DAYS 3, 4: Tuesday, Wednesday June 29-30
The 2010 American Conference on Neutron Scattering (ACNS 2010) concluded on Wednesday, June 30, in Ottawa, Canada. The conference banquet was held Tuesday evening. The major event on Wednesday morning was a plenary session including four talks. After the conclusion of the conference, a tour of Chalk River Laboratories was organized for attendees.
Ottawa's majestic Fairmont Château Laurier hotel, venue of ACNS 2010. Born in the age of grandeur in 1912, it has hosted a number of well-known people over the years. With its commanding presence in downtown Ottawa next to the parliament building and displaying a green copper roof line, it has been dubbed the "Third Chamber of Parliament" of Canada.
45 Years of Neutron Scattering at ORNL
Herbert A. Mook, 2010 Clifford G.
Shull Prize Lecture
The first plenary lecture on Wednesday was presented by Herbert A. Mook Jr. of Oak Ridge National Laboratory (ORNL), one of the pioneers in the field of neutron scattering. The talk was in recognition of his 2010 Clifford E. Shull Prize awarded by the Neutron Scattering Society of America (NSSA) in June 2010. The prize is named in honor of Clifford E. Shull, the 1994 co-winner of the Nobel Prize in Physics, and to recognize high-impact neutron science research, as well as leadership in promoting the North America neutron scattering community. Mook completed his doctorate at Harvard University under the supervision of Clifford Shull (1994 Nobel laureate in physics) . In his talk, he emphasized that Shull was wonderful to work for, had great insights and could get more physics out of a piece of data than anyone else.
His talk illustrated his outstanding contributions to the study of magnetism, superconductivity, and quantum phenomena in matter using neutrons. Mook started his career at ORNL in 1965, where he applied neutron scattering to investigate the interaction of magnetism and superconductivity. Over the years, he worked at the forefront of neutron scattering utilizing neutrons to study the nature of the magnetic structure and fluctuations in high-temperature superconductors using ORNL's High Flux Isotope Reactor. His research covers a variety of topics. In his talk overviewing 45 years of neutron scattering at ORNL, he discussed his results on magnetic excitations in transition metals and transition from magnetic to superconducting properties in rare-earth rhodium borides. He also highlighted his work on the nature of magnetic structure and fluctuations in ‘214’ and ‘Y123’ high-temperature superconducting materials.
Mook served as the first scientific director of the Spallation Neutron Source from 1995-2000 and was the director of the Center for Neutron Science from 2000-2004. He was a recipient of the DOE Award for Outstanding Scientific Accomplishments in Solid State Physics, in 1982 and 1998. He has authored over 225 papers with a total of 9000 citations, holds several patents for neutron instrumentation, two of which have received R&D 100 Awards.
Broken Symmetries of the Cuprate Pseudogap Phase Visualized by Spectroscopic Imaging STM and Detected by Inelastic Neutron Scattering
J. C. Seamus Davis
Cuprate high temperature superconductors were discovered in the 1980s and over the past nearly 25 years have seen significant research and development. A detailed understanding of their electronic structures remains incomplete. In particular, a pseudogap phase is observed in the electronic phase diagram of the cuprates close to the superconducting regime, and it is essential to understand this pseudogap phase to get a clear understanding of this class of materials. In the second plenary talk on Wednesday morning, J.C. Seamus Davis of Cornell University described the use of scanning tunneling microscopy (STM) to visualize the pseudogap phase as well as the use of inelastic neutron scattering to investigate the pseudogap phase in cuprate superconductors. In particular, it is important to understand which electronic symmetries are broken (nematic phase: rotational symmetries are broken; smectic phase: rotational and translational symmetries are broken) and what the associated order parameter might be.
Typically, STM cannot see the wave function in a material. Davis described a method that can be used to see the wave function using STM (spectroscopic imaging STM or SI-STM). He described the determination of an order parameter representing intra unit cell nematicity within each CuO2 unit cell. Focusing on the BiSrCaCu 2212 system, in the underdoped material, he demonstrated evidence for electronic nematicity of the states close to the pseudogap energy. He show how the methodology could be used to show that these phenomena arise from electronic differences between the two oxygen sites within each unit cell. The excitations seen by inelastic neutron scattering and SI-STM in the pseudogap phase have the same origin, and they represent weakly magnetic states at the oxygen sites whose electronic structure breaks the 90° rotational symmetry. Thus, these investigations by Davis demonstrate that the pseudogap energies (states) can indeed be visualized and quantified yielding a much clearer understanding of the electronic structures of cuprate superconductors. Also, the study represents the first time topological defects, essentially dislocations in the electronic structure, have been viewed.
