts to determine from first principles a detailed understanding of their chemical, electronic, and mechanical properties. Fortunately, this has now been made possible as a result of the dramatic advances in condensed matter theory made in the last decade, driven in large part by new and sophisticated experiments on high-purity materials that have been well and carefully characterized. Particularly in electronic structure, these advances may be attributable directly to the close collaboration of theoretical and experimental researchers. Indeed, the new found ability to apply fundamental theoretical concepts to real materials (rather than to simple model systems) made possible by using the continued rapid development of computer power, has served to fill the increasingly urgent demand of experimentalists for theoretical interpretation of their data. Also, in some cases, these computational efforts can be used to provide data that would be currently impossible or impractical to obtain experimentally. This development has been an essential element in the phenomenal growth in this area of materials science.

More specifically, the advent of accurate self-consistent (local) spin density functional (LSDF) calculations for surfaces, interfaces, and multilayers means that theory is no longer limited to simple, parameter-dependent models. These complex systems are of growing interest because the reduced symmetry, lower coordination number, and availability and role of highly localized surface and interface states offers the possibility of inducing new and exotic phenomena and promotes the possibility of new device applications.

Recording in the 1990s will be high density, high frequency, in-contact recording on high coercivity media. Today's state-of-the-art head materials, NiFe, Sendust and amorphous alloys, will be used in some applications. However, layered structured and artificial superlattices will become the key head technologies of the future. Use of advanced preparation techniques will allow “true” materials engineering and the fabrication of devices to incredibly accurate specifications.

These very high density recording systems will require optimal inter-play between all the components—media, head, and head/media interface. Future media will be very smooth, high coercivity, large moment thin films. The head/media spacing will be less than 50 nm, and recording at more than 100 MHz will be required. Single-track heads will be replaced by very narrow track-width multitrack devices in high data rate recorders. Thin film heads will provide the answer to most of the problems of large recording fields and high frequency response. These changes will define new requirements for head materials, some of which cannot be met by currently used materials. Conventional ferrite heads will not be found in high performance recording systems; thin film inductive and magnetoresistive (MR) heads (Figure 1) will be widely used. Thin films, metals and alloys, both crystalline and amorphous, layered structures and artificial superlattices will be the key head technologies in the future. New material preparation technologies, MBE (molecular beam epitaxy), MOCVD (molecular chemical vapor deposition), sputtering, and ion beam deposition are becoming increasingly available and less expensive. These methods will be used to fabricate future devices to incredibly accurate specifications.

Synchrotron light sources are electron storage rings that produce synchrotron radiation by accelerating electrons in a circular storage ring. The synchrotron light (photon beam) is then used to irradiate various sample materials for basic and applied research in such fields as solid state physics, biology, chemistry, surface science, and technology.

The electron storage ring must provide an ultrahigh vacuum environment for the electron beam to minimize electron residual gas collision which would shorten the beam-lifetime. This article will discuss the design of electron storage ring vacuum systems and materials, and how the choice of materials can affect the machine design.

A typical electron storage ring is shown in Figure 1. It consists of an injector (linac and booster), transport system, storage rings, and experimental photon beam lines. These machines vary in size from a few meters in circumference for a compact light source used for x-ray lithography, to a few hundred meters in circumference for high energy physics.

The vacuum system for an electron storage ring is an all-metal ultrahigh vacuum system. The operating pressure is in the low 10−9 torr range with stored electron beam, and 10−10 torr without beam.

Certain unique vacuum problems must be faced in electron storage ring design: photon-stimulated gas desorption, power dissipation in the chamber walls, impedance changes due to changes in the chamber cross section, conductance limitations, accurate placement of the chamber, and all of those sundry problems associated with a large bake-able all-metal UHV system. Some of these characteristics are illustrated schematically in Figure 2. Two excellent papers that address many of these issues have been written by N. Mistry (system design) and H. Wiedemen (impedances and instabilities).

Materials research and education is currently one of the liveliest areas of science and engineering and is likely to be so for many decades. It is an outstanding example of an interdisciplinary field; persons who call themselves materials researchers are found in departments of chemistry, physics, metallurgy, ceramics, electrical engineering, chemical engineering, and mechanical engineering, and also in many departments that now call themselves by the name “materials science and engineering.” The field has grown so rapidly that the term “materials science and engineering,” has many different meanings. In fact, most of the funding that supports materials science and engineering research is awarded to investigators in the more traditional disciplines, and the vast majority of scientists and engineers working in the field were educated in these traditional core disciplines.

There is no question that the field of materials science and engineering is a success. However, is materials science and engineering now a discipline as well as a field? Should MS&E departments exist and what should be their educational mission? Should MS&E departments offer undergraduate and graduate majors? These questions are being discussed by many university faculties as they work to devise effective research structures and educational programs to respond to the growth of interest in a field that does not fit neatly into any single traditional discipline, but is far too important to ignore.

Recently, the University Materials Council appointed a committee to consider these issues and specifically address the challenge of creating effective, attractive programs of undergraduate education in materials science and engineering.

