1. Exploring the effect of uniaxial strain in Ba(Fe 1-x Co x ) 2 As 2 Stephen D. Wilson, Boston College, DMR 1056625 Physical Review Letters 108, 087001 (2012). Within the newly discovered classes of iron-based high temperature superconductors, one of the ongoing mysteries is the origin of an electronic anisotropy thought to arise from an electronic nematic phase—a potential key signature of the underlying superconducting mechanism. Ba(Fe 1-x Co x ) 2 As 2 is an ideal system for exploring the microscopic origin of this anisotropy, where the first transport measurements suggesting a nematic anisotropy in the iron superconductors were reported. Strain must however first be applied to crystals in order to resolve this nematic phase due to structural twinning effects The direct effect of this strain and its influence on the conventional electronic and structural phases of this system was unexplored Our neutron scattering measurements (shown in the right panel) resolve the microscopic origin of the transport signature of the previously reported nematic phase in Ba(Fe 1-x Co x ) 2 As 2 Strain couples to both magnetic order and the lattice distortion, driving both transitions upward in temperature. Strain breaks both time reversal and rotational symmetry through coupling to the magnetic phase and nucleating magnetic domains at high temperature This study points toward strong spin-lattice coupling or the presence of high temperature fluctuating magnetic domains as the source of the transport “nematic” anisotropy in BaFe 2 As 2 Our most recent measurements on Co-doped samples show that this simple strain-induced effect is present across the entirety of the previously reported Ba(Fe 1-x Co x ) 2 As 2 nematic phase diagram. Neutron scattering data showing the nucleation of magnetic domains and increasing higher temperatures as increased pressure/strain is applied Pressure clamp for applying strain to BaFe 2 As 2 crystals for neutron scattering experiments

2. Inspiring and Training Future Scientists Stephen D. Wilson, Boston College, DMR 1056625 Undergraduate Research Participation Two undergraduate students at Boston College (D. Gilligan and K. Vien) were hosted in the PI’s lab and performed research in support of the project. Both students learned leading edge materials synthesis/crystal growth techniques as well as how to conduct transport characterization of new materials. Two local area high school students (A. Soloviev and M. Gould) performed summer internships in the PI’s lab through our collaboration with the nonprofit organization The Educational Cooperative (TEC). Both students learned mechanical design, basic CAD, and data analysis techniques, and both plan to pursue science-based undergraduate majors. Considerable recent investments into the nation’s neutron scattering facility infrastructure require a parallel effort to expand the U.S. user community in order to maximize their impact. Two graduate students supported by the grant (C. Dhital and Z. Ren) continued their integration into the U.S. neutron user community through participation in over 10 experiments at national and international facilities. Both students also continue to learn the state-of-the- art materials synthesis and crystal growth techniques necessary to fuel new science at these user facilities and to address the nation’s materials synthesis deficit.