1. Spin excitations in electron-doped iron superconductorsPengcheng Dai, University of Tennessee Knoxville, DMR 1063866 The discovery of high-transition temperature (high-Tc) superconductivity near antiferromagnetism in iron arsenides raised the possibility of an unconventional superconducting mechansim. Over the past several years, we have used neutron scattering as a probe to study the interplay between magnetism and superconductivity. By applying a magnetic field along the c-axis that partially suppress superconductivity, spin excitations such as neutron spin resonance are also affected. These results demonstrate that the energy of the resonance is intimately connected to the electron pairing energy, and thus indicate that the mode is a direct probe for measuring electron pairing and superconductivity in iron arsenides.

2. Spin excitations in electron-doped iron superconductors Pengcheng Dai, University of Tennessee Knoxville, DMR 1063866 There are a total of 4 graduate students and 1 undergraduate student supported by the present NSF, including Jun Zhao, Leland Harriger, Miaoyin Wang, Mengshu Liu, and undergraduate student Tucker Netherton. The participation of graduate/undergraduate students in our program has greatly enhanced interactions amongst students, and promoted neutron science in general. Both graduate and undergraduate students have played important role in our NSF supported neutron scattering work on understanding the role of magnetism in the high-temperature superconductivity of iron pnictides. Publications resulted from the NSF support: PRB 81, 180505(R) (2010); 81, 174524 (201); 83, 094516 (2011), and submitted for publication.