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Science and Technology of Crystalline Systems

The importance of symmetry in nature has been recognized since the time of the Greek philosophers. Today, new applications of symmetry continue to influence scientific thought in many fields, ranging from biology to astrophysics. In particular, the description of how symmetry in condensed matter is lowered or “broken,” such as when a liquid becomes a crystal, forms a mathematical connection with other disparate phenomena, such as the handedness of an oligomeric protein to the evolution of matter after the big bang. Indeed, the remarkable properties of graphene (see Box 2.1) originate from the unique way in which a single sheet of carbon atoms breaks spatial symmetry.

Symmetry is described mathematically through the theory of space groups, which are the set of spatial translations and rotations that leave a crystal structure unchanged. The structural symmetry is defined at high temperatures, when the crystal forms. At lower temperatures, interactions of lower energy than that of the interatomic bonds yield states such as superconductivity, ferromagnetism, and ferroelectricity, which then lower the symmetry of the interacting particles and fields even farther.

Crystal lattices can also serve as a laboratory for studying broken symmetries that arise in a much different context: for example, the generation of elementary particles. Elementary particle theorists often impose the symmetry of a lattice to calculate the consequences of subnuclear interactions such as the Higgs mechanism, at present being sought at the European Organization for Nuclear Research’s (CERN’s) Large Hadron Collider. Such theoretical concepts can also be realized in crystalline solids. For instance, certain magnetic systems on chain-like crystal



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2 Science and Technology of Crystalline Systems The importance of symmetry in nature has been recognized since the time of the Greek philosophers. Today, new applications of symmetry continue to influence scientific thought in many fields, ranging from biology to astrophysics. In particular, the description of how symmetry in condensed matter is lowered or “broken,” such as when a liquid becomes a crystal, forms a mathematical connection with other disparate phenomena, such as the handedness of an oligomeric protein to the evo- lution of matter after the big bang. Indeed, the remarkable properties of graphene (see Box 2.1) originate from the unique way in which a single sheet of carbon atoms breaks spatial symmetry. Symmetry is described mathematically through the theory of space groups, which are the set of spatial translations and rotations that leave a crystal structure unchanged. The structural symmetry is defined at high temperatures, when the crystal forms. At lower temperatures, interactions of lower energy than that of the interatomic bonds yield states such as superconductivity, ferromagnetism, and ferroelectricity, which then lower the symmetry of the interacting particles and fields even farther. Crystal lattices can also serve as a laboratory for studying broken symmetries that arise in a much different context: for example, the generation of elementary particles. Elementary particle theorists often impose the symmetry of a lattice to calculate the consequences of subnuclear interactions such as the Higgs mecha- nism, at present being sought at the European Organization for Nuclear Research’s (CERN’s) Large Hadron Collider. Such theoretical concepts can also be realized in crystalline solids. For instance, certain magnetic systems on chain-like crystal 

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frontiers c rys ta l l i n e m at t e r  in BOX 2.1 Graphene Elemental carbon adopts a variety of crystalline forms—three-dimensional diamond, layered graphite, carbon nanotubes, and C60 fullerite. In 2005, a new form of carbon, graphene, was discovered. Graphene is a single sheet of hexagonally ordered carbon atoms. The purely two-dimensional nature of graphene sheets gives rise to an astounding array of new phenomena, among which are the following: • Behavior that mimics the relativistic motion of particles in high-energy accelerators, • New states of matter in the quantum Hall regime (in Chapter 1 of this report, see the subsection entitled “Example in the Area of Thin Films: Gallium Arsenide-Based Heterostructures”), • Electronic conductivity at zero electron density, and • Extremely fast “ballistic” motion of electrons and holes even at room temperature. The latter property suggests a new class of electronic devices with switching speeds much greater than those achievable in silicon complementary metal oxide semiconductors, perhaps reaching terahertz frequencies. These properties and more result from an unusual electronic momentum-energy relation- ship. Electrons in the hexagonal crystal structure of graphene behave like massless relativistic electrons in a world with only two dimensions. Many materials possess a quasi-two-dimensional hexagonal structure in which the sheets interact slightly with neighboring sheets, such as in graphite itself, thus breaking the special momentum-energy relationship of electrons in graphene. What makes graphene special is that the sheets are only one atom or a few atoms thick. The technical breakthrough that led to an explosion of research into graphene (now more than 1,000 papers per year; see Figure 2.1.1) was the discovery that crystals with a thickness of only a nanometer can be seen under an optical microscope when the crystals are placed on a Si wafer coated with a layer of silicon dioxide (SiO2) (see Figure 2.1.2). The SiO2 layer thick- ness must be precisely engineered—300 nm will not work, whereas 315 nm will—to produce interference contrast with the graphene crystal. Thus, the discovery of graphene is an instance of the combining of a novel measure- ment approach with a prosaic “synthesis” technique. A large part of the future challenge for creating graphene-based devices will be that of replacing these techniques with a scalable manufacturing process that does not sacrifice the unique properties of this remarkable form of crystalline matter.

