This book studies the structural, magnetic and electronic properties of, as well as magnetic excitations in, high-temperature BaFe2-xNixAs2 superconductors using neutron diffraction and neutron spectroscopic methods. It describes the precise determination of the phase diagram of BaFe2-xNixAs2, which demonstrates strong magnetoelastic coupling and avoided quantum criticality driven by short-range incommensurate antiferromagnetic order, showing cluster spin glass behavior. It also identifies strong nematic spin correlations in the tetragonal state of uniaxial strained BaFe2-xNixAs2. The nematic correlations have similar temperature and doping dependence as resistivity anisotropy in detwinned samples, which suggests that they are intimately connected. Lastly, it investigates doping evolution of magnetic excitations in overdoped BaFe2-xNixAs2 and discusses the links with superconductivity. This book includes detailed neutron scattering results on BaFe2-xNixAs2 and an introduction to neutron scattering techniques, making it a useful guide for readers pursuing related research.

Xingye Lu received his B. Sc. in Physics from Hebei University, China in 2008, and completed his Ph.D. in experimental condensed matter physics at the Institute of Physics, Chinese Academy of Sciences in July 2014. His work involves the use of neutron scattering to study the magnetic ordering and spin dynamics of iron-based superconductors, supervised by Prof. Pengcheng Dai. He visited Prof. Pengcheng Dais group in Rice University from November 2014 to January 2015, prior to becoming a postdoctoral fellow (PSI fellow) working with Dr. Thorsten Schmitt at the Paul Scherrer Institut, Switzerland, where he focuses on measuring elementary excitations of correlated electron materials by means of resonant inelastic X-ray scattering.

Introduction.- Iron based superconductors.- Neutron scattering.- Phase diagram and incommensurate antiferromagnetic order of BaFe2-xNixAs2.- Origin of the incommensurate magnetic order in BaFe2-xNixAs2.- Nematic spin correlations in detwinned BaFe2-xNixAs2.- Magnetic excitations in overdoped BaFe2-xNixAs2.- Summary and outlook.- References.- Publications.- Resume.- Acknowledgements.