Integrated optical recording head concepts show promise for future light-weight and compact optical heads. Several fundamental issues remain to be explored before fully integrated optical heads. become a reality. This article reviews the basic operating principles of integrated optic heads, focusing on the materials challenges inherent in developing both partially and fully integrated optical heads.

Currently available optical mass storage systems provide a feature unavailable in magnetic direct access storage devices (DASD) — removable and interchangeable media. At present, however, this feature is bought at a price, with magnetic storage devices currently outperforming their optical counterparts in access speed and volumetric storage density. The access time of optical drives is greater than 40 ms, compared to the 10–20 ms typical of magnetic DASD. This is, in large part, a consequence of the 50–100 g moving mass of the optical head. The optical head also limits the maximum volumetric storage density, with current heads ranging from about 0.5 cm to more than 2 cm in height.

Materials Science at Trinity College, University of Dublin, Ireland, has a distinguished past as well as a promising future. Trinity College published the first book on optics in English by Molyneux (1692). The work of Hamilton, Lloyd, Fitzgerald and others in the 19th century are impressive antecedents for today's research as well, which now enjoys broader horizons and new research opportunities due to major funding by the European Community (EC) and other agencies.

In the Departments of Chemistry and Pure and Applied Physics, internationally recognized research groups are pursuing materials-oriented research in laser physics and nonlinear optics, surfaces and interfaces, magnetic materials, polymers, and theoretical solid-state physics and chemistry.

The current research, described in the following two sections, has for many years resulted from close collaboration in the materials area among researchers in both departments. Common interests have led the departments to establish an honors degree course in the science of materials. The final section discusses the aims of this course.

Fine particles can be produced via aerosol processes either by means of vapor phase reactions that produce solid or liquid particles or by reactions between a preexisting solid or liquid particle and a reactive gas. This article examines the latter processes because a strong interest has developed in the production of materials via aerosol processing. Although fine particles are frequently produced using flow systems, such as in the laminar flow aerosol reactor of McRae and his co-workers, fundamental studies of the chemical kinetics are more readily done using single microparticles or microdroplets. Design of an aerosol reactor requires knowledge of the reaction rates, for there must be a sufficient residence time of the reacting species in the reactor to complete the desired reaction.

Matijević reviewed early work on preparing well-defined and very pure metal oxides by hydrolysis of alkoxide aerosol particles, and Ingebrethsen and co-workers studied the hydrolysis rates of aerosol droplets of aluminum and titanium alkoxides and mixtures of the two alkoxides. Following Matijevic and his colleagues, Okuyama et al. used the thermal decomposition of metal alkoxide vapors to produce ultrafine particles of the oxides of titanium, silicon, and aluminum. The preparation of polymeric aerosols has been studied by Partch et al. and by Ward et al. The latter investigators used single-particle techniques (the electrodynamic balance) to obtain polymerization rate data for the photochemical polymerization of acrylamide monomer microparticles.

This section will provide a background for understanding and visualizing the microstructures observed in LDMMs. The word microcellular means small cell size, or more generally (because most low-density materials are not composed of easily recognizable “cells”) some measure of the spatial scale of the foam structure. The concept, measurement, and significance of cell size is intrinsically connected to the morphology, as are certain properties (e.g., stiffness). These matters are covered in later sections. As a background, this article describes some of the structural morphologies of low-density foams, not all of them available as LDMMs (i.e., simultaneously low-density and microcellular.) Interested readers are encouraged to also examine References 1 and 2 because a detailed discussion would take us too far afield. Instead, we shall indicate how most low-density morphologies may be considered in terms of a few simple models of morphological types encountered and how they arise. Often these are seen most cleanly in large-celled foams. Note that these are simplifications—there is no standard nomenclature.

At the 1989 MRS Fall Meeting the present guest editors and J.D. Jorgensen chaired a symposium on “Neutron Scattering for Materials Science.” Approximately 80 papers were presented, covering a great variety of topics which joined members of the materials science and neutron scattering communities. Because of that symposium's success, it was decided to bring the topic of Neutron Scattering to the wider attention of the materials community through this special issue of the MRS BULLETIN.

Our purpose is twofold. First, neutrons have increasingly come to play a crucial role, both here and especially in Europe, in our understanding of the structure and properties of materials. Through the manipulation of materials (radiation-induced effects, transmutation doping of semiconductors), nondestructive materials testing (residual stress measurements on industrial-sized objects, depth profiling of ion-implanted semiconductors) and structural and dynamical studies, academic, government, and industrial scientists and engineers are coming to recognize the broad utility of neutron methods. We would like to highlight some of the advances in this field for MRS BULLETIN readers.

Second, neutron scattering presents an excellent example of the contribution of our large research facilities to the solution of both basic and applied problems in materials science. Without the major neutron scattering centers we would be severely limited in the scope of our materials activities (no knowledge of magnetic structures and only primitive insight into polymer structures, for example).