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science technology crystalline systems  and of 1200 Number of “Graphene” Papers 1000 800 600 400 200 0 2003 2004 2005 2006 2007 2008 Year FIGURE 2.1.1 The number of papers published on an annual basis from 2003 through 2008 figure 2-1-1.eps that relate to graphene, revealed by a citation search for “graphene” on the Web of Science. FIGURE 2.1.2 Photograph of an approximately 3-nanometer-thick graphene flake on top of an oxidized silicon wafer. SOURCE: From K.S. Novoselov, A.K. Geim, S.V. Morozov et al., “Electric Field Effect in Atomically Thin Carbon Films,” Science, 306, 666 (2004); reprinted with permission from the Americanfigure 2-1-2.eps Association for the Advancement of Science. bitmap

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frontiers c rys ta l l i n e m at t e r  in lattices are described by the same mathematics used to model elementary particle mass generation. Thus, crystalline symmetry provides an intellectual connection spanning 15 decades in energy! Crystalline systems are exemplars of condensed matter. The usual image of a crystal is one of clarity, which originates from the perfect ordering of atoms. In the laboratory, symmetry is manifested in regular x-ray scattering patterns, a phenomenon discovered by Sir William Lawrence Bragg and his father, Sir William Henry Bragg, for which they shared the 1915 Nobel Prize in physics. The microscopic crystal images shown in various figures in this report are constructed mathematically from such x-ray diffraction patterns; the produced images have a type of beauty appreciated especially by the scientists studying them. An additional measure of perfection comes in the physical properties of crys- tals. Examples abound in which “fragile” states are able to form only in crystals possessing a very low density of defects. One example is the set of fractional quantum Hall states discussed in Chapter 1, which are not seen in samples with crystalline disorder. Advances in measurement technology enable the probing of ever-higher degrees of crystalline perfection, allowing scientists to better match theories with experi- ments on exotic properties of matter. Currently, no general theoretical prescription exists for many of the new states of matter that emerge with increasing crystalline perfection. Discovering and understanding such states demand continual and close collaboration among synthesis, experiment, and theory. Controlling and minimizing defects in crystalline materials also constitute an important path to device innovation. Many next-generation devices for applica- tions such as solar energy, solid-state lighting, and novel sensors require crystalline order among nontraditional atomic, molecular, or nanoscale building blocks. Con - trolling such ordering will be key to creating devices that will form the foundation of these future technologies, just as controlling covalently bonded semiconductors enabled today’s microelectronics industry. Innovation involving crystalline materials has historically benefited from a fruitful interplay between basic research and device development, as described in Chapter 1. This interplay was guided by large industrial laboratories that oper- ated significant basic research divisions whose scientific agenda was to address technology goals of the corporation to which they belonged. Research scientists and engineers outside these corporations thus had a window onto technology roadmaps through interactions with fellow basic researchers in industry. Each of these laboratories often employed 100 or more discovery and growth of crystal- line materials (DGCM) scientists, including many crystal and film growers. They provided generally stable and substantial operating funds for DGCM activities and had the ability to respond rapidly to new materials opportunities without the need to pursue new funding to do so.

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science technology crystalline systems  and of In the current business climate, U.S. corporations no longer sustain such basic research divisions, with the result that an important mechanism for communicat- ing technology needs has been greatly diminished. The impact of this loss is evi- denced by the reduction in industrial publications in physics from 998 in 1988 to 312 in 2005 (see also the discussion in the section entitled “The Role of Industry in Crystal Growth” in Chapter 3). The impact of this loss on crystalline materials could also be high, considering that most modern crystal growth techniques as well as a significant amount of materials discovery in the past 50 years were achieved in the integrated corporate laboratory environment. Achieving the scientific vision expressed in this chapter while addressing an increased need to innovate will carry with it the challenge that future research activities be organized to emulate the modalities of interaction in former industrial DGCM research environments. The recommendations of the committee presented in Chapter 4 include suggestions for new approaches that go beyond the industrial research model to accommodate the changing needs of DGCM researchers in academia, national laboratories, and industry. In the major sections immediately following, the committee presents a vision for the future of both bulk and thin-film crystalline systems in the form of three grand challenges. This chapter then concludes by discussing the needs for applied crystal growth in technology development and the role that characterization will continue to have in new crystalline materials discovery. GRAND CHALLENGES IN THE SCIENCE AND TECHNOLOGY OF CRYSTALLINE MATERIALS Advances in the science and technology of condensed matter will continue both to challenge and to enable society’s understanding of the physical world for the foreseeable future. Crystalline matter is at the heart of many of the most exciting areas of research, as shown by three grand challenges: • Grand Challenge . The Development of Next-Generation Crystalline Materials— New States of Matter and New Materials—for Future Information and Com- munications Technologies The ability to tailor both the symmetry and dimensionality of a crystalline lattice allows for the creation of a delicate balance between dissimilar ground states, especially in crystals supporting strongly correlated or quantum-mechanical interactions among the electrons. Such appropriately tailored crystals can exhibit completely new behavior: quantum phenomena such as superconductivity or quantum coherence at room temperature, the ability to switch and store multiple types of signals (electric, magnetic, and structural), or the attributes of physical

