Simple real-space picture of nodeless and nodal s-wave gap functions in iron pnictide superconductors

We propose a simple method of parameterizing the gap function in iron pnictides. The key idea is to use orbital representation, not band representation, and to assume real-space short-range pairing. Our parameterization reproduces the structure of the gap function obtained in microscopic calculation fairly well. At the same time, the present parametrization is simple enough to obtain an intuitive picture of the gap function and to develop a phenomenological theory. We also discuss the simplification of the treatment of the superconducting state. (author)