The study of Artificially Layered Superconductors (ALS) started more than 20 years ago with the search for unusual and/or high temperature superconductivity in a variety of metal-semiconducting layers. Renewed interest was motivated by the advent of novel preparation techniques that allow control of layer thicknesses close to interatomic distances. In this way layered superconductors can be used as model systems to study a variety of physical phenomena, prepare structures with improved properties and discover novel metastable phases which do not exist in nature. Examples of these studies include: a diversity of dimensional transitions, interaction between superconductivity and magnetism, interaction between superconductivity and electron localization, enhancements of critical fields and critical currents, and the study of incommensurate systems.

Recent developments in high temperature ceramic superconductors further increase the importance of studies of Artificially Layered Superconductors. The newly discovered ceramic superconductors are structurally layered and therefore many of their properties will also be determined by this structure. Because of this, particularly in the search for the mechanism of superconductivity, it is important to understand which properties are a consequence of the layered nature of the material and which are due to the presence of some unusual, yet undetermined physical phenomena.

What makes Artificially Layered Superconductors especially attractive for investigation? The main reason rests on the fact that the characteristic lengths which determine the superconducting properties, i.e., the coherence length and penetration depth, are quite long in conventional low temperature superconductors.

A litany has recently emerged which proclaims materials science and engineering a primary enabling technology for most aspects of a modern high technology economy. More materials engineers have been called for, but employers and academia have not yet agreed on what job description or curricula should define a materials engineer. While the research or science aspects of materials science have been functioning well, manpower availability and industrial materials technology for commercial, military, environmental, and other societal needs lag behind our worldwide competitors.

to nucleate the growth of a consensus and to create awareness and understanding of materials science and engineering begs an answer. In the absence of a consensus to help focus curriculum and increase awareness of materials science and engineering, the current diverse educational practices will remain, and a rare opportunity for professional development will be lost.

The importance of interface materials is based largely on their inherent inhomogeneity, i.e., that the chemical composition and physical properties at or near an interface can differ dramatically from those of the nearby bulk material. For example, the propagation of a crack along an interface — rather than through the surrounding bulk material — indicates a different mechanical strength near the interface. Also, the elastic response and thermal behavior near an interface can be highly anisotropic in an otherwise isotropic material, and can differ by orders of magnitude from those of the adjacent bulk regions. Typically, these gradients extend over only a few atomic distances.

Because relatively few atoms control the properties in the interfacial region, the inherent difficulty in the experimental investigation of buried interfaces is actually an advantage in the atomic-level study of solid interfaces by means of computer-simulation techniques. While the limitations of such simulations are well known, this article will attempt to demonstrate the unique insights they can provide on some aspects of the mechanical behavior of both buried and thin-film interfaces. While to date, relatively little simulation work has focused directly on the observation of crack extension, we will discuss two types of phenomena with particular relevance in the fracture behavior of interface materials, namely their elastic and high-temperature properties. We will conclude with an outlook, too optimistic perhaps, on how the complementary capabilities of continuum-elastic theory, atomic-level computer simulation, and experiment could (and probably should) be combined in a new strategy for tackling the difficult problem of interface fracture to elucidate the underlying complex interplay between elasticity, plasticity, and temperature.

Magneto-optical (MO) recording combines the advantages of optical recording and magnetic recording. It offers very high storage densities, it is a noncontact technique, and it allows an unlimited number of read/write cycles. Although the potential of magneto-optical recording was recognized nearly 25 years ago, suitable materials did not exist at that time. Since then, substantial efforts have been made optimizing existing materials and searching for new ones. In 1973 an important development was started by Chaudhari et al., who discovered amorphous GdCo with perpendicular magnetic anisotropy as a possible MO recording material. Today amorphous ternary rare-earth (RE) transition-metal (TM) alloys like GdTbFe and TbFeCo are the recording layers in the MO disks which appear on the market. Although these materials show good recording performance, they also exhibit some drawbacks, mainly caused by their susceptibility to oxidarion and corrosion. However, two new classes of suitable MO materials with good oxidation resistance are emerging.

This article considers a particular class of optic microstructure — multilayers for soft x-ray (SXR, 100 eV[124 Å] to 3,000 eV[4.1 Å] and extreme ultraviolet (EUV, 15 eV[800 Å] to 100 eV[124 Å] optics. Multilayers are manmade vapor-deposited depth-periodic layered micro-stractures of high enough quality to be considered synthetic crystals. Layers of two materials, A and B, having significant differences in their scattering powers for x-rays (electron densities) and of uniform thicknesses ta and tb are combined to form a sample of uniform in-depth period d0 ( = ta + tb). These microstructures are of scientific and technological significance since the high angle of incidence SXR and EUV reflectivities of single film reflectors is ~10−4) and since only a limited number of naturally occurring or synthetic large lattice constant materials efficiently diffraction reflect long wavelength radiation (SXR and EUV).

Multilayer microstructure based SXR and EUV optics are direct analogues to standard quarterwave stacks applied at longer wavelengths with the requirement that absorption must be included. They are also analogous to atomic crystalline lattices in that one multilayer period is equivalent to one lattice plane. Thus, the multilayer structure forms a superlattice that diffraction reflects incident radiation. Multilayer diffraction may be modeled using x-ray scattering theory and obeys the refraction-corrected Bragg's law of crystal lattice diffraction. As will be discussed later, many factors determine the character of the multilayer response to an incident spectrum.