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frontiers c rys ta l l i n e m at t e r  in states not anticipated by present theory. With respect to this grand challenge, the ensuing physics will be so unexpected that scientists possess a priori, besides a few general design guides such as effective dimensionality, little understanding of how atoms should be assembled. Thus, this challenge is tantamount to creating a research ecosystem for discovery-based inquiry in crystalline matter. • Grand Challenge . The Creation of New Crystalline Materials for Energy Production and Conversion Future energy technologies will require dramatic advances in crystalline mate- rials, including thermoelectric materials for heat-to-electricity conversion, solar photovoltaic materials for sunlight-to-electricity conversion, novel materials for hydrogen production and storage, new electrode and membrane materials for fuel cells, and affordable catalysts for feedstock-to-fuel conversion. Many such energy applications will require crystalline phases with low parasitic energy loss, such as low-cost solar cells with 50 percent power efficiency. Each individual topic in the area of crystalline materials for energy production and conversion requires a stretch goal for materials performance involving an unprecedented degree of atom-level control. Collectively these topics constitute a grand challenge for increasing energy availability and self-sufficiency in the United States. • Grand Challenge . Evolution in the Capacity to Create Crystalline Materials by Design In the next 10 years, advances in theory and modeling, coupled with dramatic increases in computational power, will enable the creation of completely new materials incorporating either thin-film or molecular subunits. More than ever before, these new materials will possess specific device functionalities. Realizing a materials-by-design approach would enable industry to move beyond current device limitations to provide cheaper and more efficient solar cells, high-power electronics, and devices with functionality not yet imagined. GRAND CHALLENGE 1: DEVELOPMENT OF NEXT-GENERATION CRYSTALLINE MATERIALS—NEW STATES OF MATTER AND NEW MATERIALS—FOR FUTURE INFORMATION AND COMMUNICATIONS TECHNOLOGIES In analogy to the traditional states of matter—solid, liquid, and gas— researchers have discovered a multitude of fascinating states in crystalline materials. These states depend on the effective spatial relationships between atoms within the crystal. Of particular importance is effective dimensionality and connectivity of

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science technology crystalline systems  and of substructures within the crystal; for example, materials based on lamellar sheets of atoms can be considered two-dimensional for electronic or magnetic excita- tions. These excitations interact among themselves to create patterns, or states of matter, that are highly dependent on the effective dimensionality and connectivity. Such states of matter include modified traditional states of ferromagnetism, ferro- electricity, and superconductivity. Other, more exotic states such as vortex matter, spin ice, and quantum states with no classical interpretation can occur in such sublattices. It is the ability of crystalline solids to emulate spaces of different dimensionality and connectivity that allows the realization of exotic new states of matter with novel properties. A few examples of future directions of new crystalline materials with novel properties are presented below. Scaffold Structures The challenge of creating novel states is that of using unusual and innovative combinations of elements to create an electronic structure in which there is competi- tion among possible states or forces, and then of fine-tuning the balance with chem- istry. One approach uses an atomic sublattice to serve as the scaffolding for another sublattice. Examples are shown in Figures 2.1 and 2.2. In the first example, the high-transition-temperature (Tc) superconductor yBa2Cu3O7, the superconducting phase is formed from the sublattice of copper-oxygen (Cu-O) pyramids, with the Cu-O chains and larger yttrium and barium atoms providing the necessary spac- ing to create a two-dimensional electronic structure (Figure 2.1). In addition, the Cu-O chains form a spatially distinct subsystem that acts as a charge reservoir for the two-dimensional subsystem. Another example of atomic scaffolding is in the skutterudites. Skutterudites have the general formula RM4X12, where R is a rare-earth ion; M is iron (Fe), ruthenium (Ru), or osmium (Os); and X is phosphorus (P), arsenic (As), or anti- mony (Sb). A structure is shown in Figure 2.2. Characterized by an open structure and large numbers of synthetic combinations, the skutterudites exhibit a wide array of physical properties, including metal-insulator transitions, heavy-fermion super- conductivity, and large thermoelectric power figure of merit. The latter property is aided by large voids in the crystal structure that allow large thermal vibrations for the atoms that reside in them—the atoms rattle around. This rattling is respon- sible for unusually large phonon scattering, which reduces thermal shorting in a thermoelectric cooling application. Another example of scaffolding is found in the molecular compound [BEDT- TTF]Mn[Crox3] (BEDT-TTF is bis(ethylenedithiolo)tetrathiafulvalene and ox is oxalate). Here, the coexistence of magnetic ordering and metallic-like electrical conductivity occurs due to alternating electrically conducting layers composed of [BEDT-TTF]+ and ferromagnetically ordered layers composed of {Mn[Crox3]}–.

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frontiers c rys ta l l i n e m at t e r 0 in FIGURE 2.1 Crystal structure of YBa2Cu3O7 showing the sublattice of the copper-oxygen pyramids (blue) and the larger yttrium (yellow) and barium (green) atoms providing the spacing for a two- dimensional electronic structure. SOURCE: Courtesy of M.A. Subramanian, Oregon State University. FIGURE 2.2 Filled skutterudite LaFe3CoSb12 showing the large open spaces where the lanthanum ions (yellow) rattle to scatter phonons. Iron (Fe) and cobalt (Co) (red) and antimony (Sb) (blue) are also shown. SOURCE: Courtesy of D. Mandrus, Oak Ridge National Laboratory.

