Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
3 Science and Technology at the Center Research conducted at the NIST Center for Neutron Research continues to be highly collaborative, and the facility continues to attract a diverse cross section of users from academia, national laboratories, and industry in the United States and from Europe. Whereas academic and national laboratory researchers comprise 67 and 13 percent of the participants, respectively, industrial researchers currently account for only 5 percent. Plans are being developed and implemented to increase the number of industrial participants. The formation of the nSoft consortium to involve industry in using neutron scattering in the area of soft materials is an attempt to formalize and continue collaborations with industry, and it is hoped that it will result in successful collaborations. A workshop on industrial uses of neutron scattering is being planned for industrial scientists. This is an excellent idea, with the laudable goal of improving industrial participation. Partnerships with universities and other agencies have strengthened the scientific impact and capabilities of the NCNR. For example, the Center for High Resolution Neutron Scattering (CHRNS) represents a long-standing partnership between the NCNR and the National Science Foundation. It provides funds for scientific staff to support users on the CHRNS instruments; instrument development such as the multi-angle crystal spectrometer (MACS), a best-of-its-kind, high-flux spectrometer allowing ultrahigh- sensitivity access to dynamic correlations in condensed matter on length scales from 0.1 nm to 50 nm and energy scales from 0.05 meV to 20 meV; and outreach activities to educate and serve the neutron-scattering community. Flexible partnerships with universities have enabled the NCNR to carry out leading-edge research and advance the application of neutron-scattering techniques. The maintenance of existing university partnerships and the development of new partnerships should continue to enhance the ability of the NCNR to advance neutron-measurement techniques and their application to science and engineering problems. One of the primary reasons that the NCNR continues to be one of the best places in the world for performing neutron experiments is the quality of the research staff and the availability of state-of-the-art instrumentation for measuring the structure and dynamics of diverse materials systems. The capabilities and continuous improvement of the instruments, which currently enable measurement of the structure and dynamics over a wide range of length scales (0.01 nm to 10 microns) and timescales (0.01 picoseconds to 100 nanoseconds), respectively, make the NCNR facility competitive on the world stage. The instruments continue to be oversubscribed, as evidenced by the proposal pressure, which is between two and three proposals offered for each one accepted. The research continues to be at the forefront, covering a number of areas of soft and hard condensed-matter science, as well as measurement science. These areas include structural biology (particularly membranes and vesicles), neutron radioactive decay experiments, ferroelectricity, dynamics of complex fluids, high critical-temperature (Tc) materials, and magnetism. The highest level of proposal oversubscription and the most 12
publications currently arise from experiments utilizing small angle neutron scattering (SANS) and powder diffractometry. An important metric of performance is the number and quality of publications annually and the percentage of experiments performed at the facility that lead to publications. At 325, the number of publications in 2010 remains high; 76 percent of the papers are published in journals with impact factors equal to or greater than 2. Moreover, 50 percent of the experiments performed at the facility resulted in publications. This indicates that the experiments are well planned and that the proposal review process is effective. Notably, most of the publications are in the area of soft-matter science, reflected by the fact that four of the top five journals in which publications appear emphasize topics involving soft matter. The NCNR has best-in-class instruments and capabilities in the area of soft condensed matter. A focus area in neutron-scattering measurements of membrane proteins has been significantly enhanced over previous years through collaborative partnerships involving the NCNR and other NIST laboratories and external collaborators, such as the Biomolecular Labeling Laboratory (involving the University of Maryland and the NIST Biochemical Science Division of the Material Measurement Laboratory); a joint hire of a research scientist with the Material Measurement Laboratory; and a proposal developed jointly by the Institute for Bioscience and Biotechnology Research (a collaborative institute involving NIST and the University of Maryland), JILA (located in Boulder, Colorado), and NCNR staff in this area. Continued emphasis and effort to ensure a successful realization of this partnership should greatly strengthen the capabilities and impact of biological work at the NCNR as well as tap into a high-growth community. The focus on membranes and