CONDENSED MATTER PHYSICS: SOFT MATTER
The comprehensive suite of scattering instruments at the NCNR, including uSANS, vSANS, two 30-meter SANS, MAGIK,1 Polarized Beam Reflectometer (PBR), and horizontal reflectometer (soon to be sunset), provide soft matter researchers in condensed matter physics (CMP) with a comprehensive world-class set of instruments to address both fundamental and applied problems for which the use of neutrons is necessary and critical. The novel Chromatic Analysis Neutron Diffractometer Or Reflectometer (CANDOR), when fully realized in 2019, will be a game changer for diffraction and reflectivity measurements at a reactor source. Through the simultaneous use of multiple wavelengths, large effective enhancements of the neutron flux on the sample will be achieved. The High Flux Backscattering Spectrometer (HFBS) has been enhanced by near doubling of its flux by the improvement in focusing optics. Together, HFBS and the Neutron Spin-Echo Spectrometer (NSE) enable molecular-level motion to be correlated with soft-matter structure. The partnership with the National Science Foundation to operate the Center for High Resolution Neutron Scattering (CHRNS), which supports the operation of these spectrometers, is a crucial component for the NCNR’s continued leadership in soft matter research.
The NCNR continues to carryout leading-edge research in the area of complex fluids and flow, covering diverse areas of structure, dynamics, and functionality. Scanning Narrow Aperture Flow uSANS (SNAFUSANS) is a robust sample environment available for users to measure structure along the flow gradient. Such measurements, for example, enabled concentration gradients induced by shear stress to be measured in shear-banding fluids. These flow-induced concentration gradients govern and provide desired flow properties in commercial products ranging from shampoos to fluid additives for drag reduction and enhanced oil recovery.
Neutron-scattering measurements continue to provide important new insight for controlling complex fluid systems. The SANS rheology sample environments were developed at the NCNR with the
1 Further information available at https://www.nist.gov/ncnr/spin-filters/spin-filter-instruments/magik.
University of Delaware’s Center for Neutron Science. The team has also provided assistance for SANFUSANS to be implemented on the D22 beamline at the Institut Laue-Langevin (ILL).
Research on the structure of nanoparticle grafted polymer chains2 has clearly demonstrated the power of neutron scattering to provide information that is otherwise difficult, or impossible, to obtain. The synthesis of nanoparticles may provide new ways of catalyzing a wide variety of chemical reactions in both laboratory and industrial contexts. By selectively deuterating the inner-only versus outer-only portion of the grafted polymer chains (corona), the variation in polymer chain stretching away from the nanoparticle surface was directly measured by SANS. The structure, thus determined, matched well with theoretical predictions, and these structure models are being incorporated into the SASSIE analysis software. Importantly, the integrated and complementary instruments of the NCNR CHRNS facility enabled researchers to go beyond static structure and to measure chain dynamics in two polymer-corona regions, which differed significantly, on the same samples using the NSE spectrometer. These are the first measurements of their kind.
A second research presentation on lipid membranes3—barriers that separate cells from the surrounding environment and that delimit their internal compartments—demonstrated additional efforts to couple neutron scattering measurements of structure and dynamics. It is believed the dynamical properties of these membranes play a critical role in determining their functional properties. In this case, the change in dynamic fluctuations of two-component lipid membranes as a function of composition and temperature were determined. The decrease in fluctuations could be directly mapped onto changes in the bending modulus of the membrane. Although the membrane composition was relatively simple, these measurements are compelling for continuation work to measure the impact of composition and inclusions such as raft domains, proteins, and so forth on membrane dynamics.
Opportunities and Challenges
With the recent demonstration of Dielectric RheoSANS, SANS rheological measurements can now be carried out simultaneously with dielectric spectroscopy. This novel sample environment enables (di)electric properties to be directly coupled to changes in structure/morphology and flow/stress. Syncing the three simultaneous data sets—structure/morphology, flow/stress, and dielectric properties—is well under way and will lead to fundamental insights into structure-property relationships in conducting systems under steady and dynamic flow conditions. Plans are in place to make this sample environment available to users.
CONDENSED MATTER PHYSICS: HARD MATTER
The scientific impact of the NCNR in the area of CMP is very high. Measurements being done at the NCNR on magnetism, superconductivity, and multiferroics continue to produce new insights that advance our understanding of these materials. Strong electron correlations in materials lead to emergent properties that result in unusually large material responses to external stimuli. This makes these materials of interest for the construction of transducers and non-volatile memories. A particularly exciting recent development is the verification of topological effects in materials. Neutrons have had a large impact in the study of skyrmions— topological spin-texture objects that are studied using SANS—and the NCNR has
2 M. Hore, Case Western Reserve University, “Structure and Dynamics in Polymer-grafted Nanoparticle Systems,” presentation to the panel on July 11, 2018.