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science technology crystalline systems  and of The final example is the newly discovered magnetic material of Figure 2.3, in which the presence of the second interpenetrating lattice led to anomalous magnetic switching behavior not observed before. Low-Dimensional Structures As mentioned above, crystalline materials can possess internal scaffolding structures with effective dimensionality lower than three dimensions. Such crys- talline structures possessing either chains or planes of interacting atoms modify the flow of energy for magnetic and electronic excitations, creating platforms for useful devices. This lower effective dimensionality can also lead to entirely new ground states. For instance, when local degrees of freedom are continuously variable, an atomic spin can point in any direction, and fluctuations suppress a long-range ordered state in one or two dimensions. In some cases, excitations in such systems can be “topological,” like knots in a string. Solitons, which are localized waves whose shape is unaffected by usual dispersive effects, are an FIGURE 2.3 Interpenetrating [Ru2(O2CMe)4]3[Cr(CN)6] lattices are depicted in orange and purple. SOURCE: Courtesy of J.S. Miller, University of Utah.

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frontiers c rys ta l l i n e m at t e r  in example of topological magnetic excitations in one dimension. In two dimen - sions a common topological excitation is the vortex, which resembles a swirling pattern of spins. Topological character implies insensitivity to localized defects and dispersive effects that commonly destroy the coherence of harmonic, or wave- like, excitations. Such features are important for future information technologies; for instance, topological excitations in a two-dimensional electron gas have been discussed as possible stable bits of information in a quantum computer. Magnetic topological excitations are also potential “qubit” candidates, but the understand- ing required to design a material supporting such excitations is in the very early stages of development. Low-Dimensional High-Tc Cuprates Several central themes come together in the cuprate materials that exhibit high-Tc superconductivity, with yBa2Cu3O7 (Figure 2.1) being the best-studied example. These materials have copper oxide sheets that are doped and effectively two-dimensional by means of a superlattice spatially distinct from the sheets. Absent a widely accepted microscopic theory, most contending theories feature the hybridized copper oxygen band as a key component of the superconducting as well as the normal state. The formation of patterns indicative of density varia- tions in a high-Tc compound is shown in the scanning tunneling microscopy data of Figure 2.4. Low-Dimensional High-Thermopower Cobaltates The concepts of low dimensionality, geometric frustration of spin order- ing, structural and orbital degrees of freedom, correlated electron physics, and quantum fluctuations converge to yield the unexpected physical properties of the layered oxide cobaltates. In a structural family based on hexagonal CoO2 layers, these metallically conducting compounds display thermoelectric coefficients two orders of magnitude larger than those observed in conventional metals. Further- more, superconductivity is observed in the hydrate NaxCoO2·yH2O. The origin of these phenomena is not understood but could be due to a combination of low dimensionality and strong correlations unique to the combination of cobalt and oxygen. Structures Leading to Strong Competition of Internal Forces Patterns among low-energy degrees of freedom in a solid (such as those shown in Figure 2.4) can be thought of as distinct phases of matter. New phases are often found when competing forces are finely balanced at the microscopic

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science technology crystalline systems  and of FIGURE 2.4 An example of anisotropic superconductivity in a cuprate material is seen in this scan - ning electronic micrograph in which thefigure 2lighteps dark and - 4. regions represent differing electron density. bitmap SOURCE: Courtesy of J.C. Seamus Davis, Cornell University. level, either chemically by means of a novel combination of elements, or physi- cally, for example by means of magnetic fields. The following are examples of competing states: localized versus itinerant electrons, spin alignment versus spin antialignment, or classical versus quantum fluctuations. When systems with finely balanced forces pass from one state to another, their physical response can become very large. An example is colossal magnetoresistance in manganite perovskites. Another example is the very large thermal resistance effect in vanadium oxide, used as the sensor in commercial infrared imaging systems. Physicists describe such behavior as “emerging” from the collective behavior of the large number of atoms (~1022) that comprise a solid. The synthesis scientist approaches the chal- lenge of creating such new emergent behavior by using different paradigmatic approaches to crystal growth. Geometrically Frustrated Structures One route to optimizing competing interactions is through geometrical frus- tration of magnetic interactions. In geometrically frustrated materials, the spins that constitute magnetic matter interact antiferromagnetically: interactions favor antiparallel alignment of neighboring spins, as shown in Figure 2.5. Such inter- actions cannot be simultaneously satisfied when the spins occupy a triangular lattice, as also shown in Figure 2.5. The inability to achieve a state that minimizes the energy of all two-body interactions, and the resulting low-energy entropy, are called geometrical frustration. This simple paradigm has deep consequences. When spins in anisotropic pyrochlore magnets are allowed to point only up or down, in a manner similar to that shown in Figure 2.5 for a triangle, the spin system

