Three- to Two-Dimensional Transition of the Electronic Structure in CaFe₂As₂: A Parent Compound for an Iron Arsenic High-Temperature Superconductor

We use angle-resolved photoemission spectroscopy (ARPES) to study the electronic properties of CaFe₂As₂—parent compound of a pnictide superconductor. We find that the structural and magnetic transition is accompanied by a three- to two-dimensional (3D-2D) crossover in the electronic structure. Above the transition temperature (T_s) Fermi surfaces around Γ and X points are cylindrical and quasi 2D. Below T_s, the Γ pocket forms a 3D ellipsoid, while the X pocket remains quasi 2D. This finding strongly suggests that low dimensionality plays an important role in understanding the superconducting mechanism in pnictides.