3 E. Kelley, NIST, “Insights into the Dynamics of Heterogeneous Lipid Membranes from Neutron Scattering,” presentation to the panel on July 11, 2018.
been in the forefront of these developments. An exciting study reports the generation of ground-state skyrmions created by patterning a planar vortex configuration within cobalt nanodots of diameter 560 nm and height of 30 nm that are placed on top of a magnetic thin film.4 The nanodots produce boundaries that favor a planar vortex configuration of spins in the film, but the spins lower their energy by “escaping to the third dimension” toward the center of the dot to yield a skyrmion.
The NCNR has excellent capabilities on a number of beamlines for producing polarized neutron beams that are ideally suited for measurements of spin and spin fluctuations in traditional magnets and in exotic quantum magnetic systems. The Multi-Axis Crystal Spectrometer (MACS, as noted in Chapter 0), has been operating for 8 years and is currently a world-class instrument for studying magnetic dynamics with high flux and low backgrounds, allowing a sensitivity to detecting small signals that is unmatched anywhere in the world. This instrument is critical in the search for quantum spin liquids, a phase of quantum matter that has long been predicted, but whose existence has never fully been established. Recent experiments on MACS are bringing us closer than ever before to establishing its existence.
Experiments on unconventional superconductivity have utilized the MACS to show strong interactions between itinerant electrons and magnetic spins in CeCoIn5 cause an unusually high critical temperature (Tc).5 These imply a direct role of spins in the superconductivity and a change in the magnetic exchange energy on entering the superconducting state that leads to a novel incommensurate entwined magnetic/superconducting state called Q-phase. Planned developments of MACS include lowering the background for better signal-to-noise (critical for weak magnetic signals in the quantum spin liquid for example) and the development of event-mode neutron detection, which will greatly increase experimental throughput.
Neutron scattering is used by biological scientists to obtain low-resolution information about the structural organization of biological materials that would be challenging to secure by other means. It can also be used to obtain insights into the dynamic properties of biological systems of all kinds. Many of these experiments exploit the difference in scattering lengths between hydrogen (b = −0.374 × 10−12 cm) and deuterium (b = +0.667 × 10−12 cm) either to produce differences in contrast between components of large biological structures that otherwise would not exist, or to alter those that already do. It is also used to control the contrast between these structures and the solvents in which they are dissolved. The NCNR facility, as noted, includes several kinds of instruments useful for doing experiments of this sort: SANS diffractometers, neutron reflectometers, and an NSE spectrometer (i.e., the NG-A). For example, the SANS spectrometers have been used recently to characterize the structure of nucleosomes and to investigate the time-averaged structure of immunoglobulins. Neutron reflectometers are used to determine the transverse distribution of scattering-length density in lipid membranes. This allows for better understanding of the three-dimensional structure and dynamics of macromolecules with health care and other applications.
The reflectometers at the NCNR have been used recently to investigate the organization of the complexes that lipid bilayers form with specific proteins and polypeptides. Inelastic neutron scattering is
4 D.A. Gilbert, B. Maranville, A.L. Balk, et al., 2015, Realization of ground-state artificial skyrmion lattices at room temperature, Nature Communications 6:8462.
5 C. Stock, J.A. Rodriguez-Rivera, K. Schmalzl, F. Demmel, D.K. Singh, F. Ronning, J.D. Thompson, and E.D. Bauer, 2018, From Ising resonant fluctuations to static uniaxial order in antiferromagnetic and weakly superconducting CeCo(In1 -xHgx)5 (x = 0.01 ), Physical Review Letters 121:037003.
the only experimental technique available for characterizing the large-scale conformational changes associated with the activities of biological structures of all kinds. The NSE spectrometer has been used to good effect recently to study the dynamics of both cytochrome P450, and a series model phospholipid bilayers.
Opportunities and Challenges
Biological scientists are regular consumers of beam time at neutron scattering facilities all over the world, and they have long been members of the user community at the NCNR, which has several first-class instruments suitable for biological experiments. Nevertheless, although biological scientists consume approximately 15 percent of the beam time at the NCNR, only approximately 6 percent of the most highly cited papers that have emerged from the NCNR over the past decade describe work done on biological samples, as determined by the web of Science.6 Consistent with these statistics, the leadership of the NCNR did not indicate that it believed biological science is one of the facility’s strong suits. In fact, most of the biology-related posters presented to the panel described studies directed at bioengineering issues, rather than biology per se. This work may well be useful to those concerned about the specific applications to which they pertain, but it is unlikely to attract much attention from the wider scientific community. All of the above notwithstanding, the measurements of lipid bilayer dynamics that have been done recently at the NCNR did much to reassure the panel that neutron-scattering experiments can produce important information about biological systems that cannot be obtained in any other way.