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frontiers c rys ta l l i n e m at t e r  in or xenon lamps. A high-pressure environment can also be combined with FZ growth, providing the unique opportunity to grow single crystals that are either unstable at ambient pressure or have very high oxidation states, such as those containing Cu3+, Ni3+, and/or Fe4+. It is expected that further variations on the basic FZ process will be explored in the coming years. • Charge carrier doping often plays a key role in producing novel electronic phases, including superconductivity. In many cases, carrier doping has been achieved by chemical substitution, which often suffers from structural disorder introduced by the mismatch of either atomic size or electronic structure of the substituent, or from the presence of a miscibility gap. Electrostatic doping, using a field-effect transistor (FET) structure fabricated on the surface of the crystal, is a nonchemical approach to modifying the carrier concentration while avoiding structural disorder. To date, however, the maximum number of charge carriers injected by such “FET chemistry” is limited up to ~10–19cm-3, due to the finite breakdown voltage of typical gate insulator materials. Recently, however, carrier injection using an elec- trolyte cell situated on the crystal surface, instead of a conventional FET structure, was reported. This electrochemical technique can inject carriers up to ~10–20cm-3 and has succeeded in “doping” the normally insulating SrTiO3, to induce a superconducting phase. Thus, co-joined FET structures provide a novel route to altering the electronic properties of crystalline matter, and new FET methods will likely be developed in the future. • Related to the interest in FET structures, interfaces in crystalline hetero- structures are now seen as a well-established type of crystalline matter for studying new electronic states, not only in covalent semiconductors but also in ionic oxides and van der Waals bonded organics. The need for precise control of such interfaces requires characterization of local electronic states on the atomic level using state-of-the-art spectroscopies, including angle resolved photoemission (ARPES) and scanning tunneling microscope (STM). In order to conduct interface characterization in situ during layer-by-layer growth, the integration of the thin-film chamber (either MBE or pulsed laser deposition [PLD]) with ARPES or STM should be pursued. Indeed, efforts are now under way to perform PLD growth in a chamber connected directly to a synchrotron x-ray beam line for photo- emission spectroscopy. • In order to promote the search for new materials, a technique named combinatorial chemistry (CC) has been developed. This technique was first employed in the pharmaceutical industry to reduce the time and cost associ- ated with producing effective and competitive new drugs. Efforts have been made to apply CC to oxide thin films in order to introduce dopants with a graded concentration (also called composition-spread films). This tech-

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science technology crystalline systems  and of nique has seen limited application; one example is the successful fabrication of correlated transition metal oxides such as the colossal magnetoresistance manganites. With these materials, researchers have investigated the phase diagram of the solid solution using a single film. Probes to characterize local lattice constant, resistivity, and magnetization of composition-spread film have been developed. In the future, CC is expected to be applied to an increasing number of materials problems in which compositions to opti- mize a particular property are desired. Future directions in crystal growth techniques will depend strongly on the classes of materials that define the mainstream topics in condensed-matter science. New compounds are constantly being discovered, and novel approaches to produc- ing large single crystals of these compounds are often required. ROLE OF CHARACTERIZATION FOR NEW CRYSTALLINE MATERIALS DISCOVERY The discovery of materials with novel, scientifically or technologically useful properties involves a wide range of scientific expertise, equipment, and processes in various institutions. Developments in the United States over the past 50 years have created an imbalance in this multifaceted process of new materials discovery and development. While facilities for materials characterization have increased capacity and expanded capabilities, the shrinking level of industrial basic research has led to a reduction in synthesis capabilities, as documented elsewhere in this report. This section focuses on opportunities to leverage the greater capacity for materials characterization to advance the scientific understanding and application of new crystalline materials. Laboratory-Scale Materials Characterization Tools In contrast to crystal growth, there has been extraordinary progress in laboratory- scale materials characterization tools. Mass production, automation, and informa- tion technology have greatly reduced the costs and increased the efficiency of such tools. Examples of improved materials characterization equipment in this category include x-ray diffraction instrumentation, Raman and infrared spectrometers, and cryogenic systems for specific heat, susceptibility, and transport measurements. Consider, for example, a typical activity that would follow the development of a new material: a specific heat measurement. Twenty years ago, a measurement of the low-temperature heat capacity of a single crystal required expertise that could only be gained through dedication over a career, in conjunction with technical skills in cryogenics, vacuum technology, analogue temperature control, signal process-

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frontiers c rys ta l l i n e m at t e r  in ing, and data analysis. Today a young researcher with a typical start-up package can purchase a commercial cryostat with fully automated cryogenics, vacuum, and electronics. Heat capacity is an affordable option, as are alternating-current susceptibility, magnetization, thermal conductivity, and electrical transport. In addition, data collection is fully automated and can be monitored remotely. All of these features greatly increase the individual investigator’s measurement capacity. There has also been dramatic progress in data analysis and presentation tools. Twenty years ago data analysis involved the use of unforgiving mainframe com- puters, and plotting was done by a graphic designer. These tasks are now completed faster and better by commercial software packages on desktop computers. As a result, experimentalists have the capacity to acquire, analyze, and publish compre- hensive data for a much wider range of samples than was the case just 10 years ago. The capacity to probe bulk properties of new materials has increased, perhaps by as much as an order of magnitude per experimentalist. National Facilities for Materials Characterization While bulk experiments carried out in a single-investigator laboratory are typi- cally completed first, full understanding of new materials often requires the use of national facilities that probe matter on the atomic scale. Table 2.1 provides an overview of major federally funded facilities for materials research. Experimental probes include neutrons, x-rays, and microscopy with photons and electrons. The role of facilities in the development of new materials and the sample synthesis requirements for each type of facility are discussed here. TABLE 2.1 Overview of National User Facilities for Materials Science, with FY 2007 User Statistics and Number of Publications for CY 2006 No. of Users No. of Papers in FY 2007a in CY 2006b Facility Laboratory Radiation NIST Center for Neutron NIST Neutrons, reactor 858 406 Research High Flux Isotope Reactor ORNL Neutrons, reactor 72 81 Spallation Neutron Source ORNL Neutrons, pulsed 24 68 spallation Los Alamos Neutron LANL Neutrons, pulsed 272 170 Science Center spallation

