Nematic Electronic Structure in the "Parent" State of the Iron-Based
Superconductor Ca(Fe_1-xCo_x)₂As₂

Tieng-Ming Chuang
Cornell and High Magnetic Field Laboratory of Florida State University

The mechanism of high-temperature superconductivity in the newly discovered iron-based superconductors is unresolved. We use spectroscopic imaging–scanning tunneling microscopy to study the electronic structure of a representative compound CaFe_1.94Co_0.06As₂ in  the "parent" state from which this superconductivity emerges. Static, unidirectional electronic nanostructures of dimension eight times the inter–iron-atom distance a(Fe-Fe) and aligned along the crystal a axis  are observed. In contrast, the delocalized electronic states detectable  by quasiparticle interference imaging are dispersive along the b axis  only and are consistent with a nematic alpha2 band with an apparent band  folding having wave vector q~+/-2p/8a(Fe-Fe) along the a axis. All these effects rotate through 90° at orthorhombic twin boundaries, indicating that they are bulk properties. As none of these phenomena are expected merely due to crystal symmetry, underdoped ferropnictides may exhibit a more complex electronic nematic state than originally expected.  

Science 327, 181 (2010).