HARD CONDENSED MATTER
The NCNR hosts an excellent mix of outstanding senior investigators and early-career, energetic, talented researchers. The early-career researchers who presented their research to the panel were very knowledgeable, enthusiastic, and articulate and gave clear descriptions of their research projects.
Magnetism, Superconductivity, and Correlated Electron Phenomena
The general direction of research in magnetism, superconductivity, and correlated electron physics is toward timely and important research problems at the forefront of condensed matter/materials physics. Research in this area is driven by both senior scientists and users of the NCNR facility.
The NCNR is supporting a powerful program on quantum magnetism that is based on the unique MACS II spectrometer. The focus of this research program is on frustrated two-dimensional quantum magnets, and the primary interest is on exotic states of matter, such as spinons in Kagome antiferromagnets and resonating valence bonds in a triangular lattice. In this project, frustration is used as a tuning parameter to drive a quantum phase transition from a magnetically ordered state to an exotic nonmagnetic state. Eight papers based on this program have been published in high-impact journals, such as Physical Review Letters, Nature , and Science, during the past 3 years. Highlights include the first observation of spinons in two-dimensions and the observation of a collective continuum in a molecular magnet.
The NCNR has also supported recent progress in neutron scattering studies of magnetism, superconductivity, and correlated electron phenomena in novel materials. This excellent and broad-based program has been very successful over the years and has had a significant impact in condensed matter/materials physics. One of the recent efforts focused on various phenomena in Mn-based compounds, for example (Ba-Sr)MnO3— multiferroic, (Ba-K)Mn2As2—itinerant half-filled ferromagnet, and LaMnPO—antiferromagnetic insulator. The compound LaMnPO, which has the same structure as the LaFePnO (Pn = P, As) superconductors, was studied under applied pressure to determine whether it would be possible to suppress the magnetic order and induce a correlated electron metallic state that would exhibit superconductivity. It was found that only modest pressures are required to transform LaMnPO from an insulating tetragonal structure with a large moment to a gapless orthorhombic structure with a small moment and no long-range order. However, no superconductivity was observed to emerge upon suppression of the magnetic order.
Superconductivity in Fe-pnictides and Chalcogenides
NIST was an early world leader in the area of Fe-based superconductors. They had access to the first high-quality crystals outside of Japan and China and were the first to determine the magnetic structure of one of the “122” compounds, BaFe2As2 (parent compound), revealing the novel antiferromagnetic structure of that compound. That was only a few months after the discovery of the Fe-based superconductors in Japan. The paper that reports these results, as obtained by neutron scattering, remains highly cited. The NIST Superconductivity Group continues its world leadership and has done fundamental work on over a dozen families of Fe-pnictides and Fe-chalcogenides. They continue their leadership in the growth of large single crystals of all three of the 122 families of materials, (Ba,Ca,Sr)Fe2As2, through their own work and external collaborations. In the Sr122 system, they identified early the low-energy spin waves and magnetic interactions. In the Fe-chalcogenide system (FeTeSe), they detected a spin resonance that bore a surprisingly close parallel to the magnetic resonance studied in the high-temperature superconducting cuprate Bi2212 more than 10 years earlier. This solid work over time is helping the field to compare and contrast the role of spin fluctuations in the mechanism of these two distinct families of high-temperature superconductors (cuprates and Fe-based). NIST included one of the early laboratories to start to test the S± theory for the order parameter symmetry in the Fe-based superconductors.
Related to the work on the superconductivity is the outstanding research on quantum criticality in other materials that exhibit phase diagrams (carrier concentration versus temperature) similar to the high-temperature superconductors. They all exhibit a dome under which there is strong evidence for a quantum critical point (QCP). Superconductivity may be arising from this proximity to the magnetic state at the QCP. Most of the work presented was accomplished by the MACS II spectrometer—a noteworthy success in neutron scattering design. One of the many impressive results is the detection of collective molecular magnetism in LiZn2Mo3O8 at 1.5 K. The resonating valence bond detection on a triangular lattice is equally impressive. It is not clear how far MACS will take the field of strong electron correlations in condensed matter systems, but the future is quite promising.
The entire neutron group and the condensed matter community benefit from a symbiotic relationship that substantially strengthens both. The experiments done on MACS II and SANS and neutron reflectivity experiments are impressive and deserve continued strong support.