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science technology crystalline systems  and of TABLE 2.1 (Continued) No. of Users No. of Papers in FY 2007a in CY 2006b Facility Laboratory Radiation Intense Pulsed Neutron ANL Neutrons, pulsed 173 103 Source (IPNS)c spallation National Synchrotron Light BNL X-rays, 2,219 612 Source synchrotron Advanced Photon Source ANL X-rays, 3,420 1,106 synchrotron Advanced Light Source LBNL X-rays, 1,748 593 synchrotron Stanford Synchrotron Stanford University X-rays, 1,151 320 Radiation Laboratory synchrotron Center for Microanalysis University of Illinois Electrons, 600 200 of Materialsd at Urbana-Champaign microscopy (approx.) (approx.) Electron Microscopy ANL Electrons, 199 89 Center microscopy National Center for LBNL Electrons, 183 150 Electron Microscopy microscopy National High Magnetic NHMFL High magnetic 1,144 404 Field Laboratorye fields NOTE: FY, fiscal year; CY, calendar year; NIST, National Institute of Standards and Technology; ORNL, Oak Ridge National Laboratory; LANL, Los Alamos National Laboratory; ANL, Argonne National Laboratory; BNL, Brookhaven National Laboratory; LBNL, Lawrence Berkeley National Laboratory; NHMFL, National High Magnetic Field Laboratory. a Data from the Department of Energy, available at http://www.sc.doe.gov/bes/users.htm, except as other- wise noted below in footnote e. Users are defined generally as researchers who propose and conduct peer- reviewed experiments at a scientific facility. They include remote users (researchers granted authority to remotely produce data) and offsite users (researchers to whom the facility provides custom-manufactured materials, tools or devices). An individual is counted as one user no matter how often or how long the researcher conducts experiments at the facility during the fiscal year. User data for the NIST Center for Neutron Research were obtained from private correspondence with operators of the facility. b C ommittee-collected data. c I PNS closed in February 2008. Table shows the final number of users in FY 2007 and the number of publications for CY 2006. d The Center for Microanalysis of Materials at the University of Illinois at Urbana-Champaign has not been designated a Basic Energy Sciences user facility since FY 2005; hence no current statistics are available. e U ser information regarding the National High Magnetic Field Laboratory is from the 2 007 Annual Report for NHMFL, available at http://www.magnet.fsu.edu/mediacenter/publications/reports/annualreport-2007.pdf.

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frontiers c rys ta l l i n e m at t e r  in While a laboratory-based x-ray source is the traditional route to atomic-scale structural information, synchrotron x-ray sources provide unprecedented sensi- tivity, accuracy, and detail. The much higher source brightness makes it possible to determine the structure of minute crystals (with size on the order of cubic microns) typical of the early stages of materials discovery work. With the current generation of synchrotron x-ray sources, it has even become possible to probe dynamic properties of solids and liquids such as phonons in micron-sized samples. Such information is critical for understanding physical properties, from electronic transport to thermal conductivity. Thin films and nanosize crystalline materials will be incorporated into many future technologies, from quantum computers to medical therapies. The lack of long-range order in one or more dimensions and the very small quantities of materials involved preclude the use of conventional structural probes. Synchrotron x-ray sources provide unique capabilities for prob- ing structure and dynamics under such conditions. Electron microscopy can provide direct, real-space, structural, chemical, and electronic information, resolved at the atomic scale for crystals, buried interfaces, and point or line defects. Recent examples have included the imaging of oxygen vacancies near oxide grain boundaries and in artificially grown heterostructures. Lattice distortions and strain fields can be measured to a precision of a few picometers using the 0.1 nm or better resolution images acquired on aberration- corrected transmission electron microscopes. Scanning transmission electron microscopy (STEM) offers good chemical sensitivity at a comparable resolution and can be used to detect and image individual dopant atoms inside a crystal. Electron energy loss spectroscopy can be performed simultaneously with STEM, and it provides very similar information to that obtained with x-ray absorp- tion spectroscopy, probing the local electronic structure, partitioned by chemical species and site at the atomic scale. Electron holography can measure real-time changes in the electric and magnetic fields in the thinned sample, with the sensi- tivity of a single fluxon. Sample preparation techniques developed for semiconductor failure analysis have greatly improved the quality and speed of sample preparation, which had long been a bottleneck for rapid electron characterization. As this is a real-space imaging method, very small sample quantities are adequate, and the method is well suited for initial explorations of new materials systems. Secondary phases can be identified and studied or avoided as needed. This suggests that electron microscopy can be most effective when coupled closely to sample growth for timely feedback and insight. However, a modern electron microscope can be as sophisticated and complicated to operate as a synchrotron beam line. While most microscopists are well trained in general structural analysis for materials science, very few laborato- ries, whether in the United States or abroad, are producing users trained to quan- titatively analyze and interpret the sophisticated spectra and images generated by

