neutron scattering society of america

Archive for the ‘2014 Highlights’ Category

Neutrons Reveal Structural Tunability of Magnetic Exchange Couplings

Inelastic neutron scattering experiments reveal that the effective magnetic exchange couplings in NaFeAs are smaller and more isotropic than those in the heavily studied AFe2As2 family. These results provide evidence that the energy scale of the magnetic excitations, which are thought to influence the superconducting transition temperature, is controlled by the position of As atoms above the Fe layers.

Spin waves, measured using the ARCS chopper spectrometer at SNS, were studied in the antiferromagnetic ordered phase of NaFeAs, the parent compound of NaFe1−x CoxAs family of iron pnictide superconductors. NaFeAs was chosen because it has weak AF ordered moment, low superconducting transition temperature, and large As height, significantly different from AFe2As2.

C. Zhang, L.W. Harriger, Z. Yin, W. Lv, M. Wang, G. Tan, Y. Song, D. L. Abernathy, W. Tian, T. Egami, K. Haule, G. Kotliar, and P. Dai, “Effect of Pnictogen Height on Spin Waves in Iron Pnictides”. Phys. Rev. Lett. 112 (2014): 217202.

Spatiotemporal stress and structure evolution in dynamically sheared polymer-like micellar solutions

The complex, nonlinear flow behavior of soft materials transcends industrial applications, smart material design and non-equilibrium thermodynamics. A long-standing, fundamental challenge in soft-matter science is establishing a quantitative connection between the deformation field, local microstructure and macroscopic dynamic flow properties i.e., the rheology. Here, a new experimental method is developed using simultaneous small angle neutron scattering (SANS) and nonlinear oscillatory shear rheometry to investigate the spatiotemporal microstructure evolution of a polymer-like micellar (PLM) solution. We demonstrate the novelty of nonlinear oscillatory shear experimental methods to create and interrogate metastable material states. These include a precursory state to the shear banded condition as well as a disentangled, low viscosity state with an inhomogeneous supra-molecular microstructure flowing at high shear rates. This new experimental evidence provides insight into the complexities of the shear banding phenomenon often observed in sheared complex fluids and provides valuable data for quantitatively testing non-equilibrium theory.

A. Kate Gurnon, Carlos R. Lopez-Barron, Aaron P. R. Eberle, Lionel Porcar and Norman J. Wagner, Soft Matter, 2014,10, 2889-2898