  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 (Ts) Fermi surfaces around Gamma
and X points are cylindrical and quasi-2D. Below Ts the former becomes a 3D
ellipsoid, while the latter remains quasi-2D. This finding strongly suggests
that low dimensionality plays an important role in understanding the
superconducting mechanism in pnictides.
