Pairing Operators in Multiorbital Systems*

The discovery of high-$T_c$ superconductivity in the pnictides highlights the need to understand how superconductivity arises in multiorbital systems. In a simple two orbital model we discuss how strong hybridization between orbitals may lead to both intra and interband pairing [1]. Next a three orbital model for the pnictides is considered where hybridization between the As $p$ orbitals and three of the Fe 3$d$ orbitals is described via the Slater-Koster approach [2]. Spin-singlet pairing operators allowed by lattice and orbital symmetries are constructed and indexed. It is found that the only fully gapped and purely intraband spin-singlet pairing operator arises if the electrons in different orbitals couple with equal strength to the source of pairing. Other pairing operators are discussed as well, including $S_{\pm}$. Spectral functions are calculated in both investigations to guide experimental work.\\[4pt] [1] A. Moreo {\it et. al.}, Phys. Rev. {\bf B80}, 104507 (2009). \\[0pt] [2] M. Daghofer {\it et al.},arXiv:0910.1573v1.