membrane proteins is a reasonable approach, given the number of individuals currently spearheading this effort. In addition, synergies between the NCNR and the NIST Polymer Division have historically led to highly productive science. Collaborative efforts with the NIST Material Measurement Laboratory and the Physical Measurement Laboratory should be maintained to aid the NCNR in extending its leadership in cold-neutron research. Future partnerships with the NIST Center for Nanoscale Science and Technology should be explored to strengthen the capabilities and impact of the NCNR. The NCNR continues to develop novel ancillary sample environments and equipment, developed in part by users, such as the development of the novel shear cell with the University of Delaware, the development of humidity cell for membrane studies with Carnegie Mellon University, and advances in 3He polarization capabilities. Such capabilities add to the attractiveness of the NCNR as a facility for carrying out unique studies with a convenience not yet found in other facilities. The development at the NCNR of novel ancillary sample environments and equipment should be continued. The section below contains an assessment of the potential impact of ongoing projects presented to the panel. CAPABILITY DEVELOPMENT: USANS, VSANS, AND POWDER DIFFRACTION The ultra-small angle neutron scattering (USANS) instrument at the NCNR continues to attract new users needing the world-leading capabilities of this unique instrument. New users, including those from industry, interested in topical areas such as 13
carbon sequestration, and concrete and explosives research present new opportunities for the SANS program. The development of vSANS, which will extend the momentum transfer vector (Q) range to overlap USANS and SANS, is a particularly noteworthy development. The vSANS instrument should allow for efficient measurements with low background, and, given appropriate samples, be as convenient as conventional SANS measurements. With the addition of vSANS, the NCNR instruments will provide the capability of carrying out measurements over multiple length scales, covering five orders of magnitude in a single facility. Such a capability is especially important in real-world problems; for example, size distribution of voids is important to the shock resistance of explosives and to the integrity of concrete used in nuclear waste storage. The powder diffraction program continues to be internationally competitive as well as forward looking, owing largely to excellent management support and high-quality external and internal collaborators. Currently, research is devoted to iron superconductors, hydrogen, CO2, and other gas sorption/separation experiments. A preconceptual design for a high-flux materials diffractometer was presented to the panel. This is an important development, as its existence will enable measurements such as time-resolved powder diffraction and diffraction from small samples, which might allow more routine study of hydrogenated materials. If fully realized, such capabilities are likely to be utilized by a large number of users, as it will be much more tractable to study pure samples of novel compounds when only small volumes of material are required. The new materials diffractometer also will enable the study of materials under extreme conditions of, for example, pressure (up to 2.5 GPa), temperature (30 milli-kelvin to 700 K), and magnetic field (10 and 15 tesla), with sample environments available at the NCNR with higher temporal resolution. It might well return prominence to the NCNR, where the modern era of high-pressure science began with the invention of the diamond anvil cell at NIST/National Bureau of Standards in the 1950s through the 1970s (http://nvl.nist.gov/pub/nistpubs/sp958-lide/100-103.pdf). The development of such an instrument should be supported. RHEOLOGY The work done in the area of rheology is diverse. It includes the structure and properties of polymers for fuel cells, organic materials for solar cells, and polymers for microelectronics, as well as the flow and rheology of complex fluids. Collaborations with universities through the Center for High-Resolution Neutron Scattering have been very successful. These collaborations have been responsible for the development of a commercial rheometer that enables the in situ study of complex fluids under shear, while simultaneously probing the structure using SANS. Exciting results using scanning narrow aperture flow-USANS (SNAFUSANS) were presented to the panel and were also featured in the 2010 annual report of the NCNR. The construction of a novel sample cell, now available in the pool of ancillary equipment at the NCNR, allowed observation of compositional and structural differences in complex fluids under shear. This new capability enabled the establishment of ânon-equilibrium phase diagramsâ for complex fluids that are used for a wide range of applications, from health care products to drug delivery. 14