Three conditions must be met if a biological neutron scattering experiment is to have a large impact: (1) the system examined must be scientifically significant; (2) the experiments must yield information that could not easily be obtained by any other means, if at all; and (3) the information must materially alter the way biological scientists think about the system. Many of the opportunities for doing high-impact neutron experiments on biological systems have been, and continue to be, missed. The problem is that the vast majority of the biological macromolecular systems under investigation today are studied by biological scientists well trained in X-ray techniques but not in those using neutrons and, hence, are oblivious to the opportunities it offers. Moreover, until one of the groups working on such a system is persuaded of the value of the information neutron-scattering experiments might provide, no one is going to make the investment required to do those experiments. Furthermore, it is unrealistic to expect that groups already working at the NCNR will start doing experiments on a system unrelated to the one they are already studying, just because it looks promising. The grant support system discourages such changes in direction. It would be worthwhile, therefore, for the NCNR to make a special effort to educate the larger biochemical and biophysical communities about neutrons. If just a few of the groups that are studying systems that might yield high-impact neutron data could be persuaded to do work at the NCNR, both the quantity and the quality of the biological papers produced at the NCNR would improve considerably.
Many of the systems that chemical physicists study at the NCNR are relevant to today’s economy and to technical innovation. Importantly, the pursuit of basic scientific challenges in the NCNR program has also been thriving, so it is impossible to capture the full range of the chemical physics problems addressed at the NCNR in just a few sentences. The systems investigated include materials for capturing and storing Cl2, Br2, H2, and methane. Substances that are superconducting or that have interesting magnetic properties have also been receiving considerable attention lately, as have some novel polymer physical chemistry challenges.
6 NIST, “Highly Cited Publications: 2008-2017,” background paper transmitted to the panel, June 27, 2018.
Opportunities and Challenges
It is important to note that the most important instrument at the NCNR for these investigators is BT-1, which is a powder diffractometer. This is the oldest instrument now operating at the NCNR, and while it is unquestionably true that a better powder diffractometer could be built today, there are no plans to replace it. Lack of resources, rather than lack of need, appears to be the reason.
The NCNR has an engineering physics effort focused primarily on stress analysis of industrially important materials using the RT8 Residual Stress Diffractometer. In the past few years, this effort has responded to two important new manufacturing developments: (1) the ongoing transition in passenger vehicles from steel-based structures to bodies composed partially of aluminum, magnesium, and advanced high-strength steel brought about by commitments to increase fuel efficiency and (2) the introduction of additive manufacturing (AM) (often referred to as 3D printing) into a variety of industries, most notably the aircraft industry. Both developments create internal stresses that need to be characterized and understood, because they can lead to long-term degradation. The transition to lighter metals requires methods for joining different metals beyond spot welding.
Experiments at RT8 measured stresses in joined sheets of different materials produced by three alternatives fasteners: self-piercing, flow-drill screw driving, and composite friction rivets. Another series of experiments measured internal stresses in additively manufactured steel cylinders of different sizes.
The Neutron Physics Group of the NIST Physical Measurement Laboratory performs experiments at the CNR, but being as they are from a different NIST laboratory, the group was not under review. The group leader nonetheless provided an informative presentation on the group’s work on neutron science and metrology.
The basic science could be summarized as focusing on fundamental neutron physics, neutron interferometry, and reactor and solar neutrinos.7 The neutron undergoes beta decay—into a proton, beta-minus particle (i.e., an electron), and an anti-neutrino—in accord with the Standard Model. The lifetime of the neutron is not however known to good precision; there remain discrepancies between beam- and bottle-type experiments. The physics of this electro-weak interaction is being investigated using cold beam neutrons. The aCorn experiment measures electron and proton momenta to permit inferences of the angular correlation of these two subatomic particles. The group reports the most precise measurement of the angle—the dimensionless parameter “a”—to date following a 15-month data run at the CNR.8 In its experiments using neutron interferometry, the group conducts experiments to observe phase-coherent neutron waves. The group’s work on neutrinos is searching for what are dubbed sterile neutrinos, a fourth flavor of this particle such as might explain anomalies such as the reactor neutrino deficit. The Li6-doped liquid scintillator they are developing is operational at HFIR at the Oak Ridge National Laboratory.9
7 J.S. Nico, NIST, “Recent Results in Neutron Physics,” presentation to the panel on July 10, 2018.
8 Darius, G., W.A. Byron, C.R. DeAngelis, M.T. Hassan, F.E. Wietfeldt, B. Collett, G.L. Jones, et al. 2017, Measurement of the electron-antineutrino angular correlation in neutron β decay, Physical Review Letters 119:042502.
9 J.S. Nico, NIST, “Recent Results in Neutron Physics,” presentation to the panel on July 10, 2018.
The group also undertakes investigations on the neutron magnetic dipole moment as a proof-of-concept for work on setting an upper limit on the neutron’s electric dipole moment, the ultimate value of which cannot be non-zero without violating time reversal symmetry in the Standard Model. The group also provides metrology services—for example, the recalibration of the NBS-1 national neutron standard.10 (Discussion of neutron research services at CNR may be found in Chapter 3, in the section “Neutron Imaging.”)
10 J.S. Nico, NIST, “Recent Results in Neutron Physics,” presentation to the panel on July 10, 2018.