Discovery of the Nematic Parent State and the Anisotropic Energy Gap Structure of Iron-based High-Tc Superconductors
Iron-based superconductors reveal new routes towards understanding high-temperature superconductivity. I will discuss our discovery of strong electronic nematicity in the parent state of iron-based superconductors , and the recent realization of how dopant-atom induced unidirectional impurity states in this nematic environment, and how anisotropic scattering in general likely influences the mysterious anisotropic transport characteristics in this material . Then I will describe our exploration of the superconducting energy gaps in the canonical Fe-based superconductor LiFeAs .
If strong electron-electron interactions between neighboring Fe atoms mediate the Cooper pairing in iron-pnictide superconductors, then specific and distinct anisotropic superconducting energy gaps Di(k) should appear on the different electronic bands i. We introduced intra-band Bogoliubov quasiparticle scattering interference (QPI) techniques for determination of Di(k) in such materials. We identify three hole-like bands and determine the anisotropy, magnitude and relative orientations of their Di(k). Such data could play a key role in identifying the Cooper pairing mechanism of iron-based superconductivity.
 Nematic electronic structure in the ‘parent’ state of the iron-based superconductor Ca(Fe1-xCox)2As2. T.-M. Chuang & M.P. Allan, et al Science 327, 181 (2010).
 Nanoscale electronic-dimer scattering and the mechanism of electronic anisotropy in underdoped iron-arsenides. M. P. Allan & T.-M. Chuang et al, Submitted (2011).
 Anisotropic energy-gaps of iron-based superconductivity from intra-band quasiparticle interference in LiFeAs. M. P. Allan & A.W. Rost et al, Science 336, 563 (2012).