THIN-FILM STRUCTURE Another area of active investigation involves off-specular neutron scattering to develop an understanding of the structure of two-dimensional ordered materials and the extracting of orientational nanophase information from thin films. The technique is complementary to more conventional atomic force microscopy and grazing-incidence small angle x-ray scattering (GSAXS) measurements. Software is being developed to analyze data taken from a position-sensitive detector in order to gain information laterally about the structure of the sample within the film. Success of this program should lead to more widespread use of this technique for studying two-dimensional complex fluid films, such as ordered block copolymer thin films subject to geometrical constraints, and biomimetic membranes. Reflectivity measurements continue to be used to address important research problems. CORRELATED ELECTRON AND MAGNETIC MATERIALS Research on correlated electron materials continues to be an area of strength at the NCNR for its internal researchers as well as for the vigorous national and international user community. The NCNR continues to make key advances on the iron pnictide superconductors, having made important alliances with leading sample growers. There continue to be important advances in the understanding of the magnetic excitations and structures of the 122 compounds, and the position-sensitive detectors and spin polarization on the BT-7 double focusing triple-axis spectrometer have proven instrumental for these advances. Of particular note in the past year have been the diffraction and inelastic scattering experiments on the iron chalcogenide compounds that clarify the role of iron interstitials and strain on the magnetic structure of Fe1-xTe, and also the evolution of magnetic excitations with doping. The U.S. quantum magnetism community, with interests in compounds such as heavy fermions and geometrically frustrated insulators, remains headquartered at the NCNR. The MACS is becoming increasingly important for these users. The incorporation of spin polarization analysis into inelastic scattering and diffraction experiments is just starting to make its mark, as evidenced by the recent results on the magnetic shell structure of nanoparticles and magnetic domains in a multiferroic compound. The NCNRâs continued emphasis on providing reliable sample environments makes it competitive with the SNS in retaining these users, although this situation is likely to become less tenable as SNS capabilities increase in the next few years. NCNR management is providing appropriate and unique capabilities that will keep the NCNR relevant to the correlated-electron user community, even as more intense sources come online elsewhere. The CHRNS has played an especially important role in the continued vitality of this field, both in providing resources and in recruiting and training new practitioners. The NCNR continues to attract excellent junior scientists whose research interests reside in these correlated electron materials, which bodes very well for the continued preeminence of NCNR research in this area. 15
LIPID MEMBRANES The work with the membrane diffractometer (advanced neutron diffractometer/ reflectometer [AND/R]) on lipid membrane multilayers continues to be of high quality and is geared toward studying the structure of proteins inside lipid membranes. One surprising result from recent measurements carried out by a group at the NCNR is that the disorder due to fluctuations in the multilayers is much larger than previously thought. Another group has carried out some impressive neutron spin-echo work to study the fluctuation dynamics of unilamellar vesicles to obtain their elastic moduli. There are very few results of this type in the field at present. The hiring of a leader for the joint team involving the NIST Material Measurement Laboratory and the NCNR is an excellent move that should strengthen the collaborative efforts with strong groups outside of the NCNR in this area. As the Biomolecular Labeling Laboratory with the Biochemical Science Division comes online, the ability to use neutron-scattering techniques to address biological questions of interest will grow, positioning the NCNR to serve a high-growth community in the biological sciences. In a similar vein, the transition of SASSIE to a robust, user-friendly analysis software package has the potential for significantly aiding the expansion of neutron- scattering techniques in the life sciences. The development of software for facile structural biology analysis by the general user community is likely to have significant impact and will aid in promoting the conduct of neutron-scattering measurements by nontraditional users in the life sciences. Two personnel are being hired, using funds provided under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5), for the further development and refinement of the SASSIE analysis software (which is used to create atomistic models of molecular systems and also to compare scattering data from these models directly to experimental data), although these hires are for only 1 and 2 years for the respective tasks. Continuing support in this area can ensure the completion of a robust analysis package for users as well as enhanced modeling and analysis capabilities in the future. For example, similar capability to include known chemistry, physics, and structural information in order to analyze scattering data from magnetic materials would