Beauty Is Only Skin-Deep: Probing The Surface and Beneath by Neutron Reflection
The third plenary talk of the morning was presented by Charles Majkrzak (NIST). His talk was focused on phase-sensitive reflectivity to resolve the structures of nanometer scaled layered and patterned films. In contrast to conventional specular reflectivity which limits accuracy and special resolution, the phase-sensitive method yields a real-space picture without fitting or any parameters. In his talk, he discussed advanced neutron reflectometry and the theory for polarized neutron reflectometry which has led to the development of important new methods of data analysis. He illustrated how he designed, optimized, and made use of supermirror polarizers, integrating them into neutron instruments that attain very low backgrounds and consequently the highest signal-to-noise ratios. This point is crucial in achieving the widest possible wave vector range of data, thereby providing the highest spatial resolution and thus the most detailed and reliable structural information available.
He also highlighted his work on Gd/Y rare-earth multilayers that revealed an oscillatory exchange coupling spanning non-magnetic layers. This research provided the basis for interpreting similar effects in transition metal multilayers that exhibit giant magnetoresistance (GMR), which is now at the heart of present-day magnetic read heads in hard disk drives. His research on neutron reflectometry was also extended to surface-induced ordering of block copolymers and to the structures of biological and biomimetic membranes. In recognition of his extraordinary contributions to the field of neutron reflectometry and diffraction physics and for his pioneering work in the exploration of many issues in interface science, Majkrzak was awarded the Warren Award at the American Crystallographic Association Annual Meeting in Honolulu in 2006.
Brighter and better... The Future of Neutron Sources and Instrumentation
For all the wonderful experiments carried out by neutron scientists, a critical aspect is the neutron source itself and the associated instrumentation. Ferenc Mezei, in the final plenary talk of the conference, described the development of neutron sources from a historical perspective and more importantly, current and future directions. Very recently, Mezei joined the European Spallation Source (ESS) secretariat, which will be constructing the next generation source in Lund, Sweden. Mezei is the originator of the ESS design concept, the long pulse spallation source. Starting with the neutron research universal (NRU) reactor in Chalk River, Canada, which was the first dedicated neutron source, there has been tremendous progress, culminating in the recent achievement of 1 MW power by the spallation neutron source at Oak Ridge. However, this represents an instantaneous peak flux, and the time-averaged brightness remains lower than those of other sources.
Mezei described recent efforts in refining the source technology mainly be exploiting opportunities of complementarities and interplay between accelerator design and instrumentation approaches. He focused on the use of advanced mechanical neutron beam chopper systems to shape pulses, rather than through the use of accumulator accelerator rings and extensive, delicate neutron absorber structures in the target-moderator assembly. The advanced long pulse source ESS will be the first example of this next generation breakthrough in neutron source power, according to Mezei. The 5 MW ESS will have a linac+target station+100 neutron choppers. Also, a 15 MW long pulse source at the ESS is feasible if crucial issues such as cooling of the target and heat recovery can be solved. If realized, such a source will deliver 2 orders of magnitude greater flux compared to the highest flux reactors available currently.
Chalk River Laboratories Tour
On the last day of the conference, Wednesday afternoon, attendees were invited to visit Chalk River Laboratories. The Laboratories accommodate major Canadian neutron facilities used for neutron research performed at the neutron research universal (NRU) reactor located there. The Laboratories are situated on the banks of the Ottawa River, in the upper Ottawa valley, Ontario. Approximately a 2 hour drive up river from Ottawa, the tour started at noon and took about 8 hours in total. The facilities consist of 100 buildings covering a square kilometer and employ 2,000 people.
Originally built after World War II for continued allied atomic research, the labs were a part of the National Research Council. In 1952 the laboratories' organization became a crown corporation "Atomic Energy of Canada." Today, the National Research Council - Canadian Neutron Beam Centre (NRC-CNBC) is a unique national science facility, and is a major resource for scientists. It has a strong international reputation and connects Canadian scientists to international collaborations with over 100 institutions in more than 20 countries during a typical five-year period. Since the 1950s, many hundreds of neutron scattering experiments have been carried out to solve practical problems for industry sometimes with multi-million dollar impacts, and pushing back the frontiers of human understanding of the world around us. Those experiments have contributed to the education of many hundreds of highly qualified people across the spectrum of science.