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science technology crystalline systems  and of modern instruments. As the complexity of the instrumentation increases, so must efforts to train graduate student and postdoctoral fellows in the use of the latest instrumentation for materials science. Neutron scattering offers unique sensitivity to light atoms and magnetism, momentum-resolved spectroscopy spanning 10 orders of magnitude in energy (0.1 nanoelectronvolt to 1 eV), and the ability to penetrate thick samples, sample environment, and materials processing systems for in situ studies. For fundamental reasons that are directly tied to the utility of the technique, neutron interactions with matter are much weaker than for photons and electrons. In addition, because neutrons are tightly bound in nuclei, neutron sources provide orders-of-magnitude less flux on sample than photon flux at synchrotron facilities. While these factors have limited the use of neutrons for materials science in the past, the Spallation Neutron Source at the Oak Ridge National Laboratory and improved moderators and neutron optics at existing reactor facilities are leading to several orders-of- magnitude improvement in sensitivity and capacity. As a result, a substantial expansion in the use and impact of the neutron-scattering technique in research is occurring, which parallels the transition from rotating anodes to synchrotron sources for x-rays. High magnetic fields continue to be an essential tool for discovering and understanding new materials functionality. An applied magnetic field will alter the course of an electron in a circular path called a cyclotron orbit. At fields greater than 1 tesla (T) (10,000 times Earth’s field), the cyclotron orbit approaches interatomic dimensions. Measurements of electronic and thermal properties versus applied fields greater than a tesla can thus yield information on the electronic structure of a crystalline material. The National High Magnetic Field Laboratory in Tallahassee, Florida, is the primary U.S. facility with this focus; it routinely provides static mag- netic fields in excess of 30 T and can reach 100 T in short (millisecond) pulses. As a rule of thumb, 1 T approximates 1 kelvin (k), and thus 100 T fields enable the probing of electronic interactions of 100 k in strength. The availability of such fields enables the study of novel electronic phenomena such as critical phases in the quantum regime, including new fractional quantum Hall states; characterization of high-temperature superconductors for both basic science and large-current- carrying applications; and novel phases in magnetic materials such as spin ice and low-effective-dimensionality materials. Science from this type of facility requires a steady stream of qualitatively new materials, preferably in single-crystalline form so that intrinsic anisotropies can be explored. The growing capacity for facility-based materials characterization is illustrated by recent statistics for single-crystal experiments at the National Institute of Stan- dards and Technology (NIST) Center for Neutron Research. Figure 2.20 shows the number of single-crystal neutron experiments over time, classified by instrument. While single-crystal experiments hold the potential for more detailed atomic-scale

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frontiers c rys ta l l i n e m at t e r 0 in 30 0 Reflectometer SANS TOF 250 Cold -TAS Therm-TAS 20 0 NCNR Cr ystal Experiments 150 10 0 50 0 1997 1998 1999 20 00 20 01 20 02 20 03 20 04 20 05 20 06 20 07 Year FIGURE 2.20 Number of single-crystal neutron-scattering experiments per year from 1997 through figure 2-20.eps 2007 at the NIST Center for Neutron Research (NCNR). Color coding indicates the instrument types: reflectometry, small angle neutron scattering (SANS), time of flight (TOF) spectroscopy, and cold or thermal triple axis spectroscopy (TAS). SOURCE: Data provided by Peter Gehring, NIST Center for Neutron Research. information, they also demand more of the instrumentation. The fivefold increase in the number of such experiments over the past decade is a result of progress in instrumentation and user access. With further improvements in these areas at both the NIST Center for Neutron Research and at the Oak Ridge National Laboratory, the capacity for single-crystal experiments will continue to grow dramatically in the coming decade. While single-crystal neutron scattering is seldom the first experiment to be conducted on a new material, such experiments are often necessary in order to understand and control new materials properties. In the preceding decade the supply of novel crystalline materials apparently kept up with the demand, as indi- cated by the factor-of-two average instrumentation oversubscription at NIST. But

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science technology crystalline systems  and of as capacity grows, will there continue to be an adequate supply of exciting new materials for a strong scientific program? To answer that question, the committee examined the origin of the materials discoveries underlying the rapidly expanding activity in single-crystal neutron scat- tering at NIST. Figure 2.21 shows crystal experiments over time, classified by the origin of the underlying materials discovery. Europe, the United States, and Japan are the dominant locations of origin. The strong European representation can largely be attributed to the continuing impact of Bednorz and Müller’s discovery of high-temperature superconductivity in La2-xBaxCuO4. The large impact of Japan is associated with discoveries of novel correlated electron systems and magnetism in manganese (Mn)-doped gallium arsenide (GaAs). Materials classes underlying the U.S. impact include superconducting y2BaCuO3, heavy-fermion intermetallics such as CeCoIn5, and strongly correlated oxides such as Sr3Ru2O7. However, a large frac- tion of the materials attributed to the United States could be called legacy materials with continuing scientific impact, such as LaCoO3 and LaMnO3. This indicates that Figure 2.21 is a lagging indicator of materials synthesis, not yet having picked up the decline in U.S. materials synthesis documented elsewhere in this report. Original 90 Materials Discover y: 80 Europe NCNR User Experiments on Cr ystals USA 70 Japan Other 60 Unidentified 50 40 30 20 10 0 2003 2004 2005 2006 2007 Year FIGURE 2.21 User experiments at the NIST Center for-Neutron Research (NCNR) for the years 2003 figure 2 21.eps through 2007, classified by the origin (country or region) of the materials discovery that initiated the research. SOURCE: Data provided by Peter Gehring, NIST Center for Neutron Research.