Fundamental Neutron Science: aCORN
A very interesting poster titled “aCORN [‘a’ correlation in neutron decay]: Measurement of the Electron-Antineutrino Correlation (Little “α”)” innovative fundamental physics experiment incorporating a new approach to measure a dimensionless parameter α within uncertainties of about 0.5 percent, which represents the angular correlation between the beta electron and antineutrino in neutron beta decay 
. When combined with other neutron decay parameters, the value of α for free neutron decay can be used to determine the weak vector and axial vector coupling constants gv and gA and to test the validity and self-consistency of the
Electroweak Standard Model. Previous experiments measuring α relied on precise proton spectroscopy and were limited by systematic effects at about the 5 percent level.
Crystallography
Although the NCNR suite of instruments for crystallography is not extensive (comprising the powder diffractometer BT-1 and the residual stress diffractometer BT-8) and has no dedicated instrument for single-crystal diffraction, in the hands of a team of crystallographers with a broad profile in experience and a strong and varied user community, the scientific output is nevertheless very strong. Highlights include research featured in recent highly cited papers in the fields of porous systems and functional materials such as superconductors and multiferroics. Particularly striking examples include the location of different molecules in microporous materials used to process or separate different molecular species. These provide unique insights into the absorption mechanisms to aid their exploitation in chemical processes. This research also impacts industrially relevant problems, with close links to a number of companies both in chemical crystallography and engineering.
SOFT CONDENSED MATTER
Neutron scattering is a very powerful technique for studying soft matter systems. Because scattering contrast arises from interactions between the scattering neutron and the nucleus, useful scattering intensity can be obtained from common soft materials or biomaterials. By contrast, X-ray scattering can often be too weak, and light scattering (multiple scattering) can be too strong for useful experiments. Furthermore, neutron scattering has the unique advantage that contrast can be adjusted through deuteration to limit the contrast to a specific portion of the structure, further enhancing the capabilities of the scattering. Several examples that exploited these features were presented to the panel.
By far the most widely used scattering technique of those presented was small-angle scattering, and there are three SANS instruments at NCNR. In addition, neutron reflectivity was used for interfacial studies, and spin-echo scattering provided information about dynamics.
One of the most impressive efforts in soft matter science at NCNR is the work on rheo-scattering, in which neutron scattering is combined with rheological measurements. There are several tools that enable the neutron beam to probe all three independent directions required for Couette flow in a rheometer. These are tools that have been constructed at NCNR in response to user needs, and NCNR leads the world in this field. The work on shear-thickening colloidal suspensions, which have potential as body armor, particularly benefits from neutron scattering. The NCNR staff developed a creative way to utilize all the neutrons in a SANS experiment done on an oscillatory measurement to isolate the scattering at specific times during the oscillation. This enabled the team to confirm the underlying mechanism of the shear thickening—the formation of transient hydro-clusters for which lubrication forces, which lead to the large increase in stress observed at high shear rates, become dominant. These experiments are an excellent
example of a case in which neutron scattering data provides essential insight into the underlying physical processes.
Another good example of collaboration between NCNR and users is the study of monoclonal antibodies, which is supported in part by Genentech and is an attempt to help solve an important problem that could limit the use of some new biologic, antibody-based drugs, which are becoming increasingly important and common. The NCNR studies attempted to determine the origin of a large increase in viscosity, which makes injection difficult, with increased monoclonal antibody concentration. There were studies using both small angle-scattering and spin-echo scattering to help resolve the origin of this increase. In both cases, the results support the hypothesis that at high concentrations the antibodies interact to form trimeric clusters that can further interact to form larger structures that lead to the increased viscosity. The results seem solid, but the data depend quite strongly on modeling the behavior, particularly in the case of the SANS data.
Polymer glasses and composites of polymers and small particles are important industrial materials. The transition temperature of the materials from the liquid phase to the glass phase is often smaller in thin films in the bulk. This may be a result of interactions between the polymer and the interface, making it important to investigate interfacial properties of polymer glasses. Neutron reflectivity measurements by the NCNR group from thin films with one or more deuterated layers provided a direct measure of the roughness of the layer interfaces and of their self-diffusion coefficient and, thus, provide valuable insight into the glassy properties of these films and the effects of the interface.
Membranes constitute a form of soft matter of importance both to biology and to complex fluids. Using the unique capabilities of the spin-echo technique, an NCNR-university collaboration produced high-quality measurements of dynamical correlations in lipid membranes arising from both their shape and their thickness fluctuations.
Systems of relevance to biology, including membranes and proteins, tend to be quite complex, and interpreting neutron data about them is often a challenging undertaking. NCNR staff realize this and have begun to develop in-house modeling expertise. As the soft matter effort grows, however, it would benefit from more theoretical and modeling input.