The major attraction of the tour was the NRU Reactor, Canada's largest and most productive science facility. As well as the neutron scattering activities that CINS represents (pioneered by Canadian Nobel Laureate Bertram Brockhouse), the NRU reactor is the world's largest producer of medical isotopes, used to treat more than 21 million people in 60 countries each year. It is also the test-bed for Canada's nuclear electricity industry. Overall, the NRU reactor has made substantial contributions to the science, technology, energy, health and economy of Canada. However, built in 1952, the reactor is showing its age and has been shut down for a year for repair. Tremendous efforts have been carried out for the reactor to be up and running, and the reactor is expected to start an operational cycle at the end of July.
Visitors had also a chance to see the four neutron scattering spectrometers attached to beam lines at the NRU for experiments. NRU enables hundreds of experiments to be conducted every year on materials from ceramics to biological tissue, from steel to superconductors. At the end of the tour, NRU testing facilities were demonstrated to the visitors. The facilities are used in nuclear research and development, helping to build better electricity-producing reactors. Knowledge gained from the test facilities at NRU has been an essential foundation for developing the current fleet of CANDU power stations in Canada and abroad. These power stations are an important source of electricity for Canada.
Symposium C: Soft Condensed Matter
Neutron Reflectometry: From Model Biomembranes To Living Cells
Understanding the nature of living cell membranes gives important information on cell interaction with the environment. Hillary Smith, who worked under supervision of Yaroslaw Majewski (Las Alamos National lab), focused her talk on adhesion of living cells on surfaces followed by probing their structure with neutron reflectivity. The main questions to ask are if it is possible to measure a highly complex living biological systems with neutrons and if it is achievable to visualize the interface between the living cell and solid templates with a sub-nanometer resolution. Measuring living cells is challenging because of the complexity, disordered and inhomogeneous nature of the system, and difficulty to control and obtain the consistent surface coverage. Despite those challenges she demonstrated for the first time the feasibility to perform such experiment which thus providing a unique information about organization of protein in cell membrane which is not affordable with other techniques.
Symposium G: Engineering Applications
In Situ Deformation Studies of Nanocrystalline Materials
It is now understood that nanocrystalline materials deform very differently from conventional materials that contain larger sized crystalline grains. In conventional crystalline materials, the Hall-Petch relationship is valid, wherein the yield stress increases with decreasing grain size due to reduced dislocation movement. Beyond a critical grain size in the nanoscale regime, however, an inverse Hall-Petch relationship is observed. Sheng Cheng of Oak Ridge National Lab and the University of Tennessee described in situ X-ray and neutron investigations to examine this deformation cross-over in Ni. Results showed little intergranular strains in nano-Ni under tensile deformation due to the lack of dislocation slip. However, a significant build-up of intergranular strains was observed in the nano-Ni influnced significantly by twinning. The results clearly confirm the cross-ver behavior. In a separate investigation on fatigue deformation, Cheng described how fatigue deformation of nanocrystalliine materials is different from that of conventional materials as well as computer simulation predictions. Significant grain growth was seen associated with fatigue cracks. This was speculated to be due to nano-grain rotation and consolidation.
Effects of Overload, Underload During Fatigue-Crack Propagation
An overload or underload introduced during cyclic loading can significantly retard or accelerate crack growth rate. Peter K. Liaw of the University of Tennessee described the use of neutron and electric potential measurements to investigate the effects of overloading and underloading (5 different loading conditions) of a Hastelloy (56Ni-23Cr-16Mo) alloy during fatigue loading. Neturon measurements were carried out with in situ loading. After a single overload or underload-overload, compressive stresses around the crack tip yielded large crack-growth retardation. After a single compressive underload, instantaneous acceleration of crack-growth rates occurred. Thus, distinct residual stress-strain profiles around the crack tip are closely related to different crack-growth behaviors under varying loading conditions. Neutron strain measurements and simulated lattice strain evolutions near the fatigue crack tips are in qualitative agreement.
Scanning the Conference
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