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frontiers c rys ta l l i n e m at t e r  in Opportunities Through Crystalline Matter Discovery The expanded capabilities for materials characterization both in the laboratory and at national facilities create extraordinary opportunities for accelerated progress in materials science. However, investment in and coordination with crystalline matter discovery are essential for realizing this potential. For nanoscale-structured materials, action has recently been taken in rec- ognition of this need. By colocating nanophase synthesis centers with national facilities to probe materials, the Department of Energy has created extraordinary conditions for synergy between crystalline matter discovery and characterization for nanostructured materials. An example is the Center for Nanophase Materials Science colocated with the Spallation Neutron Source. Combining deuterium- labeled polymer synthesis capabilities with small angle neutron scattering and neutron reflectometry provides unprecedented capabilities for creating and prob- ing self-assembled nanoscale structures for fundamental science and applications. Also, by combining the ability to create a thousand to a million copies of a given nanoscale structure with three-orders-of-magnitude-greater neutron brightness, it will be possible to probe phonons and magnons confined to the nanoscale. There are corresponding opportunities for extraordinary insight from x-ray diffraction and spectroscopy with microfocused synchrotron beams at the Advanced Photon Source, National Synchrotron Light Source, and Advanced Light Source on nano- structures created respectively at the colocated Center for Nanoscale Materials, Center for Functional Nanomaterials, and the Molecular Foundry. The advances in materials characterization techniques also create new oppor- tunities for understanding and controlling homogeneous crystalline materials. However, the utility and significance of the science produced depend critically on the resources devoted to discovering new materials and on producing them in suf- ficient quality and quantity for advanced characterization. For neutron-scattering experiments, the quality and depth of information produced are typically limited only by the sample’s size and quality. Examples of samples used for recent experi- ments are shown in Figure 2.22. Two specific examples of crystal growth as the rate-limiting factor for scientific progress are provided below. The first of these examples is that almost two decades ago, neutron scatter- ing experiments uncovered a spectacular spin resonance in the superconducting state of yBa2Cu3O6+δ. It is a phenomenon that continues to be at the forefront of research in high-temperature superconductivity. To correlate the resonance energy with the superconducting gap amplitude, neutron and angle-resolved photo- emission spectroscopy or scanning tunneling spectroscopy must be carried out on the same material. It took more than a decade to produce single crystals of cleavable Bi2Sr2Ca2CuO8+δ that were large enough for neutron-scattering experiments to accomplish this goal. At the time of the writing of this report, the correspond-

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science technology crystalline systems  and of FIGURE 2.22 Images of crystals and sample holders for inelastic neutron scattering experiments. (Top left) ZrW2O8 grown using a flux technique by Glen Kowach, City College of New York. (Top right) Co-aligned ZrW2O8 single crystals used at the ISIS Facility to probe phonons in this negative thermal expansion material. (Bottom left) Co-aligned copper pyrazine dinitrate crystals used to probe an extended critical phase in a quasi-one-dimensional spin-1/2 antiferromagnet as a function of applied magnetic field at the NIST Center for Neutron Research. The crystals were grown in the group of Mark Turnbull and Christopher Landee at Clarke University. (Bottom right) Co-aligned Y 2BaNiO5 single crys- tals grown by H. Takagi and used to probe the Haldane singlet phase in a quasi-one-dimensional spin-1 antiferromagnet. The chains extend approximately along the cylinder axis of the samples. SOURCES: Courtesy of (top left) Glen Kowach, City College of New York; (top right) Joost van Duijn, Universidad Complutense de Madrid, Spain; (bottom left and right) Collin Broholm, Johns Hopkins University. ing letter to Nature reporting a spin resonance in the superconducting state of Bi2Sr2Ca2CuO8+δ8 had been cited 190 times since its publication in 1999. A second example of research limited by crystal growth capabilities is the recent effort to understand charge and spin dynamics in NaxCoO2.yH2O. The fun- 8 H.F. Fong, P. Bourges, y. Sidis et al., “Neutron Scattering from Magnetic Excitations in Bi2Sr2CaCu2O8+δ,” Nature, 398, 588 (1999). Number of citations obtained from the ISI Web of Science, http://apps.isiknowledge.com. Last accessed April 2, 2009.

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frontiers c rys ta l l i n e m at t e r  in damental importance of this material and of the exploratory materials synthesis that produced it is indicated by the more than 150 citations annually of the 2003 discovery paper by k. Takada et al.9 While neutron scattering would provide key information for understanding electronic correlations in this material, adequate single crystals are at present not available. The central challenge for understanding NaxCoO2.yH2O thus arguably lies in single-crystal synthesis. In summary, there has been extraordinary progress in the ability to probe new materials through laboratory-scale instrumentation and national user facilities. The quality of the science produced is, however, critically dependent on (1) the dis- covery of new crystalline materials and (2) the production of high-quality samples with the appropriate morphology and dimensions for advanced characterization. Increased emphasis on the discovery and growth of novel crystalline materials is needed to realize the potential of facilities for new science and for materials-based applications in technologies ranging from information to energy. 9 k. Takada, H. Sakurai, E. Takayama-Muromachi et al., “Superconductivity in Two-Dimensional CoO2 Layers,” Nature, 422, 53 (2003). Number of citations obtained from the ISI Web of Science, http://apps.isiknowledge.com. Last accessed April 2, 2009.