be extremely helpful not only for the analysis of scattering data but also for the design of pertinent experiments. The positive impact of robust and user-friendly software to the existing potentially growing user community is substantial. FE-BASED SUPERCONDUCTORS The NCNR science in the area of Fe-based superconductors is competitive with efforts anywhere in the world, and the MACS and spin polarization are special capabilities that the NCNR offers. The NRCâs 2009 assessment report6 noted how rapidly the NCNR pursued this new family of Fe-based superconductors that had been discovered; the NCNR groupâs work had been based on samples brought to it by a group from the ORNL. Together, the NCNR and ORNL groups published the first report of antiferromagnetic order in one of the compounds and showed that it occurred close to, but not coincidentally with, a structural phase transition. Work on these materials has 6 National Research Council, An Assessment of the National Institute of Standards and Technology Center for Neutron Research: Fiscal Year 2009. Washington, D.C.: The National Academies Press, 2009. 16
continued, even though many other groups around the world, and at the SNS in particular, are also carrying out neutron-scattering experiments on these materials. Some of the NCNR researchers are also part of collaborations involving work carried out elsewhere. The focus at the SNS has been mainly on the study of the spin excitations by inelastic neutron scattering, whereas much diffraction work (both powder and single crystal) and some inelastic scattering is being carried out at the NCNR. The current efforts at the NCNR have been partly focused on the iron-tellurium compounds, doped with lighter chalcogenides, selenium, and sulfur, using both neutron diffraction and inelastic scattering. The powder diffraction work mentioned above concentrated on studying the consequences for nuclear and magnetic structure of controlled depopulation, by de- intercalation with I2 vapor, of the interstitial iron, which also controls the superconductivity. The inelastic scattering on the MACS has observed an energy resonance and a spin gap at the Fermi surface nesting vector. The other focus has been on the compound CaFe2As2 as a function of rare-earth doping, to simulate the effect of pressure. This compound is known to undergo a volume collapse under pressure, and Lynn and coworkers at the NCNR showed that this also exists as a function of temperature in rare-earth doped compounds. They have also studied spin-wave excitations in SrFe2As2. Overall, the NCNR maintains considerable momentum in this very competitive area of research. In the well-trodden field of ferroelectrics, the NCNR group has studied the ferroelectric phase transition in lead zirconate titanate (PZT) and identified the soft modes associated with the corresponding structural phase transitions, and clarified the differences in behavior with the well-studied relaxor ferroelectric PMN (Pb(Mg1/3Nb2/3)O3). This work is a continuation of a long tradition of studying soft modes in ferroelectrics (primarily at the Brookhaven National Laboratory); through the use of large, vertical focusing monochromators, the work has been successful in spite of the limited size of available single crystals. FUNDAMENTAL QUESTIONS IN NEUTRON SCIENCE The research group at the NCNR continues to develop unique capabilities for probing the properties of the neutron, for using the neutrons for imaging and interferometry applications, and for the development of 3He spin filters and imaging techniques. The group recently completed a new measurement of the parity-violating spin rotation of neutrons in 4He, and for the first time it has observed the radiative decay branch of the neutron with a precision that may allow the group to begin testing detailed models of the bremsstrahlung processes involved. The groupâs work with three-blade single-crystal silicon interferometers has long set the standard for precision measurements of neutron-scattering lengths and tests of quantum mechanics with neutrons. By adding two more blades to the standard configuration and employing a quantum error correction code, the researchers have shown that they can suppress vibration effects, potentially allowing more widespread use of the technique. The group achieved a world-record 85 percent polarization for a 3He glass cell and developed imaging capabilities for fuel cells and lithium batteries. 17
SUMMARY In summary, with the new instruments and improvements becoming available through the Expansion Project, there is an emphasis on the continued development and enhancement of the neutron-scattering techniques available at the facility. The new instruments will have an important impact on studies of hard and soft matter and will enable the NCNR and its research output to remain at the forefront of the field for many years. To further augment the scientific productivity of the NCNR, the development of facilities for the growth of large single crystals suitable for neutron-scattering experiments would remove a considerable hindrance to further advancement in many areas of condensed-matter science. The addition of this capacity would be an important service to the materials community. 18